path: root/doc/rfc/rfc8994.txt

Internet Engineering Task Force (IETF)                    T. Eckert, Ed.
Request for Comments: 8994                                 Futurewei USA
Category: Standards Track                              M. Behringer, Ed.
ISSN: 2070-1721                                                         
                                                            S. Bjarnason
                                                          Arbor Networks
                                                                May 2021


                    An Autonomic Control Plane (ACP)

Abstract

   Autonomic functions need a control plane to communicate, which
   depends on some addressing and routing.  This Autonomic Control Plane
   should ideally be self-managing and be as independent as possible of
   configuration.  This document defines such a plane and calls it the
   "Autonomic Control Plane", with the primary use as a control plane
   for autonomic functions.  It also serves as a "virtual out-of-band
   channel" for Operations, Administration, and Management (OAM)
   communications over a network that provides automatically configured,
   hop-by-hop authenticated and encrypted communications via
   automatically configured IPv6 even when the network is not configured
   or is misconfigured.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8994.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction (Informative)
     1.1.  Applicability and Scope
   2.  Acronyms and Terminology (Informative)
   3.  Use Cases for an Autonomic Control Plane (Informative)
     3.1.  An Infrastructure for Autonomic Functions
     3.2.  Secure Bootstrap over an Unconfigured Network
     3.3.  Permanent Reachability Independent of the Data Plane
   4.  Requirements (Informative)
   5.  Overview (Informative)
   6.  Self-Creation of an Autonomic Control Plane (ACP) (Normative)
     6.1.  Requirements for the Use of Transport Layer Security (TLS)
     6.2.  ACP Domain, Certificate, and Network
       6.2.1.  ACP Certificates
       6.2.2.  ACP Certificate AcpNodeName
         6.2.2.1.  AcpNodeName ASN.1 Module
       6.2.3.  ACP Domain Membership Check
         6.2.3.1.  Realtime Clock and Time Validation
       6.2.4.  Trust Anchors (TA)
       6.2.5.  Certificate and Trust Anchor Maintenance
         6.2.5.1.  GRASP Objective for EST Server
         6.2.5.2.  Renewal
         6.2.5.3.  Certificate Revocation Lists (CRLs)
         6.2.5.4.  Lifetimes
         6.2.5.5.  Reenrollment
         6.2.5.6.  Failing Certificates
     6.3.  ACP Adjacency Table
     6.4.  Neighbor Discovery with DULL GRASP
     6.5.  Candidate ACP Neighbor Selection
     6.6.  Channel Selection
     6.7.  Candidate ACP Neighbor Verification
     6.8.  Security Association (Secure Channel) Protocols
       6.8.1.  General Considerations
       6.8.2.  Common Requirements
       6.8.3.  ACP via IPsec
         6.8.3.1.  Native IPsec
           6.8.3.1.1.  RFC 8221 (IPsec/ESP)
           6.8.3.1.2.  RFC 8247 (IKEv2)
         6.8.3.2.  IPsec with GRE Encapsulation
       6.8.4.  ACP via DTLS
       6.8.5.  ACP Secure Channel Profiles
     6.9.  GRASP in the ACP
       6.9.1.  GRASP as a Core Service of the ACP
       6.9.2.  ACP as the Security and Transport Substrate for GRASP
         6.9.2.1.  Discussion
     6.10. Context Separation
     6.11. Addressing inside the ACP
       6.11.1.  Fundamental Concepts of Autonomic Addressing
       6.11.2.  The ACP Addressing Base Scheme
       6.11.3.  ACP Zone Addressing Sub-Scheme (ACP-Zone)
       6.11.4.  ACP Manual Addressing Sub-Scheme (ACP-Manual)
       6.11.5.  ACP Vlong Addressing Sub-Scheme (ACP-Vlong-8/
               ACP-Vlong-16)
       6.11.6.  Other ACP Addressing Sub-Schemes
       6.11.7.  ACP Registrars
         6.11.7.1.  Use of BRSKI or Other Mechanisms or Protocols
         6.11.7.2.  Unique Address/Prefix Allocation
         6.11.7.3.  Addressing Sub-Scheme Policies
         6.11.7.4.  Address/Prefix Persistence
         6.11.7.5.  Further Details
     6.12. Routing in the ACP
       6.12.1.  ACP RPL Profile
         6.12.1.1.  Overview
           6.12.1.1.1.  Single Instance
           6.12.1.1.2.  Reconvergence
         6.12.1.2.  RPL Instances
         6.12.1.3.  Storing vs. Non-Storing Mode
         6.12.1.4.  DAO Policy
         6.12.1.5.  Path Metrics
         6.12.1.6.  Objective Function
         6.12.1.7.  DODAG Repair
         6.12.1.8.  Multicast
         6.12.1.9.  Security
         6.12.1.10. P2P Communications
         6.12.1.11. IPv6 Address Configuration
         6.12.1.12. Administrative Parameters
         6.12.1.13. RPL Packet Information
         6.12.1.14. Unknown Destinations
     6.13. General ACP Considerations
       6.13.1.  Performance
       6.13.2.  Addressing of Secure Channels
       6.13.3.  MTU
       6.13.4.  Multiple Links between Nodes
       6.13.5.  ACP Interfaces
         6.13.5.1.  ACP Loopback Interfaces
         6.13.5.2.  ACP Virtual Interfaces
           6.13.5.2.1.  ACP Point-to-Point Virtual Interfaces
           6.13.5.2.2.  ACP Multi-Access Virtual Interfaces
   7.  ACP Support on L2 Switches/Ports (Normative)
     7.1.  Why (Benefits of ACP on L2 Switches)
     7.2.  How (per L2 Port DULL GRASP)
   8.  Support for Non-ACP Components (Normative)
     8.1.  ACP Connect
       8.1.1.  Non-ACP Controller and/or Network Management System
               (NMS)
       8.1.2.  Software Components
       8.1.3.  Autoconfiguration
       8.1.4.  Combined ACP and Data Plane Interface (VRF Select)
       8.1.5.  Use of GRASP
     8.2.  Connecting ACP Islands over Non-ACP L3 Networks (Remote ACP
           Neighbors)
       8.2.1.  Configured Remote ACP Neighbor
       8.2.2.  Tunneled Remote ACP Neighbor
       8.2.3.  Summary
   9.  ACP Operations (Informative)
     9.1.  ACP (and BRSKI) Diagnostics
       9.1.1.  Secure Channel Peer Diagnostics
     9.2.  ACP Registrars
       9.2.1.  Registrar Interactions
       9.2.2.  Registrar Parameters
       9.2.3.  Certificate Renewal and Limitations
       9.2.4.  ACP Registrars with Sub-CA
       9.2.5.  Centralized Policy Control
     9.3.  Enabling and Disabling the ACP and/or the ANI
       9.3.1.  Filtering for Non-ACP/ANI Packets
       9.3.2.  "admin down" State
         9.3.2.1.  Security
         9.3.2.2.  Fast State Propagation and Diagnostics
         9.3.2.3.  Low-Level Link Diagnostics
         9.3.2.4.  Power Consumption Issues
       9.3.3.  Enabling Interface-Level ACP and ANI
       9.3.4.  Which Interfaces to Auto-Enable?
       9.3.5.  Enabling Node-Level ACP and ANI
         9.3.5.1.  Brownfield Nodes
         9.3.5.2.  Greenfield Nodes
       9.3.6.  Undoing "ANI/ACP enable"
       9.3.7.  Summary
     9.4.  Partial or Incremental Adoption
     9.5.  Configuration and the ACP (Summary)
   10. Summary: Benefits (Informative)
     10.1.  Self-Healing Properties
     10.2.  Self-Protection Properties
       10.2.1.  From the Outside
       10.2.2.  From the Inside
     10.3.  The Administrator View
   11. Security Considerations
   12. IANA Considerations
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Appendix A.  Background and Future (Informative)
     A.1.  ACP Address Space Schemes
     A.2.  BRSKI Bootstrap (ANI)
     A.3.  ACP Neighbor Discovery Protocol Selection
       A.3.1.  LLDP
       A.3.2.  mDNS and L2 Support
       A.3.3.  Why DULL GRASP?
     A.4.  Choice of Routing Protocol (RPL)
     A.5.  ACP Information Distribution and Multicast
     A.6.  CAs, Domains, and Routing Subdomains
     A.7.  Intent for the ACP
     A.8.  Adopting ACP Concepts for Other Environments
     A.9.  Further (Future) Options
       A.9.1.  Auto-Aggregation of Routes
       A.9.2.  More Options for Avoiding IPv6 Data Plane Dependencies
       A.9.3.  ACP APIs and Operational Models (YANG)
       A.9.4.  RPL Enhancements
       A.9.5.  Role Assignments
       A.9.6.  Autonomic L3 Transit
       A.9.7.  Diagnostics
       A.9.8.  Avoiding and Dealing with Compromised ACP Nodes
       A.9.9.  Detecting ACP Secure Channel Downgrade Attacks
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction (Informative)

   Autonomic Networking is a concept of self-management: autonomic
   functions self-configure, and negotiate parameters and settings
   across the network.  "Autonomic Networking: Definitions and Design
   Goals" [RFC7575] defines the fundamental ideas and design goals of
   Autonomic Networking.  A gap analysis of Autonomic Networking is
   given in "General Gap Analysis for Autonomic Networking" [RFC7576].
   The reference architecture for Autonomic Networking in the IETF is
   specified in the document "A Reference Model for Autonomic
   Networking" [RFC8993].

   Autonomic functions need an autonomically built communications
   infrastructure.  This infrastructure needs to be secure, resilient,
   and reusable by all autonomic functions.  Section 5 of [RFC7575]
   introduces that infrastructure and calls it the Autonomic Control
   Plane (ACP).  More descriptively, it could be called the "Autonomic
   communications infrastructure for OAM and control".  For naming
   consistency with that prior document, this document continues to use
   the name ACP.

   Today, the OAM and control plane of IP networks is what is typically
   called in-band management and/or signaling: its management and
   control protocol traffic depends on the routing and forwarding
   tables, security, policy, QoS, and potentially other configuration
   that first has to be established through the very same management and
   control protocols.  Misconfigurations, including unexpected side
   effects or mutual dependencies, can disrupt OAM and control
   operations and especially disrupt remote management access to the
   affected node itself and potentially disrupt access to a much larger
   number of nodes for which the affected node is on the network path.

   For an example of in-band management failing in the face of operator-
   induced misconfiguration, see [FCC], for example, Section III.B.15 on
   page 8:

   |  ...engineers almost immediately recognized that they had
   |  misdiagnosed the problem.  However, they were unable to resolve
   |  the issue by restoring the link because the network management
   |  tools required to do so remotely relied on the same paths they had
   |  just disabled.

   Traditionally, physically separate, so-called out-of-band
   (management) networks have been used to avoid these problems or at
   least to allow recovery from such problems.  In the worst-case
   scenario, personnel are sent on site to access devices through out-
   of-band management ports (also called craft ports, serial consoles,
   or management Ethernet ports).  However, both options are expensive.

   In increasingly automated networks, both centralized management
   systems and distributed autonomic service agents in the network
   require a control plane that is independent of the configuration of
   the network they manage, to avoid impacting their own operations
   through the configuration actions they take.

   This document describes a modular design for a self-forming, self-
   managing, and self-protecting ACP, which is a virtual out-of-band
   network designed to be as independent as possible of configuration,
   addressing, and routing to avoid the self-dependency problems of
   current IP networks while still operating in-band on the same
   physical network that it is controlling and managing.  The ACP design
   is therefore intended to combine as well as possible the resilience
   of out-of-band management networks with the low cost of traditional
   IP in-band network management.  The details of how this is achieved
   are described in Section 6.

   In a fully Autonomic Network without legacy control or management
   functions and/or protocols, the data plane would be just a forwarding
   plane for "data" IPv6 packets, which are packets other than those
   control and management plane packets forwarded by the ACP itself.  In
   such a network, there would be no non-autonomous control of nodes nor
   a non-autonomous management plane.

   Routing protocols would be built inside the ACP as autonomous
   functions via autonomous service agents, leveraging the following ACP
   functions instead of implementing them separately for each protocol:
   discovery; automatically established, authenticated, and encrypted
   local and distant peer connectivity for control and management
   traffic; and common session and presentation functions of the control
   and management protocol.

   When ACP functionality is added to nodes that do not have autonomous
   management plane and/or control plane functions (henceforth called
   non-autonomous nodes), the ACP instead is best abstracted as a
   special Virtual Routing and Forwarding (VRF) instance (or virtual
   router), and the complete, preexisting, non-autonomous management
   and/or control plane is considered to be part of the data plane to
   avoid introducing more complex terminology only for this case.

   Like the forwarding plane for "data" packets, the non-autonomous
   control and management plane functions can then be managed and/or
   used via the ACP.  This terminology is consistent with preexisting
   documents such as "Using an Autonomic Control Plane for Stable
   Connectivity of Network Operations, Administration, and Maintenance
   (OAM)" [RFC8368].

   In both autonomous and non-autonomous instances, the ACP is built
   such that it operates in the absence of the data plane.  The ACP also
   operates in the presence of any (mis)configured non-autonomous
   management and/or control components in the data plane.

   The ACP serves several purposes simultaneously:

   1.  Autonomic functions communicate over the ACP.  The ACP therefore
       directly supports Autonomic Networking functions, as described in
       [RFC8993].  For example, GRASP ("GeneRic Autonomic Signaling
       Protocol (GRASP)" [RFC8990]) runs securely inside the ACP and
       depends on the ACP as its "security and transport substrate".

   2.  A controller or network management system can use ACP to securely
       bootstrap network devices in remote locations, even if the (data
       plane) network in between is not yet configured; no bootstrap
       configuration that is dependent on the data plane is required.
       An example of such a secure bootstrap process is described in
       "Bootstrapping Remote Secure Key Infrastructure (BRSKI)"
       [RFC8995].

   3.  An operator can use ACP to access remote devices using protocols
       such as Secure SHell (SSH) or Network Configuration Protocol
       (NETCONF), even if the network is misconfigured or unconfigured.

   This document describes these purposes as use cases for the ACP in
   Section 3, and it defines the requirements in Section 4.  Section 5
   gives an overview of how the ACP is constructed.

   The normative part of this document starts with Section 6, where the
   ACP is specified.  Section 7 explains how to support ACP on Layer 2
   (L2) switches (normative).  Section 8 explains how non-ACP nodes and
   networks can be integrated (normative).

   The remaining sections are non-normative.  Section 10 reviews the
   benefits of the ACP (after all the details have been defined).
   Section 9 provides operational recommendations.  Appendix A provides
   additional background and describes possible extensions that were not
   applicable for this specification but were considered important to
   document.  There are no dependencies on Appendix A in order to build
   a complete working and interoperable ACP according to this document.

   The ACP provides secure IPv6 connectivity; therefore, it can be used
   for secure connectivity not only for self-management as required for
   the ACP in [RFC7575] but also for traditional (centralized)
   management.  The ACP can be implemented and operated without any
   other components of Autonomic Networks, except for GRASP.  ACP relies
   on per-link Discovery Unsolicited Link-Local (DULL) GRASP (see
   Section 6.4) to auto-discover ACP neighbors and includes the ACP
   GRASP instance to provide service discovery for clients of the ACP
   (see Section 6.9), including for its own maintenance of ACP
   certificates.

   The document [RFC8368] describes how the ACP can be used alone to
   provide secure and stable connectivity for autonomic and non-
   autonomic OAM applications, specifically for the case of current non-
   autonomic networks and/or nodes.  That document also explains how
   existing management solutions can leverage the ACP in parallel with
   traditional management models, when to use the ACP, and how to
   integrate with potentially IPv4-only OAM backends.

   Combining ACP with Bootstrapping Remote Secure Key Infrastructure
   (BRSKI) (see [RFC8995]) results in the "Autonomic Network
   Infrastructure" (ANI) as defined in [RFC8993], which provides
   autonomic connectivity (from ACP) with secure zero-touch (automated)
   bootstrap from BRSKI.  The ANI itself does not constitute an
   Autonomic Network, but it allows the building of more or less
   Autonomic Networks on top of it, using either centralized automation
   in SDN style (see "Software-Defined Networking (SDN): Layers and
   Architecture Terminology" [RFC7426]) or distributed automation via
   Autonomic Service Agents (ASA) and/or Autonomic Functions (AF), or a
   mixture of both.  See [RFC8993] for more information.

1.1.  Applicability and Scope

   Please see the following Terminology section (Section 2) for
   explanations of terms used in this section.

   The design of the ACP as defined in this document is considered to be
   applicable to all types of "professionally managed" networks: Service
   Provider, Local Area Network (LAN), Metropolitan Area Network (MAN/
   Metro), Wide Area Network (WAN), Enterprise Information Technology
   (IT) and Operational Technology (OT) networks.  The ACP can operate
   equally on Layer 3 (L3) equipment and on L2 equipment such as bridges
   (see Section 7).  The hop-by-hop authentication, integrity
   protection, and confidentiality mechanism used by the ACP is defined
   to be negotiable; therefore, it can be extended to environments with
   different protocol preferences.  The minimum implementation
   requirements in this document attempt to achieve maximum
   interoperability by requiring support for multiple options depending
   on the type of device: IPsec (see "Security Architecture for the
   Internet Protocol" [RFC4301]) and Datagram Transport Layer Security
   (DTLS, see Section 6.8.4).

   The implementation footprint of the ACP consists of Public Key
   Infrastructure (PKI) code for the ACP certificate including EST (see
   "Enrollment over Secure Transport" [RFC7030]), GRASP, UDP, TCP, and
   Transport Layer Security (TLS, see Section 6.1).  For more
   information regarding the security and reliability of GRASP and for
   EST, the ACP secure channel protocol used (such as IPsec or DTLS),
   and an instance of IPv6 packet forwarding and routing via RPL, see
   "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks"
   [RFC6550], which is separate from routing and forwarding for the data
   plane (user traffic).

   The ACP uses only IPv6 to avoid the complexity of dual-stack (both
   IPv6 and IPv4) ACP operations.  Nevertheless, it can be integrated
   without any changes to otherwise IPv4-only network devices.  The data
   plane itself would not need to change, and it could continue to be
   IPv4 only.  For such IPv4-only devices, IPv6 itself would be
   additional implementation footprint that is only required for the
   ACP.

   The protocol choices of the ACP are primarily based on wide use and
   support in networks and devices, well-understood security properties,
   and required scalability.  The ACP design is an attempt to produce
   the lowest risk combination of existing technologies and protocols to
   build a widely applicable, operational network management solution.

   RPL was chosen because it requires a smaller routing table footprint
   in large networks compared to other routing protocols with an
   autonomically configured single area.  The deployment experience of
   large-scale Internet of Things (IoT) networks serves as the basis for
   wide deployment experience with RPL.  The profile chosen for RPL in
   the ACP does not leverage any RPL-specific forwarding plane features
   (IPv6 extension headers), making its implementation a pure control
   plane software requirement.

   GRASP is the only completely novel protocol used in the ACP, and this
   choice was necessary because there is no existing protocol suitable
   for providing the necessary functions to the ACP, so GRASP was
   developed to fill that gap.

   The ACP design can be applicable to devices constrained with respect
   to CPU and memory, and to networks constrained with respect to
   bitrate and reliability, but this document does not attempt to define
   the most constrained type of devices or networks to which the ACP is
   applicable.  RPL and DTLS for ACP secure channels are two protocol
   choices already making ACP more applicable to constrained
   environments.  Support for constrained devices in this specification
   is opportunistic, but not complete, because the reliable transport
   for GRASP (see Section 6.9.2) only specifies TCP/TLS.  See
   Appendix A.8 for discussions about how future standards or
   proprietary extensions and/or variations of the ACP could better meet
   expectations that are different from those upon which the current
   design is based, including supporting constrained devices better.

2.  Acronyms and Terminology (Informative)

   This document serves both as a normative specification for ACP node
   behavior as well as an explanation of the context by providing
   descriptions of requirements, benefits, architecture, and operational
   aspects.  Normative sections are labeled "(Normative)" and use BCP 14
   keywords.  Other sections are labeled "(Informative)" and do not use
   those normative keywords.

   In the rest of the document, we will refer to systems that use the
   ACP as "nodes".  Typically, such a node is a physical (network
   equipment) device, but it can equally be some virtualized system.
   Therefore, we do not refer to them as devices unless the context
   specifically calls for a physical system.

   This document introduces or uses the following terms (sorted
   alphabetically).  Introduced terms are explained on first use, so
   this list is for reference only.

   ACP:  Autonomic Control Plane.  The autonomic function as defined in
      this document.  It provides secure, zero-touch (automated)
      transitive (network-wide) IPv6 connectivity for all nodes in the
      same ACP domain as well as a GRASP instance running across this
      ACP IPv6 connectivity.  The ACP is primarily meant to be used as a
      component of the ANI to enable Autonomic Networks, but it can
      equally be used in simple ANI networks (with no other autonomic
      functions) or completely by itself.

   ACP address:  An IPv6 address assigned to the ACP node.  It is stored
      in the acp-node-name of the ACP certificate.

   ACP address range or set:  The ACP address may imply a range or set
      of addresses that the node can assign for different purposes.
      This address range or set is derived by the node from the format
      of the ACP address called the addressing sub-scheme.

   ACP certificate:  A Local Device IDentity (LDevID) certificate
      conforming to "Internet X.509 Public Key Infrastructure
      Certificate and Certificate Revocation List (CRL) Profile"
      [RFC5280] that carries the acp-node-name, which is used by the ACP
      to learn its address in the ACP and to derive and
      cryptographically assert its membership in the ACP domain.  In the
      context of the ANI, the ACP certificate is also called the ANI
      certificate.  In the context of AN, the ACP certificate is also
      called the AN certificate.

   ACP connect interface:  An interface on an ACP node that provides
      access to the ACP for non-ACP-capable nodes without using an ACP
      secure channel.  See Section 8.1.1.

   ACP domain:  The ACP domain is the set of nodes with ACP certificates
      that allow them to authenticate each other as members of the ACP
      domain.  See also Section 6.2.3.

   ACP loopback interface:  The loopback interface in the ACP VRF that
      has the ACP address assigned to it.  See Section 6.13.5.1.

   ACP network:  The ACP network comprises all the nodes that have
      access to the ACP.  It is the set of active and transitively
      connected nodes of an ACP domain plus all nodes that get access to
      the ACP of that domain via ACP edge nodes.

   ACP (ULA) prefix(es):  The /48 IPv6 address prefixes used across the
      ACP.  In the normal or simple case, the ACP has one Unique Local
      Address (ULA) prefix, see Section 6.11.  The ACP routing table may
      include multiple ULA prefixes if the rsub option is used to create
      addresses from more than one ULA prefix.  See Section 6.2.2.  The
      ACP may also include non-ULA prefixes if those are configured on
      ACP connect interfaces.  See Section 8.1.1.

   ACP secure channel:  A channel authenticated via ACP certificates
      providing integrity protection and confidentiality through
      encryption.  These channels are established between (normally)
      adjacent ACP nodes to carry ACP VRF traffic in-band over the same
      links and paths as data plane traffic but isolate it from the data
      plane traffic and secure it.

   ACP secure channel protocol:  The protocol used to build an ACP
      secure channel, e.g., Internet Key Exchange Protocol version 2
      (IKEv2) with IPsec or DTLS.

   ACP virtual interface:  An interface in the ACP VRF mapped to one or
      more ACP secure channels.  See Section 6.13.5.

   acp-node-name field:  An information field in the ACP certificate in
      which the following ACP-relevant information is encoded: the ACP
      domain name, the ACP IPv6 address of the node, and optional
      additional role attributes about the node.

   AN:  Autonomic Network.  A network according to [RFC8993].  Its main
      components are ANI, autonomic functions, and Intent.

   (AN) Domain Name:  An FQDN (Fully Qualified Domain Name) in the acp-
      node-name of the domain certificate.  See Section 6.2.2.

   ANI (nodes/network):  Autonomic Network Infrastructure.  The ANI is
      the infrastructure to enable Autonomic Networks.  It includes ACP,
      BRSKI, and GRASP.  Every Autonomic Network includes the ANI, but
      not every ANI network needs to include autonomic functions beyond
      the ANI (nor Intent).  An ANI network without further autonomic
      functions can, for example, support secure zero-touch (automated)
      bootstrap and stable connectivity for SDN networks, see [RFC8368].

   ANIMA:  Autonomic Networking Integrated Model and Approach.  ACP,
      BRSKI, and GRASP are specifications of the IETF ANIMA Working
      Group.

   ASA:  Autonomic Service Agent.  Autonomic software modules running on
      an ANI device.  The components making up the ANI (BRSKI, ACP, and
      GRASP) are also described as ASAs.

   autonomic function:  A function and/or service in an Autonomic
      Network (AN) composed of one or more ASAs across one or more ANI
      nodes.

   BRSKI:  Bootstrapping Remote Secure Key Infrastructure [RFC8995].  A
      protocol extending EST to enable secure zero-touch bootstrap in
      conjunction with ACP.  ANI nodes use ACP, BRSKI, and GRASP.

   CA:  Certification Authority.  An entity that issues digital
      certificates.  A CA uses its private key to sign the certificates
      it issues.  Relying parties use the public key in the CA
      certificate to validate the signature.

   CRL:  Certificate Revocation List.  A list of revoked certificates is
      required to revoke certificates before their lifetime expires.

   data plane:  The counterpoint to the ACP VRF in an ACP node: the
      forwarding of user traffic in non-autonomous nodes and/or networks
      and also any non-autonomous control and/or management plane
      functions.  In a fully Autonomic Network node, the data plane is
      managed autonomically via autonomic functions and Intent.  See
      Section 1 for more details.

   device:  A physical system or physical node.

   enrollment:  The process by which a node authenticates itself to a
      network with an initial identity, which is often called an Initial
      Device IDentity (IDevID) certificate, and acquires from the
      network a network-specific identity, which is often called an
      LDevID certificate, and certificates of one or more trust
      anchor(s).  In the ACP, the LDevID certificate is called the ACP
      certificate.

   EST:  Enrollment over Secure Transport [RFC7030].  IETF Standards
      Track protocol for enrollment of a node with an LDevID
      certificate.  BRSKI is based on EST.

   GRASP:  GeneRic Autonomic Signaling Protocol.  An extensible
      signaling protocol required by the ACP for ACP neighbor discovery.

      The ACP also provides the "security and transport substrate" for
      the "ACP instance of GRASP".  This instance of GRASP runs across
      the ACP secure channels to support BRSKI and other NOC and/or OAM
      or autonomic functions.  See [RFC8990].

   IDevID:  An Initial Device IDentity X.509 certificate installed by
      the vendor on new equipment.  The IDevID certificate contains
      information that establishes the identity of the node in the
      context of its vendor and/or manufacturer such as device model
      and/or type and serial number.  See [AR8021].  The IDevID
      certificate cannot be used as a node identifier for the ACP
      because they are not provisioned by the owner of the network, so
      they can not directly indicate an ACP domain they belong to.

   in-band (as in management or signaling):  In-band management traffic
      and/or control plane signaling uses the same network resources
      such as routers and/or switches and network links that it manages
      and/or controls.  In-band is the standard management and signaling
      mechanism in IP networks.  Compared to out-of-band, the in-band
      mechanism requires no additional physical resources, but it
      introduces potentially circular dependencies for its correct
      operations.  See Section 1.

   Intent:  The policy language of an Autonomic Network according to
      [RFC8993].

   Loopback interface:  See ACP loopback interface.

   LDevID:  A Local Device IDentity is an X.509 certificate installed
      during enrollment.  The domain certificate used by the ACP is an
      LDevID certificate.  See [AR8021].

   management:  Used in this document as another word for OAM.

   MASA (service):  Manufacturer Authorized Signing Authority.  A vendor
      and/or manufacturer or delegated cloud service on the Internet
      used as part of the BRSKI protocol.

   MIC:  Manufacturer Installed Certificate.  A synonym for an IDevID in
      referenced materials.  This term is not used in this document.

   native interface:  Interfaces existing on a node without
      configuration of the already running node.  On physical nodes,
      these are usually physical interfaces; on virtual nodes, their
      equivalent.

   NOC:  Network Operations Center.

   node:  A system supporting the ACP according to this document.  A
      node can be virtual or physical.  Physical nodes are called
      devices.

   Node-ID:  The identifier of an ACP node inside that ACP.  It is
      either the last 64 bits (see Section 6.11.3) or 78 bits (see
      Section 6.11.5) of the ACP address.

   OAM:  Operations, Administration, and Management.  Includes network
      monitoring.

   Operational Technology (OT):  "The hardware and software dedicated to
      detecting or causing changes in physical processes through direct
      monitoring and/or control of physical devices such as valves,
      pumps, etc."  [OP-TECH].  In most cases today, OT networks are
      well separated from Information Technology (IT) networks.

   out-of-band (management) network:  An out-of-band network is a
      secondary network used to manage a primary network.  The equipment
      of the primary network is connected to the out-of-band network via
      dedicated management ports on the primary network equipment.
      Serial (console) management ports were historically most common;
      however, higher-end network equipment now also has Ethernet ports
      dedicated only to management.  An out-of-band network provides
      management access to the primary network independent of the
      configuration state of the primary network.  See Section 1.

   out-of-band network, virtual:  The ACP can be called a virtual out-
      of-band network for management and control because it attempts to
      provide the benefits of a (physical) out-of-band network even
      though it is physically carried in-band.  See Section 1.

   root CA:  root Certification Authority.  A CA for which the root CA
      key update procedures of [RFC7030], Section 4.4, can be applied.

   RPL:  IPv6 Routing Protocol for Low-Power and Lossy Networks.  The
      routing protocol used in the ACP.  See [RFC6550].

   registrar (ACP, ANI/BRSKI):  An ACP registrar is an entity (software
      and/or person) that orchestrates the enrollment of ACP nodes with
      the ACP certificate.  ANI nodes use BRSKI, so ANI registrars are
      also called BRSKI registrars.  For non-ANI ACP nodes, the
      registrar mechanisms are not defined in this document.  See
      Section 6.11.7.  Renewal and other maintenance (such as
      revocation) of ACP certificates may be performed by entities other
      than registrars.  EST must be supported for ACP certificate
      renewal (see Section 6.2.5).  BRSKI is an extension of EST, so
      ANI/BRSKI registrars can easily support ACP domain certificate
      renewal in addition to initial enrollment.

   RPI:  RPL Packet Information.  Network extension headers for use with
      RPL.  Not used with RPL in the ACP.  See Section 6.12.1.13.

   RPL:  Routing Protocol for Low-Power and Lossy Networks.  The routing
      protocol used in the ACP.  See Section 6.12.

   sUDI:  secured Unique Device Identifier.  This is a synonym of IDevID
      in referenced material.  This term is not used in this document.

   TA:  Trust Anchor.  A TA is an entity that is trusted for the purpose
      of certificate validation.  TA information such as self-signed
      certificate(s) of the TA is configured into the ACP node as part
      of enrollment.  See [RFC5280], Section 6.1.1.

   UDI:  Unique Device Identifier.  In the context of this document,
      unsecured identity information of a node typically consists of at
      least a device model and/or type and a serial number, often in a
      vendor-specific format.  See sUDI and LDevID.

   ULA (Global ID prefix):  A Unique Local Address is an IPv6 address in
      the block fc00::/7, defined in "Unique Local IPv6 Unicast
      Addresses" [RFC4193].  ULA is the IPv6 successor of the IPv4
      private address space ("Address Allocation for Private Internets"
      [RFC1918]).  ULAs have important differences over IPv4 private
      addresses that are beneficial for and exploited by the ACP, such
      as the locally assigned Global ID prefix, which is the first 48
      bits of a ULA address [RFC4193], Section 3.2.1.  In this document,
      this prefix is abbreviated as "ULA prefix".

   (ACP) VRF:  The ACP is modeled in this document as a Virtual Routing
      and Forwarding instance.  This means that it is based on a
      "virtual router" consisting of a separate IPv6 forwarding table to
      which the ACP virtual interfaces are attached and an associated
      IPv6 routing table separate from the data plane.  Unlike the VRFs
      on MPLS/VPN Provider Edge ("BGP/MPLS IP Virtual Private Networks
      (VPNs)" [RFC4364]) or LISP xTR ("The Locator/ID Separation
      Protocol (LISP)" [RFC6830]), the ACP VRF does not have any special
      "core facing" functionality or routing and/or mapping protocols
      shared across multiple VRFs.  In vendor products, a VRF such as
      the ACP VRF may also be referred to as a VRF-lite.

   (ACP) Zone:  An ACP zone is a set of ACP nodes using the same zone
      field value in their ACP address according to Section 6.11.3.
      Zones are a mechanism to support structured addressing of ACP
      addresses within the same /48 ULA prefix.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Use Cases for an Autonomic Control Plane (Informative)

   This section summarizes the use cases that are intended to be
   supported by an ACP.  To understand how these are derived from and
   relate to the larger set of use cases for Autonomic Networks, please
   refer to "Autonomic Networking Use Case for Distributed Detection of
   Service Level Agreement (SLA) Violations" [RFC8316].

3.1.  An Infrastructure for Autonomic Functions

   Autonomic functions need a stable infrastructure to run on, and all
   autonomic functions should use the same infrastructure to minimize
   the complexity of the network.  In this way, there is only need for a
   single discovery mechanism, a single security mechanism, and single
   instances of other processes that distributed functions require.

3.2.  Secure Bootstrap over an Unconfigured Network

   Today, bootstrapping a new node typically requires all nodes between
   a controlling node such as an SDN controller (see [RFC7426]) and the
   new node to be completely and correctly addressed, configured, and
   secured.  Bootstrapping and configuration of a network happens in
   rings around the controller -- configuring each ring of devices
   before the next one can be bootstrapped.  Without console access (for
   example, through an out-of-band network), it is not possible today to
   make devices securely reachable before having configured the entire
   network leading up to them.

   With the ACP, secure bootstrap of new devices and whole new networks
   can happen without requiring any configuration of unconfigured
   devices along the path.  As long as all devices along the path
   support ACP and a zero-touch bootstrap mechanism such as BRSKI, the
   ACP across a whole network of unconfigured devices can be brought up
   without operator and/or provisioning intervention.  The ACP also
   offers additional security for any bootstrap mechanism because it can
   provide the encrypted discovery (via ACP GRASP) of registrars or
   other bootstrap servers by bootstrap proxies connecting to nodes that
   are to be bootstrapped.  The ACP encryption hides the identities of
   the communicating entities (pledge and registrar), making it more
   difficult to learn which network node might be attackable.  The ACP
   certificate can also be used to end-to-end encrypt the bootstrap
   communication between such proxies and server.  Note that
   bootstrapping here includes not only the first step that can be
   provided by BRSKI (secure keys), but also later stages where
   configuration is bootstrapped.

3.3.  Permanent Reachability Independent of the Data Plane

   Today, most critical control plane protocols and OAM protocols use
   the data plane of the network.  This leads to often undesirable
   dependencies between the control and OAM plane on one side and the
   data plane on the other: only if the forwarding and control plane of
   the data plane are configured correctly, will the data plane and the
   OAM and/or control plane work as expected.

   Data plane connectivity can be affected by errors and faults.
   Examples include misconfigurations that make AAA (Authentication,
   Authorization, and Accounting) servers unreachable or that can lock
   an administrator out of a device; routing or addressing issues can
   make a device unreachable; and shutting down interfaces over which a
   current management session is running can lock an administrator
   irreversibly out of the device.  Traditionally only out-of-band
   access via a serial console or Ethernet management port can help
   recover from such issues.

   Data plane dependencies also affect applications in a NOC such as SDN
   controller applications: certain network changes are hard to
   implement today because the change itself may affect reachability of
   the devices.  Examples include address or mask changes, routing
   changes, or security policies.  Today such changes require precise,
   hop-by-hop planning.

   Note that specific control plane functions for the data plane often
   depend on the ability to forward their packets via the data plane:
   sending aliveness and routing protocol signaling packets across the
   data plane to verify reachability, using IPv4 signaling packets for
   IPv4 routing and IPv6 signaling packets for IPv6 routing.

   Assuming appropriate implementation (see Section 6.13.2 for more
   details), the ACP provides reachability that is independent of the
   data plane.  This allows the control plane and OAM plane to operate
   more robustly:

   *  For management plane protocols, the ACP provides the functionality
      of a Virtual out-of-Band (VooB) channel, by providing connectivity
      to all nodes regardless of their data plane configuration, and
      routing and forwarding tables.

   *  For control plane protocols, the ACP allows their operation even
      when the data plane is temporarily faulty, or during transitional
      events, such as routing changes, which may affect the control
      plane at least temporarily.  This is specifically important for
      autonomic service agents, which could affect data plane
      connectivity.

   The document "Using Autonomic Control Plane for Stable Connectivity
   of Network OAM" [RFC8368] explains this use case for the ACP in
   significantly more detail and explains how the ACP can be used in
   practical network operations.

4.  Requirements (Informative)

   The following requirements were identified for the design of the ACP
   based on the above use cases (Section 3).  These requirements are
   informative.  The ACP as specified in the normative parts of this
   document is meeting or exceeding these use case requirements:

   ACP1:     The ACP should provide robust connectivity: as far as
             possible, it should be independent of configured
             addressing, configuration, and routing.  Requirements 2 and
             3 build on this requirement, but they also have value on
             their own.

   ACP2:     The ACP must have a separate address space from the data
             plane.  This separate address space provides traceability,
             ease of debugging, separation from data plane, and
             infrastructure security (filtering based on known address
             space).

   ACP3:     The ACP must use an autonomically managed address space.
             An autonomically managed address space provides ease of
             bootstrap and setup ("autonomic"), and robustness (the
             administrator cannot break network easily).  This document
             uses ULA for this purpose, see [RFC4193].

   ACP4:     The ACP must be generic, that is, it must be usable by all
             the functions and protocols of the ANI.  Clients of the ACP
             must not be tied to a particular application or transport
             protocol.

   ACP5:     The ACP must provide security: messages coming through the
             ACP must be authenticated to be from a trusted node, and it
             is very strongly recommended that they be encrypted.

   The explanation for ACP4 is as follows: in a fully Autonomic Network
   (AN), all newly written ASAs could potentially communicate with each
   other exclusively via GRASP, and if that were the only requirement
   for the ACP, it would not need to provide IPv6-layer connectivity
   between nodes, but only GRASP connectivity.  Nevertheless, because
   ACP also intends to support non-autonomous networks, it is crucial to
   support IPv6-layer connectivity across the ACP to support any
   transport-layer and application-layer protocols.

   The ACP operates hop-by-hop because this interaction can be built on
   IPv6 link-local addressing, which is autonomic, and has no dependency
   on configuration (requirement ACP1).  It may be necessary to have ACP
   connectivity across non-ACP nodes, for example, to link ACP nodes
   over the general Internet.  This is possible, but it introduces a
   dependency on stable and/or resilient routing over the non-ACP hops
   (see Section 8.2).

5.  Overview (Informative)

   When a node has an ACP certificate (see Section 6.2.1) and is enabled
   to bring up the ACP (see Section 9.3.5), it will create its ACP
   without any configuration as follows.  For details, see Section 6 and
   following sections:

   1.  The node creates a VRF instance or a similar virtual context for
       the ACP.

   2.  The node assigns its ULA IPv6 address (prefix) (see
       Section 6.11), which is learned from the acp-node-name (see
       Section 6.2.2) of its ACP certificate (see Section 6.2.1), to an
       ACP loopback interface (see Section 6.11) and connects this
       interface to the ACP VRF.

   3.  The node establishes a list of candidate peer adjacencies and
       candidate channel types to try for the adjacency.  This is
       automatic for all candidate link-local adjacencies (see
       Section 6.4) across all native interfaces (see Section 9.3.4).
       If a candidate peer is discovered via multiple interfaces, this
       will result in one adjacency per interface.  If the ACP node has
       multiple interfaces connecting to the same subnet across which it
       is also operating as an L2 switch in the data plane, it employs
       methods for ACP with L2 switching, see Section 7.

   4.  For each entry in the candidate adjacency list, the node
       negotiates a secure tunnel using the candidate channel types.
       See Section 6.6.

   5.  The node authenticates the peer node during secure channel setup
       and authorizes it to become part of the ACP according to
       Section 6.2.3.

   6.  Unsuccessful authentication of a candidate peer results in
       throttled connection retries for as long as the candidate peer is
       discoverable.  See Section 6.7.

   7.  Each successfully established secure channel is mapped to an ACP
       virtual interface, which is placed into the ACP VRF.  See
       Section 6.13.5.2.

   8.  Each node runs a lightweight routing protocol (see Section 6.12)
       to announce reachability of the ACP loopback address (or prefix)
       across the ACP.

   9.  This completes the creation of the ACP with hop-by-hop secure
       tunnels, auto-addressing, and auto-routing.  The node is now an
       ACP node with a running ACP.

   Note:

   *  None of the above operations (except the following explicitly
      configured ones) are reflected in the configuration of the node.

   *  Non-ACP network management systems (NMS) or SDN controllers have
      to be explicitly configured for connection to the ACP.

   *  Additional candidate peer adjacencies for ACP connections across
      non-ACP Layer 3 clouds requires explicit configuration.  See
      Section 8.2.

   Figure 1 illustrates the ACP.

             ACP Node 1                          ACP Node 2
          ...................               ...................
   secure .                 .   secure      .                 .  secure
   channel:  +-----------+  :   channel     :  +-----------+  : channel
   ..--------| ACP VRF   |---------------------| ACP VRF   |---------..
          : / \         / \   <--routing-->   / \         / \ :
          : \ /         \ /                   \ /         \ / :
   ..--------| loopback  |---------------------| loopback  |---------..
          :  | interface |  :               :  | interface |  :
          :  +-----------+  :               :  +-----------+  :
          :                 :               :                 :
          :   Data Plane    :...............:   Data Plane    :
          :                 :    link       :                 :
          :.................:               :.................:

                   Figure 1: ACP VRF and Secure Channels

   The resulting overlay network is normally based exclusively on hop-
   by-hop tunnels.  This is because addressing used on links is IPv6
   link-local addressing, which does not require any prior setup.  In
   this way, the ACP can be built even if there is no configuration on
   the node, or if the data plane has issues such as addressing or
   routing problems.

6.  Self-Creation of an Autonomic Control Plane (ACP) (Normative)

   This section specifies the components and steps to set up an ACP.
   The ACP is automatically self-creating, which makes it
   "indestructible" against most changes to the data plane, including
   misconfigurations of routing, addressing, NAT, firewall, or any other
   traffic policy filters that would inadvertently or otherwise
   unavoidably also impact the management plane traffic, such as the
   actual operator command-line interface (CLI) session or controller
   NETCONF session through which the configuration changes to the data
   plane are executed.

   Physical misconfiguration of wiring between ACP nodes will also not
   break the ACP.  As long as there is a transitive physical path
   between ACP nodes, the ACP should be able to recover given that it
   automatically operates across all interfaces of the ACP nodes and
   automatically determines paths between them.

   Attacks against the network via incorrect routing or addressing
   information for the data plane will not impact the ACP.  Even
   impaired ACP nodes will have a significantly reduced attack surface
   against malicious misconfiguration because only very limited ACP or
   interface up/down configuration can affect the ACP, and depending on
   their specific designs, these types of attacks could also be
   eliminated.  See more in Section 9.3 and Section 11.

   An ACP node can be a router, switch, controller, NMS host, or any
   other IPv6-capable node.  Initially, it MUST have its ACP
   certificate, as well as an (empty) ACP adjacency table (described in
   Section 6.3).  It then can start to discover ACP neighbors and build
   the ACP.  This is described step by step in the following sections.

6.1.  Requirements for the Use of Transport Layer Security (TLS)

   The following requirements apply to TLS that is required or used by
   ACP components.  Applicable ACP components include ACP certificate
   maintenance via EST (see Section 6.2.5), TLS connections for CRL
   Distribution Point (CRLDP) or Online Certificate Status Protocol
   (OCSP) responder (if used, see Section 6.2.3), and ACP GRASP (see
   Section 6.9.2).  On ANI nodes, these requirements also apply to
   BRSKI.

   TLS MUST comply with "Recommendations for Secure Use of Transport
   Layer Security (TLS) and Datagram Transport Layer Security (DTLS)"
   [RFC7525] except that TLS 1.2 ("The Transport Layer Security (TLS)
   Protocol Version 1.2" [RFC5246]) is REQUIRED and that older versions
   of TLS MUST NOT be used.  TLS 1.3 ("The Transport Layer Security
   (TLS) Protocol Version 1.3" [RFC8446]) SHOULD be supported.  The
   choice for TLS 1.2 as the lowest common denominator for the ACP is
   based on the currently expected and most likely availability across
   the wide range of candidate ACP node types, potentially with non-
   agile operating system TCP/IP stacks.

   TLS MUST offer TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 and
   TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 and MUST NOT offer options
   with less than 256-bit symmetric key strength or hash strength of
   less than 384 bits.  When TLS 1.3 is supported,
   TLS_AES_256_GCM_SHA384 MUST be offered and
   TLS_CHACHA20_POLY1305_SHA256 MAY be offered.

   TLS MUST also include the "Supported Elliptic Curves" extension, and
   it MUST support the NIST P-256 (secp256r1(22)) and P-384
   (secp384r1(24)) curves "Elliptic Curve Cryptography (ECC) Cipher
   Suites for Transport Layer Security (TLS) Versions 1.2 and Earlier"
   [RFC8422].  In addition, TLS 1.2 clients SHOULD send an
   ec_point_format extension with a single element, "uncompressed".

6.2.  ACP Domain, Certificate, and Network

   The ACP relies on group security.  An ACP domain is a group of nodes
   that trust each other to participate in ACP operations such as
   creating ACP secure channels in an autonomous, peer-to-peer fashion
   between ACP domain members via protocols such as IPsec.  To
   authenticate and authorize another ACP member node with access to the
   ACP domain, each ACP member requires keying material: an ACP node
   MUST have an LDevID certificate and information about one or more TAs
   as required for the ACP domain membership check (Section 6.2.3).

   Manual keying via shared secrets is not usable for an ACP domain
   because it would require a single shared secret across all current
   and future ACP domain members to meet the expectation of autonomous,
   peer-to-peer establishment of ACP secure channels between any ACP
   domain members.  Such a single shared secret would be an unacceptable
   security weakness.  Asymmetric keying material (public keys) without
   certificates does not provide the mechanism to authenticate ACP
   domain membership in an autonomous, peer-to-peer fashion for current
   and future ACP domain members.

   The LDevID certificate is henceforth called the ACP certificate.  The
   TA is the CA root certificate of the ACP domain.

   The ACP does not mandate specific mechanisms by which this keying
   material is provisioned into the ACP node.  It only requires that the
   certificate comply with Section 6.2.1, specifically that it have the
   acp-node-name as specified in Section 6.2.2 in its domain certificate
   as well as those of candidate ACP peers.  See Appendix A.2 for more
   information about enrollment or provisioning options.

   This document uses the term ACP in many places where the Autonomic
   Networking reference documents [RFC7575] and [RFC8993] use the word
   autonomic.  This is done because those reference documents consider
   (only) fully Autonomic Networks and nodes, but the support of ACP
   does not require the support for other components of Autonomic
   Networks except for the reliance on GRASP and the providing of
   security and transport for GRASP.  Therefore, the word autonomic
   might be misleading to operators interested in only the ACP.

   [RFC7575] defines the term "autonomic domain" as a collection of
   autonomic nodes.  ACP nodes do not need to be fully autonomic, but
   when they are, then the ACP domain is an autonomic domain.  Likewise,
   [RFC8993] defines the term "domain certificate" as the certificate
   used in an autonomic domain.  The ACP certificate is that domain
   certificate when ACP nodes are (fully) autonomic nodes.  Finally,
   this document uses the term ACP network to refer to the network
   created by active ACP nodes in an ACP domain.  The ACP network itself
   can extend beyond ACP nodes through the mechanisms described in
   Section 8.1.

6.2.1.  ACP Certificates

   ACP certificates MUST be [RFC5280] compliant X.509 v3 [X.509]
   certificates.

   ACP nodes MUST support handling ACP certificates, TA certificates,
   and certificate chain certificates (henceforth just called
   certificates in this section) with RSA public keys and certificates
   with Elliptic Curve Cryptography (ECC) public keys.

   ACP nodes MUST NOT support certificates with RSA public keys of less
   than a 2048-bit modulus or curves with group order of less than 256
   bits.  They MUST support certificates with RSA public keys with
   2048-bit modulus and MAY support longer RSA keys.  They MUST support
   certificates with ECC public keys using NIST P-256 curves and SHOULD
   support P-384 and P-521 curves.

   ACP nodes MUST NOT support certificates with RSA public keys whose
   modulus is less than 2048 bits, or certificates whose ECC public keys
   are in groups whose order is less than 256 bits.  RSA signing
   certificates with 2048-bit public keys MUST be supported, and such
   certificates with longer public keys MAY be supported.  ECDSA
   certificates using the NIST P-256 curve MUST be supported, and such
   certificates using the P-384 and P-521 curves SHOULD be supported.

   ACP nodes MUST support RSA certificates that are signed by RSA
   signatures over the SHA-256 digest of the contents and SHOULD
   additionally support SHA-384 and SHA-512 digests in such signatures.
   The same requirements for digest usage in certificate signatures
   apply to Elliptic Curve Digital Signature Algorithm (ECDSA)
   certificates, and additionally, ACP nodes MUST support ECDSA
   signatures on ECDSA certificates.

   The ACP certificate SHOULD use an RSA key and an RSA signature when
   the ACP certificate is intended to be used not only for ACP
   authentication but also for other purposes.  The ACP certificate MAY
   use an ECC key and an ECDSA signature if the ACP certificate is only
   used for ACP and ANI authentication and authorization.

   Any secure channel protocols used for the ACP as specified in this
   document or extensions of this document MUST therefore support
   authentication (e.g., signing), starting with these types of
   certificates.  See [RFC8422] for more information.

   The reason for these choices are as follows: as of 2020, RSA is still
   more widely used than ECC, therefore the MUST-level requirements for
   RSA.  ECC offers equivalent security at (logarithmically) shorter key
   lengths (see [RFC8422]).  This can be beneficial especially in the
   presence of constrained bandwidth or constrained nodes in an ACP/ANI
   network.  Some ACP functions such as GRASP peer-to-peer across the
   ACP require end-to-end/any-to-any authentication and authorization,
   therefore ECC can only reliably be used in the ACP when it MUST be
   supported on all ACP nodes.  RSA signatures are mandatory to be
   supported also for ECC certificates because the CAs themselves may
   not support ECC yet.

   The ACP certificate SHOULD be used for any authentication between
   nodes with ACP domain certificates (ACP nodes and NOC nodes) where a
   required authorization condition is ACP domain membership, such as
   ACP node to NOC/OAM end-to-end security and ASA to ASA end-to-end
   security.  Section 6.2.3 defines this "ACP domain membership check".
   The uses of this check that are standardized in this document are for
   the establishment of hop-by-hop ACP secure channels (Section 6.8) and
   for ACP GRASP (Section 6.9.2) end to end via TLS.

   The ACP domain membership check requires a minimum number of elements
   in a certificate as described in Section 6.2.3.  The identity of a
   node in the ACP is carried via the acp-node-name as defined in
   Section 6.2.2.

   To support Elliptic Curve Diffie-Hellman (ECDH) directly with the key
   in the ACP certificate, ACP certificates with ECC keys need to
   indicate that they are ECDH capable: if the X.509 v3 keyUsage
   extension is present, the keyAgreement bit must then be set.  Note
   that this option is not required for any of the required ciphersuites
   in this document and may not be supported by all CAs.

   Any other fields of the ACP certificate are to be populated as
   required by [RFC5280].  As long as they are compliant with [RFC5280],
   any other field of an ACP certificate can be set as desired by the
   operator of the ACP domain through the appropriate ACP registrar and/
   or ACP CA procedures.  For example, other fields may be required for
   purposes other than those that the ACP certificate is intended to be
   used for (such as elements of a SubjectName).

   For further certificate details, ACP certificates may follow the
   recommendations from [CABFORUM].

   For diagnostic and other operational purposes, it is beneficial to
   copy the device-identifying fields of the node's IDevID certificate
   into the ACP certificate, such as the "serialNumber" attribute
   ([X.520], Section 6.2.9) in the subject field distinguished name
   encoding.  Note that this is not the certificate serial-number.  See
   also [RFC8995], Section 2.3.1.  This can be done, for example, if it
   would be acceptable for the device's "serialNumber" to be signaled
   via the Link Layer Discovery Protocol [LLDP] because, like LLDP-
   signaled information, the ACP certificate information can be
   retrieved by neighboring nodes without further authentication and can
   be used either for beneficial diagnostics or for malicious attacks.
   Retrieval of the ACP certificate is possible via a (failing) attempt
   to set up an ACP secure channel, and the "serialNumber" usually
   contains device type information that may help to more quickly
   determine working exploits/attacks against the device.

   Note that there is no intention to constrain authorization within the
   ACP or Autonomic Networks using the ACP to just the ACP domain
   membership check as defined in this document.  It can be extended or
   modified with additional requirements.  Such future authorizations
   can use and require additional elements in certificates or policies
   or even additional certificates.  See Section 6.2.5 for the
   additional check against the id-kp-cmcRA extended key usage attribute
   ("Certificate Management over CMS (CMC) Updates" [RFC6402]), and see
   Appendix A.9.5 for possible future extensions.

6.2.2.  ACP Certificate AcpNodeName

     acp-node-name = local-part "@" acp-domain-name
     local-part = [ acp-address ] [ "+" rsub extensions ]
     acp-address = 32HEXDIG / "0" ; HEXDIG as of [RFC5234], Appendix B.1
     rsub = [ <subdomain> ] ; <subdomain> as of [RFC1034], Section 3.5
     acp-domain-name = <domain> ; as of [RFC1034], Section 3.5
     extensions = *( "+" extension )
     extension  = 1*etext  ; future standard definition.
     etext      = ALPHA / DIGIT /  ; Printable US-ASCII
                  "!" / "#" / "$" / "%" / "&" / "'" /
                  "*" / "-" / "/" / "=" / "?" / "^" /
                  "_" / "`" / "{" / "|" / "}" / "~"

     routing-subdomain = [ rsub "." ] acp-domain-name

                        Figure 2: ACP Node Name ABNF

   Example:

   Given an ACP address of fd89:b714:f3db:0:200:0:6400:0000, an ACP
   domain name of acp.example.com, and an rsub extension of
   area51.research, then this results in the following:

     acp-node-name      = fd89b714f3db00000200000064000000
                          +area51.research@acp.example.com
     acp-domain-name    = acp.example.com
     routing-subdomain  = area51.research.acp.example.com

   The acp-node-name in Figure 2 is the ABNF definition ("Augmented BNF
   for Syntax Specifications: ABNF" [RFC5234]) of the ACP Node Name.  An
   ACP certificate MUST carry this information.  It MUST contain an
   otherName field in the X.509 Subject Alternative Name extension, and
   the otherName MUST contain an AcpNodeName as described in
   Section 6.2.2.

   Nodes complying with this specification MUST be able to receive their
   ACP address through the domain certificate, in which case their own
   ACP certificate MUST have a 32HEXDIG acp-address field.  The acp-
   address field is case insensitive because ABNF HEXDIG is.  It is
   recommended to encode acp-address with lowercase letters.  Nodes
   complying with this specification MUST also be able to authenticate
   nodes as ACP domain members or ACP secure channel peers when they
   have a zero-value acp-address field and as ACP domain members (but
   not as ACP secure channel peers) when the acp-address field is
   omitted from their AcpNodeName.  See Section 6.2.3.

   The acp-domain-name is used to indicate the ACP domain across which
   ACP nodes authenticate and authorize each other, for example, to
   build ACP secure channels to each other, see Section 6.2.3.  The acp-
   domain-name SHOULD be the FQDN of an Internet domain owned by the
   network administration of the ACP and ideally reserved to only be
   used for the ACP.  In this specification, it serves as a name for the
   ACP that ideally is globally unique.  When acp-domain-name is a
   globally unique name, collision of ACP addresses across different ACP
   domains can only happen due to ULA hash collisions (see
   Section 6.11.2).  Using different acp-domain-names, operators can
   distinguish multiple ACPs even when using the same TA.

   To keep the encoding simple, there is no consideration for
   internationalized acp-domain-names.  The acp-node-name is not
   intended for end-user consumption.  There is no protection against an
   operator picking any domain name for an ACP whether or not the
   operator can claim to own the domain name.  Instead, the domain name
   only serves as a hash seed for the ULA and for diagnostics for the
   operator.  Therefore, any operator owning only an internationalized
   domain name should be able to pick an equivalently unique 7-bit ASCII
   acp-domain-name string representing the internationalized domain
   name.

   The routing-subdomain is a string that can be constructed from the
   acp-node-name, and it is used in the hash creation of the ULA (see
   Section 6.11.2).  The presence of the rsub component allows a single
   ACP domain to employ multiple /48 ULA prefixes.  See Appendix A.6 for
   example use cases.

   The optional extensions field is used for future standardized
   extensions to this specification.  It MUST be ignored if present and
   not understood.

   The following points explain and justify the encoding choices
   described:

   1.  Formatting notes:

       1.1  The rsub component needs to be in the local-part: if the
            format just had routing-subdomain as the domain part of the
            acp-node-name, rsub and acp-domain-name could not be
            separated from each other to determine in the ACP domain
            membership check which part is the acp-domain-name and which
            is solely for creating a different ULA prefix.

       1.2  If both acp-address and rsub are omitted from AcpNodeName,
            the local-part will have the format "++extension(s)".  The
            two plus characters are necessary so the node can
            unambiguously parse that both acp-address and rsub are
            omitted.

   2.  The encoding of the ACP domain name and ACP address as described
       in this section is used for the following reasons:

       2.1  The acp-node-name is the identifier of a node's ACP.  It
            includes the necessary components to identify a node's ACP
            both from within the ACP as well as from the outside of the
            ACP.

       2.2  For manual and/or automated diagnostics and backend
            management of devices and ACPs, it is necessary to have an
            easily human-readable and software-parsable standard, single
            string representation of the information in the acp-node-
            name.  For example, inventory or other backend systems can
            always identify an entity by one unique string field but not
            by a combination of multiple fields, which would be
            necessary if there were no single string representation.

       2.3  If the encoding was not such a string, it would be necessary
            to define a second standard encoding to provide this format
            (standard string encoding) for operator consumption.

       2.4  Addresses of the form <local>@<domain> have become the
            preferred format for identifiers of entities in many
            systems, including the majority of user identifiers in web
            or mobile applications such as multi-domain single-sign-on
            systems.

   3.  Compatibilities:

       3.1  It should be possible to use the ACP certificate as an
            LDevID certificate on the system for uses besides the ACP.
            Therefore, the information element required for the ACP
            should be encoded so that it minimizes the possibility of
            creating incompatibilities with other such uses.  The
            attributes of the subject field, for example, are often used
            in non-ACP applications and therefore should not be occupied
            by new ACP values.

       3.2  The element should not require additional ASN.1 encoding
            and/or decoding because libraries for accessing certificate
            information, especially for embedded devices, may not
            support extended ASN.1 decoding beyond predefined, mandatory
            fields. subjectAltName / otherName is already used with a
            single string parameter for several otherNames (see
            "Extensible Messaging and Presence Protocol (XMPP): Core"
            [RFC6120], "Dynamic Peer Discovery for RADIUS/TLS and
            RADIUS/DTLS Based on the Network Access Identifier (NAI)"
            [RFC7585], "Internet X.509 Public Key Infrastructure Subject
            Alternative Name for Expression of Service Name" [RFC4985],
            "Internationalized Email Addresses in X.509 Certificates"
            [RFC8398]).

       3.3  The element required for the ACP should minimize the risk of
            being misinterpreted by other uses of the LDevID
            certificate.  It also must not be misinterpreted as an email
            address, hence the use of the otherName / rfc822Name option
            in the certificate would be inappropriate.

   See Section 4.2.1.6 of [RFC5280] for details on the subjectAltName
   field.

6.2.2.1.  AcpNodeName ASN.1 Module

   The following ASN.1 module normatively specifies the AcpNodeName
   structure.  This specification uses the ASN.1 definitions from "New
   ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)"
   [RFC5912] with the 2002 ASN.1 notation used in that document.
   [RFC5912] updates normative documents using older ASN.1 notation.

   ANIMA-ACP-2020
     { iso(1) identified-organization(3) dod(6)
       internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-anima-acpnodename-2020(97) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   IMPORTS
     OTHER-NAME
     FROM PKIX1Implicit-2009
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-implicit-02(59) }

     id-pkix
     FROM PKIX1Explicit-2009
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-explicit-02(51) } ;

     id-on OBJECT IDENTIFIER ::= { id-pkix 8 }

     AcpNodeNameOtherNames OTHER-NAME ::= { on-AcpNodeName, ... }

     on-AcpNodeName OTHER-NAME ::= {
         AcpNodeName IDENTIFIED BY id-on-AcpNodeName
     }

     id-on-AcpNodeName OBJECT IDENTIFIER ::= { id-on 10 }

     AcpNodeName ::= IA5String (SIZE (1..MAX))
      -- AcpNodeName as specified in this document carries the
      -- acp-node-name as specified in the ABNF in Section 6.2.2

   END

                     Figure 3: AcpNodeName ASN.1 Module

6.2.3.  ACP Domain Membership Check

   The following points constitute the ACP domain membership check of a
   candidate peer via its certificate:

   1.  The peer has proved ownership of the private key associated with
       the certificate's public key.  This check is performed by the
       security association protocol used, for example, Section 2.15 of
       "Internet Key Exchange Protocol Version 2 (IKEv2)" [RFC7296].

   2.  The peer's certificate passes certificate path validation as
       defined in [RFC5280], Section 6, against one of the TAs
       associated with the ACP node's ACP certificate (see
       Section 6.2.4).  This includes verification of the validity
       (lifetime) of the certificates in the path.

   3.  If the peer's certificate indicates a CRLDP ([RFC5280],
       Section 4.2.1.13) or OCSP responder ([RFC5280], Section 4.2.2.1),
       then the peer's certificate MUST be valid according to those
       mechanisms when they are available: an OCSP check for the peer's
       certificate across the ACP must succeed, or the peer's
       certificate must not be listed in the CRL retrieved from the
       CRLDP.  These mechanisms are not available when the ACP node has
       no ACP or non-ACP connectivity to retrieve a current CRL or has
       no access an OCSP responder, and the security association
       protocol itself also has no way to communicate the CRL or OCSP
       check.

       Retries to learn revocation via OCSP or CRL SHOULD be made using
       the same backoff as described in Section 6.7.  If and when the
       ACP node then learns that an ACP peer's certificate is invalid
       for which Rule 3 had to be skipped during ACP secure channel
       establishment, then the ACP secure channel to that peer MUST be
       closed even if this peer is the only connectivity to access CRL/
       OCSP.  This applies (of course) to all ACP secure channels to
       this peer if there are multiple.  The ACP secure channel
       connection MUST be retried periodically to support the case that
       the neighbor acquires a new, valid certificate.

   4.  The peer's certificate has a syntactically valid acp-node-name
       field, and the acp-domain-name in that peer's acp-node-name is
       the same as in this ACP node's certificate (lowercase
       normalized).

   When checking a candidate peer's certificate for the purpose of
   establishing an ACP secure channel, one additional check is
   performed:

   5.  The acp-address field of the candidate peer certificate's
       AcpNodeName is not omitted but is either 32HEXDIG or 0, according
       to Figure 2.

   Technically, ACP secure channels can only be built with nodes that
   have an acp-address.  Rule 5 ensures that this is taken into account
   during ACP domain membership check.

   Nodes with an omitted acp-address field can only use their ACP domain
   certificate for non-ACP secure channel authentication purposes.  This
   includes, for example, NMS type nodes permitted to communicate into
   the ACP via ACP connect (Section 8.1)

   The special value "0" in an ACP certificate's acp-address field is
   used for nodes that can and should determine their ACP address
   through mechanisms other than learning it through the acp-address
   field in their ACP certificate.  These ACP nodes are permitted to
   establish ACP secure channels.  Mechanisms for those nodes to
   determine their ACP address are outside the scope of this
   specification, but this option is defined here so that any ACP nodes
   can build ACP secure channels to them according to Rule 5.

   The optional rsub field of the AcpNodeName is not relevant to the ACP
   domain membership check because it only serves to structure routing
   and addressing within an ACP but not to segment mutual authentication
   and authorization (hence the name "routing subdomain").

   In summary:

   *  Steps 1 through 4 constitute standard certificate validity
      verification and private key authentication as defined by
      [RFC5280] and security association protocols (such as IKEv2
      [RFC7296]) when leveraging certificates.

   *  Except for its public key, Steps 1 through 4 do not include the
      verification of any preexisting form of a certificate's identity
      elements, such as a web server's domain name prefix, which is
      often encoded in the certificate common name.  Step 5 is an
      equivalent step for the AcpNodeName.

   *  Step 4 constitutes standard CRL and OCSP checks refined for the
      case of missing connectivity and limited-functionality security
      association protocols.

   *  Steps 1 through 4 authorize the building of any secure connection
      between members of the same ACP domain except for ACP secure
      channels.

   *  Step 5 is the additional verification of the presence of an ACP
      address as necessary for ACP secure channels.

   *  Steps 1 through 5 therefore authorize the building of an ACP
      secure channel.

   For brevity, the remainder of this document refers to this process
   only as authentication instead of as authentication and
   authorization.

6.2.3.1.  Realtime Clock and Time Validation

   An ACP node with a realtime clock in which it has confidence MUST
   check the timestamps when performing an ACP domain membership check,
   such as checking the certificate validity period in Step 1 and the
   respective times in Step 4 for revocation information (e.g.,
   signingTimes in Cryptographic Message Syntax (CMS) signatures).

   An ACP node without such a realtime clock MAY ignore those timestamp
   validation steps if it does not know the current time.  Such an ACP
   node SHOULD obtain the current time in a secured fashion, such as via
   NTP signaled through the ACP.  It then ignores timestamp validation
   only until the current time is known.  In the absence of implementing
   a secured mechanism, such an ACP node MAY use a current time learned
   in an insecure fashion in the ACP domain membership check.

   Current time MAY be learned in an unsecured fashion, for example, via
   NTP ("Network Time Protocol Version 4: Protocol and Algorithms
   Specification" [RFC5905]) over the same link-local IPv6 addresses
   used for the ACP from neighboring ACP nodes.  ACP nodes that do
   provide unsecured NTP over their link-local addresses SHOULD
   primarily run NTP across the ACP and provide NTP time across the ACP
   only when they have a trusted time source.  Details for such NTP
   procedures are beyond the scope of this specification.

   Besides the ACP domain membership check, the ACP itself has no
   dependency on knowing the current time, but protocols and services
   using the ACP, for example, event logging, will likely need to know
   the current time.

6.2.4.  Trust Anchors (TA)

   ACP nodes need TA information according to [RFC5280], Section 6.1.1
   (d), typically in the form of one or more certificates of the TA to
   perform certificate path validation as required by Section 6.2.3,
   Rule 2.  TA information MUST be provisioned to an ACP node (together
   with its ACP domain certificate) by an ACP registrar during initial
   enrollment of a candidate ACP node.  ACP nodes MUST also support the
   renewal of TA information via EST as described in Section 6.2.5.

   The required information about a TA can consist of one or more
   certificates as required for dealing with CA certificate renewals as
   explained in Section 4.4 of "Internet X.509 Public Key Infrastructure
   Certificate Management Protocol (CMP)" [RFC4210]).

   A certificate path is a chain of certificates starting at the ACP
   certificate (the leaf and/or end entity), followed by zero or more
   intermediate CA certificates, and ending with the TA information,
   which is typically one or two self-signed certificates of the TA.
   The CA that signs the ACP certificate is called the assigning CA.  If
   there are no intermediate CAs, then the assigning CA is the TA.
   Certificate path validation authenticates that the TA associated with
   the ACP permits the ACP certificate, either directly or indirectly
   via one or more intermediate CA.

   Note that different ACP nodes may have different intermediate CAs in
   their certificate path and even different TA.  The set of TAs for an
   ACP domain must be consistent across all ACP members so that any ACP
   node can authenticate any other ACP node.  The protocols through
   which the ACP domain membership check Rules 1 through 3 are performed
   need to support the exchange not only of the ACP nodes certificates
   but also the exchange of the intermediate TA.

   For the ACP domain membership check, ACP nodes MUST support
   certificate path validation with zero or one intermediate CA.  They
   SHOULD support two intermediate CAs and two TAs (to permit migration
   from one TA to another TA).

   Certificates for an ACP MUST only be given to nodes that are allowed
   to be members of that ACP.  When the signing CA relies on an ACP
   registrar, the CA MUST only sign certificates with acp-node-name
   through trusted ACP registrars.  In this setup, any existing CA,
   unaware of the formatting of acp-node-name, can be used.

   These requirements can be achieved by using a TA private to the owner
   of the ACP domain or potentially through appropriate contractual
   agreements between the involved parties (registrar and CA).  Using a
   public CA is out of scope of this document.

   A single owner can operate multiple, independent ACP domains from the
   same set of TAs.  Registrars must then know into which ACP a node
   needs to be enrolled.

6.2.5.  Certificate and Trust Anchor Maintenance

   ACP nodes MUST support renewal of their certificate and TA
   information via EST and MAY support other mechanisms.  See
   Section 6.1 for TLS requirements.  An ACP network MUST have at least
   one ACP node supporting EST server functionality across the ACP so
   that EST renewal is usable.

   ACP nodes SHOULD remember the GRASP O_IPv6_LOCATOR parameters of the
   EST server with which they last renewed their ACP certificate.  They
   SHOULD provide the ability for these EST server parameters to also be
   set by the ACP registrar (see Section 6.11.7) that initially enrolled
   the ACP device with its ACP certificate.  When BRSKI is used (see
   [RFC8995]), the IPv6 locator of the BRSKI registrar from the BRSKI
   TLS connection SHOULD be remembered and used for the next renewal via
   EST if that registrar also announces itself as an EST server via
   GRASP on its ACP address (see Section 6.2.5.1).

   The EST server MUST present a certificate that passes the ACP domain
   membership check in its TLS connection setup (Section 6.2.3, rules 1
   through 4, not rule 5 as this is not for an ACP secure channel
   setup).  The EST server certificate MUST also contain the id-kp-cmcRA
   extended key usage attribute [RFC6402], and the EST client MUST check
   its presence.

   The additional check against the id-kp-cmcRA extended key usage
   extension field ensures that clients do not fall prey to an illicit
   EST server.  While such illicit EST servers should not be able to
   support certificate signing requests (as they are not able to elicit
   a signing response from a valid CA), such an illicit EST server would
   be able to provide faked CA certificates to EST clients that need to
   renew their CA certificates when they expire.

   Note that EST servers supporting multiple ACP domains will need to
   have a separate certificate for each of these ACP domains and respond
   on a different transport address (IPv6 address and/or TCP port).
   This is easily automated on the EST server if the CA allows
   registrars to request certificates with the id-kp-cmcRA extended
   usage extension for themselves.

6.2.5.1.  GRASP Objective for EST Server

   ACP nodes that are EST servers MUST announce their service in the ACP
   via GRASP Flood Synchronization (M_FLOOD) messages.  See [RFC8990],
   Section 2.8.11 for the definition of this message type and Figure 4
   for an example.

      [M_FLOOD, 12340815, h'fd89b714f3db0000200000064000001', 210000,
          [["SRV.est", 4, 255 ],
          [O_IPv6_LOCATOR,
               h'fd89b714f3db0000200000064000001', IPPROTO_TCP, 443]]
      ]

                Figure 4: GRASP "SRV.est" Objective Example

   The formal definition of the objective in CDDL (see "Concise Data
   Definition Language (CDDL): A Notational Convention to Express
   Concise Binary Object Representation (CBOR) and JSON Data Structures"
   [RFC8610]) is as follows:

   flood-message = [M_FLOOD, session-id, initiator, ttl,
                    +[objective, (locator-option / [])]]
                                ; See example above and explanation
                                ; below for initiator and ttl.

   objective = ["SRV.est", objective-flags, loop-count,
                                          objective-value]

   objective-flags = sync-only  ; As in [RFC8990].
   sync-only       = 4          ; M_FLOOD only requires synchronization.
   loop-count      = 255        ; Recommended as there is no mechanism
                                ; to discover network diameter.
   objective-value = any        ; Reserved for future extensions.

                    Figure 5: GRASP "SRV.est" Definition

   The objective name "SRV.est" indicates that the objective is an EST
   server compliant with [RFC7030] because "est" is a registered service
   name ("Internet Assigned Numbers Authority (IANA) Procedures for the
   Management of the Service Name and Transport Protocol Port Number
   Registry" [RFC6335]) for [RFC7030].  The 'objective-value' field MUST
   be ignored if present.  Backward-compatible extensions to [RFC7030]
   MAY be indicated through 'objective-value'.  Certificate renewal
   options that are incompatible with [RFC7030] MUST use a different
   'objective-name'.  Unrecognized 'objective-value' fields (or parts
   thereof if it is a partially understood structure) MUST be ignored.

   The M_FLOOD message MUST be sent periodically.  The default SHOULD be
   60 seconds; the value SHOULD be operator configurable but SHOULD be
   not smaller than 60 seconds.  The frequency of sending MUST be such
   that the aggregate amount of periodic M_FLOODs from all flooding
   sources cause only negligible traffic across the ACP.  The time-to-
   live (ttl) parameter SHOULD be 3.5 times the period so that up to
   three consecutive messages can be dropped before an announcement is
   considered expired.  In the example above, the ttl is 210000 msec,
   that is, 3.5 times 60 seconds.  When a service announcer using these
   parameters unexpectedly dies immediately after sending the M_FLOOD,
   receivers would consider it expired 210 seconds later.  When a
   receiver tries to connect to this dead service before this timeout,
   it will experience a failing connection and use that as an indication
   that the service instance is dead and select another instance of the
   same service instead (from another GRASP announcement).

   The "SRV.est" objective(s) SHOULD only be announced when the ACP node
   knows that it can successfully communicate with a CA to perform the
   EST renewal and/or rekeying operations for the ACP domain.  See also
   Section 11.

6.2.5.2.  Renewal

   When performing renewal, the node SHOULD attempt to connect to the
   remembered EST server.  If that fails, it SHOULD attempt to connect
   to an EST server learned via GRASP.  The server with which
   certificate renewal succeeds SHOULD be remembered for the next
   renewal.

   Remembering the last renewal server and preferring it provides
   stickiness that can help diagnostics.  It also provides some
   protection against off-path, compromised ACP members announcing bogus
   information into GRASP.

   Renewal of certificates SHOULD start after less than 50% of the
   domain certificate lifetime so that network operations have ample
   time to investigate and resolve any problems that cause a node to not
   renew its domain certificate in time, and to allow prolonged periods
   of running parts of a network disconnected from any CA.

6.2.5.3.  Certificate Revocation Lists (CRLs)

   The ACP node SHOULD support revocation through CRL(s) via HTTP from
   one or more CRL Distribution Points (CRLDP).  The CRLDP(s) MUST be
   indicated in the domain certificate when used.  If the CRLDP URL uses
   an IPv6 address (ULA address when using the addressing rules
   specified in this document), the ACP node will connect to the CRLDP
   via the ACP.  If the CRLDP uses a domain name, the ACP node will
   connect to the CRLDP via the data plane.

   It is common to use domain names for CRLDP(s), but there is no
   requirement for the ACP to support DNS.  Any DNS lookup in the data
   plane is not only a possible security issue, but it would also not
   indicate whether the resolved address is meant to be reachable across
   the ACP.  Therefore, the use of an IPv6 address versus the use of a
   DNS name doubles as an indicator whether or not to reach the CRLDP
   via the ACP.

   A CRLDP can be reachable across the ACP either by running it on a
   node with ACP or by connecting its node via an ACP connect interface
   (see Section 8.1).

   When using a private PKI for ACP certificates, the CRL may be need-
   to-know, for example, to prohibit insight into the operational
   practices of the domain by tracking the growth of the CRL.  In this
   case, HTTPS may be chosen to provide confidentiality, especially when
   making the CRL available via the data plane.  Authentication and
   authorization SHOULD use ACP certificates and the ACP domain
   membership check (Section 6.2.3).  The CRLDP MAY omit the CRL
   verification during authentication of the peer to permit CRL
   retrieval by an ACP node with a revoked ACP certificate, which can
   allow the (ex) ACP node to quickly discover its ACP certificate
   revocation.  This may violate the desired need-to-know requirement,
   though.  ACP nodes MAY support CRLDP operations via HTTPS.

6.2.5.4.  Lifetimes

   The certificate lifetime may be set to shorter lifetimes than
   customary (one year) because certificate renewal is fully automated
   via ACP and EST.  The primary limiting factor for shorter certificate
   lifetimes is the load on the EST server(s) and CA.  It is therefore
   recommended that ACP certificates are managed via a CA chain where
   the assigning CA has enough performance to manage short-lived
   certificates.  See also Section 9.2.4 for a discussion about an
   example setup achieving this.  See also "Support for Short-Term,
   Automatically Renewed (STAR) Certificates in the Automated
   Certificate Management Environment (ACME)" [RFC8739].

   When certificate lifetimes are sufficiently short, such as few hours,
   certificate revocation may not be necessary, allowing the
   simplification of the overall certificate maintenance infrastructure.

   See Appendix A.2 for further optimizations of certificate maintenance
   when BRSKI can be used [RFC8995].

6.2.5.5.  Reenrollment

   An ACP node may determine that its ACP certificate has expired, for
   example, because the ACP node was powered down or disconnected longer
   than its certificate lifetime.  In this case, the ACP node SHOULD
   convert to a role of a reenrolling candidate ACP node.

   In this role, the node maintains the TA and certificate chain
   associated with its ACP certificate exclusively for the purpose of
   reenrollment, and it attempts (or waits) to get reenrolled with a new
   ACP certificate.  The details depend on the mechanisms and protocols
   used by the ACP registrars.

   Please refer to Section 6.11.7 and [RFC8995] for explanations about
   ACP registrars and vouchers as used in the following text.  When ACP
   is intended to be used without BRSKI, the details about BRSKI and
   vouchers in the following text can be skipped.

   When BRSKI is used (i.e., on ACP nodes that are ANI nodes), the
   reenrolling candidate ACP node attempts to enroll like a candidate
   ACP node (BRSKI pledge), but instead of using the ACP node's IDevID
   certificate, it SHOULD first attempt to use its ACP domain
   certificate in the BRSKI TLS authentication.  The BRSKI registrar MAY
   honor this certificate beyond its expiration date purely for the
   purpose of reenrollment.  Using the ACP node's domain certificate
   allows the BRSKI registrar to learn that node's acp-node-name so that
   the BRSKI registrar can reassign the same ACP address information to
   the ACP node in the new ACP certificate.

   If the BRSKI registrar denies the use of the old ACP certificate, the
   reenrolling candidate ACP node MUST reattempt reenrollment using its
   IDevID certificate as defined in BRSKI during the TLS connection
   setup.

   When the BRSKI connection is attempted with either with the old ACP
   certificate or the IDevID certificate, the reenrolling candidate ACP
   node SHOULD authenticate the BRSKI registrar during TLS connection
   setup based on its existing TA certificate chain information
   associated with its old ACP certificate.  The reenrolling candidate
   ACP node SHOULD only fall back to requesting a voucher from the BRSKI
   registrar when this authentication fails during TLS connection setup.
   As a countermeasure against attacks that attempt to force the ACP
   node to forget its prior (expired) certificate and TA, the ACP node
   should alternate between attempting to reenroll using its old keying
   material and attempting to reenroll with its IDevID and requesting a
   voucher.

   When mechanisms other than BRSKI are used for ACP certificate
   enrollment, the principles of the reenrolling candidate ACP node are
   the same.  The reenrolling candidate ACP node attempts to
   authenticate any ACP registrar peers using reenrollment protocols
   and/or mechanisms via its existing certificate chain and/or TA
   information and provides its existing ACP certificate and other
   identification (such as the IDevID certificate) as necessary to the
   registrar.

   Maintaining existing TA information is especially important when
   using enrollment mechanisms that do not leverage a mechanism to
   authenticate the ACP registrar (such as the voucher in BRSKI), and
   when the injection of certificate failures could otherwise make the
   ACP vulnerable to remote attacks by returning the ACP node to a
   "duckling" state in which it accepts enrollment by any network it
   connects to.  The (expired) ACP certificate and ACP TA SHOULD
   therefore be maintained and attempted to be used as one possible
   credential for reenrollment until new keying material is acquired.

   When using BRSKI or other protocols and/or mechanisms that support
   vouchers, maintaining existing TA information allows for lighter-
   weight reenrollment of expired ACP certificates, especially in
   environments where repeated acquisition of vouchers during the
   lifetime of ACP nodes may be operationally expensive or otherwise
   undesirable.

6.2.5.6.  Failing Certificates

   An ACP certificate is called failing in this document if or when the
   ACP node to which the certificate was issued can determine that it
   was revoked (or explicitly not renewed), or in the absence of such
   explicit local diagnostics, when the ACP node fails to connect to
   other ACP nodes in the same ACP domain using its ACP certificate.  To
   determine that the ACP certificate is the culprit of a connection
   failure, the peer should pass the domain membership check
   (Section 6.2.3), and connection error diagnostics should exclude
   other reasons for the connection failure.

   This type of failure can happen during the setup or refreshment of a
   secure ACP channel connection or during any other use of the ACP
   certificate, such as for the TLS connection to an EST server for the
   renewal of the ACP domain certificate.

   The following are examples of failing certificates that the ACP node
   can only discover through connection failure: the domain certificate
   or any of its signing certificates could have been revoked or may
   have expired, but the ACP node cannot diagnose this condition
   directly by itself.  Revocation information or clock synchronization
   may only be available across the ACP, but the ACP node cannot build
   ACP secure channels because the ACP peers reject the ACP node's
   domain certificate.

   An ACP node SHOULD support the option to determine whether its ACP
   certificate is failing, and when it does, put itself into the role of
   a reenrolling candidate ACP node as explained in Section 6.2.5.5.

6.3.  ACP Adjacency Table

   To know to which nodes to establish an ACP channel, every ACP node
   maintains an adjacency table.  The adjacency table contains
   information about adjacent ACP nodes, at a minimum: Node-ID (the
   identifier of the node inside the ACP, see Section 6.11.3 and
   Section 6.11.5), the interface on which neighbor was discovered (by
   GRASP as explained below), the link-local IPv6 address of the
   neighbor on that interface, and the certificate (including acp-node-
   name).  An ACP node MUST maintain this adjacency table.  This table
   is used to determine to which neighbor an ACP connection is
   established.

   When the next ACP node is not directly adjacent (i.e., not on a link
   connected to this node), the information in the adjacency table can
   be supplemented by configuration.  For example, the Node-ID and IP
   address could be configured.  See Section 8.2.

   The adjacency table MAY contain information about the validity and
   trust of the adjacent ACP node's certificate.  However, subsequent
   steps MUST always start with the ACP domain membership check against
   the peer (see Section 6.2.3).

   The adjacency table contains information about adjacent ACP nodes in
   general, independent of their domain and trust status.  The next step
   determines to which of those ACP nodes an ACP connection should be
   established.

6.4.  Neighbor Discovery with DULL GRASP

   Discovery Unsolicited Link-Local (DULL) GRASP is a limited subset of
   GRASP intended to operate across an insecure link-local scope.  See
   Section 2.5.2 of [RFC8990] for its formal definition.  The ACP uses
   one instance of DULL GRASP for every L2 interface of the ACP node to
   discover candidate ACP neighbors that are link-level adjacent.
   Unless modified by policy as noted earlier (Section 5, bullet point
   2), native interfaces (e.g., physical interfaces on physical nodes)
   SHOULD be initialized automatically to a state in which ACP discovery
   can be performed, and any native interfaces with ACP neighbors can
   then be brought into the ACP even if the interface is otherwise
   unconfigured.  Reception of packets on such otherwise unconfigured
   interfaces MUST be limited so that at first only SLAAC ("IPv6
   Stateless Address Autoconfiguration" [RFC4862]) and DULL GRASP work,
   and then only the following ACP secure channel setup packets work,
   but not any other unnecessary traffic (e.g., no other link-local IPv6
   transport stack responders, for example).

   Note that the use of the IPv6 link-local multicast address
   (ALL_GRASP_NEIGHBORS) implies the need to use MLDv2 (see "Multicast
   Listener Discovery Version 2 (MLDv2) for IPv6" [RFC3810]) to announce
   the desire to receive packets for that address.  Otherwise, DULL
   GRASP could fail to operate correctly in the presence of MLD-snooping
   switches ("Considerations for Internet Group Management Protocol
   (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches"
   [RFC4541]) that either do not support ACP or are not ACP enabled
   because those switches would stop forwarding DULL GRASP packets.
   Switches that do not support MLD snooping simply need to operate as
   pure L2 bridges for IPv6 multicast packets for DULL GRASP to work.

   ACP discovery SHOULD NOT be enabled by default on non-native
   interfaces.  In particular, ACP discovery MUST NOT run inside the ACP
   across ACP virtual interfaces.  See Section 9.3 for further non-
   normative suggestions on how to enable and disable ACP at the node
   and interface level.  See Section 8.2.2 for more details about
   tunnels (typical non-native interfaces).  See Section 7 for extending
   ACP on devices operating (also) as L2 bridges.

   Note: if an ACP node also implements BRSKI to enroll its ACP
   certificate (see Appendix A.2 for a summary), then the above
   considerations also apply to GRASP discovery for BRSKI.  Each DULL
   instance of GRASP set up for ACP is then also used for the discovery
   of a bootstrap proxy via BRSKI when the node does not have a domain
   certificate.  Discovery of ACP neighbors happens only when the node
   does have the certificate.  The node therefore never needs to
   discover both a bootstrap proxy and an ACP neighbor at the same time.

   An ACP node announces itself to potential ACP peers by use of the
   "AN_ACP" objective.  This is a synchronization objective intended to
   be flooded on a single link using the GRASP Flood Synchronization
   (M_FLOOD) message.  In accordance with the design of the Flood
   Synchronization message, a locator consisting of a specific link-
   local IP address, IP protocol number, and port number will be
   distributed with the flooded objective.  An example of the message is
   informally:

      [M_FLOOD, 12340815, h'fe80000000000000c0011001feef0000', 210000,
        [["AN_ACP", 4, 1, "IKEv2" ],
         [O_IPv6_LOCATOR,
              h'fe80000000000000c0011001feef0000', IPPROTO_UDP, 15000]]
        [["AN_ACP", 4, 1, "DTLS" ],
         [O_IPv6_LOCATOR,
              h'fe80000000000000c0011001feef0000', IPPROTO_UDP, 17000]]
      ]

                 Figure 6: GRASP "AN_ACP" Objective Example

   The formal CDDL definition is:

     flood-message = [M_FLOOD, session-id, initiator, ttl,
                      +[objective, (locator-option / [])]]

     objective = ["AN_ACP", objective-flags, loop-count,
                                            objective-value]

     objective-flags = sync-only ; as in [RFC8990]
     sync-only =  4    ; M_FLOOD only requires synchronization
     loop-count = 1    ; limit to link-local operation

     objective-value = method-name / [ method, *extension ]
     method = method-name / [ method-name, *method-param ]
     method-name = "IKEv2" / "DTLS" / id
     extension = any
     method-param = any
     id = text .regexp "[A-Za-z@_$]([-.]*[A-Za-z0-9@_$])*"

                    Figure 7: GRASP "AN_ACP" Definition

   The 'objective-flags' field is set to indicate synchronization.

   The 'loop-count' is fixed at 1 since this is a link-local operation.

   In the above example, the RECOMMENDED period of sending of the
   objective is 60 seconds.  The indicated 'ttl' of 210000 msec means
   that the objective would be cached by ACP nodes even when two out of
   three messages are dropped in transit.

   The 'session-id' is a random number used for loop prevention
   (distinguishing a message from a prior instance of the same message).
   In DULL this field is irrelevant but has to be set according to the
   GRASP specification.

   The originator MUST be the IPv6 link-local address of the originating
   ACP node on the sending interface.

   The 'method-name' in the 'objective-value' parameter is a string
   indicating the protocol available at the specified or implied
   locator.  It is a protocol supported by the node to negotiate a
   secure channel.  "IKEv2" as shown in Figure 6 is the protocol used to
   negotiate an IPsec secure channel.

   The 'method-param' parameter allows method-specific parameters to be
   carried.  This specification does not define any 'method-param'(s)
   for "IKEv2" or "DTLS".  Any method-params for these two methods that
   are not understood by an ACP node MUST be ignored by it.

   The 'extension' parameter allows the definition of method-independent
   parameters.  This specification does not define any extensions.
   Extensions not understood by an ACP node MUST be ignored by it.

   The 'locator-option' is optional and is only required when the secure
   channel protocol is not offered at a well-defined port number, or if
   there is no well-defined port number.

   IKEv2 is the actual protocol used to negotiate an IPsec connection.
   GRASP therefore indicates "IKEv2" and not "IPsec".  If "IPsec" was
   used, this could mean the use of the obsolete, older version IKE (v1)
   ("The Internet Key Exchange (IKE)" [RFC2409]).  IKEv2 has an IANA-
   assigned port number 500, but in Figure 6, the candidate ACP neighbor
   is offering ACP secure channel negotiation via IKEv2 on port 15000
   (purely to show through the example that GRASP allows the indication
   of a port number, and it does not have to be IANA assigned).

   There is no default UDP port for DTLS, it is always locally assigned
   by the node.  For further details about the "DTLS" secure channel
   protocol, see Section 6.8.4.

   If a locator is included, it MUST be an O_IPv6_LOCATOR, and the IPv6
   address MUST be the same as the initiator address (these are DULL
   requirements to minimize third-party DoS attacks).

   The secure channel methods defined in this document use "IKEv2" and
   "DTLS" for 'objective-value'.  There is no distinction between IKEv2
   native and GRE-IKEv2 because this is purely negotiated via IKEv2.

   A node that supports more than one secure channel protocol method
   needs to flood multiple versions of the "AN_ACP" objective so that
   each method can be accompanied by its own 'locator-option'.  This can
   use a single GRASP M_FLOOD message as shown in Figure 6.

   The primary use of DULL GRASP is to discover the link-local IPv6
   address of candidate ACP peers on subnets.  The signaling of the
   supported secure channel option is primarily for diagnostic purposes,
   but it is also necessary for discovery when the protocol has no well-
   known transport address, such as in the case of DTLS.

   Note that a node serving both as an ACP node and BRSKI Join Proxy may
   choose to distribute the "AN_ACP" objective and the respective BRSKI
   in the same M_FLOOD message, since GRASP allows multiple objectives
   in one message.  This may be impractical, though, if ACP and BRSKI
   operations are implemented via separate software modules and/or ASAs.

   The result of the discovery is the IPv6 link-local address of the
   neighbor as well as its supported secure channel protocols (and the
   non-standard port they are running on).  It is stored in the ACP
   adjacency table (see Section 6.3), which then drives the further
   building of the ACP to that neighbor.

   Note that the described DULL GRASP objective intentionally does not
   include the ACP node's ACP certificate, even though this would be
   useful for diagnostics and to simplify the security exchange in ACP
   secure channel security association protocols (see Section 6.8).  The
   reason is that DULL GRASP messages are periodically multicast across
   IPv6 subnets, and full certificates could easily lead to fragmented
   IPv6 DULL GRASP multicast packets due to the size of a certificate.
   This would be highly undesirable.

6.5.  Candidate ACP Neighbor Selection

   An ACP node determines to which other ACP nodes in the adjacency
   table it should attempt to build an ACP connection.  This is based on
   the information in the ACP adjacency table.

   The ACP is established exclusively between nodes in the same domain.
   This includes all routing subdomains.  Appendix A.6 explains how ACP
   connections across multiple routing subdomains are special.

   The result of the candidate ACP neighbor selection process is a list
   of adjacent or configured autonomic neighbors to which an ACP channel
   should be established.  The next step begins that channel
   establishment.

6.6.  Channel Selection

   To avoid attacks, the initial discovery of candidate ACP peers cannot
   include any unprotected negotiation.  To avoid reinventing and
   validating security association mechanisms, the next step after
   discovering the address of a candidate neighbor is to establish a
   security association with that neighbor using a well-known security
   association method.

   It seems clear from the use cases that not all types of ACP nodes can
   or need to connect directly to each other, nor are they able to
   support or prefer all possible mechanisms.  For example, IoT devices
   that are codespace limited may only support DTLS because that code
   exists already on them for end-to-end security, but low-end, in-
   ceiling L2 switches may only want to support Media Access Control
   Security (MacSec, see 802.1AE [MACSEC]) because that is also
   supported in their chips.  Only a flexible gateway device may need to
   support both of these mechanisms and potentially more.  Note that
   MacSec is not required by any profiles of the ACP in this
   specification.  Instead, MacSec is mentioned as an interesting
   potential secure channel protocol.  Note also that the security model
   allows and requires any-to-any authentication and authorization
   between all ACP nodes because there is not only hop-by-hop but also
   end-to-end authentication for secure channels.

   To support extensible selection of the secure channel protocol
   without a single common mandatory-to-implement (MTI) protocol, an ACP
   node MUST try all the ACP secure channel protocols it supports and
   that are also announced by the candidate ACP neighbor via its
   "AN_ACP" GRASP parameters (these are called the "feasible" ACP secure
   channel protocols).

   To ensure that the selection of the secure channel protocols always
   succeeds in a predictable fashion without blocking, the following
   rules apply:

   *  An ACP node may choose to attempt to initiate the different
      feasible ACP secure channel protocols it supports according to its
      local policies sequentially or in parallel, but it MUST support
      acting as a responder to all of them in parallel.

   *  Once the first ACP secure channel protocol connection to a
      specific peer IPv6 address passes peer authentication, the two
      peers know each other's certificate because those ACP certificates
      are used by all secure channel protocols for mutual
      authentication.  The peer with the higher Node-ID in the
      AcpNodeName of its ACP certificate takes on the role of the
      Decider towards the peer.  The other peer takes on the role of the
      Follower.  The Decider selects which secure channel protocol to
      ultimately use.

   *  The Follower becomes passive: it does not attempt to further
      initiate ACP secure channel protocol connections with the Decider
      and does not consider it to be an error when the Decider closes
      secure channels.  The Decider becomes the active party, continuing
      to attempt the setup of secure channel protocols with the
      Follower.  This process terminates when the Decider arrives at the
      "best" ACP secure channel connection option that also works with
      the Follower ("best" from the Decider's point of view).

   *  A peer with a "0" acp-address in its AcpNodeName takes on the role
      of Follower when peering with a node that has a non-"0" acp-
      address (note that this specification does not fully define the
      behavior of ACP secure channel negotiation for nodes with a "0"
      ACP address field, it only defines interoperability with such ACP
      nodes).

   In a simple example, ACP peer Node 1 attempts to initiate an IPsec
   connection via IKEv2 to peer Node 2.  The IKEv2 authentication
   succeeds.  Node 1 has the lower ACP address and becomes the Follower.
   Node 2 becomes the Decider.  IKEv2 might not be the preferred ACP
   secure channel protocol for the Decider Node 2.  Node 2 would
   therefore proceed to attempt secure channel setups with more
   preferred protocol options (in its view, e.g., DTLS/UDP).  If any
   such preferred ACP secure channel connection of the Decider succeeds,
   it would close the IPsec connection.  If Node 2 has no preferred
   protocol option over IPsec, or no such connection attempt from Node 2
   to Node 1 succeeds, Node 2 would keep the IPsec connection and use
   it.

   The Decider SHOULD NOT send actual payload packets across a secure
   channel until it has decided to use it.  The Follower MAY delay
   linking the ACP secure channel to the ACP virtual interface until it
   sees the first payload packet from the Decider up to a maximum of 5
   seconds to avoid unnecessarily linking a secure channel that will be
   terminated as undesired by the Decider shortly afterward.

   The following sequence of steps show this example in more detail.
   Each step is tagged with [<step#>{:<connection>}].  The connection is
   included to more easily distinguish which of the two competing
   connections the step belongs to, one initiated by Node 1, one
   initiated by Node 2.

   [1]       Node 1 sends GRASP "AN_ACP" message to announce itself.

   [2]       Node 2 sends GRASP "AN_ACP" message to announce itself.

   [3]       Node 2 receives [1] from Node 1.

   [4:C1]    Because of [3], Node 2 starts as initiator on its preferred
             secure channel protocol towards Node 1.  Connection C1.

   [5]       Node 1 receives [2] from Node 2.

   [6:C2]    Because of [5], Node 1 starts as initiator on its preferred
             secure channel protocol towards Node 2.  Connection C2.

   [7:C1]    Node 1 and Node 2 have authenticated each other's
             certificate on connection C1 as valid ACP peers.

   [8:C1]    Node 1's certificate has a lower ACP Node-ID than Node 2's,
             therefore Node 1 considers itself the Follower and Node 2
             the Decider on connection C1.  Connection setup C1 is
             completed.

   [9]       Node 1 refrains from attempting any further secure channel
             connections to Node 2 (the Decider) as learned from [2]
             because it knows from [8:C1] that it is the Follower
             relative to Node 2.

   [10:C2]   Node 1 and Node 2 have authenticated each other's
             certificate on connection C2 (like [7:C1]).

   [11:C2]   Node 1's certificate has a lower ACP Node-ID than Node 2's,
             therefore Node 1 considers itself the Follower and Node 2
             the Decider on connection C2, but they also identify that
             C2 is to the same mutual peer as their C1, so this has no
             further impact: the roles Decider and Follower where
             already assigned between these two peers by [8:C1].

   [12:C2]   Node 2 (the Decider) closes C1.  Node 1 is fine with this,
             because of its role as the Follower (from [8:C1]).

   [13]      Node 2 (the Decider) and Node 1 (the Follower) start data
             transfer across C2, which makes it become a secure channel
             for the ACP.

   All this negotiation is in the context of an L2 interface.  The
   Decider and Follower will build ACP connections to each other on
   every L2 interface that they both connect to.  An autonomic node MUST
   NOT assume that neighbors with the same L2 or link-local IPv6
   addresses on different L2 interfaces are the same node.  This can
   only be determined after examining the certificate after a successful
   security association attempt.

   The Decider SHOULD NOT suppress attempting a particular ACP secure
   channel protocol connection on one L2 interface because this type of
   ACP secure channel connection has failed to the peer with the same
   ACP certificate on another L2 interface: not only may the supported
   ACP secure channel protocol options be different on the same ACP peer
   across different L2 interfaces, but also error conditions may cause
   inconsistent failures across different L2 interfaces.  Avoiding such
   connection attempt optimizations can therefore help to increase
   robustness in the case of errors.

6.7.  Candidate ACP Neighbor Verification

   Independent of the security association protocol chosen, candidate
   ACP neighbors need to be authenticated based on their domain
   certificate.  This implies that any secure channel protocol MUST
   support certificate-based authentication that can support the ACP
   domain membership check as defined in Section 6.2.3.  If it fails,
   the connection attempt is aborted and an error logged.  Attempts to
   reconnect MUST be throttled.  The RECOMMENDED default is exponential
   base-two backoff with an initial retransmission time (IRT) of 10
   seconds and a maximum retransmission time (MRT) of 640 seconds.

   Failure to authenticate an ACP neighbor when acting in the role of a
   responder of the security authentication protocol MUST NOT impact the
   attempts of the ACP node to attempt establishing a connection as an
   initiator.  Only failed connection attempts as an initiator must
   cause throttling.  This rule is meant to increase resilience of
   secure channel creation.  Section 6.6 shows how simultaneous mutual
   secure channel setup collisions are resolved.

6.8.  Security Association (Secure Channel) Protocols

   This section describes how ACP nodes establish secured data
   connections to automatically discovered or configured peers in the
   ACP.  Section 6.4 describes how peers that are adjacent on an IPv6
   subnet are discovered automatically.  Section 8.2 describes how to
   configure peers that are not adjacent on an IPv6 subnet.

   Section 6.13.5.2 describes how secure channels are mapped to virtual
   IPv6 subnet interfaces in the ACP.  The simple case is to map every
   ACP secure channel to a separate ACP point-to-point virtual interface
   (Section 6.13.5.2.1).  When a single subnet has multiple ACP peers,
   this results in multiple ACP point-to-point virtual interfaces across
   that underlying multiparty IPv6 subnet.  This can be optimized with
   ACP multi-access virtual interfaces (Section 6.13.5.2.2), but the
   benefits of that optimization may not justify the complexity of that
   option.

6.8.1.  General Considerations

   Due to channel selection (Section 6.6), ACP can support an evolving
   set of security association protocols and does not require support
   for a single network-wide MTI.  ACP nodes only need to implement
   those protocols required to interoperate with their candidate peers,
   not with potentially any node in the ACP domain.  See Section 6.8.5
   for an example of this.

   The degree of security required on every hop of an ACP network needs
   to be consistent across the network so that there is no designated
   "weakest link" because it is that "weakest link" that would otherwise
   become the designated point of attack.  When the secure channel
   protection on one link is compromised, it can be used to send and/or
   receive packets across the whole ACP network.  Therefore, even though
   the security association protocols can be different, their minimum
   degree of security should be comparable.

   Secure channel protocols do not need to always support arbitrary
   Layer 3 (L3) connectivity between peers, but can leverage the fact
   that the standard use case for ACP secure channels is an L2
   adjacency.  Hence, L2 dependent mechanisms could be adopted for use
   as secure channel association protocols.

   L2 mechanisms such as strong encrypted radio technologies or [MACSEC]
   may offer equivalent encryption, and the ACP security association
   protocol may only be required to authenticate ACP domain membership
   of a peer and/or derive a key for the L2 mechanism.  Mechanisms that
   leverage such underlying L2 security to auto-discover and associate
   ACP peers are possible and desirable to avoid duplication of
   encryption, but none are specified in this document.

   Strong physical security of a link may stand in where cryptographic
   security is infeasible.  As there is no secure mechanism to
   automatically discover strong physical security solely between two
   peers, it can only be used with explicit configuration, and that
   configuration too could become an attack vector.  This document
   therefore specifies with ACP connect (Section 8.1) only one
   explicitly configured mechanism without any secure channel
   association protocol for the case where both the link and the nodes
   attached to it have strong physical security.

6.8.2.  Common Requirements

   The authentication of peers in any security association protocol MUST
   use the ACP certificate according to Section 6.2.3.  Because auto-
   discovery of candidate ACP neighbors via GRASP (see Section 6.4) as
   specified in this document does not communicate the neighbor's ACP
   certificate, and ACP nodes may not (yet) have any other network
   connectivity to retrieve certificates, any security association
   protocol MUST use a mechanism to communicate the certificate directly
   instead of relying on a referential mechanism such as communicating
   only a hash and/or URL for the certificate.

   A security association protocol MUST use Forward Secrecy (whether
   inherently or as part of a profile of the security association
   protocol).

   Because the ACP payload of legacy protocol payloads inside the ACP
   and hop-by-hop ACP flooded GRASP information is unencrypted, the ACP
   secure channel protocol requires confidentiality.  Symmetric
   encryption for the transmission of secure channel data MUST use
   encryption schemes considered to be security wise equal to or better
   than 256-bit key strength, such as AES-256.  There MUST NOT be
   support for NULL encryption.

   Security association protocols typically only signal the end entity
   certificate (e.g., the ACP certificate) and any possible intermediate
   CA certificates for successful mutual authentication.  The TA has to
   be mutually known and trusted, and therefore its certificate does not
   need to be signaled for successful mutual authentication.
   Nevertheless, for use with ACP secure channel setup, there SHOULD be
   the option to include the TA certificate in the signaling to aid
   troubleshooting, see Section 9.1.1.

   Signaling of TA certificates may not be appropriate when the
   deployment relies on a security model where the TA certificate
   content is considered confidential, and only its hash is appropriate
   for signaling.  ACP nodes SHOULD have a mechanism to select whether
   the TA certificate is signaled or not, assuming that both options are
   possible with a specific secure channel protocol.

   An ACP secure channel MUST immediately be terminated when the
   lifetime of any certificate in the chain used to authenticate the
   neighbor expires or becomes revoked.  This may not be standard
   behavior in secure channel protocols because the certificate
   authentication may only influence the setup of the secure channel in
   these protocols, but may not be revalidated during the lifetime of
   the secure connection in the absence of this requirement.

   When specifying an additional security association protocol for ACP
   secure channels beyond those covered in this document, any protocol
   options that are unnecessary for the support of devices that are
   expected to be able to support the ACP SHOULD be eliminated in order
   to minimize implementation complexity.  For example, definitions for
   security protocols often include old and/or inferior security options
   required only to interoperate with existing devices that cannot
   update to the currently preferred security options.  Such old and/or
   inferior security options do not need to be supported when a security
   association protocol is first specified for the ACP, thus
   strengthening the "weakest link" and simplifying ACP implementation
   overhead.

6.8.3.  ACP via IPsec

   An ACP node announces its ability to support IPsec, negotiated via
   IKEv2, as the ACP secure channel protocol using the "IKEv2"
   'objective-value' in the "AN_ACP" GRASP objective.

   The ACP usage of IPsec and IKEv2 mandates a profile with a narrow set
   of options of the current Standards Track usage guidance for IPsec
   ("Cryptographic Algorithm Implementation Requirements and Usage
   Guidance for Encapsulating Security Payload (ESP) and Authentication
   Header (AH)" [RFC8221]) and IKEv2 ("Algorithm Implementation
   Requirements and Usage Guidance for the Internet Key Exchange
   Protocol Version 2 (IKEv2)" [RFC8247]).  These options result in
   stringent security properties and can exclude deprecated and legacy
   algorithms because there is no need for interoperability with legacy
   equipment for ACP secure channels.  Any such backward compatibility
   would lead only to an increased attack surface and implementation
   complexity, for no benefit.

6.8.3.1.  Native IPsec

   An ACP node that is supporting native IPsec MUST use IPsec in tunnel
   mode, negotiated via IKEv2, and with IPv6 payload (e.g., ESP Next
   Header of 41).  It MUST use local and peer link-local IPv6 addresses
   for encapsulation.  Manual keying MUST NOT be used, see Section 6.2.
   Traffic Selectors are:

   TSi = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
   TSr = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

   IPsec tunnel mode is required because the ACP will route and/or
   forward packets received from any other ACP node across the ACP
   secure channels, and not only its own generated ACP packets.  With
   IPsec transport mode (and no additional encapsulation header in the
   ESP payload), it would only be possible to send packets originated by
   the ACP node itself because the IPv6 addresses of the ESP must be the
   same as that of the outer IPv6 header.

6.8.3.1.1.  RFC 8221 (IPsec/ESP)

   ACP IPsec implementations MUST comply with [RFC8221] and any
   specifications that update it.  The requirements from above and this
   section amend and supersede its requirements.

   The IP Authentication Header (AH) MUST NOT be used (because it does
   not provide confidentiality).

   For the required ESP encryption algorithms in Section 5 of [RFC8221],
   the following guidance applies:

   *  ENCR_NULL AH MUST NOT be used (because it does not provide
      confidentiality).

   *  ENCR_AES_GCM_16 is the only MTI ESP encryption algorithm for ACP
      via IPsec/ESP (it is already listed as MUST in [RFC8221]).

   *  ENCR_AES_CBC with AUTH_HMAC_SHA2_256_128 (as the ESP
      authentication algorithm) and ENCR_AES_CCM_8 MAY be supported.  If
      either provides higher performance than ENCR_AES_GCM_16, it SHOULD
      be supported.

   *  ENCR_CHACHA20_POLY1305 SHOULD be supported at equal or higher
      performance than ENCR_AES_GCM_16.  If that performance is not
      feasible, it MAY be supported.

   IKEv2 indicates an order for the offered algorithms.  The algorithms
   SHOULD be ordered by performance.  The first algorithm supported by
   both sides is generally chosen.

   Explanations:

   *  There is no requirement to interoperate with legacy equipment in
      ACP secure channels, so a single MTI encryption algorithm for
      IPsec in ACP secure channels is sufficient for interoperability
      and allows for the most lightweight implementations.

   *  ENCR_AES_GCM_16 is an Authenticated Encryption with Associated
      Data (AEAD) cipher mode, so no additional ESP authentication
      algorithm is needed, simplifying the MTI requirements of IPsec for
      the ACP.

   *  There is no MTI requirement for the support of ENCR_AES_CBC
      because ENCR_AES_GCM_16 is assumed to be feasible with less cost
      and/or higher performance in modern devices' hardware-accelerated
      implementations compared to ENCR-AES_CBC.

   *  ENCR_CHACHA20_POLY1305 is mandatory in [RFC8221] because of its
      target use as a fallback algorithm in case weaknesses in AES are
      uncovered.  Unfortunately, there is currently no way to
      automatically propagate across an ACP a policy to disallow use of
      AES-based algorithms, so this target benefit of
      ENCR_CHACHA20_POLY1305 cannot fully be adopted yet for the ACP.
      Therefore, this algorithm is only recommended.  Changing from AES
      to this algorithm with a potentially big drop in performance could
      also render the ACP inoperable.  Therefore, there is a performance
      requirement against this algorithm so that it could become an
      effective security backup to AES for the ACP once a policy to
      switch over to it or prefer it is available in an ACP framework.

   [RFC8221] allows for 128-bit or 256-bit AES keys.  This document
   mandates that only 256-bit AES keys MUST be supported.

   When [RFC8221] is updated, ACP implementations will need to consider
   legacy interoperability.

6.8.3.1.2.  RFC 8247 (IKEv2)

   [RFC8247] provides a baseline recommendation for mandatory-to-
   implement ciphers, integrity checks, pseudorandom functions, and
   Diffie-Hellman mechanisms.  Those recommendations, and the
   recommendations of subsequent documents, apply as well to the ACP.
   Because IKEv2 for ACP secure channels is sufficient to be implemented
   in control plane software rather than in Application-Specific
   Integrated Circuit (ASIC) hardware, and ACP nodes supporting IKEv2
   are not assumed to be code space constrained, and because existing
   IKEv2 implementations are expected to support [RFC8247]
   recommendations, this document makes no attempt to simplify its
   recommendations for use with the ACP.

   See [IKEV2IANA] for IANA IKEv2 parameter names used in this text.

   ACP nodes supporting IKEv2 MUST comply with [RFC8247] amended by the
   following requirements, which constitute a policy statement as
   permitted by [RFC8247].

   To signal the ACP certificate chain (including TA) as required by
   Section 6.8.2, the "X.509 Certificate - Signature" payload in IKEv2
   can be used.  It is mandatory according to [RFC7296], Section 3.6.

   ACP nodes SHOULD set up IKEv2 to only use the ACP certificate and TA
   when acting as an IKEv2 responder on the IPv6 link-local address and
   port number indicated in the "AN_ACP" DULL GRASP announcements (see
   Section 6.4).

   When CERTREQ is received from a peer, and it does not indicate any of
   this ACP node's TA certificates, the ACP node SHOULD ignore the
   CERTREQ and continue sending its certificate chain including its TA
   as subject to the requirements and explanations in Section 6.8.2.
   This will not result in successful mutual authentication but assists
   diagnostics.

   Note that with IKEv2, failing authentication will only result in the
   responder receiving the certificate chain from the initiator, but not
   vice versa.  Because ACP secure channel setup is symmetric (see
   Section 6.7), every non-malicious ACP neighbor will attempt to
   connect as an initiator, though, allowing it to obtain the diagnostic
   information about the neighbor's certificate.

   In IKEv2, ACP nodes are identified by their ACP addresses.  The
   ID_IPv6_ADDR IKEv2 identification payload MUST be used and MUST
   convey the ACP address.  If the peer's ACP certificate includes a
   32HEXDIG ACP address in the acp-node-name (not "0" or omitted), the
   address in the IKEv2 identification payload MUST match it.  See
   Section 6.2.3 for more information about "0" or omitted ACP address
   fields in the acp-node-name.

   IKEv2 authentication MUST use authentication method 14 ("Digital
   Signature") for ACP certificates; this authentication method can be
   used with both RSA and ECDSA certificates, indicated by an ASN.1
   object AlgorithmIdentifier.

   The Digital Signature hash SHA2-512 MUST be supported (in addition to
   SHA2-256).

   The IKEv2 Diffie-Hellman key exchange group 19 (256-bit random ECP),
   MUST be supported.  Reason: ECC provides a similar security level to
   finite-field (modular exponentiation (MODP)) key exchange with a
   shorter key length, so is generally preferred absent other
   considerations.

6.8.3.2.  IPsec with GRE Encapsulation

   In network devices, it is often more common to implement high-
   performance virtual interfaces on top of GRE encapsulation than on
   top of a "native" IPsec association (without any other encapsulation
   than those defined by IPsec).  On those devices, it may be beneficial
   to run the ACP secure channel on top of GRE protected by the IPsec
   association.

   The requirements for ESP/IPsec/IKEv2 with GRE are the same as for
   native IPsec (see Section 6.8.3.1) except that IPsec transport mode
   and next protocol GRE (47) are to be negotiated.  Tunnel mode is not
   required because of GRE.  Traffic Selectors are:

  TSi = (47, 0-65535, Initiator-IPv6-LL-addr ... Initiator-IPv6-LL-addr)
  TSr = (47, 0-65535, Responder-IPv6-LL-addr ... Responder-IPv6-LL-addr)

   If the IKEv2 initiator and responder support IPsec over GRE, it will
   be preferred over native IPsec because of how IKEv2 negotiates
   transport mode (as used by this IPsec over GRE profile) versus tunnel
   mode as used by native IPsec (see Section 1.3.1 of [RFC7296]).  The
   ACP IPv6 traffic has to be carried across GRE according to "IPv6
   Support for Generic Routing Encapsulation (GRE)" [RFC7676].

6.8.4.  ACP via DTLS

   This document defines the use of ACP via DTLS on the assumption that
   it is likely the first transport encryption supported in some classes
   of constrained devices: DTLS is commonly used in constrained devices
   when IPsec is not.  Code space on those devices may be also be too
   limited to support more than the minimum number of required
   protocols.

   An ACP node announces its ability to support DTLS version 1.2
   ("Datagram Transport Layer Security Version 1.2" [RFC6347]) compliant
   with the requirements defined in this document as an ACP secure
   channel protocol in GRASP through the "DTLS" 'objective-value' in the
   "AN_ACP" objective (see Section 6.4).

   To run ACP via UDP and DTLS, a locally assigned UDP port is used that
   is announced as a parameter in the GRASP "AN_ACP" objective to
   candidate neighbors.  This port can also be any newer version of DTLS
   as long as that version can negotiate a DTLS 1.2 connection in the
   presence of a peer that only supports DTLS 1.2.

   All ACP nodes supporting DTLS as a secure channel protocol MUST
   adhere to the DTLS implementation recommendations and security
   considerations of BCP 195 [RFC7525] except with respect to the DTLS
   version.  ACP nodes supporting DTLS MUST support DTLS 1.2.  They MUST
   NOT support older versions of DTLS.

   Unlike for IPsec, no attempts are made to simplify the requirements
   of the recommendations in BCP 195 [RFC7525] because the expectation
   is that DTLS would use software-only implementations where the
   ability to reuse widely adopted implementations is more important
   than the ability to minimize the complexity of a hardware-accelerated
   implementation, which is known to be important for IPsec.

   DTLS 1.3 [TLS-DTLS13] is "backward compatible" with DTLS 1.2 (see
   Section 1 of [TLS-DTLS13]).  A DTLS implementation supporting both
   DTLS 1.2 and DTLS 1.3 does comply with the above requirements of
   negotiating to DTLS 1.2 in the presence of a DTLS 1.2 only peer, but
   using DTLS 1.3 when booth peers support it.

   Version 1.2 is the MTI version of DTLS in this specification because
   of the following:

   *  There is more experience with DTLS 1.2 across the spectrum of
      target ACP nodes.

   *  Firmware of lower-end, embedded ACP nodes may not support a newer
      version for a long time.

   *  There are significant changes with DTLS 1.3, such as a different
      record layer requiring time to gain implementation and deployment
      experience especially on lower-end devices with limited code
      space.

   *  The existing BCP [RFC7525] for DTLS 1.2 may take an equally longer
      time to be updated with experience from a newer DTLS version.

   *  There are no significant benefits of DTLS 1.3 over DTLS 1.2 that
      are use-case relevant in the context of the ACP options for DTLS.
      For example, signaling performance improvements for session setup
      in DTLS 1.3 is not important for the ACP given the long-lived
      nature of ACP secure channel connections and the fact that DTLS
      connections are mostly link local (short RTT).

   Nevertheless, newer versions of DTLS, such as DTLS 1.3, have stricter
   security requirements, and the use of the latest standard protocol
   version is in general recommended for IETF security standards.
   Therefore, ACP implementations are advised to support all the newer
   versions of DTLS that can still negotiate down to DTLS 1.2.

   There is no additional session setup or other security association
   besides this simple DTLS setup.  As soon as the DTLS session is
   functional, the ACP peers will exchange ACP IPv6 packets as the
   payload of the DTLS transport connection.  Any DTLS-defined security
   association mechanisms such as rekeying are used as they would be for
   any transport application relying solely on DTLS.

6.8.5.  ACP Secure Channel Profiles

   As explained in the beginning of Section 6.6, there is no single
   secure channel mechanism mandated for all ACP nodes.  Instead, this
   section defines two ACP profiles, "baseline" and "constrained", for
   ACP nodes that do introduce such requirements.

   An ACP node supporting the baseline profile MUST support IPsec
   natively and MAY support IPsec via GRE.  An ACP node supporting the
   constrained profile that cannot support IPsec MUST support DTLS.  An
   ACP node connecting an area of constrained ACP nodes with an area of
   baseline ACP nodes needs to support both IPsec and DTLS and therefore
   supports both the baseline and constrained profiles.

   Explanation: not all types of ACP nodes are able to or need to
   connect directly to each other, nor are they able to support or
   prefer all possible secure channel mechanisms.  For example, IoT
   devices with limited code space may only support DTLS because that
   code already exists on them for end-to-end security, but high-end
   core routers may not want to support DTLS because they can perform
   IPsec in accelerated hardware, but they would need to support DTLS in
   an underpowered CPU forwarding path shared with critical control
   plane operations.  This is not a deployment issue for a single ACP
   across these types of nodes as long as there are also appropriate
   gateway ACP nodes that sufficiently support many secure channel
   mechanisms to allow interconnecting areas of ACP nodes with a more
   constrained set of secure channel protocols.  On the edge between IoT
   areas and high-end core networks, general-purpose routers that act as
   those gateways and that can support a variety of secure channel
   protocols are the norm already.

   Native IPsec with tunnel mode provides the shortest encapsulation
   overhead.  GRE may be preferred by legacy implementations because, in
   the past, the virtual interfaces required by ACP design in
   conjunction with secure channels have been implemented more often for
   GRE than purely for native IPsec.

   ACP nodes need to specify the set of secure ACP mechanisms they
   support in documentation and should declare which profile they
   support according to the above requirements.

6.9.  GRASP in the ACP

6.9.1.  GRASP as a Core Service of the ACP

   The ACP MUST run an instance of GRASP inside of it.  It is a key part
   of the ACP services.  The function in GRASP that makes it fundamental
   as a service of the ACP is the ability to provide ACP-wide service
   discovery (using objectives in GRASP).

   ACP provides IP unicast routing via RPL (see Section 6.12).

   The ACP does not use IP multicast routing nor does it provide generic
   IP multicast services (the handling of GRASP link-local multicast
   messages is explained in Section 6.9.2).  Instead, the ACP provides
   service discovery via the objective discovery/announcement and
   negotiation mechanisms of the ACP GRASP instance (services are a form
   of objectives).  These mechanisms use hop-by-hop reliable flooding of
   GRASP messages for both service discovery (GRASP M_DISCOVERY
   messages) and service announcement (GRASP M_FLOOD messages).

   See Appendix A.5 for discussion about this design choice of the ACP.

6.9.2.  ACP as the Security and Transport Substrate for GRASP

   In the terminology of GRASP [RFC8990], the ACP is the security and
   transport substrate for the GRASP instance run inside the ACP ("ACP
   GRASP").

   This means that the ACP is responsible for ensuring that this
   instance of GRASP is only sending messages across the ACP GRASP
   virtual interfaces.  Whenever the ACP adds or deletes such an
   interface because of new ACP secure channels or loss thereof, the ACP
   needs to indicate this to the ACP instance of GRASP.  The ACP exists
   also in the absence of any active ACP neighbors.  It is created when
   the node has a domain certificate, and it continues to exist even if
   all of its neighbors cease operation.

   In this case, ASAs using GRASP running on the same node still need to
   be able to discover each other's objectives.  When the ACP does not
   exist, ASAs leveraging the ACP instance of GRASP via APIs MUST still
   be able to operate, and they MUST be able to understand that there is
   no ACP and that therefore the ACP instance of GRASP cannot operate.

   How the ACP acts as the security and transport substrate for GRASP is
   shown in Figure 8.

   GRASP unicast messages inside the ACP always use the ACP address.
   Link-local addresses from the ACP VRF MUST NOT be used inside
   objectives.  GRASP unicast messages inside the ACP are transported
   via TLS.  See Section 6.1 for TLS requirements.  TLS mutual
   authentication MUST use the ACP domain membership check defined in
   Section 6.2.3.

   GRASP link-local multicast messages are targeted for a specific ACP
   virtual interface (as defined Section 6.13.5), but they are sent by
   the ACP to an ACP GRASP virtual interface that is constructed from
   the TCP connection(s) to the IPv6 link-local neighbor address(es) on
   the underlying ACP virtual interface.  If the ACP GRASP virtual
   interface has two or more neighbors, the GRASP link-local multicast
   messages are replicated to all neighbor TCP connections.

   TCP and TLS connections for GRASP in the ACP use the IANA-assigned
   TCP port for GRASP (7017).  Effectively, the transport stack is
   expected to be TLS for connections to and from the ACP address (e.g.,
   global-scope address(es)) and TCP for connections to and from the
   link-local addresses on the ACP virtual interfaces.  The latter ones
   are only used for the flooding of GRASP messages.

       ..............................ACP..............................
       .                                                             .
       .         /-GRASP-flooding-\         ACP GRASP instance       .
       .        /                  \                                 A
       .    GRASP      GRASP      GRASP                              C
       .  link-local   unicast  link-local                           P
       .   multicast  messages   multicast                           .
       .   messages      |       messages                            .
       .      |          |          |                                .
       ...............................................................
       .      v          v          v    ACP security and transport  .
       .      |          |          |    substrate for GRASP         .
       .      |          |          |                                .
       .      |       ACP GRASP     |       - ACP GRASP              A
       .      |       loopback      |         loopback interface     C
       .      |       interface     |       - ACP-cert auth          P
       .      |         TLS         |                                .
       .   ACP GRASP     |       ACP GRASP  - ACP GRASP virtual      .
       .   subnet1       |       subnet2      interfaces             .
       .     TCP         |         TCP                               .
       .      |          |          |                                .
       ...............................................................
       .      |          |          |   ^^^ Users of ACP (GRASP/ASA) .
       .      |          |          |   ACP interfaces/addressing    .
       .      |          |          |                                .
       .      |          |          |                                A
       .      | ACP loopback interf.|      <- ACP loopback interface C
       .      |      ACP-address    |       - address (global ULA)   P
       .    subnet1      |        subnet2  <- ACP virtual interfaces .
       .  link-local     |      link-local  - link-local addresses   .
       ...............................................................
       .      |          |          |   ACP VRF                      .
       .      |     RPL-routing     | virtual routing and forwarding .
       .      |   /IP-Forwarding\   |                                A
       .      |  /               \  |                                C
       .  ACP IPv6 packets   ACP IPv6 packets                        P
       .      |/                   \|                                .
       .    IPsec/DTLS        IPsec/DTLS  - ACP-cert auth            .
       ...............................................................
                |                   |   Data Plane
                |                   |
                |                   |     - ACP secure channel
            link-local        link-local  - encapsulation addresses
              subnet1            subnet2  - data plane interfaces
                |                   |
             ACP-Nbr1            ACP-Nbr2

        Figure 8: ACP as Security and Transport Substrate for GRASP

6.9.2.1.  Discussion

   TCP encapsulation for GRASP M_DISCOVERY and M_FLOOD link-local
   messages is used because these messages are flooded across
   potentially many hops to all ACP nodes, and a single link with even
   temporary packet-loss issues (e.g., a Wi-Fi or Powerline link) can
   reduce the probability for loss-free transmission so much that
   applications would want to increase the frequency with which they
   send these messages.  Such shorter periodic retransmission of
   datagrams would result in more traffic and processing overhead in the
   ACP than the hop-by-hop, reliable retransmission mechanism offered by
   TCP and duplicate elimination by GRASP.

   TLS is mandated for GRASP non-link-local unicast because the ACP
   secure channel mandatory authentication and encryption protects only
   against attacks from the outside but not against attacks from the
   inside: compromised ACP members that have (not yet) been detected and
   removed (e.g., via domain certificate revocation and/or expiry).

   If GRASP peer connections were to use just TCP, compromised ACP
   members could simply eavesdrop passively on GRASP peer connections
   for which they are on-path ("man in the middle" or MITM) or intercept
   and modify messages.  With TLS, it is not possible to completely
   eliminate problems with compromised ACP members, but attacks are a
   lot more complex.

   Eavesdropping and/or spoofing by a compromised ACP node is still
   possible because in the model of the ACP and GRASP, the provider and
   consumer of an objective have initially no unique information (such
   as an identity) about the other side that would allow them to
   distinguish a benevolent from a compromised peer.  The compromised
   ACP node would simply announce the objective as well, potentially
   filter the original objective in GRASP when it is a MITM and act as
   an application-level proxy.  This of course requires that the
   compromised ACP node understand the semantics of the GRASP
   negotiation to an extent that allows the compromised node to proxy
   the messages without being detected, but in an ACP environment, this
   is quite likely public knowledge or even standardized.

   The GRASP TLS connections are run the same as any other ACP traffic
   through the ACP secure channels.  This leads to double authentication
   and encryption, which has the following benefits:

   *  Secure channel methods such as IPsec may provide protection
      against additional attacks, for example, reset attacks.

   *  The secure channel method may leverage hardware acceleration, and
      there may be little or no gain in eliminating it.

   *  The security model for ACP GRASP is no different than other ACP
      traffic.  Instead, there is just another layer of protection
      against certain attacks from the inside, which is important due to
      the role of GRASP in the ACP.

6.10.  Context Separation

   The ACP is in a separate context from the normal data plane of the
   node.  This context includes the ACP channels' IPv6 forwarding and
   routing as well as any required higher-layer ACP functions.

   In a classical network system, a dedicated VRF is one logical
   implementation option for the ACP.  If allowed by the system's
   software architecture, separation options that minimize shared
   components, such as a logical container or virtual machine instance,
   are preferred.  The context for the ACP needs to be established
   automatically during the bootstrap of a node.  As much as possible,
   it should be protected from being modified unintentionally by (data
   plane) configuration.

   Context separation improves security because the ACP is not reachable
   from the data plane routing or forwarding table(s).  Also,
   configuration errors from the data plane setup do not affect the ACP.

6.11.  Addressing inside the ACP

   The channels explained above typically only establish communication
   between two adjacent nodes.  In order for communication to happen
   across multiple hops, the Autonomic Control Plane requires ACP
   network-wide valid addresses and routing.  Each ACP node creates a
   loopback interface with an ACP network-wide unique address (prefix)
   inside the ACP context (as explained in Section 6.10).  This address
   may be used also in other virtual contexts.

   With the algorithm introduced here, all ACP nodes in the same routing
   subdomain have the same /48 ULA prefix.  Conversely, ULA Global IDs
   from different domains are unlikely to clash, such that two ACP
   networks can be merged, as long as the policy allows that merge.  See
   also Section 10.1 for a discussion on merging domains.

   Links inside the ACP only use link-local IPv6 addressing, such that
   each node's ACP only requires one routable address prefix.

6.11.1.  Fundamental Concepts of Autonomic Addressing

   *  Usage: autonomic addresses are exclusively used for self-
      management functions inside a trusted domain.  They are not used
      for user traffic.  Communications with entities outside the
      trusted domain use another address space, for example, a normally
      managed routable address space (called "data plane" in this
      document).

   *  Separation: autonomic address space is used separately from user
      address space and other address realms.  This supports the
      robustness requirement.

   *  Loopback only: only ACP loopback interfaces (and potentially those
      configured for ACP connect, see Section 8.1) carry routable
      address(es); all other interfaces (called ACP virtual interfaces)
      only use IPv6 link-local addresses.  The usage of IPv6 link-local
      addressing is discussed in "Using Only Link-Local Addressing
      inside an IPv6 Network" [RFC7404].

   *  Use of ULA: for loopback interfaces of ACP nodes, we use ULA with
      the L bit set to 1 (as defined in Section 3.1 of [RFC4193]).  Note
      that the random hash for ACP loopback addresses uses the
      definition in Section 6.11.2 and not the one in [RFC4193],
      Section 3.2.2.

   *  No external connectivity: the addresses do not provide access to
      the Internet.  If a node requires further connectivity, it should
      use another, traditionally managed addressing scheme in parallel.

   *  Addresses in the ACP are permanent and do not support temporary
      addresses as defined in "Temporary Address Extensions for
      Stateless Address Autoconfiguration in IPv6" [RFC8981].

   *  Addresses in the ACP are not considered sensitive on privacy
      grounds because ACP nodes are not expected to be end-user hosts,
      and therefore ACP addresses do not represent end users or groups
      of end users.  All ACP nodes are in one (potentially federated)
      administrative domain.  For ACP traffic, the nodes are assumed to
      be either candidate hosts or transit nodes.  There are no transit
      nodes with fewer privileges to know the identity of other hosts in
      the ACP.  Therefore, ACP addresses do not need to be pseudorandom
      as discussed in "Security and Privacy Considerations for IPv6
      Address Generation Mechanisms" [RFC7721].  Because they are not
      propagated to untrusted (non-ACP) nodes and stay within a domain
      (of trust), we also do not consider them to be subject to scanning
      attacks.

   The ACP is based exclusively on IPv6 addressing for a variety of
   reasons:

   *  Simplicity, reliability, and scale: if other network-layer
      protocols were supported, each would have to have its own set of
      security associations, routing table, and process, etc.

   *  Autonomic functions do not require IPv4: autonomic functions and
      autonomic service agents are new concepts.  They can be
      exclusively built on IPv6 from day one.  There is no need for
      backward compatibility.

   *  OAM protocols do not require IPv4: the ACP may carry OAM
      protocols.  All relevant protocols (SNMP, TFTP, SSH, SCP, RADIUS,
      Diameter, NETCONF, etc.) are available in IPv6.  See also
      [RFC8368] for how ACP could be made to interoperate with IPv4-only
      OAM.

   Further explanation about the addressing and routing-related reasons
   for the choice of the autonomous ACP addressing can be found in
   Section 6.13.5.1.

6.11.2.  The ACP Addressing Base Scheme

   The ULA addressing base scheme for ACP nodes has the following
   format:

     8      40                     2                     78
   +--+-------------------------+------+------------------------------+
   |fd| hash(routing-subdomain) | Type |     (sub-scheme)             |
   +--+-------------------------+------+------------------------------+

                    Figure 9: ACP Addressing Base Scheme

   The first 48 bits follow the ULA scheme as defined in [RFC4193], to
   which a Type field is added.

   fd:  Identifies a locally defined ULA address.

   hash(routing-subdomain):  The 40-bit ULA Global ID (a term from
      [RFC4193]) for ACP addresses carried in the acp-node-name in the
      ACP certificates are the first 40 bits of the SHA-256 hash of the
      routing-subdomain from the same acp-node-name.  In the example of
      Section 6.2.2, the routing-subdomain is
      "area51.research.acp.example.com", and the 40-bit ULA Global ID is
      89b714f3db.

      When creating a new routing-subdomain for an existing Autonomic
      Network, it MUST be ensured that rsub is selected so the resulting
      hash of the routing-subdomain does not collide with the hash of
      any preexisting routing-subdomains of the Autonomic Network.  This
      ensures that ACP addresses created by registrars for different
      routing subdomains do not collide with each other.

      To allow for extensibility, the fact that the ULA Global ID is a
      hash of the routing-subdomain SHOULD NOT be assumed by any ACP
      node during normal operations.  The hash function is only executed
      during the creation of the certificate.  If BRSKI is used, then
      the BRSKI registrar will create the acp-node-name in response to
      the EST Certificate Signing Request (CSR) Attributes Request
      message sent by the pledge.

      Establishing connectivity between different ACPs (different acp-
      domain-names) is outside the scope of this specification.  If it
      is being done through future extensions, then the rsub of all
      routing-subdomains across those Autonomic Networks needs to be
      selected so that the resulting routing-subdomain hashes do not
      collide.  For example, a large cooperation with its own private TA
      may want to create different Autonomic Networks that initially do
      not connect but where the option to do so should be kept open.
      When taking this possibility into account, it is always easy to
      select rsub so that no collisions happen.

   Type:  This field allows different addressing sub-schemes.  This
      addresses the "upgradability" requirement.  Assignment of types
      for this field will be maintained by IANA.

   (sub-scheme):  The sub-scheme may imply a range or set of addresses
      assigned to the node.  This is called the ACP address range/set
      and explained in each sub-scheme.

   Please refer to Section 6.11.7 and Appendix A.1 for further
   explanations for why the following addressing sub-schemes are used
   and why multiple are necessary.

   The following summarizes the addressing sub-schemes:

         +======+==============+=======+=====+=========+========+
         | Type | Name         | F-bit | Z   | V-bits  | Prefix |
         +======+==============+=======+=====+=========+========+
         | 0    | ACP-Zone     | N/A   | 0   | 1 bit   | /127   |
         +------+--------------+-------+-----+---------+--------+
         | 0    | ACP-Manual   | N/A   | 1   | N/A     | /64    |
         +------+--------------+-------+-----+---------+--------+
         | 1    | ACP-Vlong-8  | 0     | N/A | 8 bits  | /120   |
         +------+--------------+-------+-----+---------+--------+
         | 1    | ACP-Vlong-16 | 1     | N/A | 16 bits | /112   |
         +------+--------------+-------+-----+---------+--------+
         | 2    | Reserved / For future definition/allocation   |
         +------+-----------------------------------------------+
         | 3    | Reserved / For future definition/allocation   |
         +------+-----------------------------------------------+

                     Table 1: Addressing Sub-Schemes

   The F-bit (format bit, Section 6.11.5) and Z (Section 6.11.4) are two
   encoding fields that are explained in the sections covering the sub-
   schemes that use them.  V-bits is the number of bits of addresses
   allocated to the ACP node.  Prefix is the prefix that the ACP node is
   announcing into RPL.

6.11.3.  ACP Zone Addressing Sub-Scheme (ACP-Zone)

   This sub-scheme is used when the Type field of the base scheme is 0
   and the Z bit is 0.

                    64                             64
   +-----------------+---+---------++-----------------------------+---+
   |  (base scheme)  | Z | Zone-ID ||           Node-ID               |
   |                 |   |         || Registrar-ID |   Node-Number| V |
   +-----------------+---+---------++--------------+--------------+---+
            50         1     13            48           15          1

                 Figure 10: ACP Zone Addressing Sub-Scheme

   The fields are defined as follows:

   Type:  MUST be 0.

   Z:  MUST be 0.

   Zone-ID:  A value for a network zone.

   Node-ID:  A unique value for each node.

      The 64-bit Node-ID must be unique across the ACP domain for each
      node.  It is derived and composed as follows:

      Registrar-ID (48 bits):  A number unique inside the domain
         identifying the ACP registrar that assigned the Node-ID to the
         node.  One or more domain-wide unique identifiers of the ACP
         registrar can be used for this purpose.  See Section 6.11.7.2.

      Node-Number:  A number to make the Node-ID unique.  This can be
         sequentially assigned by the ACP registrar owning the
         Registrar-ID.

   V (1 bit):  Virtualization bit:

      0:  Indicates the ACP itself ("ACP node base system)

      1:  Indicates the optional "host" context on the ACP node (see
         below).

   In the Zone Addressing Sub-Scheme, the ACP address in the certificate
   has V field as all zero bits.

   The ACP address set of the node includes addresses with any Zone-ID
   value and any V value.  Therefore, no two nodes in the same ACP and
   the same hash(routing-subdomain) can have the same Node-ID with the
   Zone Addressing Sub-Scheme, for example, by differing only in their
   Zone-ID.

   The Virtualization bit in this sub-scheme allows the easy addition of
   the ACP as a component to existing systems without causing problems
   in the port number space between the services in the ACP and the
   existing system.  V=0 is the ACP router (autonomic node base system),
   V=1 is the host with preexisting transport endpoints on it that could
   collide with the transport endpoints used by the ACP router.  The ACP
   host could, for example, have a P2P (peer-to-peer) virtual interface
   with the V=0 address as its router to the ACP.  Depending on the
   software design of ASAs, which is outside the scope of this
   specification, they may use the V=0 or V=1 address.

   The location of the V bit(s) at the end of the address allows the
   announcement of a single prefix for each ACP node.  For example, in a
   network with 20,000 ACP nodes, this avoids 20,000 additional routes
   in the routing table.

   It is RECOMMENDED that only Zone-ID 0 is used unless it is meant to
   be used in conjunction with operational practices for partial or
   incremental adoption of the ACP as described in Section 9.4.

   Note: Zones and Zone-ID as defined here are not related to zones or
   zone_id defined in "IPv6 Scoped Address Architecture" [RFC4007].  ACP
   zone addresses are not scoped (i.e., reachable only from within a
   zone as defined by [RFC4007]) but are reachable across the whole ACP.
   A zone_id is a zone index that has only local significance on a node
   [RFC4007], whereas an ACP Zone-ID is an identifier for an ACP zone
   that is unique across that ACP.

6.11.4.  ACP Manual Addressing Sub-Scheme (ACP-Manual)

   This sub-scheme is used when the Type field of the base scheme is 0
   and the Z bit is 1.

                   64                             64
   +---------------------+---+----------++-----------------------------+
   |    (base scheme)    | Z | Subnet-ID||     Interface Identifier    |
   +---------------------+---+----------++-----------------------------+
            50             1    13

                Figure 11: ACP Manual Addressing Sub-Scheme

   The fields are defined as follows:

   Type:  MUST be 0.

   Z:  MUST be 1.

   Subnet-ID:  Configured subnet identifier.

   Interface Identifier:  Interface identifier according to [RFC4291].

   This sub-scheme is meant for the "manual" allocation to subnets where
   the other addressing schemes cannot be used.  The primary use case is
   for assignment to ACP connect subnets (see Section 8.1.1).

   "Manual" means that allocations of the Subnet-ID need to be done with
   preexisting, non-autonomic mechanisms.  Every subnet that uses this
   addressing sub-scheme needs to use a unique Subnet-ID (unless some
   anycast setup is done).

   The Z bit field was added to distinguish between the Zone Addressing
   Sub-Scheme and the Manual Addressing Sub-Scheme without requiring one
   more bit in the base scheme and therefore allowing for the Vlong
   Addressing Sub-Scheme (described in Section 6.11.5) to have one more
   bit available.

   The Manual Addressing Sub-Scheme addresses SHOULD NOT be used in ACP
   certificates.  Any node capable of building ACP secure channels and
   is permitted by registrar policy to participate in building ACP
   secure channels SHOULD receive an ACP address (prefix) from one of
   the other ACP addressing sub-schemes.  A node that cannot or is not
   permitted to participate in ACP secure channels can connect to the
   ACP via ACP connect interfaces of ACP edge nodes (see Section 8.1)
   without setting up an ACP secure channel.  Its ACP certificate MUST
   omit the acp-address field to indicate that its ACP certificate is
   only usable for non-ACP secure channel authentication, such as end-
   to-end transport connections across the ACP or data plane.

   Address management of ACP connect subnets is done using traditional
   assignment methods and existing IPv6 protocols.  See Section 8.1.3
   for details.  Therefore, the notion of /V-bits multiple addresses
   assigned to the ACP nodes does not apply to this sub-scheme.

6.11.5.  ACP Vlong Addressing Sub-Scheme (ACP-Vlong-8/ACP-Vlong-16)

   This addressing sub-scheme is used when the Type field of the base
   scheme is 1.

             50                              78
   +---------------------++-----------------------------+----------+
   |    (base scheme)    ||           Node-ID                      |
   |                     || Registrar-ID |F| Node-Number|        V |
   +---------------------++--------------+--------------+----------+
             50                46         1   23/15          8/16

                 Figure 12: ACP Vlong Addressing Sub-Scheme

   This addressing sub-scheme foregoes the Zone-ID field
   (Section 6.11.3) to allow for larger, flatter routed networks (e.g.,
   as in IoT) with 8,421,376 Node-Numbers (2^23 + 2^15).  It also allows
   for up to 2^16 (i.e., 65,536) different virtualized addresses within
   a node, which could be used to address individual software components
   in an ACP node.

   The fields are the same as in the Zone Addressing Sub-Scheme
   (Section 6.11.3) with the following refinements:

   F:  Format bit.  This bit determines the format of the subsequent
      bits.

   V:  Virtualization bit: this is a field that is either 8 or 16 bits.
      For F=0, it is 8 bits, for F=1 it is 16 bits.  The V-bits are
      assigned by the ACP node.  In the ACP certificate's ACP address
      (Section 6.2.2), the V-bits are always set to 0.

   Registrar-ID:  To maximize Node-Number and V, the Registrar-ID is
      reduced to 46 bits.  One or more domain-wide unique identifiers of
      the ACP registrar can be used for this purpose.  See
      Section 6.11.7.2.

   Node-Number:  The Node-Number is unique to each ACP node.  There are
      two formats for the Node-Number.  When F=0, the Node-Number is 23
      bits, for F=1, it is 15 bits.  Each format of Node-Number is
      considered to be in a unique number space.

   The F=0 bit format addresses are intended to be used for "general
   purpose" ACP nodes that would potentially have a limited number (less
   than 256) of clients (ASA and/or autonomic functions or legacy
   services) of the ACP that require separate V(irtual) addresses.

   The F=1 bit Node-Numbers are intended for ACP nodes that are ACP edge
   nodes (see Section 8.1.1) or that have a large number of clients
   requiring separate V(irtual) addresses, for example, large SDN
   controllers with container modular software architecture (see
   Section 8.1.2).

   In the Vlong Addressing Sub-Scheme, the ACP address in the
   certificate has all V field bits as zero.  The ACP address set for
   the node includes any V value.

6.11.6.  Other ACP Addressing Sub-Schemes

   Before further addressing sub-schemes are defined, experience with
   the schemes defined here should be collected.  The schemes defined in
   this document have been devised to allow hopefully a sufficiently
   flexible setup of ACPs for a variety of situations.  These reasons
   also lead to the fairly liberal use of address space: the Zone
   Addressing Sub-Scheme is intended to enable optimized routing in
   large networks by reserving bits for Zone-IDs.  The Vlong Addressing
   Sub-Scheme enables the allocation of 8/16-bit of addresses inside
   individual ACP nodes.  Both address spaces allow distributed,
   uncoordinated allocation of node addresses by reserving bits for the
   Registrar-ID field in the address.

6.11.7.  ACP Registrars

   ACP registrars are responsible for enrolling candidate ACP nodes with
   ACP certificates and associated trust anchor(s).  They are also
   responsible for including an acp-node-name field in the ACP
   certificate.  This field carries the ACP domain name and the ACP
   node's ACP address prefix.  This address prefix is intended to
   persist unchanged through the lifetime of the ACP node.

   Because of the ACP addressing sub-schemes, an ACP domain can have
   multiple distributed ACP registrars that do not need to coordinate
   for address assignment.  ACP registrars can also be sub-CAs, in which
   case they can also assign ACP certificates without dependencies
   against a (shared) TA (except during renewals of their own
   certificates).

   ACP registrars are PKI registration authorities (RA) enhanced with
   the handling of the ACP certificate-specific fields.  They request
   certificates for ACP nodes from a CA through any appropriate
   mechanism (out of scope in this document, but this mechanism is
   required to be BRSKI for ANI registrars).  Only nodes that are
   trusted to be compliant with the registrar requirements described in
   this section can be given the necessary credentials to perform this
   RA function, such as the credential for the ACP registrar to connect
   to the CA as a registrar.

6.11.7.1.  Use of BRSKI or Other Mechanisms or Protocols

   Any protocols or mechanisms may be used by ACP registrars as long as
   the resulting ACP certificate and TA certificate(s) can be used by
   other domain members to perform the ACP domain membership check
   described in Section 6.2.3, and the acp-node-name meets the ACP
   addressing requirements described in the next three sections.

   An ACP registrar could be a person deciding whether to enroll a
   candidate ACP node and then orchestrating the enrollment of the ACP
   certificate and associated TA, using command line or web-based
   commands on the candidate ACP node and TA to generate and sign the
   ACP certificate and configure certificate and TA onto the node.

   The only currently defined protocol for ACP registrars is BRSKI
   [RFC8995].  When BRSKI is used, the ACP nodes are called ANI nodes,
   and the ACP registrars are called BRSKI or ANI registrars.  The BRSKI
   specification does not define the handling of the acp-node-name field
   because the rules do not depend on BRSKI but apply equally to any
   protocols or mechanisms that an ACP registrar may use.

6.11.7.2.  Unique Address/Prefix Allocation

   ACP registrars MUST NOT allocate ACP address prefixes to ACP nodes
   via the acp-node-name that would collide with the ACP address
   prefixes of other ACP nodes in the same ACP domain.  This includes
   both prefixes allocated by the same ACP registrar to different ACP
   nodes as well as prefixes allocated by other ACP registrars for the
   same ACP domain.

   To support such unique address allocation, an ACP registrar MUST have
   one or more 46-bit identifiers, called the Registrar-ID, that are
   unique across the ACP domain.  Allocation of Registrar-ID(s) to an
   ACP registrar can happen through OAM mechanisms in conjunction with
   some database and/or allocation orchestration.

   ACP registrars running on physical devices with known globally unique
   EUI-48 MAC address(es) (EUI stands for "Extended Unique Identifier")
   can use the lower 46 bits of those address(es) as unique Registrar-
   IDs without requiring any external signaling and/or configuration
   (the upper two bits, V and U, are not uniquely assigned but are
   functional).  This approach is attractive for distributed, non-
   centrally administered, lightweight ACP registrar implementations.
   There is no mechanism to deduce from a MAC address itself whether it
   is actually uniquely assigned.  Implementations need to consult
   additional offline information before making this assumption, for
   example, by knowing that a particular physical product or Network
   Interface Controller (NIC) chip is guaranteed to use globally unique
   assigned EUI-48 MAC address(es).

   When the candidate ACP device (called pledge in BRSKI) is to be
   enrolled into an ACP domain, the ACP registrar needs to allocate a
   unique ACP address to the node and ensure that the ACP certificate
   gets an acp-node-name field (Section 6.2.2) with the appropriate
   information: ACP domain name, ACP address, and so on.  If the ACP
   registrar uses BRSKI, it signals the ACP acp-node-name field to the
   pledge via EST CSR Attributes (see [RFC8995], Section 5.9.2, "EST CSR
   Attributes").

6.11.7.3.  Addressing Sub-Scheme Policies

   The ACP registrar selects for the candidate ACP node a unique address
   prefix from an appropriate ACP addressing sub-scheme, either a Zone
   Addressing Sub-Scheme prefix (see Section 6.11.3), or a Vlong
   Addressing Sub-Scheme prefix (see Section 6.11.5).  The assigned ACP
   address prefix encoded in the acp-node-name field of the ACP
   certificate indicates to the ACP node its ACP address information.
   The addressing sub-scheme indicates the prefix length: /127 for the
   Zone Addressing Sub-Scheme, /120 or /112 for the Vlong Addressing
   Sub-Scheme.  The first address of the prefix is the ACP address.  All
   other addresses in the prefix are for other uses by the ACP node as
   described in the Zone Addressing Sub-Scheme and Vlong Addressing Sub-
   Scheme sections.  The ACP address prefix itself is then signaled by
   the ACP node into the ACP routing protocol (see Section 6.12) to
   establish IPv6 reachability across the ACP.

   The choice of addressing sub-scheme and prefix length in the Vlong
   Addressing Sub-Scheme is subject to ACP registrar policy.  It could
   be an ACP domain-wide policy, or a per ACP node or per ACP node type
   policy.  For example, in BRSKI, the ACP registrar is aware of the
   IDevID certificate of the candidate ACP node, which typically
   contains a "serialNumber" attribute in the subject field
   distinguished name encoding that often indicates the node's vendor
   and device type, and it can be used to drive a policy for selecting
   an appropriate addressing sub-scheme for the (class of) node(s).

   ACP registrars SHOULD default to allocating Zone Addressing Sub-
   Scheme addresses with Zone-ID 0.

   ACP registrars that are aware of the IDevID certificate of a
   candidate ACP device SHOULD be able to choose the Zone vs. Vlong
   Addressing Sub-Scheme for ACP nodes based on the "serialNumber"
   attribute [X.520] in the subject field distinguished name encoding of
   the IDevID certificate, for example, by the PID (Product Identifier)
   part, which identifies the product type, or by the complete
   "serialNumber".  The PID, for example, could identify nodes that
   allow for specialized ASA requiring multiple addresses or for non-
   autonomic virtual machines (VMs) for services, and those nodes could
   receive Vlong Addressing Sub-Scheme ACP addresses.

   In a simple allocation scheme, an ACP registrar remembers
   persistently across reboots its currently used Registrar-ID and, for
   each addressing scheme (Zone with Zone-ID 0, Vlong with /112, Vlong
   with /120), the next Node-Number available for allocation, and it
   increases the next Node-Number during successful enrollment of an ACP
   node.  In this simple allocation scheme, the ACP registrar would not
   recycle ACP address prefixes from ACP nodes that are no longer used.

   If allocated addresses cannot be remembered by registrars, then it is
   necessary either to use a new value for the Register-ID field in the
   ACP addresses or to determine allocated ACP addresses by determining
   the addresses of reachable ACP nodes, which is not necessarily the
   set of all ACP nodes.  Untracked ACP addresses can be reclaimed by
   revoking or not renewing their certificates and instead handing out
   new certificates with new addresses (for example, with a new
   Registrar-ID value).  Note that such strategies may require
   coordination amongst registrars.

6.11.7.4.  Address/Prefix Persistence

   When an ACP certificate is renewed or rekeyed via EST or other
   mechanisms, the ACP address/prefix in the acp-node-name field MUST be
   maintained unless security issues or violations of the unique address
   assignment requirements exist or are suspected by the ACP registrar.

   ACP address information SHOULD be maintained even when the renewing
   and/or rekeying ACP registrar is not the same as the one that
   enrolled the prior ACP certificate.  See Section 9.2.4 for an
   example.

   ACP address information SHOULD also be maintained even after an ACP
   certificate expires or fails.  See Section 6.2.5.5 and
   Section 6.2.5.6.

6.11.7.5.  Further Details

   Section 9.2 discusses further informative details of ACP registrars:
   needed interactions, required parameters, certificate renewal and
   limitations, use of sub-CAs on registrars, and centralized policy
   control.

6.12.  Routing in the ACP

   Once ULA addresses are set up, all autonomic entities should run a
   routing protocol within the ACP context.  This routing protocol
   distributes the ULA created in the previous section for reachability.
   The use of the ACP-specific context eliminates the probable clash
   with data plane routing tables and also secures the ACP from
   interference from configuration mismatch or incorrect routing
   updates.

   The establishment of the routing plane and its parameters are
   automatic and strictly within the confines of the ACP.  Therefore, no
   explicit configuration is required.

   All routing updates are automatically secured in transit as the
   channels of the ACP are encrypted, and this routing runs only inside
   the ACP.

   The routing protocol inside the ACP is RPL [RFC6550].  See
   Appendix A.4 for more details on the choice of RPL.

   RPL adjacencies are set up across all ACP channels in the same domain
   including all its routing subdomains.  See Appendix A.6 for more
   details.

6.12.1.  ACP RPL Profile

   The following is a description of the RPL profile that ACP nodes need
   to support by default.  The format of this section is derived from
   [ROLL-APPLICABILITY].

6.12.1.1.  Overview

   RPL Packet Information (RPI), defined in [RFC6550], Section 11.2,
   defines the data packet artifacts required or beneficial in the
   forwarding of packets routed by RPL.  This profile does not use RPI
   for better compatibility with accelerated hardware forwarding planes,
   which most often do not support the Hop-by-Hop headers used for RPI,
   but also to avoid the overhead of the RPI header on the wire and cost
   of adding and/or removing them.

6.12.1.1.1.  Single Instance

   To avoid the need for RPI, the ACP RPL profile uses a simple routing/
   forwarding table based on destination prefix.  To achieve this, the
   profile uses only one RPL instanceID.  This single instanceID can
   contain only one Destination-Oriented Directed Acyclic Graph (DODAG),
   and the routing/forwarding table can therefore only calculate a
   single class of service ("best effort towards the primary NOC/root")
   and cannot create optimized routing paths to accomplish latency or
   energy goals between any two nodes.

   This choice is a compromise.  Consider a network that has multiple
   NOCs in different locations.  Only one NOC will become the DODAG
   root.  Traffic to and from other NOCs has to be sent through the
   DODAG (shortest path tree) rooted in the primary NOC.  Depending on
   topology, this can be an annoyance from a point of view of latency or
   minimizing network path resources, but this is deemed to be
   acceptable given how ACP traffic is "only" network management/control
   traffic.  See Appendix A.9.4 for more details.

   Using a single instanceID/DODAG does not introduce a single point of
   failure, as the DODAG will reconfigure itself when it detects data
   plane forwarding failures, including choosing a different root when
   the primary one fails.

   The benefit of this profile, especially compared to other IGPs, is
   that it does not calculate routes for nodes reachable through the
   same interface as the DODAG root.  This RPL profile can therefore
   scale to a much larger number of ACP nodes in the same amount of
   computation and memory than other routing protocols, especially on
   nodes that are leafs of the topology or those close to those leafs.

6.12.1.1.2.  Reconvergence

   In RPL profiles where RPI (see Section 6.12.1.13) is present, it is
   also used to trigger reconvergence when misrouted, for example,
   looping packets, which are recognized because of their RPI data.
   This helps to minimize RPL signaling traffic, especially in networks
   without stable topology and slow links.

   The ACP RPL profile instead relies on quickly reconverging the DODAG
   by recognizing link state change (down/up) and triggering
   reconvergence signaling as described in Section 6.12.1.7.  Since
   links in the ACP are assumed to be mostly reliable (or have link-
   layer protection against loss) and because there is no stretch
   according to Section 6.12.1.7, loops caused by loss of RPL signaling
   packets should be exceedingly rare.

   In addition, there are a variety of mechanisms possible in RPL to
   further avoid temporary loops that are RECOMMENDED to be used for the
   ACP RPL profile: DODAG Information Objects (DIOs) SHOULD be sent two
   or three times to inform children when losing the last parent.  The
   technique in [RFC6550], Section 8.2.2.6 (Detaching) SHOULD be favored
   over that in Section 8.2.2.5 (Poisoning) because it allows local
   connectivity.  Nodes SHOULD select more than one parent, at least
   three if possible, and send Destination Advertisement Objects (DAOs)
   to all of them in parallel.

   Additionally, failed ACP tunnels can be quickly discovered through
   the secure channel protocol mechanisms such as IKEv2 dead peer
   detection.  This can function as a replacement for a Low-power and
   Lossy Network's (LLN's) Expected Transmission Count (ETX) feature,
   which is not used in this profile.  A failure of an ACP tunnel should
   immediately signal the RPL control plane to pick a different parent.

6.12.1.2.  RPL Instances

   There is a single RPL instance.  The default RPLInstanceID is 0.

6.12.1.3.  Storing vs. Non-Storing Mode

   The RPL Mode of Operation (MOP) MUST support mode 2, "Storing Mode of
   Operations with no multicast support".  Implementations MAY support
   mode 3 ("... with multicast support") as that is a superset of mode
   2.  Note: Root indicates mode in DIO flow.

6.12.1.4.  DAO Policy

   The DAO policy is proactive, aggressive DAO state maintenance:

   *  Use the 'K' flag in unsolicited DAO to indicate change from
      previous information (to require DAO-ACK).

   *  Retry such DAO DAO-RETRIES(3) times with DAO-ACK_TIME_OUT(256ms)
      in between.

6.12.1.5.  Path Metrics

   Use Hop Count according to "Routing Metrics Used for Path Calculation
   in Low-Power and Lossy Networks" [RFC6551].  Note that this is solely
   for diagnostic purposes as it is not used by the Objective Function.

6.12.1.6.  Objective Function

   Objective Function (OF):  Use Objective Function Zero (OF0)
      ("Objective Function Zero for the Routing Protocol for Low-Power
      and Lossy Networks (RPL)" [RFC6552]).  Metric containers are not
      used.

   rank_factor:  Derived from link speed: if less than or equal to 100
      Mbps, LOW_SPEED_FACTOR(5), else HIGH_SPEED_FACTOR(1).

      This is a simple rank differentiation between typical "low speed"
      or IoT links that commonly max out at 100 Mbps and typical
      infrastructure links with speeds of 1 Gbps or higher.  Given how
      the path selection for the ACP focuses only on reachability but
      not on path cost optimization, no attempts at finer-grained path
      optimization are made.

6.12.1.7.  DODAG Repair

   Global Repair:  We assume stable links and ranks (metrics), so there
      is no need to periodically rebuild the DODAG.  The DODAG version
      is only incremented under catastrophic events (e.g.,
      administrative action).

   Local Repair:  As soon as link breakage is detected, the ACP node
      sends a No-Path DAO for all the targets that were reachable only
      via this link.  As soon as link repair is detected, the ACP node
      validates if this link provides a better parent.  If so, a new
      rank is computed by the ACP node, and it sends a new DIO that
      advertises the new rank.  Then it sends a DAO with a new path
      sequence about itself.

      When using ACP multi-access virtual interfaces, local repair can
      be triggered directly by peer breakage, see Section 6.13.5.2.2.

   stretch_rank:  None provided ("not stretched").

   Data-Path Validation:  Not used.

   Trickle:  Not used.

6.12.1.8.  Multicast

   Multicast is not used yet, but it is possible because of the selected
   mode of operations.

6.12.1.9.  Security

   RPL security [RFC6550] is not used, and ACP security is substituted.

   Because the ACP links already include provisions for confidentiality
   and integrity protection, their usage at the RPL layer would be
   redundant, and so RPL security is not used.

6.12.1.10.  P2P Communications

   Not used.

6.12.1.11.  IPv6 Address Configuration

   Every ACP node (RPL node) announces an IPv6 prefix covering the
   addresses assigned to the ACP node via the AcpNodeName.  The prefix
   length depends on the addressing sub-scheme of the acp-address, /127
   for the Zone Addressing Sub-Scheme and /112 or /120 for the Vlong
   Addressing Sub-Scheme.  See Section 6.11 for more details.

   Every ACP node MUST install a black hole route (also known as a null
   route) if there are unused parts of the ACP address space assigned to
   the ACP node via its AcpNodeName.  This is superseded by longer
   prefixes assigned to interfaces for the address space actually used
   by the node.  For example, when the node has an ACP-Vlong-8 address
   space, it installs a /120 black hole route.  If it then only uses the
   ACP address (first address from the space), for example, it would
   assign that address via a /128 address prefix to the ACP loopback
   interface (see Section 6.13.5.1).  None of those longer prefixes are
   announced into RPL.

   For ACP-Manual address prefixes configured on an ACP node, for
   example, for ACP connect subnets (see Section 8.1.1), the node
   announces the /64 subnet prefix.

6.12.1.12.  Administrative Parameters

   Administrative Preference ([RFC6550], Section 3.2.6 --to become
   root):  The preference is indicated in the DODAGPreference field of
      DIO message.

      Explicitly configured "root":  0b100

      ACP registrar (default):  0b011

      ACP connect (non-registrar):  0b010

      Default:  0b001

6.12.1.13.  RPL Packet Information

   RPI is not required in the ACP RPL profile for the following reasons.

   One RPI option is the RPL Source Routing Header (SRH) ("An IPv6
   Routing Header for Source Routes with the Routing Protocol for
   Low-Power and Lossy Networks (RPL)" [RFC6554]), which is not
   necessary because the ACP RPL profile uses storing mode where each
   hop has the necessary next-hop forwarding information.

   The simpler RPL Option header "The Routing Protocol for Low-Power and
   Lossy Networks (RPL) Option for Carrying RPL Information in
   Data-Plane Datagrams" [RFC6553] is also not necessary in this
   profile, because it uses a single RPL instance and data-path
   validation is also not used.

6.12.1.14.  Unknown Destinations

   Because RPL minimizes the size of the routing and forwarding table,
   prefixes reachable through the same interface as the RPL root are not
   known on every ACP node.  Therefore, traffic to unknown destination
   addresses can only be discovered at the RPL root.  The RPL root
   SHOULD have attach-safe mechanisms to operationally discover and log
   such packets.

   As this requirement places additional constraints on the data plane
   functionality of the RPL root, it does not apply to "normal" nodes
   that are not configured to have special functionality (i.e., the
   administrative parameter from Section 6.12.1.12 has value 0b001).  If
   the ACP network is degraded to the point where there are no nodes
   that could be configured as root, registrar, or ACP connect nodes, it
   is possible that the RPL root (and thus the ACP as a whole) would be
   unable to detect traffic to unknown destinations.  However, in the
   absence of nodes with administrative preference other than 0b001,
   there is also unlikely to be a way to get diagnostic information out
   of the ACP, so detection of traffic to unknown destinations would not
   be actionable anyway.

6.13.  General ACP Considerations

   Since channels are established between adjacent neighbors by default,
   the resulting overlay network does hop-by-hop encryption.  Each node
   decrypts incoming traffic from the ACP and encrypts outgoing traffic
   to its neighbors in the ACP.  Routing is discussed in Section 6.12.

6.13.1.  Performance

   There are no performance requirements for ACP implementations defined
   in this document because the performance requirements depend on the
   intended use case.  It is expected that a fully autonomic node with a
   wide range of ASA can require high forwarding plane performance in
   the ACP, for example, for telemetry.  Implementations of ACP that
   solely support traditional or SDN-style use cases can benefit from
   ACP at lower performance, especially if the ACP is used only for
   critical operations, e.g., when the data plane is not available.  The
   design of the ACP as specified in this document is intended to
   support a wide range of performance options: it is intended to allow
   software-only implementations at potentially low performance, but the
   design can also support high-performance options.  See [RFC8368] for
   more details.

6.13.2.  Addressing of Secure Channels

   In order to be independent of the data plane routing and addressing,
   the ACP secure channels discovered via GRASP use IPv6 link-local
   addresses between adjacent neighbors.  Note: Section 8.2 specifies
   extensions in which secure channels are configured tunnels operating
   over the data plane, so those secure channels cannot be independent
   of the data plane.

   To avoid impacting the operations of the IPv6 (link-local) interface/
   address used for ACP channels when configuring the data plane,
   appropriate implementation considerations are required.  If the IPv6
   interface/link-local address is shared with the data plane, it needs
   to be impossible to unconfigure and/or disable it through
   configuration.  Instead of sharing the IPv6 interface/link-local
   address, a separate (virtual) interface with a separate IPv6 link-
   local address can be used.  For example, the ACP interface could be
   run over a separate MAC address of an underlying L2 (Ethernet)
   interface.  For more details and options, see Appendix A.9.2.

   Note that other (nonideal) implementation choices may introduce
   additional, undesired dependencies against the data plane, for
   example, shared code and configuration of the secure channel
   protocols (IPsec and/or DTLS).

6.13.3.  MTU

   The MTU for ACP secure channels MUST be derived locally from the
   underlying link MTU minus the secure channel encapsulation overhead.

   ACP secure channel protocols do not need to perform MTU discovery
   because they are built across L2 adjacencies: the MTUs on both sides
   connecting to the L2 connection are assumed to be consistent.
   Extensions to ACP where the ACP is, for example, tunneled need to
   consider how to guarantee MTU consistency.  This is an issue of
   tunnels, not an issue of running the ACP across a tunnel.  Transport
   stacks running across ACP can perform normal PMTUD (Path MTU
   Discovery).  Because the ACP is meant to prioritize reliability over
   performance, they MAY opt to only expect IPv6 minimum MTU (1280
   octets) to avoid running into PMTUD implementation bugs or underlying
   link MTU mismatch problems.

6.13.4.  Multiple Links between Nodes

   If two nodes are connected via several links, the ACP SHOULD be
   established across every link, but it is possible to establish the
   ACP only on a subset of links.  Having an ACP channel on every link
   has a number of advantages, for example, it allows for a faster
   failover in case of link failure, and it reflects the physical
   topology more closely.  Using a subset of links (for example, a
   single link), reduces resource consumption on the node because state
   needs to be kept per ACP channel.  The negotiation scheme explained
   in Section 6.6 allows the Decider (the node with the higher ACP
   address) to drop all but the desired ACP channels to the Follower,
   and the Follower will not retry to build these secure channels from
   its side unless the Decider appears with a previously unknown GRASP
   announcement (e.g., on a different link or with a different address
   announced in GRASP).

6.13.5.  ACP Interfaces

   Conceptually, the ACP VRF has two types of interfaces: the "ACP
   loopback interface(s)" to which the ACP ULA address(es) are assigned
   and the "ACP virtual interfaces" that are mapped to the ACP secure
   channels.

6.13.5.1.  ACP Loopback Interfaces

   For autonomous operations of the ACP, as described in Section 6 and
   Section 7, the ACP node uses the first address from the N bit ACP
   prefix assigned to the node.  N = (128 - number of Vbits of the ACP
   address).  This address is assigned with an address prefix of N or
   larger to a loopback interface.

   Other addresses from the prefix can be used by the ACP of the node as
   desired.  The autonomous operations of the ACP do not require
   additional global-scope IPv6 addresses, they are instead intended for
   ASA or non-autonomous functions.  Components of the ACP that are not
   fully autonomic, such as ACP connect interfaces (see Figure 14), may
   also introduce additional global-scope IPv6 addresses on other types
   of interfaces to the ACP.

   The use of loopback interfaces for global-scope addresses is common
   operational configuration practice on routers, for example, in
   Internal BGP (IBGP) connections since BGP4 (see "A Border Gateway
   Protocol 4 (BGP-4)" [RFC1654]) or earlier.  The ACP adopts and
   automates this operational practice.

   A loopback interface for use with the ACP as described above is an
   interface that behaves according to Section 4 of "Default Address
   Selection for Internet Protocol Version 6 (IPv6)" [RFC6724],
   paragraph 2.  Packets sent by the host of the node from the loopback
   interface behave as if they are looped back by the interface so that
   they look as if they originated from the loopback interface, are then
   received by the node and forwarded by it towards the destination.

   The term "loopback only" indicates this behavior, but not the actual
   name of the interface type chosen in an actual implementation.  A
   loopback interface for use with the ACP can be a virtual and/or
   software construct without any associated hardware, or it can be a
   hardware interface operating in loopback mode.

   A loopback interface used for the ACP MUST NOT have connectivity to
   other nodes.

   The following list reviews the reasons for the choice of loopback
   addresses for ACP addresses, which is based on the IPv6 address
   architecture and common challenges:

   1.  IPv6 addresses are assigned to interfaces, not nodes.  IPv6
       continues the IPv4 model that a subnet prefix is associated with
       one link, see Section 2.1 of "IP Version 6 Addressing
       Architecture" [RFC4291].

   2.  IPv6 implementations commonly do not allow assignment of the same
       IPv6 global-scope address in the same VRF to more than one
       interface.

   3.  Global-scope addresses assigned to interfaces that connect to
       other nodes (external interfaces) may not be stable addresses for
       communications because any such interface could fail due to
       reasons external to the node.  This could render the addresses
       assigned to that interface unusable.

   4.  If failure of the subnet does not bring down the interface and
       make the addresses unusable, it could result in unreachability of
       the address because the shortest path to the node might go
       through one of the other nodes on the same subnet, which could
       equally consider the subnet to be operational even though it is
       not.

   5.  Many OAM service implementations on routers cannot deal with more
       than one peer address, often because they already expect that a
       single loopback address can be used, especially to provide a
       stable address under failure of external interfaces or links.

   6.  Even when an application supports multiple addresses to a peer,
       it can only use one address at a time for a connection with the
       most widely deployed transport protocols, TCP and UDP.  While
       "TCP Extensions for Multipath Operation with Multiple Addresses"
       [RFC6824]/[RFC8684] solves this problem, it is not widely adopted
       by implementations of router OAM services.

   7.  To completely autonomously assign global-scope addresses to
       subnets connecting to other nodes, it would be necessary for
       every node to have an amount of prefix address space on the order
       of the maximum number of subnets that the node could connect to,
       and then the node would have to negotiate with adjacent nodes
       across those subnets which address space to use for each subnet.

   8.  Using global-scope addresses for subnets between nodes is
       unnecessary if those subnets only connect routers, such as ACP
       secure channels, because they can communicate to remote nodes via
       their global-scope loopback addresses.  Using global-scope
       addresses for those external subnets is therefore wasteful for
       the address space and also unnecessarily increases the size of
       the routing and forwarding tables, which, especially for the ACP,
       is highly undesirable because it should attempt to minimize the
       per-node overhead of the ACP VRF.

   9.  For all these reasons, the ACP addressing sub-schemes do not
       consider ACP addresses for subnets connecting ACP nodes.

   Note that "Segment Routing Architecture" [RFC8402] introduces the
   term Node-SID to refer to IGP prefix segments that identify a
   specific router, for example, on a loopback interface.  An ACP
   loopback address prefix may similarly be called an ACP Node
   Identifier.

6.13.5.2.  ACP Virtual Interfaces

   Any ACP secure channel to another ACP node is mapped to ACP virtual
   interfaces in one of the following ways.  This is independent of the
   chosen secure channel protocol (IPsec, DTLS, or other future
   protocol, either standardized or not).

   Note that all the considerations described here assume point-to-point
   secure channel associations.  Mapping multiparty secure channel
   associations, such as "The Group Domain of Interpretation" [RFC6407],
   is out of scope.

6.13.5.2.1.  ACP Point-to-Point Virtual Interfaces

   In this option, each ACP secure channel is mapped to a separate
   point-to-point ACP virtual interface.  If a physical subnet has more
   than two ACP-capable nodes (in the same domain), this implementation
   approach will lead to a full mesh of ACP virtual interfaces between
   them.

   When the secure channel protocol determines a peer to be dead, this
   SHOULD result in indicating link breakage to trigger RPL DODAG
   repair, see Section 6.12.1.7.

6.13.5.2.2.  ACP Multi-Access Virtual Interfaces

   In a more advanced implementation approach, the ACP will construct a
   single multi-access ACP virtual interface for all ACP secure channels
   to ACP-capable nodes reachable across the same underlying (physical)
   subnet.  IPv6 link-local multicast packets sent to an ACP multi-
   access virtual interface are replicated to every ACP secure channel
   mapped to the ACP multi-access virtual interface.  IPv6 unicast
   packets sent to an ACP multi-access virtual interface are sent to the
   ACP secure channel that belongs to the ACP neighbor that is the next
   hop in the ACP forwarding table entry used to reach the packets'
   destination address.

   When the secure channel protocol determines that a peer is dead for a
   secure channel mapped to an ACP multi-access virtual interface, this
   SHOULD result in signaling breakage of that peer to RPL, so it can
   trigger RPL DODAG repair, see Section 6.12.1.7.

   There is no requirement for all ACP nodes on the same multi-access
   subnet to use the same type of ACP virtual interface.  This is purely
   a node-local decision.

   ACP nodes MUST perform standard IPv6 operations across ACP virtual
   interfaces including SLAAC [RFC4862] to assign their IPv6 link-local
   address on the ACP virtual interface and ND ("Neighbor Discovery for
   IP version 6 (IPv6)" [RFC4861]) to discover which IPv6 link-local
   neighbor address belongs to which ACP secure channel mapped to the
   ACP virtual interface.  This is independent of whether the ACP
   virtual interface is point-to-point or multi-access.

   Optimistic Duplicate Address Detection (DAD) according to "Optimistic
   Duplicate Address Detection (DAD) for IPv6" [RFC4429] is RECOMMENDED
   because the likelihood for duplicates between ACP nodes is highly
   improbable as long as the address can be formed from a globally
   unique, locally assigned identifier (e.g., EUI-48/EUI-64, see below).

   ACP nodes MAY reduce the amount of link-local IPv6 multicast packets
   from ND by learning the IPv6 link-local neighbor address to ACP
   secure channel mapping from other messages, such as the source
   address of IPv6 link-local multicast RPL messages, and therefore
   forego the need to send Neighbor Solicitation messages.

   The ACP virtual interface IPv6 link-local address can be derived from
   any appropriate local mechanism, such as node-local EUI-48 or EUI-64.
   It MUST NOT depend on something that is attackable from the data
   plane, such as the IPv6 link-local address of the underlying physical
   interface, which can be attacked by SLAAC, or parameters of the
   secure channel encapsulation header that may not be protected by the
   secure channel mechanism.

   The link-layer address of an ACP virtual interface is the address
   used for the underlying interface across which the secure tunnels are
   built, typically Ethernet addresses.  Because unicast IPv6 packets
   sent to an ACP virtual interface are not sent to a link-layer
   destination address but rather to an ACP secure channel, the link-
   layer address fields SHOULD be ignored on reception, and instead the
   ACP secure channel from which the message was received should be
   remembered.

   Multi-access ACP virtual interfaces are preferable implementations
   when the underlying interface is a (broadcast) multi-access subnet
   because they reflect the presence of the underlying multi-access
   subnet to the virtual interfaces of the ACP.  This makes it, for
   example, simpler to build services with topology awareness inside the
   ACP VRF in the same way as they could have been built running
   natively on the multi-access interfaces.

   Consider also the impact of point-to-point vs. multi-access virtual
   interfaces on the efficiency of flooding via link-local multicast
   messages.

   Assume a LAN with three ACP neighbors, Alice, Bob, and Carol.
   Alice's ACP GRASP wants to send a link-local GRASP multicast message
   to Bob and Carol.  If Alice's ACP emulates the LAN as per-peer,
   point-to-point virtual interfaces, one to Bob and one to Carol,
   Alice's ACP GRASP will send two copies of multicast GRASP messages:
   one to Bob and one to Carol.  If Alice's ACP emulates a LAN via a
   multipoint virtual interface, Alice's ACP GRASP will send one packet
   to that interface, and the ACP multipoint virtual interface will
   replicate the packet to each secure channel, one to Bob, one to
   Carol.  The result is the same.  The difference happens when Bob and
   Carol receive their packets.  If they use ACP point-to-point virtual
   interfaces, their GRASP instance would forward the packet from Alice
   to each other as part of the GRASP flooding procedure.  These packets
   are unnecessary and would be discarded by GRASP on receipt as
   duplicates (by use of the GRASP Session ID).  If Bob and Carol's ACP
   emulated a multi-access virtual interface, then this would not happen
   because GRASP's flooding procedure does not replicate packets back to
   the interface from which they were received.

   Note that link-local GRASP multicast messages are not sent directly
   as IPv6 link-local multicast UDP messages to ACP virtual interfaces,
   but instead to ACP GRASP virtual interfaces that are layered on top
   of ACP virtual interfaces to add TCP reliability to link-local
   multicast GRASP messages.  Nevertheless, these ACP GRASP virtual
   interfaces perform the same replication of messages and therefore
   have the same impact on flooding.  See Section 6.9.2 for more
   details.

   RPL does support operations and correct routing table construction
   across non-broadcast multi-access (NBMA) subnets.  This is common
   when using many radio technologies.  When such NBMA subnets are used,
   they MUST NOT be represented as ACP multi-access virtual interfaces
   because the replication of IPv6 link-local multicast messages will
   not reach all NBMA subnet neighbors.  As a result, GRASP message
   flooding would fail.  Instead, each ACP secure channel across such an
   interface MUST be represented as an ACP point-to-point virtual
   interface.  See also Appendix A.9.4.

   Care needs to be taken when creating multi-access ACP virtual
   interfaces across ACP secure channels between ACP nodes in different
   domains or routing subdomains.  If, for example, future inter-domain
   ACP policies are defined as "peer-to-peer" policies, it is easier to
   create ACP point-to-point virtual interfaces for these inter-domain
   secure channels.

7.  ACP Support on L2 Switches/Ports (Normative)

7.1.  Why (Benefits of ACP on L2 Switches)

       ANrtr1 ------ ANswitch1 --- ANswitch2 ------- ANrtr2
                 .../   \                   \  ...
       ANrtrM ------     \                   ------- ANrtrN
                          ANswitchM ...

                  Figure 13: Topology with L2 ACP Switches

   Consider a large L2 LAN with routers ANrtr1 through ANrtrN connected
   via some topology of L2 switches.  Examples include large enterprise
   campus networks with an L2 core, IoT networks, or broadband
   aggregation networks, which often have a multilevel L2-switched
   topology.

   If the discovery protocol used for the ACP operates at the subnet
   level, every ACP router will see all other ACP routers on the LAN as
   neighbors, and a full mesh of ACP channels will be built.  If some or
   all of the AN switches are autonomic with the same discovery
   protocol, then the full mesh would include those switches as well.

   A full mesh of ACP connections can create fundamental scale
   challenges.  The number of security associations of the secure
   channel protocols will likely not scale arbitrarily, especially when
   they leverage platform-accelerated encryption/decryption.  Likewise,
   any other ACP operations (such as routing) need to scale to the
   number of direct ACP neighbors.  An ACP router with just four
   physical interfaces might be deployed into a LAN with hundreds of
   neighbors connected via switches.  Introducing such a new,
   unpredictable scaling factor requirement makes it harder to support
   the ACP on arbitrary platforms and in arbitrary deployments.

   Predictable scaling requirements for ACP neighbors can most easily be
   achieved if, in topologies such as these, ACP-capable L2 switches can
   ensure that discovery messages terminate on them so that neighboring
   ACP routers and switches will only find the physically connected ACP
   L2 switches as their candidate ACP neighbors.  With such a discovery
   mechanism in place, the ACP and its security associations will only
   need to scale to the number of physical interfaces instead of a
   potentially much larger number of "LAN-connected" neighbors, and the
   ACP topology will follow directly the physical topology, something
   that can then also be leveraged in management operations or by ASAs.

   In the example above, consider that ANswitch1 and ANswitchM are ACP
   capable, and ANswitch2 is not ACP capable.  The desired ACP topology
   is that ANrtr1 and ANrtrM only have an ACP connection to ANswitch1,
   and that ANswitch1, ANrtr2, and ANrtrN have a full mesh of ACP
   connections amongst each other.  ANswitch1 also has an ACP connection
   with ANswitchM, and ANswitchM has ACP connections to anything else
   behind it.

7.2.  How (per L2 Port DULL GRASP)

   To support ACP on L2 switches or L2-switched ports of an L3 device,
   it is necessary to make those L2 ports look like L3 interfaces for
   the ACP implementation.  This primarily involves the creation of a
   separate DULL GRASP instance/domain on every such L2 port.  Because
   GRASP has a dedicated link-local IPv6 multicast address
   (ALL_GRASP_NEIGHBORS), it is sufficient that all packets for this
   address are extracted at the port level and passed to that DULL GRASP
   instance.  Likewise, the IPv6 link-local multicast packets sent by
   that DULL GRASP instance need to be sent only towards the L2 port for
   this DULL GRASP instance (instead of being flooded across all ports
   of the VLAN to which the port belongs).

   When the ports/interfaces across which the ACP is expected to operate
   in an ACP-aware L2 switch or L2/L3 switch/router are L2-bridged,
   packets for the ALL_GRASP_NEIGHBORS multicast address MUST never be
   forwarded between these ports.  If MLD snooping is used, it MUST be
   prohibited from bridging packets for the ALL_GRASP_NEIGHBORS IPv6
   multicast address.

   On hybrid L2/L3 switches, multiple L2 ports are assigned to a single
   L3 VLAN interface.  With the aforementioned changes for DULL GRASP,
   ACP can simply operate on the L3 VLAN interfaces, so no further
   (hardware) forwarding changes are required to make ACP operate on L2
   ports.  This is possible because the ACP secure channel protocols
   only use link-local IPv6 unicast packets, and these packets will be
   sent to the correct L2 port towards the peer by the VLAN logic of the
   device.

   This is sufficient when P2P ACP virtual interfaces are established to
   every ACP peer.  When it is desired to create multi-access ACP
   virtual interfaces (see Section 6.13.5.2.2), it is REQUIRED not to
   coalesce all the ACP secure channels on the same L3 VLAN interface,
   but only all those on the same L2 port.

   If VLAN tagging is used, then the logic described above only applies
   to untagged GRASP packets.  For the purpose of ACP neighbor discovery
   via GRASP, no VLAN-tagged packets SHOULD be sent or received.  In a
   hybrid L2/L3 switch, each VLAN would therefore only create ACP
   adjacencies across those ports where the VLAN is carried untagged.

   As a result, the simple logic is that ACP secure channels would
   operate over the same L3 interfaces that present a single, flat
   bridged network across all routers, but because DULL GRASP is
   separated on a per-port basis, no full mesh of ACP secure channels is
   created, but only per-port ACP secure channels to per-port
   L2-adjacent ACP node neighbors.

   For example, in the above picture, ANswitch1 would run separate DULL
   GRASP instances on its ports to ANrtr1, ANswitch2, and ANswitchI,
   even though all three ports may be in the data plane in the same
   (V)LAN and perform L2 switching between these ports, ANswitch1 would
   perform ACP L3 routing between them.

   The description in the previous paragraph is specifically meant to
   illustrate that, on hybrid L3/L2 devices that are common in
   enterprise, IoT, and broadband aggregation, there is only the GRASP
   packet extraction (by Ethernet address) and GRASP link-local
   multicast per L2-port packet injection that has to consider L2 ports
   at the hardware-forwarding level.  The remaining operations are
   purely ACP control plane and setup of secure channels across the L3
   interface.  This hopefully makes support for per-L2 port ACP on those
   hybrid devices easy.

   In devices without such a mix of L2 port/interfaces and L3 interfaces
   (to terminate any transport-layer connections), implementation
   details will differ.  Logically and most simply every L2 port is
   considered and used as a separate L3 subnet for all ACP operations.
   The fact that the ACP only requires IPv6 link-local unicast and
   multicast should make support for it on any type of L2 devices as
   simple as possible.

   A generic issue with ACP in L2-switched networks is the interaction
   with the Spanning Tree Protocol (STP).  Without further L2
   enhancements, the ACP would run only across the active STP topology,
   and the ACP would be interrupted and reconverge with STP changes.
   Ideally, ACP peering SHOULD be built also across ports that are
   blocked in STP so that the ACP does not depend on STP and can
   continue to run unaffected across STP topology changes, where
   reconvergence can be quite slow.  The above described simple
   implementation options are not sufficient to achieve this.

8.  Support for Non-ACP Components (Normative)

8.1.  ACP Connect

8.1.1.  Non-ACP Controller and/or Network Management System (NMS)

   The ACP can be used by management systems, such as controllers or NMS
   hosts, to connect to devices (or other type of nodes) through it.
   For this, an NMS host needs to have access to the ACP.  The ACP is a
   self-protecting overlay network, which allows access only to trusted,
   autonomic systems by default.  Therefore, a traditional, non-ACP NMS
   does not have access to the ACP by default, such as any other
   external node.

   If the NMS host is not autonomic, i.e., it does not support autonomic
   negotiation of the ACP, then it can be brought into the ACP by
   explicit configuration.  To support connections to adjacent non-ACP
   nodes, an ACP node SHOULD support "ACP connect" (sometimes also
   called "autonomic connect").

   "ACP connect" is an interface-level, configured workaround for
   connection of trusted non-ACP nodes to the ACP.  The ACP node on
   which ACP connect is configured is called an "ACP edge node".  With
   ACP connect, the ACP is accessible from those non-ACP nodes (such as
   NOC systems) on such an interface without those non-ACP nodes having
   to support any ACP discovery or ACP channel setup.  This is also
   called "native" access to the ACP because, to those NOC systems, the
   interface looks like a normal network interface without any ACP
   secure channel that is encapsulating the traffic.

                                    Data Plane "native" (no ACP)
                                             .
   +--------+       +----------------+       .        +-------------+
   | ACP    |       |ACP Edge Node   |       .        |             |
   | Node   |       |                |       v        |             |
   |        |-------|...[ACP VRF]....+----------------|             |+
   |        |   ^   |.               |                | NOC Device  ||
   |        |   .   | .[Data Plane]..+----------------| "NMS hosts" ||
   |        |   .   |  [          ]  | .         ^    |             ||
   +--------+   .   +----------------+  .        .    +-------------+|
                .                        .       .     +-------------+
                .                        .       .
             Data Plane "native"         .   ACP "native" (unencrypted)
           + ACP auto-negotiated         .   "ACP connect subnet"
             and encrypted               .
                                       ACP connect interface
                                       e.g., "VRF ACP native" (config)

                           Figure 14: ACP Connect

   ACP connect has security consequences: all systems and processes
   connected via ACP connect have access to all ACP nodes on the entire
   ACP, without further authentication.  Thus, the ACP connect interface
   and the NOC systems connected to it need to be physically controlled
   and/or secured.  For this reason, the mechanisms described here
   explicitly do not include options to allow for a non-ACP router to be
   connected across an ACP connect interface and addresses behind such a
   router routed inside the ACP.

   Physically controlled and/or secured means that attackers cannot gain
   access to the physical device hosting the ACP edge node, the physical
   interfaces and links providing the ACP connect link, nor the physical
   devices hosting the NOC device.  In a simple case, ACP edge node and
   NOC device are colocated in an access-controlled room, such as a NOC,
   to which attackers cannot gain physical access.

   An ACP connect interface provides exclusive access to only the ACP.
   This is likely insufficient for many NMS hosts.  Instead, they would
   require a second "data plane" interface outside the ACP for
   connections between the NMS host and administrators, or Internet-
   based services, or for direct access to the data plane.  The document
   "Using Autonomic Control Plane for Stable Connectivity of Network
   OAM" [RFC8368] explains in more detail how the ACP can be integrated
   in a mixed NOC environment.

   An ACP connect interface SHOULD use an IPv6 address/prefix from the
   Manual Addressing Sub-Scheme (Section 6.11.4), letting the operator
   configure, for example, only the Subnet-ID and having the node
   automatically assign the remaining part of the prefix/address.  It
   SHOULD NOT use a prefix that is also routed outside the ACP so that
   the addresses clearly indicate whether it is used inside the ACP or
   not.

   The prefix of ACP connect subnets MUST be distributed by the ACP edge
   node into the ACP routing protocol, RPL.  The NMS host MUST connect
   to prefixes in the ACP routing table via its ACP connect interface.
   In the simple case where the ACP uses only one ULA prefix, and all
   ACP connect subnets have prefixes covered by that ULA prefix, NMS
   hosts can rely on [RFC6724] to determine longest match prefix routes
   towards its different interfaces, ACP and data plane.  With
   [RFC6724], the NMS host will select the ACP connect interface for all
   addresses in the ACP because any ACP destination address is longest
   matched by the address on the ACP connect interface.  If the NMS
   host's ACP connect interface uses another prefix, or if the ACP uses
   multiple ULA prefixes, then the NMS host requires (static) routes
   towards the ACP interface for these prefixes.

   When an ACP edge node receives a packet from an ACP connect
   interface, the ACP edge node MUST only forward the packet to the ACP
   if the packet has an IPv6 source address from that interface (this is
   sometimes called Reverse Path Forwarding (RPF) filtering).  This
   filtering rule MAY be changed through administrative measures.  The
   more any such administrative action enables reachability of non-ACP
   nodes to the ACP, the more this may cause security issues.

   To limit the security impact of ACP connect, nodes supporting it
   SHOULD implement a security mechanism to allow configuration and/or
   use of ACP connect interfaces only on nodes explicitly targeted to be
   deployed with it (those in physically secure locations such as a
   NOC).  For example, the registrar could disable the ability to enable
   ACP connect on devices during enrollment, and that property could
   only be changed through reenrollment.  See also Appendix A.9.5.

   ACP edge nodes SHOULD have a configurable option to prohibit packets
   with RPI headers (see Section 6.12.1.13) across an ACP connect
   interface.  These headers are outside the scope of the RPL profile in
   this specification but may be used in future extensions of this
   specification.

8.1.2.  Software Components

   The previous section assumed that the ACP edge node and NOC devices
   are separate physical devices and that the ACP connect interface is a
   physical network connection.  This section discusses the implication
   when these components are instead software components running on a
   single physical device.

   The ACP connect mechanism can be used not only to connect physically
   external systems (NMS hosts) to the ACP but also other applications,
   containers, or virtual machines.  In fact, one possible way to
   eliminate the security issue of the external ACP connect interface is
   to colocate an ACP edge node and an NMS host by making one a virtual
   machine or container inside the other; therefore converting the
   unprotected external ACP subnet into an internal virtual subnet in a
   single device.  This would ultimately result in a fully ACP-enabled
   NMS host with minimum impact to the NMS host's software architecture.
   This approach is not limited to NMS hosts but could equally be
   applied to devices consisting of one or more VNF (virtual network
   functions): an internal virtual subnet connecting out-of-band
   management interfaces of the VNFs to an ACP edge router VNF.

   The core requirement is that the software components need to have a
   network stack that permits access to the ACP and optionally also to
   the data plane.  Like in the physical setup for NMS hosts, this can
   be realized via two internal virtual subnets: one that connects to
   the ACP (which could be a container or virtual machine by itself),
   and one (or more) connecting to the data plane.

   This "internal" use of the ACP connect approach should not be
   considered to be a "workaround" because, in this case, it is possible
   to build a correct security model: it is not necessary to rely on
   unprovable, external physical security mechanisms as in the case of
   external NMS hosts.  Instead, the orchestration of the ACP, the
   virtual subnets, and the software components can be done by trusted
   software that could be considered to be part of the ANI (or even an
   extended ACP).  This software component is responsible for ensuring
   that only trusted software components will get access to that virtual
   subnet and that only even more trusted software components will get
   access to both the ACP virtual subnet and the data plane (because
   those ACP users could leak traffic between ACP and data plane).  This
   trust could be established, for example, through cryptographic means
   such as signed software packages.

8.1.3.  Autoconfiguration

   ACP edge nodes, NMS hosts, and software components that, as described
   in the previous section, are meant to be composed via virtual
   interfaces SHOULD support SLAAC [RFC4862] on the ACP connect subnet
   and route autoconfiguration according to "Default Router Preferences
   and More-Specific Routes" [RFC4191].

   The ACP edge node acts as the router towards the ACP on the ACP
   connect subnet, providing the (auto)configured prefix for the ACP
   connect subnet and (auto)configured routes to the ACP to NMS hosts
   and/or software components.

   The ACP edge node uses the Route Information Option (RIO) of
   [RFC4191] to announce aggregated prefixes for address prefixes used
   in the ACP (with normal RIO lifetimes).  In addition, the ACP edge
   node also uses a RIO to announce the default route (::/0) with a
   lifetime of 0.

   These RIOs allow the connecting of type C hosts to the ACP via an ACP
   connect subnet on one interface and another network (Data Plane and/
   or NMS network) on the same or another interface of the type C host,
   relying on routers other than the ACP edge node.  The RIOs ensure
   that these hosts will only route the prefixes used in the ACP to the
   ACP edge node.

   Type A and B hosts ignore the RIOs and will consider the ACP node to
   be their default router for all destinations.  This is sufficient
   when the type A or type B host only needs to connect to the ACP but
   not to other networks.  Attaching a type A or type B host to both the
   ACP and other networks requires explicit ACP prefix route
   configuration on either the host or the combined ACP and data plane
   interface on the ACP edge node, see Section 8.1.4.

   Aggregated prefix means that the ACP edge node needs to only announce
   the /48 ULA prefixes used in the ACP but none of the actual /64
   (Manual Addressing Sub-Scheme), /127 (Zone Addressing Sub-Scheme),
   /112 or /120 (Vlong Addressing Sub-Scheme) routes of actual ACP
   nodes.  If ACP interfaces are configured with non-ULA prefixes, then
   those prefixes cannot be aggregated without further configured policy
   on the ACP edge node.  This explains the above recommendation to use
   ACP ULA prefixes for ACP connect interfaces: they allow for a shorter
   list of prefixes to be signaled via [RFC4191] to NMS hosts and
   software components.

   The ACP edge nodes that have a Vlong ACP address MAY allocate a
   subset of their /112 or /120 address prefix to ACP connect
   interface(s) to eliminate the need to non-autonomically configure
   and/or provision the address prefixes for such ACP connect
   interfaces.

8.1.4.  Combined ACP and Data Plane Interface (VRF Select)

                        Combined ACP and data plane interface
                                                .
     +--------+       +--------------------+    .   +--------------+
     | ACP    |       |ACP Edge No         |    .   | NMS Host(s)  |
     | Node   |       |                    |    .   | / Software   |
     |        |       |  [ACP  ].          |    .   |              |+
     |        |       | .[VRF  ] .[VRF   ] |    v   | "ACP Address"||
     |        +-------+.         .[Select].+--------+ "Data Plane  ||
     |        |   ^   | .[Data ].          |        |  Address(es)"||
     |        |   .   |  [Plane]           |        |              ||
     |        |   .   |  [     ]           |        +--------------+|
     +--------+   .   +--------------------+         +--------------+
                  .
           Data plane "native" and + ACP auto-negotiated/encrypted

                           Figure 15: VRF Select

   Using two physical and/or virtual subnets (and therefore interfaces)
   to NMS hosts (as per Section 8.1.1) or software (as per
   Section 8.1.2) may be seen as additional complexity, for example,
   with legacy NMS hosts that support only one IP interface, or it may
   be insufficient to support type A or type B hosts [RFC4191] (see
   Section 8.1.3).

   To provide a single subnet to both the ACP and Data plane, the ACP
   edge node needs to demultiplex packets from NMS hosts into ACP VRF
   and data plane.  This is sometimes called "VRF select".  If the ACP
   VRF has no overlapping IPv6 addresses with the data plane (it should
   have no overlapping addresses), then this function can use the IPv6
   destination address.  The problem is source address selection on the
   NMS host(s) according to [RFC6724].

   Consider the simple case: the ACP uses only one ULA prefix, and the
   ACP IPv6 prefix for the combined ACP and data plane interface is
   covered by that ULA prefix.  The ACP edge node announces both the ACP
   IPv6 prefix and one (or more) prefixes for the data plane.  Without
   further policy configurations on the NMS host(s), it may select its
   ACP address as a source address for data plane ULA destinations
   because of Rule 8 (Section 5 of [RFC6724]).  The ACP edge node can
   pass on the packet to the data plane, but the ACP source address
   should not be used for data plane traffic, and return traffic may
   fail.

   If the ACP carries multiple ULA prefixes or non-ULA ACP connect
   prefixes, then the correct source address selection becomes even more
   problematic.

   With separate ACP connect and data plane subnets and prefix
   announcements [RFC4191] that are to be routed across the ACP connect
   interface, the source address selection of Rule 5 (use address of
   outgoing interface) (Section 5 of [RFC6724]) will be used, so that
   above problems do not occur, even in more complex cases of multiple
   ULA and non-ULA prefixes in the ACP routing table.

   To achieve the same behavior with a combined ACP and data plane
   interface, the ACP edge node needs to behave as two separate routers
   on the interface: one link-local IPv6 address/router for its ACP
   reachability, and one link-local IPv6 address/router for its data
   plane reachability.  The Router Advertisements for both are as
   described in Section 8.1.3: for the ACP, the ACP prefix is announced
   together with the prefix option [RFC4191] routed across the ACP, and
   the lifetime is set to zero to disqualify this next hop as a default
   router.  For the data plane, the data plane prefix(es) are announced
   together with whatever default router parameters are used for the
   data plane.

   As a result, source address selection Rule 5.5 (Section 5 of
   [RFC6724]) may result in the same correct source address selection
   behavior of NMS hosts without further configuration as the separate
   ACP connect and data plane interfaces on the host.  As described in
   the text for Rule 5.5 (Section 5 of [RFC6724]), this is only a MAY
   because IPv6 hosts are not required to track next-hop information.
   If an NMS host does not do this, then separate ACP connect and data
   plane interfaces are the preferable method of attachment.  Hosts
   implementing "First-Hop Router Selection by Hosts in a Multi-Prefix
   Network" [RFC8028] should (instead of may) implement Rule 5.5
   (Section 5 of [RFC6724]), so it is preferred for hosts to support
   [RFC8028].

   ACP edge nodes MAY support the combined ACP and data plane interface.

8.1.5.  Use of GRASP

   GRASP can and should be possible to use across ACP connect
   interfaces, especially in the architecturally correct solution when
   it is used as a mechanism to connect software (e.g., ASA or legacy
   NMS applications) to the ACP.

   Given how the ACP is the security and transport substrate for GRASP,
   the requirements are that those devices connected via ACP connect are
   equivalently (if not better) secured against attacks than ACP nodes
   that do not use ACP connect, and they run only software that is
   equally (if not better) protected, known (or trusted) not to be
   malicious, and accordingly designed to isolate access to the ACP
   against external equipment.

   The difference in security is that cryptographic security of the ACP
   secure channel is replaced by required physical security and/or
   control of the network connection between an ACP edge node and the
   NMS or other host reachable via the ACP connect interface.  See
   Section 8.1.1.

   When using the combined ACP and data plane interface, care has to be
   taken that only GRASP messages received from software or NMS hosts
   and intended for the ACP GRASP domain are forwarded by ACP edge
   nodes.  Currently there is no definition for a GRASP security and
   transport substrate beside the ACP, so there is no definition how
   such software/NMS host could participate in two separate GRASP
   domains across the same subnet (ACP and data plane domains).
   Currently it is assumed that all GRASP packets on a combined ACP and
   data plane interface belong to the GRASP ACP domain.  They SHOULD all
   use the ACP IPv6 addresses of the software/NMS hosts.  The link-local
   IPv6 addresses of software/NMS hosts (used for GRASP M_DISCOVERY and
   M_FLOOD messages) are also assumed to belong to the ACP address
   space.

8.2.  Connecting ACP Islands over Non-ACP L3 Networks (Remote ACP
      Neighbors)

   Not all nodes in a network may support the ACP.  If non-ACP L2
   devices are between ACP nodes, the ACP will work across them since it
   is IP based.  However, the autonomic discovery of ACP neighbors via
   DULL GRASP is only intended to work across L2 connections, so it is
   not sufficient to autonomically create ACP connections across non-ACP
   L3 devices.

8.2.1.  Configured Remote ACP Neighbor

   On the ACP node, remote ACP neighbors are configured explicitly.  The
   parameters of such a "connection" are described in the following
   ABNF.  The syntax shown is non-normative (as there are no standards
   for configuration) but only meant to illustrate the parameters and
   which ones can be optional.

     connection = method "," local-addr "," remote-addr [ "," pmtu ]
     method =    "any"
                / ( "IKEv2" [ ":" port ] )
                / (  "DTLS"  [ ":"  port ] )
     port = 1*DIGIT
     local-addr  = [ address  [ ":" vrf  ] ]
     remote-addr =   address
     address =  "any"
                / IPv4address / IPv6address  ; From [RFC5954]
     vrf = system-dependent ; Name of VRF for local-address

               Figure 16: Parameters for Remote ACP Neighbors

   Explicit configuration of a remote peer according to this ABNF
   provides all the information to build a secure channel without
   requiring a tunnel to that peer and running DULL GRASP inside of it.

   The configuration includes the parameters otherwise signaled via DULL
   GRASP: local address, remote (peer) locator, and method.  The
   differences over DULL GRASP local neighbor discovery and secure
   channel creation are as follows:

   *  The local and remote address can be IPv4 or IPv6 and are typically
      global-scope addresses.

   *  The VRF across which the connection is built (and in which local-
      addr exists) can be specified.  If vrf is not specified, it is the
      default VRF on the node.  In DULL GRASP, the VRF is implied by the
      interface across which DULL GRASP operates.

   *  If local address is "any", the local address used when initiating
      a secure channel connection is decided by source address selection
      ([RFC6724] for IPv6).  As a responder, the connection listens on
      all addresses of the node in the selected VRF.

   *  Configuration of port is only required for methods where no
      defaults exist (e.g., "DTLS").

   *  If the remote address is "any", the connection is only a
      responder.  It is a "hub" that can be used by multiple remote
      peers to connect simultaneously -- without having to know or
      configure their addresses, for example, a hub site for remote
      "spoke" sites reachable over the Internet.

   *  The pmtu parameter should be configurable to overcome issues or
      limitations of Path MTU Discovery (PMTUD).

   *  IKEv2/IPsec to remote peers should support the optional NAT
      Traversal (NAT-T) procedures.

8.2.2.  Tunneled Remote ACP Neighbor

   An IP-in-IP, GRE, or other form of preexisting tunnel is configured
   between two remote ACP peers, and the virtual interfaces representing
   the tunnel are configured for "ACP enable".  This will enable IPv6
   link-local addresses and DULL on this tunnel.  As a result, the
   tunnel is used for normal "L2 adjacent" candidate ACP neighbor
   discovery with DULL and secure channel setup procedures described in
   this document.

   Tunneled Remote ACP Neighbor requires two encapsulations: the
   configured tunnel and the secure channel inside of that tunnel.  This
   makes it in general less desirable than Configured Remote ACP
   Neighbor.  Benefits of tunnels are that it may be easier to implement
   because there is no change to the ACP functionality - just running it
   over a virtual (tunnel) interface instead of only native interfaces.
   The tunnel itself may also provide PMTUD while the secure channel
   method may not.  Or the tunnel mechanism is permitted/possible
   through some firewall while the secure channel method may not.

   Tunneling using an insecure tunnel encapsulation increases, on
   average, the risk of a MITM downgrade attack somewhere along the
   underlay path.  In such an attack, the MITM filters packets for all
   but the most easily attacked ACP secure channel option to force use
   of that option.  ACP nodes supporting Tunneled Remote ACP Neighbors
   SHOULD support configuration on such tunnel interfaces to restrict or
   explicitly select the available ACP secure channel protocols (if the
   ACP node supports more than one ACP secure channel protocol in the
   first place).

8.2.3.  Summary

   Configured and Tunneled Remote ACP Neighbors are less
   "indestructible" than L2 adjacent ACP neighbors based on link-local
   addressing, since they depend on more correct data plane operations,
   such as routing and global addressing.

   Nevertheless, these options may be crucial to incrementally deploying
   the ACP, especially if it is meant to connect islands across the
   Internet.  Implementations SHOULD support at least Tunneled Remote
   ACP Neighbors via GRE tunnels, which is likely the most common
   router-to-router tunneling protocol in use today.

9.  ACP Operations (Informative)

   The following sections document important operational aspects of the
   ACP.  They are not normative because they do not impact the
   interoperability between components of the ACP, but they include
   recommendations and/or requirements for the internal operational
   model that are beneficial or necessary to achieve the desired use-
   case benefits of the ACP (see Section 3).

   *  Section 9.1 describes the recommended capabilities of operator
      diagnostics of ACP nodes.

   *  Section 9.2 describes at a high level how an ACP registrar needs
      to work, what its configuration parameters are, and specific
      issues impacting the choices of deployment design due to renewal
      and revocation issues.  It describes a model where ACP registrars
      have their own sub-CA to provide the most distributed deployment
      option for ACP registrars, and it describes considerations for
      centralized policy control of ACP registrar operations.

   *  Section 9.3 describes suggested ACP node behavior and operational
      interfaces (configuration options) to manage the ACP in so-called
      greenfield devices (previously unconfigured) and brownfield
      devices (preconfigured).

   The recommendations and suggestions of this chapter were derived from
   operational experience gained with a commercially available pre-
   standard ACP implementation.

9.1.  ACP (and BRSKI) Diagnostics

   Even though ACP and ANI in general are removing many manual
   configuration mistakes through their automation, it is important to
   provide good diagnostics for them.

   Basic standardized diagnostics would require support for (YANG)
   models representing the complete (auto)configuration and operational
   state of all components: GRASP, ACP, and the infrastructure used by
   them, such as TLS/DTLS, IPsec, certificates, TA, time, VRF, and so
   on.  While necessary, this is not sufficient.

   Simply representing the state of components does not allow operators
   to quickly take action -- unless they understand how to interpret the
   data, which can mean a requirement for deep understanding of all
   components and how they interact in the ACP/ANI.

   Diagnostic supports should help to quickly answer the questions
   operators are expected to ask, such as "Is the ACP working
   correctly?" or "Why is there no ACP connection to a known neighboring
   node?"

   In current network management approaches, the logic to answer these
   questions is most often built into centralized diagnostics software
   that leverages the above mentioned data models.  While this approach
   is feasible for components utilizing the ANI, it is not sufficient to
   diagnose the ANI itself:

   *  Developing the logic to identify common issues requires
      operational experience with the components of the ANI.  Letting
      each management system define its own analysis is inefficient.

   *  When the ANI is not operating correctly, it may not be possible to
      run diagnostics remotely because of missing connectivity.  The ANI
      should therefore have diagnostic capabilities available locally on
      the nodes themselves.

   *  Certain operations are difficult or impossible to monitor in real
      time, such as initial bootstrap issues in a network location where
      no capabilities exist to attach local diagnostics.  Therefore, it
      is important to also define how to capture (log) diagnostics
      locally for later retrieval.  Ideally, these captures are also
      nonvolatile so that they can survive extended power-off
      conditions, for example, when a device that fails to be brought up
      zero-touch is sent for diagnostics at a more appropriate location.

   The simplest form of diagnostics for answering questions such as the
   above is to represent the relevant information sequentially in
   dependency order, so that the first unexpected and/or nonoperational
   item is the most likely root cause, or just log and/or highlight that
   item.  For example:

   Question: Is the ACP operational to accept neighbor connections?

   *  Check if the necessary configurations to make ACP/ANI operational
      are correct (see Section 9.3 for a discussion of such commands).

   *  Does the system time look reasonable, or could it be the default
      system time after battery failure of the clock chip?  Certificate
      checks depend on reasonable notion of time.

   *  Does the node have keying material, such as domain certificate, TA
      certificates, etc.?

   *  If there is no keying material and the ANI is supported/enabled,
      check the state of BRSKI (not detailed in this example).

   *  Check the validity of the domain certificate:

      -  Does the certificate validate against the TA?

      -  Has it been revoked?

      -  Was the last scheduled attempt to retrieve a CRL successful?
         (e.g., do we know that our CRL information is up to date?)

      -  Is the certificate valid?  The validity start time is in the
         past, and the expiration time is in the future?

      -  Does the certificate have a correctly formatted acp-node-name
         field?

   *  Was the ACP VRF successfully created?

   *  Is ACP enabled on one or more interfaces that are up and running?

   If all of the above looks good, the ACP should be running "fine"
   locally, but we did not check any ACP neighbor relationships.

   Question: Why does the node not create a working ACP connection to a
   neighbor on an interface?

   *  Is the interface physically up?  Does it have an IPv6 link-local
      address?

   *  Is it enabled for ACP?

   *  Do we successfully send DULL GRASP messages to the interface?
      (Are there link-layer errors?)

   *  Do we receive DULL GRASP messages on the interface?  If not, some
      intervening L2 equipment performing bad MLD snooping could have
      caused problems.  Provide, e.g., diagnostics of the MLD querier
      IPv6 and MAC address.

   *  Do we see the ACP objective in any DULL GRASP message from that
      interface?  Diagnose the supported secure channel methods.

   *  Do we know the MAC address of the neighbor with the ACP objective?
      If not, diagnose SLAAC/ND state.

   *  When did we last attempt to build an ACP secure channel to the
      neighbor?

   *  If it failed:

      -  Did the neighbor close the connection on us, or did we close
         the connection on it because the domain certificate membership
         failed?

      -  If the neighbor closed the connection on us, provide any error
         diagnostics from the secure channel protocol.

      -  If we failed the attempt, display our local reason:

         o  There was no common secure channel protocol supported by the
            two neighbors (this could not happen on nodes supporting
            this specification because it mandates common support for
            IPsec).

         o  Did the ACP certificate membership check (Section 6.2.3)
            fail?

            +  The neighbor's certificate is not signed directly or
               indirectly by one of the node's TA.  Provide diagnostics
               which TA it has (can identify whom the device belongs
               to).

            +  The neighbor's certificate does not have the same domain
               (or no domain at all).  Diagnose acp-domain-name and
               potentially other cert info.

            +  The neighbor's certificate has been revoked or could not
               be authenticated by OCSP.

            +  The neighbor's certificate has expired, or it is not yet
               valid.

      -  Are there any other connection issues, e.g., IKEv2/IPsec, DTLS?

   Question: Is the ACP operating correctly across its secure channels?

   *  Are there one or more active ACP neighbors with secure channels?

   *  Is RPL for the ACP running?

   *  Is there a default route to the root in the ACP routing table?

   *  Is there, for each direct ACP neighbor not reachable over the ACP
      virtual interface to the root, a route in the ACP routing table?

   *  Is ACP GRASP running?

   *  Is at least one "SRV.est" objective cached (to support certificate
      renewal)?

   *  Is there at least one BRSKI registrar objective cached?  (If BRSKI
      is supported.)

   *  Is the BRSKI proxy operating normally on all interfaces where ACP
      is operating?

   These lists are not necessarily complete, but they illustrate the
   principle and show that there are variety of issues ranging from
   normal operational causes (a neighbor in another ACP domain) to
   problems in the credentials management (certificate lifetimes), to
   explicit security actions (revocation) or unexpected connectivity
   issues (intervening L2 equipment).

   The items so far illustrate how the ANI operations can be diagnosed
   with passive observation of the operational state of its components
   including historic, cached, and/or counted events.  This is not
   necessarily sufficient to provide good enough diagnostics overall.

   The components of ACP and BRSKI are designed with security in mind,
   but they do not attempt to provide diagnostics for building the
   network itself.  Consider two examples:

   1.  BRSKI does not allow for a neighboring device to identify the
       pledge's IDevID certificate.  Only the selected BRSKI registrar
       can do this, but it may be difficult to disseminate information
       from those BRSKI registrars about undesired pledges to locations
       and/or nodes where information about those pledges is desired.

   2.  LLDP disseminates information about nodes, such as node model,
       type, and/or software and interface name and/or number of the
       connection, to their immediate neighbors.  This information is
       often helpful or even necessary in network diagnostics.  It can
       equally be considered too insecure to make this information
       available unprotected to all possible neighbors.

   An "interested adjacent party" can always determine the IDevID
   certificate of a BRSKI pledge by behaving like a BRSKI proxy/
   registrar.  Therefore, the IDevID certificate of a BRSKI pledge is
   not meant to be protected -- it just has to be queried and is not
   signaled unsolicited (as it would be in LLDP) so that other observers
   on the same subnet can determine who is an "interested adjacent
   party".

9.1.1.  Secure Channel Peer Diagnostics

   When using mutual certificate authentication, the TA certificate is
   not required to be signaled explicitly because its hash is sufficient
   for certificate chain validation.  In the case of ACP secure channel
   setup, this leads to limited diagnostics when authentication fails
   because of TA mismatch.  For this reason, Section 6.8.2 recommends
   also including the TA certificate in the secure channel signaling.
   This should be possible to do without modifying the security
   association protocols used by the ACP.  For example, while [RFC7296]
   does not mention this, it also does not prohibit it.

   One common use case where diagnostics through the signaled TA of a
   candidate peer are very helpful is the multi-tenant environment, such
   as an office building, where different tenants run their own networks
   and ACPs.  Each tenant is given supposedly disjoint L2 connectivity
   through the building infrastructure.  In these environments, there
   are various common errors through which a device may receive L2
   connectivity into the wrong tenant's network.

   While the ACP itself is not impacted by this, the data plane to be
   built later may be impacted.  Therefore, it is important to be able
   to diagnose such undesirable connectivity from the ACP so that any
   autonomic or non-autonomic mechanisms to configure the data plane can
   treat such interfaces accordingly.  The information in the TA of the
   peer can then ease troubleshooting of such issues.

   Another use case is the intended or accidental reactivation of
   equipment, such as redundant gear taken from storage, whose TA
   certificate has long expired.

   A third use case is when, in a merger and acquisition case, ACP nodes
   have not been correctly provisioned with the mutual TA of a
   previously disjoint ACP.  This assumes that the ACP domain names were
   already aligned so that the ACP domain membership check is only
   failing on the TA.

   A fourth use case is when multiple registrars are set up for the same
   ACP but are not correctly set up with the same TA.  For example, when
   registrars support also being CAs themselves but are misconfigured to
   become TAs instead of intermediate CAs.

9.2.  ACP Registrars

   As described in Section 6.11.7, the ACP addressing mechanism is
   designed to enable lightweight, distributed, and uncoordinated ACP
   registrars that provide ACP address prefixes to candidate ACP nodes
   by enrolling them with an ACP certificate into an ACP domain via any
   appropriate mechanism and/or protocol, automated or not.

   This section discusses informatively more details and options for ACP
   registrars.

9.2.1.  Registrar Interactions

   This section summarizes and discusses the interactions with other
   entities required by an ACP registrar.

   In a simple instance of an ACP network, no central NOC component
   beside a TA is required.  Typically, this is a root CA.  One or more
   uncoordinated acting ACP registrars can be set up, performing the
   following interactions.

   To orchestrate enrolling a candidate ACP node autonomically, the ACP
   registrar can rely on the ACP and use proxies to reach the candidate
   ACP node, therefore allowing minimal, preexisting (auto)configured
   network services on the candidate ACP node.  BRSKI defines the BRSKI
   proxy, a design that can be adopted for various protocols that
   pledges and/or candidate ACP nodes could want to use, for example,
   BRSKI over CoAP (Constrained Application Protocol) or the proxying of
   NETCONF.

   To reach a TA that has no ACP connectivity, the ACP registrar uses
   the data plane.  The ACP and data plane in an ACP registrar could
   (and by default should) be completely isolated from each other at the
   network level.  Only applications such as the ACP registrar would
   need the ability for their transport stacks to access both.

   In non-autonomic enrollment options, the data plane between an ACP
   registrar and the candidate ACP node needs to be configured first.
   This includes the ACP registrar and the candidate ACP node.  Then any
   appropriate set of protocols can be used between the ACP registrar
   and the candidate ACP node to discover the other side, and then
   connect and enroll (configure) the candidate ACP node with an ACP
   certificate.  For example, NETCONF Zero Touch ("Secure Zero Touch
   Provisioning (SZTP)" [RFC8572]) is a protocol that could be used for
   this.  BRSKI using optional discovery mechanisms is equally a
   possibility for candidate ACP nodes attempting to be enrolled across
   non-ACP networks, such as the Internet.

   When a candidate ACP node, such as a BRSKI pledge, has secure
   bootstrap, it will not trust being configured and/or enrolled across
   the network unless it is presented with a voucher (see "A Voucher
   Artifact for Bootstrapping Protocols" [RFC8366]) authorizing the
   network to take possession of the node.  An ACP registrar will then
   need a method to retrieve such a voucher, either offline or online
   from a MASA (Manufacturer Authorized Signing Authority).  BRSKI and
   NETCONF Zero Touch are two protocols that include capabilities to
   present the voucher to the candidate ACP node.

   An ACP registrar could operate EST for ACP certificate renewal and/or
   act as a CRL Distribution Point.  A node performing these services
   does not need to support performing (initial) enrollment, but it does
   require the same above described connectivity as an ACP registrar:
   via the ACP to the ACP nodes and via the data plane to the TA and
   other sources of CRL information.

9.2.2.  Registrar Parameters

   The interactions of an ACP registrar outlined in Section 6.11.7 and
   Section 9.2.1 depend on the following parameters:

   *  A URL to the TA and credentials so that the ACP registrar can let
      the TA sign candidate ACP node certificates.

   *  The ACP domain name.

   *  The Registrar-ID to use.  This could default to a MAC address of
      the ACP registrar.

   *  For recovery, the next usable Node-IDs for the Zone Addressing
      Sub-Scheme (Zone-ID 0) and for the Vlong Addressing Sub-Scheme
      (/112 and /120).  These IDs would only need to be provisioned
      after recovering from a crash.  Some other mechanism would be
      required to remember these IDs in a backup location or to recover
      them from the set of currently known ACP nodes.

   *  Policies on whether the candidate ACP nodes should receive a
      domain certificate or not, for example, based on the device's
      IDevID certificate as in BRSKI.  The ACP registrar may whitelist
      or blacklist based on a device's "serialNumber" attribute [X.520]
      in the subject field distinguished name encoding of its IDevID
      certificate.

   *  Policies on what type of address prefix to assign to a candidate
      ACP device, likely based on the same information.

   *  For BRSKI or other mechanisms using vouchers: parameters to
      determine how to retrieve vouchers for specific types of secure
      bootstrap candidate ACP nodes (such as MASA URLs), unless this
      information is automatically learned, such as from the IDevID
      certificate of candidate ACP nodes (as defined in BRSKI).

9.2.3.  Certificate Renewal and Limitations

   When an ACP node renews and/or rekeys its certificate, it may end up
   doing so via a different registrar (e.g., EST server) than the one it
   originally received its ACP certificate from, for example, because
   that original ACP registrar is gone.  The ACP registrar through which
   the renewal/rekeying is performed would by default trust the acp-
   node-name from the ACP node's current ACP certificate and maintain
   this information so that the ACP node maintains its ACP address
   prefix.  In EST renewal/rekeying, the ACP node's current ACP
   certificate is signaled during the TLS handshake.

   This simple scenario has two limitations:

   1.  The ACP registrar cannot directly assign certificates to nodes
       and therefore needs an "online" connection to the TA.

   2.  Recovery from a compromised ACP registrar is difficult.  When an
       ACP registrar is compromised, it can insert, for example, a
       conflicting acp-node-name and thereby create an attack against
       other ACP nodes through the ACP routing protocol.

   Even when such a malicious ACP registrar is detected, resolving the
   problem may be difficult because it would require identifying all the
   wrong ACP certificates assigned via the ACP registrar after it was
   compromised.  Without additional centralized tracking of assigned
   certificates, there is no way to do this.

9.2.4.  ACP Registrars with Sub-CA

   In situations where either of the above two limitations are an issue,
   ACP registrars could also be sub-CAs.  This removes the need for
   connectivity to a TA whenever an ACP node is enrolled, and it reduces
   the need for connectivity of such an ACP registrar to a TA to only
   those times when it needs to renew its own certificate.  The ACP
   registrar would also now use its own (sub-CA) certificate to enroll
   and sign the ACP node's certificates, and therefore it is only
   necessary to revoke a compromised ACP registrar's sub-CA certificate.
   Alternatively, one can let it expire and not renew it when the
   certificate of the sub-CA is appropriately short-lived.

   As the ACP domain membership check verifies a peer ACP node's ACP
   certificate trust chain, it will also verify the signing certificate,
   which is the compromised and/or revoked sub-CA certificate.
   Therefore, ACP domain membership for an ACP node enrolled by a
   compromised and discovered ACP registrar will fail.

   ACP nodes enrolled by a compromised ACP registrar would automatically
   fail to establish ACP channels and ACP domain certificate renewal via
   EST and therefore revert to their role as candidate ACP members and
   attempt to get a new ACP certificate from an ACP registrar, for
   example, via BRSKI.  As a result, ACP registrars that have an
   associated sub-CA make isolating and resolving issues with
   compromised registrars easier.

   Note that ACP registrars with sub-CA functionality also can control
   the lifetime of ACP certificates more easily and therefore can be
   used as a tool to introduce short-lived certificates and to no longer
   rely on CRL, whereas the certificates for the sub-CAs themselves
   could be longer lived and subject to CRL.

9.2.5.  Centralized Policy Control

   When using multiple, uncoordinated ACP registrars, several advanced
   operations are potentially more complex than with a single, resilient
   policy control backend, for example, including but not limited to the
   following:

   *  Deciding which candidate ACP node is permitted or not permitted
      into an ACP domain.  This may not be a decision to be made
      upfront, so that a policy per "serialNumber" attribute in the
      subject field distinguished name encoding can be loaded into every
      ACP registrar.  Instead, it may better be decided in real time,
      potentially including a human decision in a NOC.

   *  Tracking all enrolled ACP nodes and their certificate information.
      For example, in support of revoking an individual ACP node's
      certificates.

   *  Needing more flexible policies as to which type of address prefix
      or even which specific address prefix to assign to a candidate ACP
      node.

   These and other operations could be introduced more easily by
   introducing a centralized Policy Management System (PMS) and
   modifying ACP registrar behavior so that it queries the PMS for any
   policy decision occurring during the candidate ACP node enrollment
   process and/or the ACP node certificate renewal process, for example,
   which ACP address prefix to assign.  Likewise, the ACP registrar
   would report any relevant state change information to the PMS as
   well, for example, when a certificate was successfully enrolled onto
   a candidate ACP node.

9.3.  Enabling and Disabling the ACP and/or the ANI

   Both ACP and BRSKI require interfaces to be operational enough to
   support the sending and receiving of their packets.  In node types
   where interfaces are enabled by default (e.g., without operator
   configuration), such as most L2 switches, this would be less of a
   change in behavior than in most L3 devices (e.g., routers), where
   interfaces are disabled by default.  In almost all network devices,
   though, it is common for configuration to change interfaces to a
   physically disabled state, and this would break the ACP.

   In this section, we discuss a suggested operational model to enable
   and disable interfaces and nodes for ACP/ANI in a way that minimizes
   the risk of breaking the ACP due to operator action and also
   minimizes operator surprise when the ACP/ANI becomes supported in
   node software.

9.3.1.  Filtering for Non-ACP/ANI Packets

   Whenever this document refers to enabling an interface for ACP (or
   BRSKI), it only requires permitting the interface to send and receive
   packets necessary to operate ACP (or BRSKI) -- but not any other data
   plane packets.  Unless the data plane is explicitly configured and
   enabled, all packets that are not required for ACP/BRSKI should be
   filtered on input and output.

   Both BRSKI and ACP require link-local-only IPv6 operations on
   interfaces and DULL GRASP.  IPv6 link-local operations mean the
   minimum signaling to auto-assign an IPv6 link-local address and talk
   to neighbors via their link-local addresses: SLAAC [RFC4862] and ND
   [RFC4861].  When the device is a BRSKI pledge, it may also require
   TCP/TLS connections to BRSKI proxies on the interface.  When the
   device has keying material, and the ACP is running, it requires DULL
   GRASP packets and packets necessary for the secure channel mechanism
   it supports, e.g., IKEv2 and IPsec ESP packets or DTLS packets to the
   IPv6 link-local address of an ACP neighbor on the interface.  It also
   requires TCP/TLS packets for its BRSKI proxy functionality if it
   supports BRSKI.

9.3.2.  "admin down" State

   Interfaces on most network equipment have at least two states: "up"
   and "down".  These may have product-specific names.  For example,
   "down" could be called "shutdown", and "up" could be called "no
   shutdown".  The "down" state disables all interface operations down
   to the physical level.  The "up" state enables the interface enough
   for all possible L2/L3 services to operate on top of it, and it may
   also auto-enable some subset of them.  More commonly, the operations
   of various L2/L3 services are controlled via additional node-wide or
   interface-level options, but they all become active only when the
   interface is not "down".  Therefore, an easy way to ensure that all
   L2/L3 operations on an interface are inactive is to put the interface
   into "down" state.  The fact that this also physically shuts down the
   interface is just a side effect in many cases, but it may be
   important in other cases (see Section 9.3.2.2).

   A common problem of remote management is the operator or SDN
   controller cutting its own connectivity to the remote node via
   configuration, impacting its own management connection to the node.
   The ACP itself should have no dedicated configuration other than the
   aforementioned enabling of the ACP on brownfield ACP nodes.  This
   leaves configuration that cannot distinguish between the ACP and data
   plane as sources of configuration mistakes as these commands will
   impact the ACP even though they should only impact the data plane.

   The one ubiquitous type of command that does this on many types of
   routers is the interface "down" command/configuration.  When such a
   command is applied to the interface through which the ACP provides
   access for remote management, it cuts the remote management
   connection through the ACP because, as outlined above, the "down"
   command typically impacts the physical layer, too, and not only the
   data plane services.

   To provide ACP/ANI resilience against such operator misconfiguration,
   this document recommends separating the "down" state of interfaces
   into an "admin down" state, where the physical layer is kept running
   and the ACP/ANI can use the interface, and a "physical down" state.
   Any existing "down" configurations would map to "admin down".  In
   "admin down", any existing L2/L3 services of the data plane should
   see no difference to "physical down" state.  To ensure that no data
   plane packets could be sent or received, packet filtering could be
   established automatically as described in Section 9.3.1.

   An example of ANI, but not ACP, traffic that should be permitted to
   pass even in "admin down" state is BRSKI enrollment traffic between a
   BRSKI pledge and a BRSKI proxy.

   New configuration options could be introduced as necessary (see
   discussion below) to issue "physical down".  The options should be
   provided with additional checks to minimize the risk of issuing them
   in a way that breaks the ACP without automatic restoration.  Examples
   of checks include not allowing the option to be issued from a control
   connection (NETCONF/SSH) that goes across the interface itself ("do
   not disconnect yourself") or only applying the option after
   additional reconfirmation.

   The following subsections discuss important aspects of the
   introduction of "admin down" state.

9.3.2.1.  Security

   Interfaces are physically brought down (or left in default "down"
   state) as a form of security.  The "admin down" state as described
   above also provides also a high level of security because it only
   permits ACP/ANI operations, which are both well secured.  Ultimately,
   it is subject to the deployment's security review whether "admin
   down" is a feasible replacement for "physical down".

   The need to trust the security of ACP/ANI operations needs to be
   weighed against the operational benefits of permitting the following:
   consider the typical example of a CPE (customer premises equipment)
   with no on-site network expert.  User ports are in "physical down"
   state unless explicitly configured not to be.  In a misconfiguration
   situation, the uplink connection is incorrectly plugged into such a
   user port.  The device is disconnected from the network, and
   therefore diagnostics from the network side are no longer possible.
   Alternatively, all ports default to "admin down".  The ACP (but not
   the data plane) would still automatically form.  Diagnostics from the
   network side are possible, and operator reaction could include either
   to make this port the operational uplink port or to instruct re-
   cabling.  Security wise, only the ACP/ANI could be attacked, all
   other functions are filtered on interfaces in "admin down" state.

9.3.2.2.  Fast State Propagation and Diagnostics

   The "physical down" state propagates on many interface types (e.g.,
   Ethernet) to the other side.  This can trigger fast L2/L3 protocol
   reaction on the other side, and "admin down" would not have the same
   (fast) result.

   Bringing interfaces to "physical down" state is, to the best of our
   knowledge, always a result of operator action and, today, never the
   result of autonomic L2/L3 services running on the nodes.  Therefore,
   one option is to end the operator's reliance on interface state
   propagation via the subnet link or physical layer.  This may not be
   possible when both sides are under the control of different
   operators, but in that case, it is unlikely that the ACP is running
   across the link, and actually putting the interface into "physical
   down" state may still be a good option.

   Ideally, fast physical state propagation is replaced by fast
   software-driven state propagation.  For example, a DULL GRASP "admin-
   state" objective could be used to autoconfigure a BFD session
   ("Bidirectional Forwarding Detection (BFD)" [RFC5880]) between the
   two sides of the link that would be used to propagate the "up" vs.
   "admin down" state.

   Triggering "physical down" state may also be used as a means of
   diagnosing cabling issues in the absence of easier methods.  It is
   more complex than automated neighbor diagnostics because it requires
   coordinated remote access to (likely) both sides of a link to
   determine whether up/down toggling will cause the same reaction on
   the remote side.

   See Section 9.1 for a discussion about how LLDP and/or diagnostics
   via GRASP could be used to provide neighbor diagnostics and therefore
   hopefully eliminate the need for "physical down" for neighbor
   diagnostics -- as long as both neighbors support ACP/ANI.

9.3.2.3.  Low-Level Link Diagnostics

   The "physical down" state is used to diagnose low-level interface
   behavior when higher-layer services (e.g., IPv6) are not working.
   Ethernet links are especially subject to a wide variety of possible
   incorrect configurations/cablings if they do not support automatic
   selection of variable parameters such as speed (10/100/1000 Mbps),
   crossover (automatic medium-dependent interface crossover (MDI-X)),
   and connector (fiber, copper -- when interfaces have multiple but can
   only enable one at a time).  The need for low-level link diagnostics
   can therefore be minimized by using fully autoconfiguring links.

   In addition to the "physical down" state, low-level diagnostics of
   Ethernet or other interfaces also involve the creation of other
   states on interfaces, such as physical loopback (internal and/or
   external) or the bringing down of all packet transmissions for
   reflection and/or cable-length measurements.  Any of these options
   would disrupt ACP as well.

   In cases where such low-level diagnostics of an operational link are
   desired but where the link could be a single point of failure for the
   ACP, the ASA on both nodes of the link could perform a negotiated
   diagnostic that automatically terminates in a predetermined manner
   without dependence on external input, ensuring the link will become
   operational again.

9.3.2.4.  Power Consumption Issues

   Power consumption of "physical down" interfaces may be significantly
   lower than those in "admin down" state, for example, on long-range
   fiber interfaces.  Bringing up interfaces, for example, to probe
   reachability may also consume additional power.  This can make these
   types of interfaces inappropriate to operate purely for the ACP when
   they are not currently needed for the data plane.

9.3.3.  Enabling Interface-Level ACP and ANI

   The interface-level configuration option "ACP enable" enables ACP
   operations on an interface, starting with ACP neighbor discovery via
   DULL GRASP.  The interface-level configuration option "ANI enable" on
   nodes supporting BRSKI and ACP starts with BRSKI pledge operations
   when there is no domain certificate on the node.  On ACP/BRSKI nodes,
   only "ANI enable" may need to be supported and not "ACP enable".
   Unless overridden by global configuration options (see
   Section 9.3.4), either "ACP enable" or "ANI enable" (both abbreviated
   as "ACP/ANI enable") will result in the "down" state on an interface
   behaving as "admin down".

9.3.4.  Which Interfaces to Auto-Enable?

   Section 6.4 requires that "ACP enable" is automatically set on native
   interfaces, but not on non-native interfaces (reminder: a native
   interface is one that exists without operator configuration action,
   such as physical interfaces in physical devices).

   Ideally, "ACP enable" is set automatically on all interfaces that
   provide access to additional connectivity, which allows more nodes of
   the ACP domain to be reached.  The best set of interfaces necessary
   to achieve this is not possible to determine automatically.  Native
   interfaces are the best automatic approximation.

   Consider an ACP domain of ACP nodes transitively connected via native
   interfaces.  A data plane tunnel between two of these nodes that are
   nonadjacent is created, and "ACP enable" is set for that tunnel.  ACP
   RPL sees this tunnel as just as a single hop.  Routes in the ACP
   would use this hop as an attractive path element to connect regions
   adjacent to the tunnel nodes.  As a result, the actual hop-by-hop
   paths used by traffic in the ACP can become worse.  In addition,
   correct forwarding in the ACP now depends on correct data plane
   forwarding configuration including QoS, filtering, and other security
   on the data plane path across which this tunnel runs.  This is the
   main reason why "ACP/ANI enable" should not be set automatically on
   non-native interfaces.

   If the tunnel would connect two previously disjoint ACP regions, then
   it likely would be useful for the ACP.  A data plane tunnel could
   also run across nodes without ACP and provide additional connectivity
   for an already connected ACP network.  The benefit of this additional
   ACP redundancy has to be weighed against the problems of relying on
   the data plane.  If a tunnel connects two separate ACP regions, how
   many tunnels should be created to connect these ACP regions reliably
   enough?  Between which nodes?  These are all standard tunneled
   network design questions not specific to the ACP, and there are no
   generic, fully automated answers.

   Instead of automatically setting "ACP enable" on these types of
   interfaces, the decision needs to be based on the use purpose of the
   non-native interface, and "ACP enable" needs to be set in conjunction
   with the mechanism through which the non-native interface is created
   and/or configured.

   In addition to the explicit setting of "ACP/ANI enable", non-native
   interfaces also need to support configuration of the ACP RPL cost of
   the link to avoid the problems of attracting too much traffic to the
   link as described above.

   Even native interfaces may not be able to automatically perform BRSKI
   or ACP because they may require additional operator input to become
   operational.  Examples include DSL interfaces requiring Point-to-
   Point Protocol over Ethernet (PPPoE) credentials or mobile interfaces
   requiring credentials from a SIM card.  Whatever mechanism is used to
   provide the necessary configuration to the device to enable the
   interface can also be expanded to decide whether or not to set "ACP/
   ANI enable".

   The goal of automatically setting "ACP/ANI enable" on interfaces
   (native or not) is to eliminate unnecessary "touches" to the node to
   make its operation as much as possible "zero-touch" with respect to
   ACP/ANI.  If there are "unavoidable touches" such a creating and/or
   configuring a non-native interface or provisioning credentials for a
   native interface, then "ACP/ANI enable" should be added as an option
   to that "touch".  If an erroneous "touch" is easily fixed (does not
   create another high-cost touch), then the default should be not to
   enable ANI/ACP, and if it is potentially expensive or slow to fix
   (e.g., parameters on SIM card shipped to remote location), then the
   default should be to enable ACP/ANI.

9.3.5.  Enabling Node-Level ACP and ANI

   A node-level command "ACP/ANI enable [up-if-only]" enables ACP or ANI
   on the node (ANI = ACP + BRSKI).  Without this command set, any
   interface-level "ACP/ANI enable" is ignored.  Once set, ACP/ANI will
   operate an interface where "ACP/ANI enable" is set.  Setting of
   interface-level "ACP/ANI enable" is either automatic (default) or
   explicit through operator action as described in Section 9.3.4.

   If the option "up-if-only" is selected, the behavior of "down"
   interfaces is unchanged, and ACP/ANI will only operate on interfaces
   where "ACP/ANI enable" is set and that are "up".  When it is not set,
   then "down" state of interfaces with "ACP/ANI enable" is modified to
   behave as "admin down".

9.3.5.1.  Brownfield Nodes

   A "brownfield" node is one that already has a configured data plane.

   Executing global "ACP/ANI enable [up-if-only]" on each node is the
   only command necessary to create an ACP across a network of
   brownfield nodes once all the nodes have a domain certificate.  When
   BRSKI is used ("ANI enable"), provisioning of the certificates only
   requires the setup of a single BRSKI registrar node, which could also
   implement a CA for the network.  This is the simplest way to
   introduce ACP/ANI into existing (i.e., brownfield) networks.

   The need to explicitly enable ACP/ANI is especially important in
   brownfield nodes because otherwise software updates may introduce
   support for ACP/ANI.  The automatic enabling of ACP/ANI in networks
   where the operator does not want ACP/ANI or has likely never even
   heard of it could be quite irritating to the operator, especially
   when "down" behavior is changed to "admin down".

   Automatically setting "ANI enable" on brownfield nodes where the
   operator is unaware of BRSKI and MASA operations could also be an
   unlikely, but critical, security issue.  If an attacker could
   impersonate the operator by registering as the operator at the MASA
   or otherwise getting hold of vouchers and could get enough physical
   access to the network so pledges would register to an attacking
   registrar, then the attacker could gain access to the ACP and,
   through the ACP, gain access to the data plane.

   In networks where the operator explicitly enables the ANI, this could
   not happen because the operator would create a BRSKI registrar that
   would discover attack attempts, and the operator would set up his
   registrar with the MASA.  Nodes requiring "ownership vouchers" would
   not be subject to that attack.  See [RFC8995] for more details.  Note
   that a global "ACP enable" alone is not subject to these types of
   attacks because they always depend on some other mechanism first to
   provision domain certificates into the device.

9.3.5.2.  Greenfield Nodes

   An ACP "greenfield" node is one that does not have any prior
   configuration and that can be bootstrapped into the ACP across the
   network.  To support greenfield nodes, ACP as described in this
   document needs to be combined with a bootstrap protocol and/or
   mechanism that will enroll the node with the ACP keying material: the
   ACP certificate and the TA.  For ANI nodes, this protocol/mechanism
   is BRSKI.

   When such a node is powered on and determines that it is in
   greenfield condition, it enables the bootstrap protocol(s) and/or
   mechanism(s).  Once the ACP keying material is enrolled, the
   greenfield state ends and the ACP is started.  When BRSKI is used,
   the node's state reflects this by setting "ANI enable" upon
   determination of greenfield state when it is powered on.

   ACP greenfield nodes that, in the absence of ACP, would have their
   interfaces in "down" state SHOULD set all native interfaces into
   "admin down" state and only permit data plane traffic required for
   the bootstrap protocol and/or mechanisms.

   The ACP greenfield state ends either through the successful
   enrollment of ACP keying material (certificate and TA) or the
   detection of a permitted termination of ACP greenfield operations.

   ACP nodes supporting greenfield operations MAY want to provide
   backward compatibility with other forms of configuration and/or
   provisioning, especially when only a subset of nodes are expected to
   be deployed with ACP.  Such an ACP node SHOULD observe attempts to
   provision or configure the node via interfaces and/or methods that
   traditionally indicate physical possession of the node, such as a
   serial or USB console port or a USB memory stick with a bootstrap
   configuration.  When such an operation is observed before enrollment
   of the ACP keying material has completed, the node SHOULD put itself
   into the state the node would have been in if ACP/ANI was disabled at
   boot.  This terminates ACP greenfield operations.

   When an ACP greenfield node enables multiple, automated ACP or non-
   ACP enrollment and/or bootstrap protocols or mechanisms in parallel,
   care must be taken not to terminate any protocol/mechanism before the
   others either have succeeded in enrolling ACP keying material or have
   progressed to a point of permitted termination for ACP greenfield
   operations.

   Highly secure ACP greenfield nodes may not permit any reason to
   terminate ACP greenfield operations, including physical access.

   Nodes that claim to support ANI greenfield operations SHOULD NOT
   enable in parallel to BRSKI any enrollment/bootstrap protocol/
   mechanism that allows Trust On First Use (TOFU, "Opportunistic
   Security: Some Protection Most of the Time" [RFC7435]) over
   interfaces other than those traditionally indicating physical
   possession of the node.  Protocols/mechanisms with published default
   username/password authentication are considered to suffer from TOFU.
   Securing the bootstrap protocol/mechanism by requiring a voucher
   [RFC8366] can be used to avoid TOFU.

   In summary, the goal of ACP greenfield support is to allow remote,
   automated enrollment of ACP keying materials, and therefore automated
   bootstrap into the ACP and to prohibit TOFU during bootstrap with the
   likely exception (for backward compatibility) of bootstrapping via
   interfaces traditionally indicating physical possession of the node.

9.3.6.  Undoing "ANI/ACP enable"

   Disabling ANI/ACP by undoing "ACP/ANI enable" is a risk for the
   reliable operations of the ACP if it can be executed by mistake or
   without authorization.  This behavior could be influenced through
   some additional (future) property in the certificate (e.g., in the
   acp-node-name extension field): in an ANI deployment intended for
   convenience, disabling it could be allowed without further
   constraints.  In an ANI deployment considered to be critical, more
   checks would be required.  One very controlled option would be to not
   permit these commands unless the domain certificate has been revoked
   or is denied renewal.  Configuring this option would be a parameter
   on the BRSKI registrar(s).  As long as the node did not receive a
   domain certificate, undoing "ANI/ACP enable" should not have any
   additional constraints.

9.3.7.  Summary

   Node-wide "ACP/ANI enable [up-if-only]" commands enable the operation
   of ACP/ANI.  This is only auto-enabled on ANI greenfield devices,
   otherwise it must be configured explicitly.

   If the option "up-if-only" is not selected, interfaces enabled for
   ACP/ANI interpret the "down" state as "admin down" and not "physical
   down".  In the "admin-down" state, all non-ACP/ANI packets are
   filtered, but the physical layer is kept running to permit ACP/ANI to
   operate.

   (New) commands that result in physical interruption ("physical down",
   "loopback") of ACP/ANI-enabled interfaces should be built to protect
   continuance or reestablishment of ACP as much as possible.

   Interface-level "ACP/ANI enable" commands control per-interface
   operations.  It is enabled by default on native interfaces and has to
   be configured explicitly on other interfaces.

   Disabling "ACP/ANI enable" globally and per interface should have
   additional checks to minimize undesired breakage of ACP.  The degree
   of control could be a domain-wide parameter in the domain
   certificates.

9.4.  Partial or Incremental Adoption

   The Zone Addressing Sub-Scheme (see Section 6.11.3) allows
   incremental adoption of the ACP in a network where ACP can be
   deployed on edge areas, but not across the core that is connecting
   those edges.

   In such a setup, each edge network, such as a branch or campus of an
   enterprise network, has a disjoint ACP to which one or more unique
   Zone-IDs are assigned: ACP nodes registered for a specific ACP zone
   have to receive Zone Addressing Sub-Scheme addresses, for example, by
   virtue of configuring for each such zone one or more ACP registrars
   with that Zone-ID.  All the registrars for these ACP zones need to
   get ACP certificates from CAs relying on a common set of TAs and of
   course the same ACP domain name.

   These ACP zones can first be brought up as separate networks without
   any connection between them and/or they can be connected across a
   non-ACP enabled core network through various non-autonomic
   operational practices.  For example, each separate ACP zone can have
   an edge node that is a L3 VPN PE (MPLS or IPv6 L3VPN), where a
   complete non-autonomic ACP-Core VPN is created by using the ACP VRFs
   and exchanging the routes from those ACP VRFs across the VPN's non-
   autonomic routing protocol(s).

   While such a setup is possible with any ACP addressing sub-scheme,
   the Zone Addressing Sub-Scheme makes it easy to configure and
   scalable for any VPN routing protocols because every ACP zone only
   needs to indicate one or more /64 ACP zone addressing prefix routes
   into the ACP-Core VPN as opposed to routes for every individual ACP
   node as required in the other ACP addressing schemes.

   Note that the non-autonomous ACP-Core VPN requires additional
   extensions to propagate GRASP messages when GRASP discovery is
   desired across the zones.

   For example, one could set up on each zone edge router a remote ACP
   tunnel to a GRASP hub.  The GRASP hub could be implemented at the
   application level and could run in the NOC of the network.  It would
   serve to propagate GRASP announcements between ACP zones and/or
   generate GRASP announcements for NOC services.

   Such a partial deployment may prove to be sufficient or could evolve
   to become more autonomous through future standardized or nonstandard
   enhancements, for example, by allowing GRASP messages to be
   propagated across the L3VPN, leveraging for example L3VPN multicast
   support.

   Finally, these partial deployments can be merged into a single,
   contiguous ACP that is completely autonomous (given appropriate ACP
   support across the core) without changes in the cryptographic
   material because the node's ACP certificates are from a single ACP.

9.5.  Configuration and the ACP (Summary)

   There is no desirable configuration for the ACP.  Instead, all
   parameters that need to be configured in support of the ACP are
   limitations of the solution, but they are only needed in cases where
   not all components are made autonomic.  Wherever this is necessary,
   it relies on preexisting mechanisms for configuration such as CLI or
   YANG data models ("The YANG 1.1 Data Modeling Language" [RFC7950]).

   The most important examples of such configuration include:

   *  When ACP nodes do not support an autonomic way to receive an ACP
      certificate, for example, BRSKI, then such a certificate needs to
      be configured via some preexisting mechanisms outside the scope of
      this specification.  Today, routers typically have a variety of
      mechanisms to do this.

   *  Certificate maintenance requires PKI functions.  Discovery of
      these functions across the ACP is automated (see Section 6.2.5),
      but their configuration is not.

   *  When non-ACP-capable nodes such as preexisting NMS need to be
      physically connected to the ACP, the ACP node to which they attach
      needs to be configured with ACP connect according to Section 8.1.
      It is also possible to use that single physical connection to
      connect both to the ACP and the data plane of the network as
      explained in Section 8.1.4.

   *  When devices are not autonomically bootstrapped, explicit
      configuration to enable the ACP needs to be applied.  See
      Section 9.3.

   *  When the ACP needs to be extended across interfaces other than L2,
      the ACP as defined in this document cannot auto-discover candidate
      neighbors automatically.  Remote neighbors need to be configured,
      see Section 8.2.

   Once the ACP is operating, any further configuration for the data
   plane can be done more reliably across the ACP itself because the ACP
   provides addressing and connectivity (routing) independent of the
   data plane.  For this, the configuration methods simply need to allow
   operating across the ACP VRF, for example, with NETCONF, SSH, or any
   other method.

   The ACP also provides additional security through its hop-by-hop
   encryption for any such configuration operations.  Some legacy
   configuration methods (for example, SNMP, TFTP, or HTTP) may not use
   end-to-end encryption, and most of the end-to-end secured
   configuration methods still allow for easy, passive observation along
   the path of the configuration taking place (for example, transport
   flows, port numbers, and/or IP addresses).

   The ACP can and should be used equally as the transport to configure
   any of the aforementioned non-autonomic components of the ACP, but in
   that case, the same caution needs to be exercised as with data plane
   configuration without the ACP.  Misconfiguration may cause the
   configuring entity to be disconnected from the node it configures,
   for example, when incorrectly unconfiguring a remote ACP neighbor
   through which the configured ACP node is reached.

10.  Summary: Benefits (Informative)

10.1.  Self-Healing Properties

   The ACP is self-healing:

   *  New neighbors will automatically join the ACP after successful
      validation and will become reachable using their unique ULA
      address across the ACP.

   *  When any changes happen in the topology, the routing protocol used
      in the ACP will automatically adapt to the changes and will
      continue to provide reachability to all nodes.

   *  The ACP tracks the validity of peer certificates and tears down
      ACP secure channels when a peer certificate has expired.  When
      short-lived certificates with lifetimes on the order of OCSP/CRL
      refresh times are used, then this allows for removal of invalid
      peers (whose certificate was not renewed) at similar speeds as
      when using OCSP/CRL.  The same benefit can be achieved when using
      CRL/OCSP, periodically refreshing the revocation information and
      also tearing down ACP secure channels when the peer's (long-lived)
      certificate is revoked.  There is no requirement for ACP
      implementations to require this enhancement, though, in order to
      keep the mandatory implementations simpler.

   The ACP can also sustain network partitions and mergers.  Practically
   all ACP operations are link local, where a network partition has no
   impact.  Nodes authenticate each other using the domain certificates
   to establish the ACP locally.  Addressing inside the ACP remains
   unchanged, and the routing protocol inside both parts of the ACP will
   lead to two working (although partitioned) ACPs.

   There are a few central dependencies: a CRL may not be available
   during a network partition.  This can be addressed by a suitable
   policy to not immediately disconnect neighbors when no CRL is
   available.  Also, an ACP registrar or CA might not be available
   during a partition.  This may delay renewal of certificates that are
   to expire in the future, and it may prevent the enrollment of new
   nodes during the partition.

   Highly resilient ACP designs can be built by using ACP registrars
   with embedded sub-CAs, as outlined in Section 9.2.4.  As long as a
   partition is left with one or more of such ACP registrars, it can
   continue to enroll new candidate ACP nodes as long as the ACP
   registrar's sub-CA certificate does not expire.  Because the ACP
   addressing relies on unique Registrar-IDs, a later merging of
   partitions will not cause problems with ACP addresses assigned during
   partitioning.

   After a network partition, merging will just establish the previous
   status: certificates can be renewed, the CRL is available, and new
   nodes can be enrolled everywhere.  Since all nodes use the same TA,
   the merging will be smooth.

   Merging two networks with different TAs requires the ACP nodes to
   trust the union of TAs.  As long as the routing-subdomain hashes are
   different, the addressing will not overlap.  Overlaps will only
   happen accidentally in the unlikely event of a 40-bit hash collision
   in SHA-256 (see Section 6.11).  Note that the complete mechanisms to
   merge networks is out of scope of this specification.

   It is also highly desirable for an implementation of the ACP to be
   able to run it over interfaces that are administratively down.  If
   this is not feasible, then it might instead be possible to request
   explicit operator override upon administrative actions that would
   administratively bring down an interface across which the ACP is
   running, especially if bringing down the ACP is known to disconnect
   the operator from the node.  For example, any such administrative
   down action could perform a dependency check to see if the transport
   connection across which this action is performed is affected by the
   down action (with default RPL routing used, packet forwarding will be
   symmetric, so this is actually possible to check).

10.2.  Self-Protection Properties

10.2.1.  From the Outside

   As explained in Section 6, the ACP is based on secure channels built
   between nodes that have mutually authenticated each other with their
   domain certificates.  The channels themselves are protected using
   standard encryption technologies such as DTLS or IPsec, which provide
   additional authentication during channel establishment, data
   integrity, and data confidentiality protection inside the ACP, and
   also provide replay protection.

   An attacker will not be able to join the ACP unless it has a valid
   ACP certificate.  An on-path attacker without a valid ACP certificate
   cannot inject packets into the ACP due to ACP secure channels.  An
   attacker also cannot decrypt ACP traffic unless it can crack the
   encryption.  It can attempt behavioral traffic analysis on the
   encrypted ACP traffic.

   The degree to which compromised ACP nodes can impact the ACP depends
   on the implementation of the ACP nodes and their impairment.  When an
   attacker has only gained administrative privileges to configure ACP
   nodes remotely, the attacker can disrupt the ACP only through one of
   the few configuration options to disable it (see Section 9.3) or by
   the configuring of non-autonomic ACP options if those are supported
   on the impaired ACP nodes (see Section 8).  Injecting traffic into or
   extracting traffic from an impaired ACP node is only possible when an
   impaired ACP node supports ACP connect (see Section 8.1), and the
   attacker can control traffic into/from one of the ACP node's
   interfaces, such as by having physical access to the ACP node.

   The ACP also serves as protection (through authentication and
   encryption) for protocols relevant to OAM that may not have secured
   protocol stack options or where implementation or deployment of those
   options fail due to some vendor, product, or customer limitations.
   This includes protocols such as SNMP ("An Architecture for Describing
   Simple Network Management Protocol (SNMP) Management Frameworks"
   [RFC3411]), NTP [RFC5905], PTP (Precision Time Protocol
   [IEEE-1588-2008]), DNS ("DNS Extensions to Support IP Version 6"
   [RFC3596]), DHCPv6 ("Dynamic Host Configuration Protocol for IPv6
   (DHCPv6)" [RFC3315]), syslog ("The BSD Syslog Protocol" [RFC3164]),
   RADIUS ("Remote Authentication Dial In User Service (RADIUS)"
   [RFC2865]), Diameter ("Diameter Base Protocol" [RFC6733]), TACACS
   ("An Access Control Protocol, Sometimes Called TACACS" [RFC1492]),
   IPFIX ("Specification of the IP Flow Information Export (IPFIX)
   Protocol for the Exchange of Flow Information" [RFC7011]), NetFlow
   ("Cisco Systems NetFlow Services Export Version 9" [RFC3954]) -- just
   to name a few.  Not all of these protocol references are necessarily
   the latest version of protocols, but they are versions that are still
   widely deployed.

   Protection via the ACP secure hop-by-hop channels for these protocols
   is meant to be only a stopgap, though: the ultimate goal is for these
   and other protocols to use end-to-end encryption utilizing the domain
   certificate and to rely on the ACP secure channels primarily for
   zero-touch reliable connectivity, but not primarily for security.

   The remaining attack vector would be to attack the underlying ACP
   protocols themselves, either via directed attacks or by denial-of-
   service attacks.  However, as the ACP is built using link-local IPv6
   addresses, remote attacks from the data plane are impossible as long
   as the data plane has no facilities to remotely send IPv6 link-local
   packets.  The only exceptions are ACP-connected interfaces, which
   require greater physical protection.  The ULA addresses are only
   reachable inside the ACP context and therefore unreachable from the
   data plane.  Also, the ACP protocols should be implemented to be
   attack resistant and to not consume unnecessary resources even while
   under attack.

10.2.2.  From the Inside

   The security model of the ACP is based on trusting all members of the
   group of nodes that receive an ACP certificate for the same domain.
   Attacks from the inside by a compromised group member are therefore
   the biggest challenge.

   Group members must be protected against attackers so that there is no
   easy way to compromise them or use them as a proxy for attacking
   other devices across the ACP.  For example, management plane
   functions (transport ports) should be reachable only from the ACP and
   not from the data plane.  This applies especially to those management
   plane functions that lack secure end-to-end transport and to which
   the ACP provides both automatic, reliable connectivity and protection
   against attacks.  Protection across all potential attack vectors is
   typically easier to do in devices whose software is designed from the
   beginning with the ACP in mind than in legacy, software-based systems
   where the ACP is added on as another feature.

   As explained above, traffic across the ACP should still be end-to-end
   encrypted whenever possible.  This includes traffic such as GRASP,
   EST, and BRSKI inside the ACP.  This minimizes man-in-the-middle
   attacks by compromised ACP group members.  Such attackers cannot
   eavesdrop or modify communications, but they can just filter them
   (which is unavoidable by any means).

   See Appendix A.9.8 for further considerations on avoiding and dealing
   with compromised nodes.

10.3.  The Administrator View

   An ACP is self-forming, self-managing, and self-protecting;
   therefore, it has minimal dependencies on the administrator of the
   network.  Specifically, since it is (intended to be) independent of
   configuration, there is only limited scope for configuration errors
   on the ACP itself.  The administrator may have the option to enable
   or disable the entire approach, but detailed configuration is not
   possible.  This means that the ACP must not be reflected in the
   running configuration of nodes, except for a possible on/off switch
   (and even that is undesirable).

   While configuration (except for Section 8 and Section 9.2) is not
   possible, an administrator must have full visibility into the ACP and
   all its parameters to be able to troubleshoot.  Therefore, an ACP
   must support all show and debug options, as with any other network
   function.  Specifically, an NMS or controller must be able to
   discover the ACP and monitor its health.  This visibility into ACP
   operations must clearly be separated from the visibility of the data
   plane so automated systems will never have to deal with ACP aspects
   unless they explicitly desire to do so.

   Since an ACP is self-protecting, a node that does not support the ACP
   or that does not have a valid domain certificate cannot connect to
   it.  This means that by default a traditional controller or NMS
   cannot connect to an ACP.  See Section 8.1.1 for details on how to
   connect an NMS host to the ACP.

11.  Security Considerations

   A set of ACP nodes with ACP certificates for the same ACP domain and
   with ACP functionality enabled is automatically "self-building": the
   ACP is automatically established between neighboring ACP nodes.  It
   is also self-protecting: the ACP secure channels are authenticated
   and encrypted.  No configuration is required for this.

   The self-protecting property does not include workarounds for non-
   autonomic components as explained in Section 8.  See Section 10.2 for
   details of how the ACP protects itself against attacks from the
   outside and, to a more limited degree, from the inside as well.

   However, the security of the ACP depends on a number of other
   factors:

   *  The usage of domain certificates depends on a valid supporting PKI
      infrastructure.  If the chain of trust of this PKI infrastructure
      is compromised, the security of the ACP is also compromised.  This
      is typically under the control of the network administrator.

   *  ACP nodes receive their certificates from ACP registrars.  These
      ACP registrars are security-critical dependencies of the ACP.
      Procedures and protocols for ACP registrars are outside the scope
      of this specification as explained in Section 6.11.7.1; only the
      requirements for the resulting ACP certificates are specified.

   *  Every ACP registrar (for enrollment of ACP certificates) and ACP
      EST server (for renewal of ACP certificates) is a security-
      critical entity and its protocols are security-critical protocols.
      Both need to be hardened against attacks, similar to a CA and its
      protocols.  A malicious registrar can enroll malicious nodes to an
      ACP network (if the CA delegates this policy to the registrar) or
      break ACP routing, for example, by assigning duplicate ACP
      addresses to ACP nodes via their ACP certificates.

   *  ACP nodes that are ANI nodes rely on BRSKI as the protocol for ACP
      registrars.  For ANI-type ACP nodes, the security considerations
      of BRSKI apply.  It enables automated, secure enrollment of ACP
      certificates.

   *  BRSKI and potentially other ACP registrar protocol options require
      that nodes have an (X.509 v3 based) IDevID.  IDevIDs are an option
      for ACP registrars to securely identify candidate ACP nodes that
      should be enrolled into an ACP domain.

   *  For IDevIDs to securely identify the node to which its IDevID is
      assigned, the node needs (1) to utilize hardware support such as a
      Trusted Platform Module (TPM) to protect against extraction and/or
      cloning of the private key of the IDevID and (2) a hardware/
      software infrastructure to prohibit execution of unauthenticated
      software to protect against malicious use of the IDevID.

   *  Like the IDevID, the ACP certificate should equally be protected
      from extraction or other abuse by the same ACP node
      infrastructure.  This infrastructure for IDevID and ACP
      certificate is beneficial independent of the ACP registrar
      protocol used (BRSKI or other).

   *  Renewal of ACP certificates requires support for EST; therefore,
      the security considerations of [RFC7030] related to certificate
      renewal and/or rekeying and TP renewal apply to the ACP.  EST
      security considerations when using other than mutual certificate
      authentication do not apply, nor do considerations for initial
      enrollment via EST apply, except for ANI-type ACP nodes because
      BRSKI leverages EST.

   *  A malicious ACP node could declare itself to be an EST server via
      GRASP across the ACP if malicious software could be executed on
      it.  The CA should therefore authenticate only known trustworthy
      EST servers, such as nodes with hardware protections against
      malicious software.  When registrars use their ACP certificate to
      authenticate towards a CA, the id-kp-cmcRA [RFC6402] extended key
      usage attribute allows the CA to determine that the ACP node was
      permitted during enrollment to act as an ACP registrar.  Without
      the ability to talk to the CA, a malicious EST server can still
      attract ACP nodes attempting to renew their keying material, but
      they will fail to perform successful renewal of a valid ACP
      certificate.  The ACP node attempting to use the malicious EST
      server can then continue to use a different EST server and log a
      failure against a malicious EST server.

   *  Malicious on-path ACP nodes may filter valid EST server
      announcements across the ACP, but such malicious ACP nodes could
      equally filter any ACP traffic such as the EST traffic itself.
      Either attack requires the ability to execute malicious software
      on an impaired ACP node, though.

   *  In the absence of malicious software injection, an attacker that
      can misconfigure an ACP node that supports EST server
      functionality could attempt to configure a malicious CA.  This
      would not result in the ability to successfully renew ACP
      certificates, but it could result in DoS attacks by becoming an
      EST server and by making ACP nodes attempt their ACP certificate
      renewal via this impaired ACP node.  This problem can be avoided
      when the EST server implementation can verify that the configured
      CA is indeed providing renewal for certificates of the node's ACP.
      The ability to do so depends on the protocol between the EST
      server and the CA, which is outside the scope of this document.

   In summary, attacks against the PKI/certificate dependencies of the
   ACP can be minimized by a variety of hardware and/or software
   components, including options such as TPM for IDevID and/or ACP
   certificate, prohibitions against the execution of untrusted
   software, and design aspects of the EST server functionality for the
   ACP that eliminate configuration-level impairment.

   Because ACP peers select one out of potentially more than one
   mutually supported ACP secure channel protocols via the approach
   described in Section 6.6, ACP secure channel setup is subject to
   downgrade attacks by MITM attackers.  This can be discovered after
   such an attack by additional mechanisms described in Appendix A.9.9.
   Alternatively, more advanced channel selection mechanisms can be
   devised.

   The security model of the ACP as defined in this document is tailored
   for use with private PKI.  The TA of a private PKI provides the
   security against maliciously created ACP certificates that give
   access to an ACP.  Such attacks can create fake ACP certificates with
   correct-looking AcpNodeNames, but those certificates would not pass
   the certificate path validation of the ACP domain membership check
   (see Section 6.2.3, point 2).

   There is no prevention of source-address spoofing inside the ACP.
   This implies that if an attacker gains access to the ACP, it can
   spoof all addresses inside the ACP and fake messages from any other
   node.  New protocols and/or services running across the ACP should
   therefore use end-to-end authentication inside the ACP.  This is
   already done by GRASP as specified in this document.

   The ACP is designed to enable automation of current network
   management and the management of future autonomic peer-to-peer/
   distributed networks.  Any ACP member can send ACP IPv6 packets to
   other ACP members and announce via ACP GRASP services to all ACP
   members without depending on centralized components.

   The ACP relies on peer-to-peer authentication and authorization using
   ACP certificates.  This security model is necessary to enable the
   autonomic ad hoc, any-to-any connectivity between ACP nodes.  It
   provides infrastructure protection through hop-by-hop authentication
   and encryption -- without relying on third parties.  For any services
   where this complete autonomic peer-to-peer group security model is
   appropriate, the ACP certificate can also be used unchanged, for
   example, for any type of data plane routing protocol security.

   This ACP security model is designed primarily to protect against
   attack from the outside, not against attacks from the inside.  To
   protect against spoofing attacks from compromised on-path ACP nodes,
   end-to-end encryption inside the ACP is used by new ACP signaling:
   GRASP across the ACP using TLS.  The same is expected from any non-
   legacy services or protocols using the ACP.  Because no group keys
   are used, there is no risk of impacted nodes accessing end-to-end
   encrypted traffic from other ACP nodes.

   Attacks from impacted ACP nodes against the ACP are more difficult
   than against the data plane because of the autoconfiguration of the
   ACP and the absence of configuration options that could be abused to
   change or break ACP behavior.  This is excluding configuration for
   workaround in support of non-autonomic components.

   Mitigation against compromised ACP members is possible through
   standard automated certificate management mechanisms including
   revocation and nonrenewal of short-lived certificates.  In this
   specification, there are no further optimizations of these mechanisms
   defined for the ACP (but see Appendix A.9.8).

   Higher-layer service built using ACP certificates should not solely
   rely on undifferentiated group security when another model is more
   appropriate or more secure.  For example, central network
   configuration relies on a security model where only a few especially
   trusted nodes are allowed to configure the data plane of network
   nodes (CLI, NETCONF).  This can be done through ACP certificates by
   differentiating them and introducing roles.  See Appendix A.9.5.

   Operators and developers of provisioning software need to be aware of
   how the provisioning and configuration of network devices impacts the
   ability of the operator and/or provisioning software to remotely
   access the network nodes.  By using the ACP, most of the issues of
   provisioning/configuration causing connectivity loss of remote
   provisioning and configuration will be eliminated, see Section 6.
   Only a few exceptions, such as explicit physical interface down
   configuration, will be left.  See Section 9.3.2.

   Many details of ACP are designed with security in mind and discussed
   elsewhere in the document.

   IPv6 addresses used by nodes in the ACP are covered as part of the
   node's domain certificate as described in Section 6.2.2.  This allows
   even verification of ownership of a peer's IPv6 address when using a
   connection authenticated with the domain certificate.

   The ACP acts as a security (and transport) substrate for GRASP inside
   the ACP such that GRASP is not only protected by attacks from the
   outside, but also by attacks from compromised inside attackers -- by
   relying not only on hop-by-hop security of ACP secure channels, but
   also by adding end-to-end security for those GRASP messages.  See
   Section 6.9.2.

   ACP provides for secure, resilient zero-touch discovery of EST
   servers for certificate renewal.  See Section 6.2.5.

   ACP provides extensible, autoconfiguring hop-by-hop protection of the
   ACP infrastructure via the negotiation of hop-by-hop secure channel
   protocols.  See Section 6.6.

   The ACP is designed to minimize attacks from the outside by
   minimizing its dependency on any non-ACP (data plane) operations and/
   or configuration on a node.  See also Section 6.13.2.

   In combination with BRSKI, ACP enables a resilient, fully zero-touch
   network solution for short-lived certificates that can be renewed or
   reenrolled even after unintentional expiry (e.g., due to interrupted
   connectivity).  See Appendix A.2.

   Because ACP secure channels can be long lived, but certificates used
   may be short-lived, secure channels, for example, built via IPsec,
   need to be terminated when peer certificates expire.  See
   Section 6.8.5.

   Section 7.2 describes how to implement a routed ACP topology
   operating on what effectively is a large bridge domain when using L3/
   L2 routers that operate at L2 in the data plane.  In this case, the
   ACP is subject to a much higher likelihood of attacks by other nodes
   "stealing" L2 addresses than in the actual routed case, especially
   when the bridged network includes untrusted devices such as hosts.
   This is a generic issue in L2 LANs.  L2/L3 devices often already have
   some form of "port security" to prohibit this.  They rely on Neighbor
   Discovery Protocol (NDP) or DHCP learning which port/MAC-address and
   IPv6 address belong together and blocking MAC/IPv6 source addresses
   from wrong ports.  This type of function needs to be enabled to
   prohibit DoS attacks and specifically to protect the ACP.  Likewise,
   the GRASP DULL instance needs to ensure that the IPv6 address in the
   locator-option matches the source IPv6 address of the DULL GRASP
   packet.

12.  IANA Considerations

   This document defines the "Autonomic Control Plane".

   For the ANIMA-ACP-2020 ASN.1 module, IANA has assigned value 97 for
   "id-mod-anima-acpnodename-2020" in the "SMI Security for PKIX Module
   Identifier" (1.3.6.1.5.5.7.0) registry.

   For the otherName / AcpNodeName, IANA has assigned value 10 for id-
   on-AcpNodeName in the "SMI Security for PKIX Other Name Forms"
   (1.3.6.1.5.5.7.8) registry.

   IANA has registered the names in Table 2 in the "GRASP Objective
   Names" subregistry of the "GeneRic Autonomic Signaling Protocol
   (GRASP) Parameters" registry.

               +================+==========================+
               | Objective Name | Reference                |
               +================+==========================+
               | AN_ACP         | RFC 8994 (Section 6.4)   |
               +----------------+--------------------------+
               | SRV.est        | RFC 8994 (Section 6.2.5) |
               +----------------+--------------------------+

                       Table 2: GRASP Objective Names

   Explanation: this document chooses the initially strange looking
   format "SRV.<service-name>" because these objective names would be in
   line with potential future simplification of the GRASP objective
   registry.  Today, every name in the GRASP objective registry needs to
   be explicitly allocated by IANA.  In the future, this type of
   objective names could be considered to be automatically registered in
   that registry for the same service for which a <service-name> is
   registered according to [RFC6335].  This explanation is solely
   informational and has no impact on the requested registration.

   IANA has created an "Autonomic Control Plane (ACP)" registry with the
   subregistry, "ACP Address Type" (Table 3).

      +=======+==================================+==================+
      | Value | Description                      | Reference        |
      +=======+==================================+==================+
      | 0     | ACP Zone Addressing Sub-Scheme/  | RFC 8994         |
      |       | ACP Manual Addressing Sub-Scheme | (Section 6.11.3, |
      |       |                                  | Section 6.11.4)  |
      +-------+----------------------------------+------------------+
      | 1     | ACP Vlong Addressing Sub-Scheme  | RFC 8994         |
      |       |                                  | (Section 6.11.5) |
      +-------+----------------------------------+------------------+
      | 2-3   | Unassigned                       |                  |
      +-------+----------------------------------+------------------+

       Table 3: Initial Values in the "ACP Address Type" Subregistry

   The values in the "ACP Address Type" subregistry are numeric values
   0..3 paired with a name (string).  Future values MUST be assigned
   using the Standards Action policy defined by "Guidelines for Writing
   an IANA Considerations Section in RFCs" [RFC8126].

13.  References

13.1.  Normative References

   [IKEV2IANA]
              IANA, "Internet Key Exchange Version 2 (IKEv2)
              Parameters",
              <https://www.iana.org/assignments/ikev2-parameters>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and
              More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
              November 2005, <https://www.rfc-editor.org/info/rfc4191>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <https://www.rfc-editor.org/info/rfc4862>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5954]  Gurbani, V., Ed., Carpenter, B., Ed., and B. Tate, Ed.,
              "Essential Correction for IPv6 ABNF and URI Comparison in
              RFC 3261", RFC 5954, DOI 10.17487/RFC5954, August 2010,
              <https://www.rfc-editor.org/info/rfc5954>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,
              <https://www.rfc-editor.org/info/rfc6551>.

   [RFC6552]  Thubert, P., Ed., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)",
              RFC 6552, DOI 10.17487/RFC6552, March 2012,
              <https://www.rfc-editor.org/info/rfc6552>.

   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              DOI 10.17487/RFC6553, March 2012,
              <https://www.rfc-editor.org/info/rfc6553>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/info/rfc7030>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7676]  Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
              for Generic Routing Encapsulation (GRE)", RFC 7676,
              DOI 10.17487/RFC7676, October 2015,
              <https://www.rfc-editor.org/info/rfc7676>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8221]  Wouters, P., Migault, D., Mattsson, J., Nir, Y., and T.
              Kivinen, "Cryptographic Algorithm Implementation
              Requirements and Usage Guidance for Encapsulating Security
              Payload (ESP) and Authentication Header (AH)", RFC 8221,
              DOI 10.17487/RFC8221, October 2017,
              <https://www.rfc-editor.org/info/rfc8221>.

   [RFC8247]  Nir, Y., Kivinen, T., Wouters, P., and D. Migault,
              "Algorithm Implementation Requirements and Usage Guidance
              for the Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 8247, DOI 10.17487/RFC8247, September 2017,
              <https://www.rfc-editor.org/info/rfc8247>.

   [RFC8422]  Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
              Curve Cryptography (ECC) Cipher Suites for Transport Layer
              Security (TLS) Versions 1.2 and Earlier", RFC 8422,
              DOI 10.17487/RFC8422, August 2018,
              <https://www.rfc-editor.org/info/rfc8422>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8990]  Bormann, C., Carpenter, B., Ed., and B. Liu, Ed., "GeneRic
              Autonomic Signaling Protocol (GRASP)", RFC 8990,
              DOI 10.17487/RFC8990, May 2021,
              <https://www.rfc-editor.org/info/rfc8990>.

   [RFC8995]  Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
              May 2021, <https://www.rfc-editor.org/info/rfc8995>.

13.2.  Informative References

   [AR8021]   IEEE, "IEEE Standard for Local and metropolitan area
              networks - Secure Device Identity", IEEE 802.1AR,
              <https://1.ieee802.org/security/802-1ar>.

   [CABFORUM] CA/Browser Forum, "Certificate Contents for Baseline SSL",
              November 2019, <https://cabforum.org/baseline-
              requirements-certificate-contents/>.

   [FCC]      FCC, "June 15, 2020 T-Mobile Network Outage Report", A
              Report of the Public Safety and Homeland Security Bureau
              Federal Communications Commission, PS Docket No. 20-183,
              October 2020, <https://docs.fcc.gov/public/attachments/
              DOC-367699A1.docx>.

   [IEEE-1588-2008]
              IEEE, "IEEE Standard for a Precision Clock Synchronization
              Protocol for Networked Measurement and Control Systems",
              DOI 10.1109/IEEESTD.2008.4579760, IEEE 1588-2008, July
              2008,
              <https://standards.ieee.org/standard/1588-2008.html>.

   [IEEE-802.1X]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks--Port-Based Network Access Control",
              DOI 10.1109/IEEESTD.2010.5409813, IEEE 802.1X-2010,
              February 2010,
              <https://standards.ieee.org/standard/802_1X-2010.html>.

   [LLDP]     IEEE, "IEEE Standard for Local and metropolitan area
              networks: Station and Media Access Control Connectivity
              Discovery", DOI 10.1109/IEEESTD.2016.7433915, IEEE 
              802.1AB-2016, March 2016,
              <https://standards.ieee.org/standard/802_1AB-2016.html>.

   [MACSEC]   IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks: Media Access Control (MAC) Security",
              DOI 10.1109/IEEESTD.2006.245590, IEEE 802.1AE-2006, August
              2006,
              <https://standards.ieee.org/standard/802_1AE-2006.html>.

   [NOC-AUTOCONFIG]
              Eckert, T., Ed., "Autoconfiguration of NOC services in ACP
              networks via GRASP", Work in Progress, Internet-Draft,
              draft-eckert-anima-noc-autoconfig-00, 2 July 2018,
              <https://tools.ietf.org/html/draft-eckert-anima-noc-
              autoconfig-00>.

   [OP-TECH]  Wikipedia, "Operational technology", October 2020,
              <https://en.wikipedia.org/w/
              index.php?title=Operational_technology&oldid=986363045>.

   [RFC1112]  Deering, S., "Host extensions for IP multicasting", STD 5,
              RFC 1112, DOI 10.17487/RFC1112, August 1989,
              <https://www.rfc-editor.org/info/rfc1112>.

   [RFC1492]  Finseth, C., "An Access Control Protocol, Sometimes Called
              TACACS", RFC 1492, DOI 10.17487/RFC1492, July 1993,
              <https://www.rfc-editor.org/info/rfc1492>.

   [RFC1654]  Rekhter, Y., Ed. and T. Li, Ed., "A Border Gateway
              Protocol 4 (BGP-4)", RFC 1654, DOI 10.17487/RFC1654, July
              1994, <https://www.rfc-editor.org/info/rfc1654>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2315]  Kaliski, B., "PKCS #7: Cryptographic Message Syntax
              Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998,
              <https://www.rfc-editor.org/info/rfc2315>.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
              <https://www.rfc-editor.org/info/rfc2409>.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, DOI 10.17487/RFC2865, June 2000,
              <https://www.rfc-editor.org/info/rfc2865>.

   [RFC3164]  Lonvick, C., "The BSD Syslog Protocol", RFC 3164,
              DOI 10.17487/RFC3164, August 2001,
              <https://www.rfc-editor.org/info/rfc3164>.

   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              DOI 10.17487/RFC3411, December 2002,
              <https://www.rfc-editor.org/info/rfc3411>.

   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", STD 88,
              RFC 3596, DOI 10.17487/RFC3596, October 2003,
              <https://www.rfc-editor.org/info/rfc3596>.

   [RFC3954]  Claise, B., Ed., "Cisco Systems NetFlow Services Export
              Version 9", RFC 3954, DOI 10.17487/RFC3954, October 2004,
              <https://www.rfc-editor.org/info/rfc3954>.

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              DOI 10.17487/RFC4007, March 2005,
              <https://www.rfc-editor.org/info/rfc4007>.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,
              <https://www.rfc-editor.org/info/rfc4210>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC4429]  Moore, N., "Optimistic Duplicate Address Detection (DAD)
              for IPv6", RFC 4429, DOI 10.17487/RFC4429, April 2006,
              <https://www.rfc-editor.org/info/rfc4429>.

   [RFC4541]  Christensen, M., Kimball, K., and F. Solensky,
              "Considerations for Internet Group Management Protocol
              (IGMP) and Multicast Listener Discovery (MLD) Snooping
              Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
              <https://www.rfc-editor.org/info/rfc4541>.

   [RFC4604]  Holbrook, H., Cain, B., and B. Haberman, "Using Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Protocol Version 2 (MLDv2) for Source-
              Specific Multicast", RFC 4604, DOI 10.17487/RFC4604,
              August 2006, <https://www.rfc-editor.org/info/rfc4604>.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
              <https://www.rfc-editor.org/info/rfc4607>.

   [RFC4610]  Farinacci, D. and Y. Cai, "Anycast-RP Using Protocol
              Independent Multicast (PIM)", RFC 4610,
              DOI 10.17487/RFC4610, August 2006,
              <https://www.rfc-editor.org/info/rfc4610>.

   [RFC4985]  Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, DOI 10.17487/RFC4985, August 2007,
              <https://www.rfc-editor.org/info/rfc4985>.

   [RFC5790]  Liu, H., Cao, W., and H. Asaeda, "Lightweight Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Version 2 (MLDv2) Protocols", RFC 5790,
              DOI 10.17487/RFC5790, February 2010,
              <https://www.rfc-editor.org/info/rfc5790>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/info/rfc5912>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC6402]  Schaad, J., "Certificate Management over CMS (CMC)
              Updates", RFC 6402, DOI 10.17487/RFC6402, November 2011,
              <https://www.rfc-editor.org/info/rfc6402>.

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
              October 2011, <https://www.rfc-editor.org/info/rfc6407>.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              DOI 10.17487/RFC6554, March 2012,
              <https://www.rfc-editor.org/info/rfc6554>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [RFC6733]  Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
              Ed., "Diameter Base Protocol", RFC 6733,
              DOI 10.17487/RFC6733, October 2012,
              <https://www.rfc-editor.org/info/rfc6733>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
              <https://www.rfc-editor.org/info/rfc6824>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,
              <https://www.rfc-editor.org/info/rfc6830>.

   [RFC7011]  Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
              "Specification of the IP Flow Information Export (IPFIX)
              Protocol for the Exchange of Flow Information", STD 77,
              RFC 7011, DOI 10.17487/RFC7011, September 2013,
              <https://www.rfc-editor.org/info/rfc7011>.

   [RFC7404]  Behringer, M. and E. Vyncke, "Using Only Link-Local
              Addressing inside an IPv6 Network", RFC 7404,
              DOI 10.17487/RFC7404, November 2014,
              <https://www.rfc-editor.org/info/rfc7404>.

   [RFC7426]  Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
              Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
              Defined Networking (SDN): Layers and Architecture
              Terminology", RFC 7426, DOI 10.17487/RFC7426, January
              2015, <https://www.rfc-editor.org/info/rfc7426>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <https://www.rfc-editor.org/info/rfc7435>.

   [RFC7575]  Behringer, M., Pritikin, M., Bjarnason, S., Clemm, A.,
              Carpenter, B., Jiang, S., and L. Ciavaglia, "Autonomic
              Networking: Definitions and Design Goals", RFC 7575,
              DOI 10.17487/RFC7575, June 2015,
              <https://www.rfc-editor.org/info/rfc7575>.

   [RFC7576]  Jiang, S., Carpenter, B., and M. Behringer, "General Gap
              Analysis for Autonomic Networking", RFC 7576,
              DOI 10.17487/RFC7576, June 2015,
              <https://www.rfc-editor.org/info/rfc7576>.

   [RFC7585]  Winter, S. and M. McCauley, "Dynamic Peer Discovery for
              RADIUS/TLS and RADIUS/DTLS Based on the Network Access
              Identifier (NAI)", RFC 7585, DOI 10.17487/RFC7585, October
              2015, <https://www.rfc-editor.org/info/rfc7585>.

   [RFC7721]  Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
              Considerations for IPv6 Address Generation Mechanisms",
              RFC 7721, DOI 10.17487/RFC7721, March 2016,
              <https://www.rfc-editor.org/info/rfc7721>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC8028]  Baker, F. and B. Carpenter, "First-Hop Router Selection by
              Hosts in a Multi-Prefix Network", RFC 8028,
              DOI 10.17487/RFC8028, November 2016,
              <https://www.rfc-editor.org/info/rfc8028>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8316]  Nobre, J., Granville, L., Clemm, A., and A. Gonzalez
              Prieto, "Autonomic Networking Use Case for Distributed
              Detection of Service Level Agreement (SLA) Violations",
              RFC 8316, DOI 10.17487/RFC8316, February 2018,
              <https://www.rfc-editor.org/info/rfc8316>.

   [RFC8366]  Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
              "A Voucher Artifact for Bootstrapping Protocols",
              RFC 8366, DOI 10.17487/RFC8366, May 2018,
              <https://www.rfc-editor.org/info/rfc8366>.

   [RFC8368]  Eckert, T., Ed. and M. Behringer, "Using an Autonomic
              Control Plane for Stable Connectivity of Network
              Operations, Administration, and Maintenance (OAM)",
              RFC 8368, DOI 10.17487/RFC8368, May 2018,
              <https://www.rfc-editor.org/info/rfc8368>.

   [RFC8398]  Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
              Email Addresses in X.509 Certificates", RFC 8398,
              DOI 10.17487/RFC8398, May 2018,
              <https://www.rfc-editor.org/info/rfc8398>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8572]  Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
              Touch Provisioning (SZTP)", RFC 8572,
              DOI 10.17487/RFC8572, April 2019,
              <https://www.rfc-editor.org/info/rfc8572>.

   [RFC8684]  Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
              Paasch, "TCP Extensions for Multipath Operation with
              Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
              2020, <https://www.rfc-editor.org/info/rfc8684>.

   [RFC8739]  Sheffer, Y., Lopez, D., Gonzalez de Dios, O., Pastor
              Perales, A., and T. Fossati, "Support for Short-Term,
              Automatically Renewed (STAR) Certificates in the Automated
              Certificate Management Environment (ACME)", RFC 8739,
              DOI 10.17487/RFC8739, March 2020,
              <https://www.rfc-editor.org/info/rfc8739>.

   [RFC8981]  Gont, F., Krishnan, S., Narten, T., and R. Draves,
              "Temporary Address Extensions for Stateless Address
              Autoconfiguration in IPv6", RFC 8981,
              DOI 10.17487/RFC8981, February 2021,
              <https://www.rfc-editor.org/info/rfc8981>.

   [RFC8992]  Jiang, S., Ed., Du, Z., Carpenter, B., and Q. Sun,
              "Autonomic IPv6 Edge Prefix Management in Large-Scale
              Networks", RFC 8992, DOI 10.17487/RFC8992, May 2021,
              <https://www.rfc-editor.org/info/rfc8992>.

   [RFC8993]  Behringer, M., Ed., Carpenter, B., Eckert, T., Ciavaglia,
              L., and J. Nobre, "A Reference Model for Autonomic
              Networking", RFC 8993, DOI 10.17487/RFC8993, May 2021,
              <https://www.rfc-editor.org/info/rfc8993>.

   [ROLL-APPLICABILITY]
              Richardson, M., "ROLL Applicability Statement Template",
              Work in Progress, Internet-Draft, draft-ietf-roll-
              applicability-template-09, 3 May 2016,
              <https://tools.ietf.org/html/draft-ietf-roll-
              applicability-template-09>.

   [SR]       Wikipedia, "Single-root input/output virtualization",
              September 2020, <https://en.wikipedia.org/w/
              index.php?title=Single-root_input/
              output_virtualization&oldid=978867619>.

   [TLS-DTLS13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021,
              <https://tools.ietf.org/html/draft-ietf-tls-dtls13-43>.

   [X.509]    ITU-T, "Information technology - Open Systems
              Interconnection - The Directory: Public-key and attribute
              certificate frameworks", ITU-T Recommendation X.509,
              October 2016, <https://www.itu.int/rec/T-REC-X.509>.

   [X.520]    ITU-T, "Information technology - Open Systems
              Interconnection - The Directory: Selected attribute
              types", ITU-T Recommendation X.520, October 2016,
              <https://www.itu.int/rec/T-REC-X.520>.

Appendix A.  Background and Future (Informative)

   The following sections provide background information about aspects
   of the normative parts of this document or associated mechanisms such
   as BRSKI (such as why specific choices were made by the ACP), and
   they discuss possible future variations of the ACP.

A.1.  ACP Address Space Schemes

   This document defines the Zone, Vlong, and Manual Addressing Sub-
   Schemes primarily to support address prefix assignment via
   distributed, potentially uncoordinated ACP registrars as defined in
   Section 6.11.7.  This costs a 48/46-bit identifier so that these ACP
   registrars can assign nonconflicting address prefixes.  This design
   does not leave enough bits to simultaneously support a large number
   of nodes (Node-ID), plus a large prefix of local addresses for every
   node, plus a large enough set of bits to identify a routing zone.  As
   a result, the Zone and Vlong 8/16 Addressing Sub-Schemes attempt to
   support all features but via separate prefixes.

   In networks that expect always to rely on a centralized PMS as
   described Section 9.2.5, the 48/46-bits for the Registrar-ID could be
   saved.  Such variations of the ACP addressing mechanisms could be
   introduced through future work in different ways.  If a new otherName
   was introduced, incompatible ACP variations could be created where
   every design aspect of the ACP could be changed, including all
   addressing choices.  If instead a new addressing sub-scheme would be
   defined, it could be a backward-compatible extension of this ACP
   specification.  Information such as the size of a zone prefix and the
   length of the prefix assigned to the ACP node itself could be encoded
   via the extension field of the acp-node-name.

   Note that an explicitly defined Manual Addressing Sub-Scheme is
   always beneficial to provide an easy way for ACP nodes to prohibit
   incorrect non-autonomic configuration of any non-"Manual" ACP address
   spaces and therefore ensure that such non-autonomic operations will
   never impact correct routing for any non-"Manual" ACP addresses
   assigned via ACP certificates.

A.2.  BRSKI Bootstrap (ANI)

   BRSKI describes how nodes with an IDevID certificate can securely and
   zero-touch enroll with an LDevID certificate to support the ACP.
   BRSKI also leverages the ACP to enable zero-touch bootstrap of new
   nodes across networks without any configuration requirements across
   the transit nodes (e.g., no DHCP, DNS forwarding, and/or server
   setup).  This includes otherwise unconfigured networks as described
   in Section 3.2.  Therefore, BRSKI in conjunction with ACP provides
   for a secure and zero-touch management solution for complete
   networks.  Nodes supporting such an infrastructure (BRSKI and ACP)
   are called ANI nodes (Autonomic Networking Infrastructure), see
   [RFC8993].  Nodes that do not support an IDevID certificate but only
   an (insecure) vendor-specific Unique Device Identifier (UDI) or nodes
   whose manufacturer does not support a MASA could use some future,
   reduced-security version of BRSKI.

   When BRSKI is used to provision a domain certificate (which is called
   enrollment), the BRSKI registrar (acting as an enhanced EST server)
   must include the otherName / AcpNodeName encoded ACP address and
   domain name to the enrolling node (called a pledge) via its response
   to the pledge's EST CSR Attributes Request that is mandatory in
   BRSKI.

   The CA in an ACP network must not change the otherName / AcpNodeName
   in the certificate.  The ACP nodes can therefore find their ACP
   addresses and domain using this field in the domain certificate, both
   for themselves as well as for other nodes.

   The use of BRSKI in conjunction with the ACP can also help to further
   simplify maintenance and renewal of domain certificates.  Instead of
   relying on CRL, the lifetime of certificates can be made extremely
   small, for example, on the order of hours.  When a node fails to
   connect to the ACP within its certificate lifetime, it cannot connect
   to the ACP to renew its certificate across it (using just EST), but
   it can still renew its certificate as an "enrolled/expired pledge"
   via the BRSKI bootstrap proxy.  This requires only that the BRSKI
   registrar honors expired domain certificates and that the pledge
   attempts to perform TLS authentication for BRSKI bootstrap using its
   expired domain certificate before falling back to attempting to use
   its IDevID certificate for BRSKI.  This mechanism could also render
   CRLs unnecessary because the BRSKI registrar in conjunction with the
   CA would not renew revoked certificates -- only a "Do-not-renew" list
   would be necessary on the BRSKI registrar/CA.

   In the absence of BRSKI or less secure variants thereof, the
   provisioning of certificates may involve one or more touches or
   nonstandardized automation.  Node vendors usually support the
   provisioning of certificates into nodes via PKCS #7 (see "PKCS #7:
   Cryptographic Message Syntax Version 1.5" [RFC2315]) and may support
   this provisioning through vendor-specific models via NETCONF
   ("Network Configuration Protocol (NETCONF)" [RFC6241]).  If such
   nodes also support NETCONF Zero Touch [RFC8572], then this can be
   combined with zero-touch provisioning of domain certificates into
   nodes.  Unless there is the equivalent integration of NETCONF
   connections across the ACP as there is in BRSKI, this combination
   would not support zero-touch bootstrap across an unconfigured
   network, though.

A.3.  ACP Neighbor Discovery Protocol Selection

   This section discusses why GRASP DULL was chosen as the discovery
   protocol for L2-adjacent candidate ACP neighbors.  The contenders
   that were considered were GRASP, mDNS, and LLDP.

A.3.1.  LLDP

   LLDP and Cisco's earlier Cisco Discovery Protocol (CDP) are examples
   of L2 discovery protocols that terminate their messages on L2 ports.
   If those protocols had been chosen for ACP neighbor discovery, ACP
   neighbor discovery would also have terminated on L2 ports.  This
   would have prevented ACP construction over non-ACP-capable, but LLDP-
   or CDP-enabled L2 switches.  LLDP has extensions using different MAC
   addresses, and this could have been an option for ACP discovery as
   well, but the additional required IEEE standardization and definition
   of a profile for such a modified instance of LLDP seemed to be more
   work than the benefit of "reusing the existing protocol" LLDP for
   this very simple purpose.

A.3.2.  mDNS and L2 Support

   Multicast DNS (mDNS) "Multicast DNS" [RFC6762] with DNS Service
   Discovery (DNS-SD) Resource Records (RRs) as defined in "DNS-Based
   Service Discovery" [RFC6763] was a key contender as an ACP discovery
   protocol.  Because it relies on link-local IP multicast, it operates
   at the subnet level and is also found in L2 switches.  The authors of
   this document are not aware of an mDNS implementation that terminates
   its mDNS messages on L2 ports instead of on the subnet level.  If
   mDNS was used as the ACP discovery mechanism on an ACP-capable
   (L3)/L2 switch as outlined in Section 7, then this would be necessary
   to implement.  It is likely that termination of mDNS messages could
   only be applied to all mDNS messages from such a port, which would
   then make it necessary to software forward any non-ACP-related mDNS
   messages to maintain prior non-ACP mDNS functionality.  Adding
   support for ACP to such L2 switches with mDNS could therefore create
   regression problems for prior mDNS functionality on those nodes.
   With low performance of software forwarding in many L2 switches, this
   could also make the ACP risky to support on such L2 switches.

A.3.3.  Why DULL GRASP?

   LLDP was not considered because of the above mentioned issues. mDNS
   was not selected because of the above L2 mDNS considerations and
   because of the following additional points.

   If mDNS was not already existing in a node, it would be more work to
   implement than DULL GRASP, and if an existing implementation of mDNS
   was used, it would likely be more code space than a separate
   implementation of DULL GRASP or a shared implementation of DULL GRASP
   and GRASP in the ACP.

A.4.  Choice of Routing Protocol (RPL)

   This section motivates why RPL ("IPv6 Routing Protocol for Low-Power
   and Lossy Networks [RFC6550]) was chosen as the default (and in this
   specification only) routing protocol for the ACP.  The choice and
   above explained profile were derived from a pre-standard
   implementation of ACP that was successfully deployed in operational
   networks.

   The requirements for routing in the ACP are the following:

   *  Self-management: the ACP must build automatically, without human
      intervention.  Therefore, the routing protocol must also work
      completely automatically.  RPL is a simple, self-managing
      protocol, which does not require zones or areas; it is also self-
      configuring, since configuration is carried as part of the
      protocol (see Section 6.7.6 of [RFC6550]).

   *  Scale: the ACP builds over an entire domain, which could be a
      large enterprise or service provider network.  The routing
      protocol must therefore support domains of 100,000 nodes or more,
      ideally without the need for zoning or separation into areas.  RPL
      has this scale property.  This is based on extensive use of
      default routing.

   *  Low resource consumption: the ACP supports traditional network
      infrastructure, thus runs in addition to traditional protocols.
      The ACP, and specifically the routing protocol, must have low
      resource consumption requirements, both in terms of memory and
      CPU.  Specifically, at edge nodes, where memory and CPU are
      scarce, consumption should be minimal.  RPL builds a DODAG, where
      the main resource consumption is at the root of the DODAG.  The
      closer to the edge of the network, the less state needs to be
      maintained.  This adapts nicely to the typical network design.
      Also, all changes below a common parent node are kept below that
      parent node.

   *  Support for unstructured address space: in the ANI, node addresses
      are identifiers, they and may not be assigned in a topological
      way.  Also, nodes may move topologically, without changing their
      address.  Therefore, the routing protocol must support completely
      unstructured address space.  RPL is specifically made for mobile,
      ad hoc networks, with no assumptions on topologically aligned
      addressing.

   *  Modularity: to keep the initial implementation small, yet allow
      for more complex methods later, it is highly desirable that the
      routing protocol has a simple base functionality, but can import
      new functional modules if needed.  RPL has this property with the
      concept of "Objective Function", which is a plugin to modify
      routing behavior.

   *  Extensibility: since the ANI is a new concept, it is likely that
      changes to the way of operation will happen over time.  RPL allows
      for new Objective Functions to be introduced later, which allow
      changes to the way the routing protocol creates the DAGs.

   *  Multi-topology support: it may become necessary in the future to
      support more than one DODAG for different purposes, using
      different Objective Functions.  RPL allow for the creation of
      several parallel DODAGs should this be required.  This could be
      used to create different topologies to reach different roots.

   *  No need for path optimization: RPL does not necessarily compute
      the optimal path between any two nodes.  However, the ACP does not
      require this today, since it carries mainly delay-insensitive
      feedback loops.  It is possible that different optimization
      schemes will become necessary in the future, but RPL can be
      expanded (see "Extensibility" above).

A.5.  ACP Information Distribution and Multicast

   IP multicast is not used by the ACP because the ANI itself does not
   require IP multicast but only service announcement/discovery.  Using
   IP multicast for that would have made it necessary to develop a zero-
   touch autoconfiguring solution for ASM (Any Source Multicast - the
   original form of IP multicast defined in "Host extensions for IP
   multicasting" [RFC1112]), which would be quite complex and difficult
   to justify.  One aspect of complexity where no attempt at a solution
   has been described in IETF documents is the automatic selection of
   routers that should be PIM Sparse Mode (PIM-SM) Rendezvous Points
   (RPs) (see "Protocol Independent Multicast - Sparse Mode (PIM-SM):
   Protocol Specification (Revised)" [RFC7761]).  The other aspects of
   complexity are the implementation of MLD ("Using Internet Group
   Management Protocol Version 3 (IGMPv3) and Multicast Listener
   Discovery Protocol Version 2 (MLDv2) for Source-Specific Multicast"
   [RFC4604]), PIM-SM, and Anycast-RP (see "Anycast-RP Using Protocol
   Independent Multicast (PIM)" [RFC4610]).  If those implementations
   already exist in a product, then they would be very likely tied to
   accelerated forwarding, which consumes hardware resources, and that
   in turn is difficult to justify as a cost of performing only service
   discovery.

   Some future ASA may need high-performance, in-network data
   replication.  That is the case when the use of IP multicast is
   justified.  Such an ASA can then use service discovery from ACP
   GRASP, and then they do not need ASM but only SSM (see
   "Source-Specific Multicast for IP" [RFC4607]) for the IP multicast
   replication.  SSM itself can simply be enabled in the data plane (or
   even in an update to the ACP) without any configuration other than
   just enabling it on all nodes, and it only requires a simpler version
   of MLD (see "Lightweight Internet Group Management Protocol Version 3
   (IGMPv3) and Multicast Listener Discovery Version 2 (MLDv2)
   Protocols" [RFC5790]).

   IGP routing protocols based on LSP (Link State Protocol) typically
   have a mechanism to flood information, and such a mechanism could be
   used to flood GRASP objectives by defining them to be information of
   that IGP.  This would be a possible optimization in future variations
   of the ACP that do use an LSP-based routing protocol.  Note though
   that such a mechanism would not work easily for GRASP M_DISCOVERY
   messages, which are intelligently (constrained) flooded not across
   the whole ACP, but only up to a node where a responder is found.  We
   expect that many future services in the ASA will have only a few
   consuming ASAs, and for those cases, the M_DISCOVERY method is more
   efficient than flooding across the whole domain.

   Because the ACP uses RPL, one desirable future extension is to use
   RPL's existing notion of DODAG, which are loop-free distribution
   trees, to make GRASP flooding more efficient both for M_FLOOD and
   M_DISCOVERY.  See Section 6.13.5 for how this will be specifically
   beneficial when using NBMA interfaces.  This is not currently
   specified in this document because it is not quite clear yet what
   exactly the implications are to make GRASP flooding depend on RPL
   DODAG convergence and how difficult it would be to let GRASP flooding
   access the DODAG information.

A.6.  CAs, Domains, and Routing Subdomains

   There is a wide range of setting up different ACP solutions by
   appropriately using CAs and the domain and rsub elements in the acp-
   node-name in the domain certificate.  We summarize these options here
   as they have been explained in different parts of the document and
   discuss possible and desirable extensions.

   An ACP domain is the set of all ACP nodes that can authenticate each
   other as belonging to the same ACP network using the ACP domain
   membership check (Section 6.2.3).  GRASP inside the ACP is run across
   all transitively connected ACP nodes in a domain.

   The rsub element in the acp-node-name permits the use of addresses
   from different ULA prefixes.  One use case is the creation of
   multiple physical networks that initially may be separated with one
   ACP domain but different routing subdomains, so that all nodes can
   mutually trust their ACP certificates (not depending on rsub) and so
   that they could connect later together into a contiguous ACP network.

   One instance of such a use case is an ACP for regions interconnected
   via a non-ACP enabled core, for example, due to the absence of
   product support for ACP on the core nodes.  ACP connect
   configurations as defined in this document can be used to extend and
   interconnect those ACP islands to the NOC and merge them into a
   single ACP when later that product support gap is closed.

   Note that RPL scales very well.  It is not necessary to use multiple
   routing subdomains to scale ACP domains in a way that would be
   required if other routing protocols where used.  They exist only as
   options for the above mentioned reasons.

   If ACP domains need to be created that are not allowed to connect to
   each other by default, simply use different domain elements in the
   acp-node-name.  These domain elements can be arbitrary, including
   subdomains of one another: domains "example.com" and
   "research.example.com" are separate domains if both are domain
   elements in the acp-node-name of certificates.

   It is not necessary to have a separate CA for different ACP domains:
   an operator can use a single CA to sign certificates for multiple ACP
   domains that are not allowed to connect to each other because the
   checks for ACP adjacencies include the comparison of the domain part.

   If multiple, independent networks chose the same domain name but had
   their own CAs, these would not form a single ACP domain because of CA
   mismatch.  Therefore, there is no problem in choosing domain names
   that are potentially also used by others.  Nevertheless, it is highly
   recommended to use domain names that have a high probability of being
   unique.  It is recommended to use domain names that start with a DNS
   domain name owned by the assigning organization and unique within it,
   for example, "acp.example.com" if you own "example.com".

A.7.  Intent for the ACP

   Intent is the architecture component of Autonomic Networks according
   to [RFC8993] that allows operators to issue policies to the network.
   Its applicability for use is quite flexible and freeform, with
   potential applications including policies flooded across ACP GRASP
   and interpreted on every ACP node.

   One concern for future definitions of Intent solutions is the problem
   of circular dependencies when expressing Intent policies about the
   ACP itself.

   For example, Intent could indicate the desire to build an ACP across
   all domains that have a common parent domain (without relying on the
   rsub/routing-subdomain solution defined in this document): ACP nodes
   with the domains "example.com", "access.example.com",
   "core.example.com", and "city.core.example.com" should all establish
   one single ACP.

   If each domain has its own source of Intent, then the Intent would
   simply have to allow adding the peer domain's TA and domain names to
   the parameters for the ACP domain membership check (Section 6.2.3) so
   that nodes from those other domains are accepted as ACP peers.

   If this Intent was to be originated only from one domain, it could
   likely not be made to work because the other domains will not build
   any ACP connections amongst each other, whether they use the same or
   different CA due to the ACP domain membership check.

   If the domains use the same CA, one could change the ACP setup to
   permit the ACP to be established between two ACP nodes with different
   acp-domain-names, but only for the purpose of disseminating limited
   information, such as Intent, but not to set up full ACP connectivity,
   specifically not RPL routing and passing of arbitrary GRASP
   information, unless the Intent policies permit this to happen across
   domain boundaries.

   This type of approach, where the ACP first allows Intent to operate
   and only then sets up the rest of ACP connectivity based on Intent
   policy, could also be used to enable Intent policies that would limit
   functionality across the ACP inside a domain, as long as no policy
   would disturb the distribution of Intent, for example, to limit
   reachability across the ACP to certain types of nodes or locations of
   nodes.

A.8.  Adopting ACP Concepts for Other Environments

   The ACP as specified in this document is very explicit about the
   choice of options to allow interoperable implementations.  The
   choices made may not be the best for all environments, but the
   concepts used by the ACP can be used to build derived solutions.

   The ACP specifies the use of ULA and the derivation of its prefix
   from the domain name so that no address allocation is required to
   deploy the ACP.  The ACP will equally work using any other /48 IPv6
   prefix and not ULA.  This prefix could simply be a configuration of
   the ACP registrars (for example, when using BRSKI) to enroll the
   domain certificates, instead of the ACP registrar deriving the /48
   ULA prefix from the AN domain name.

   Some solutions may already have an auto-addressing scheme, for
   example, derived from existing, unique device identifiers (e.g., MAC
   addresses).  In those cases, it may not be desirable to assign
   addresses to devices via the ACP address information field in the way
   described in this document.  The certificate may simply serve to
   identify the ACP domain, and the address field could be omitted.  The
   only fix required in the remaining way the ACP operates is to define
   another element in the domain certificate for the two peers to decide
   who is the Decider and who is the Follower during secure channel
   building.  Note though that future work may leverage the ACP address
   to authenticate "ownership" of the address by the device.  If the ACP
   address used by a device is derived from some preexisting, permanent
   local ID (such as MAC address), then it would be useful to store that
   local ID also in the certificate.

   The ACP is defined as a separate VRF because it intends to support
   well-managed networks with a wide variety of configurations.
   Therefore, reliable, configuration-indestructible connectivity cannot
   be achieved from the data plane itself.  In solutions where all
   functions that impact transit connectivity are fully automated
   (including security), indestructible, and resilient, it would be
   possible to eliminate the need for the ACP to be a separate VRF.
   Consider the most simple example system in which there is no separate
   data plane, but the ACP is the data plane.  Add BRSKI, and it becomes
   a fully Autonomic Network -- except that it does not support
   automatic addressing for user equipment.  This gap can then be
   closed, for example, by adding a solution derived from "Autonomic
   IPv6 Edge Prefix Management in Large-Scale Networks" [RFC8992].

   TCP/TLS as the protocols to provide reliability and security to GRASP
   in the ACP may not be the preferred choice in constrained networks.
   For example, CoAP/DTLS (Constrained Application Protocol) may be
   preferred where they are already used, which would reduce the
   additional code space footprint for the ACP on those devices.  Hop-
   by-hop reliability for ACP GRASP messages could be made to support
   protocols like DTLS by adding the same type of negotiation as defined
   in this document for ACP secure channel protocol negotiation.  In
   future ACP extensions meant to better support constrained devices,
   end-to-end GRASP connections can be made to select their transport
   protocol by indicating the supported transport protocols (e.g.  TLS/
   DTLS) via GRASP parameters of the GRASP objective through which the
   transport endpoint is discovered.

   RPL, the routing protocol used for the ACP, explicitly does not
   optimize for shortest paths and fastest convergence.  Variations of
   the ACP may want to use a different routing protocol or introduce
   more advanced RPL profiles.

   Variations such as which routing protocol to use, or whether to
   instantiate an ACP in a VRF or (as suggested above) as the actual
   data plane, can be automatically chosen in implementations built to
   support multiple options by deriving them from future parameters in
   the certificate.  Parameters in certificates should be limited to
   those that would not need to be changed more often than that
   certificates would need to be updated, or it should be ensured that
   these parameters can be provisioned before the variation of an ACP is
   activated in a node.  Using BRSKI, this could be done, for example,
   as additional follow-up signaling directly after the certificate
   enrollment, still leveraging the BRSKI TLS connection and therefore
   not introducing any additional connectivity requirements.

   Last but not least, secure channel protocols including their
   encapsulations are easily added to ACP solutions.  ACP hop-by-hop
   network-layer secure channels could also be replaced by end-to-end
   security plus other means for infrastructure protection.  Any future
   network OAM should always use end-to-end security.  By leveraging the
   domain certificates, it would not be dependent on security provided
   by ACP secure channels.

A.9.  Further (Future) Options

A.9.1.  Auto-Aggregation of Routes

   Routing in the ACP according to this specification only leverages the
   standard RPL mechanism of route optimization, e.g., keeping only the
   routes that are not towards the RPL root.  This is known to scale to
   networks with 20,000 or more nodes.  There is no auto-aggregation of
   routes for /48 ULA prefixes (when using rsub in the acp-node-name)
   and/or Zone-ID based prefixes.

   Automatic assignment of Zone-ID and auto-aggregation of routes could
   be achieved, for example, by configuring zone boundaries, announcing
   via GRASP into the zones the zone parameters (Zone-ID and /48 ULA
   prefix), and auto-aggregating routes on the zone boundaries.  Nodes
   would assign their Zone-ID and potentially even the /48 prefix based
   on the GRASP announcements.

A.9.2.  More Options for Avoiding IPv6 Data Plane Dependencies

   As described in Section 6.13.2, the ACP depends on the data plane to
   establish IPv6 link-local addressing on interfaces.  Using a separate
   MAC address for the ACP allows the full isolation of the ACP from the
   data plane in a way that is compatible with this specification.  It
   is also an ideal option when using single-root input/output
   virtualization (SR-IOV, see [SR]) in an implementation to isolate the
   ACP because different SR-IOV interfaces use different MAC addresses.

   When additional MAC address(es) are not available, separation of the
   ACP could be done at different demux points.  The same subnet
   interface could have a separate IPv6 interface for the ACP and data
   plane and therefore separate link-local addresses for both, where the
   ACP interface is not configurable on the data plane.  This too would
   be compatible with this specification and not impact
   interoperability.

   An option that would require additional specification is to use a
   different Ethertype from 0x86DD (IPv6) to encapsulate IPv6 packets
   for the ACP.  This would be a similar approach as used for IP
   authentication packets in [IEEE-802.1X], which uses the Extensible
   Authentication Protocol over Local Area Network (EAPoL) Ethertype
   (0x88A2).

   Note that in the case of ANI nodes, all of the above considerations
   equally apply to the encapsulation of BRSKI packets including GRASP
   used for BRSKI.

A.9.3.  ACP APIs and Operational Models (YANG)

   Future work should define a YANG data model [RFC7950] and/or node-
   internal APIs to monitor and manage the ACP.

   Support for the ACP adjacency table (Section 6.3) and ACP GRASP needs
   to be included in such model and/or API.

A.9.4.  RPL Enhancements

      ..... USA ......              ..... Europe ......

           NOC1                           NOC2
            |                              |
            |            metric 100        |
          ACP1 --------------------------- ACP2  .
            |                              |     . WAN
            | metric 10          metric 20 |     . Core
            |                              |     .
          ACP3 --------------------------- ACP4  .
            |            metric 100        |
            |                              |     .
            |                              |     . Sites
          ACP10                           ACP11  .

                            Figure 17: Dual NOC

   The profile for RPL specified in this document builds only one
   spanning-tree path set to a root, typically a registrar in one NOC.
   In the presence of multiple NOCs, routing toward the non-root NOCs
   may be suboptimal.  Figure 17 shows an extreme example.  Assuming
   that node ACP1 becomes the RPL root, traffic between ACP11 and NOC2
   will pass through ACP4-ACP3-ACP1-ACP2 instead of ACP4-ACP2 because
   the RPL-calculated DODAG and routes are shortest paths towards the
   RPL root.

   To overcome these limitations, extensions and/or modifications to the
   RPL profile can optimize for multiple NOCs.  This requires utilizing
   data plane artifacts, including IP-in-IP encapsulation/decapsulation
   on ACP routers and processing of IPv6 RPI headers.  Alternatively,
   (Src,Dst) routing table entries could be used.

   Flooding of ACP GRASP messages can be further constrained and
   therefore optimized by flooding only via links that are part of the
   RPL DODAG.

A.9.5.  Role Assignments

   ACP connect is an explicit mechanism to "leak" ACP traffic explicitly
   (for example, in a NOC).  It is therefore also a possible security
   gap when it is easy to enable ACP connect on arbitrary compromised
   ACP nodes.

   One simple solution is to define an extension in the ACP
   certificate's ACP information field indicating the permission for ACP
   connect to be configured on that ACP node.  This could similarly be
   done to decide whether a node is permitted to be a registrar or not.

   Tying the permitted "roles" of an ACP node to the ACP certificate
   provides fairly strong protection against misconfiguration, but it is
   still subject to code modifications.

   Another interesting role to assign to certificates is that of a NOC
   node.  This would allow the limiting of certain types of connections,
   such as OAM TLS connections to only the NOC initiators or responders.

A.9.6.  Autonomic L3 Transit

   In this specification, the ACP can only establish autonomic
   connectivity across L2 hops but requires non-autonomic configuration
   to tunnel across L3 paths.  Future work should specify mechanisms to
   automatically tunnel ACP across L3 networks.  A hub-and-spoke option
   would allow tunneling across the Internet to a cloud or central
   instance of the ACP; a peer-to-peer tunneling mechanism could tunnel
   ACP islands across an L3VPN infrastructure.

A.9.7.  Diagnostics

   Section 9.1 describes diagnostics options that can be applied without
   changing the external, interoperability-affecting characteristics of
   ACP implementations.

   Even better diagnostics of ACP operations are possible with
   additional signaling extensions, such as the following:

   1.  Consider if LLDP should be a recommended functionality for ANI
       devices to improve diagnostics, and if so, which information
       elements it should signal (noting that such information is
       conveyed in an insecure manner).  This includes potentially new
       information elements.

   2.  As an alternative to LLDP, a DULL GRASP diagnostics objective
       could be defined to carry these information elements.

   3.  The IDevID certificate of BRSKI pledges should be included in the
       selected insecure diagnostics option.  This may be undesirable
       when exposure of device information is seen as too much of a
       security issue (the ability to deduce possible attack vectors
       from device model, for example).

   4.  A richer set of diagnostics information should be made available
       via the secured ACP channels, using either single-hop GRASP or
       network-wide "topology discovery" mechanisms.

A.9.8.  Avoiding and Dealing with Compromised ACP Nodes

   Compromised ACP nodes pose the biggest risk to the operations of the
   network.  The most common types of compromise are the leakage of
   credentials to manage and/or configure the device and the application
   of malicious configuration, including the change of access
   credentials, but not the change of software.  Most of today's
   networking equipment should have secure boot/software infrastructure
   anyhow, so attacks that introduce malicious software should be a lot
   harder.

   The most important aspect of security design against these types of
   attacks is to eliminate password-based configuration access methods
   and instead rely on certificate-based credentials handed out only to
   nodes where it is clear that the private keys cannot leak.  This
   limits unexpected propagation of credentials.

   If password-based credentials to configure devices still need to be
   supported, they must not be locally configurable, but only be
   remotely provisioned or verified (through protocols like RADIUS or
   Diameter), and there must be no local configuration permitting the
   change of these authentication mechanisms, but ideally they should be
   autoconfiguring across the ACP.  See [NOC-AUTOCONFIG].

   Without physical access to the compromised device, attackers with
   access to configuration should not be able to break the ACP
   connectivity, even when they can break or otherwise manipulate
   (spoof) the data plane connectivity through configuration.  To
   achieve this, it is necessary to avoid providing configuration
   options for the ACP, such as enabling/disabling it on interfaces.
   For example, there could be an ACP configuration that locks down the
   current ACP configuration unless factory reset is done.

   With such means, the valid administration has the best chances to
   maintain access to ACP nodes, to discover malicious configuration
   though ongoing configuration tracking from central locations, for
   example, and to react accordingly.

   The primary reaction is to withdraw or change credentials, terminate
   malicious existing management sessions, and fix the configuration.
   Ensuring that management sessions using invalidated credentials are
   terminated automatically without recourse will likely require new
   work.

   Only when these steps are infeasible, would it be necessary to revoke
   or expire the ACP certificate credentials and consider the node
   kicked off the network until the situation can be further rectified,
   likely requiring direct physical access to the node.

   Without extensions, compromised ACP nodes can only be removed from
   the ACP at the speed of CRL/OCSP information refresh or expiry (and
   non-removal) of short-lived certificates.  Future extensions to the
   ACP could, for example, use the GRASP flooding distribution of
   triggered updates of CRL/OCSP or the explicit removal indication of
   the compromised node's domain certificate.

A.9.9.  Detecting ACP Secure Channel Downgrade Attacks

   The following text proposes a mechanism to protect against downgrade
   attacks without introducing a new specialized GRASP secure channel
   mechanism.  Instead, it relies on running GRASP after establishing a
   secure channel protocol to verify if the established secure channel
   option could have been the result of a MITM downgrade attack.

   MITM attackers can force downgrade attacks for ACP secure channel
   selection by filtering and/or modifying DULL GRASP messages and/or
   actual secure channel data packets.  For example, if at some point in
   time, DTLS traffic could be more easily decrypted than traffic of
   IKEv2, the MITM could filter all IKEv2 packets to force ACP nodes to
   use DTLS (assuming that the ACP nodes in question supported both DTLS
   and IKEv2).

   For cases where such MITM attacks are not capable of injecting
   malicious traffic (but only of decrypting the traffic), a downgrade
   attack could be discovered after a secure channel connection is
   established, for example, by using the following type of mechanism.

   After the secure channel connection is established, the two ACP peers
   negotiate, via an appropriate (to be defined) GRASP negotiation,
   which ACP secure channel protocol should have been selected between
   them (in the absence of a MITM attacker).  This negotiation would
   signal the ACP secure channel options announced by DULL GRASP by each
   peer followed by an announcement of the preferred secure channel
   protocol by the ACP peer that is the Decider in the secure channel
   setup, i.e., the ACP peer that decides which secure channel protocol
   to use.  If that chosen secure channel protocol is different from the
   one that actually was chosen, then this mismatch is an indication
   that there is a MITM attacker or other similar issue (e.g., a
   firewall prohibiting the use of specific protocols) that caused a
   non-preferred secure channel protocol to be chosen.  This discovery
   could then result in mitigation options such as logging and ensuing
   investigations.

Acknowledgements

   This work originated from an Autonomic Networking project at Cisco
   Systems, which started in early 2010.  Many people contributed to
   this project and the idea of the Autonomic Control Plane, amongst
   whom (in alphabetical order): Ignas Bagdonas, Parag Bhide, Balaji BL,
   Alex Clemm, Yves Hertoghs, Bruno Klauser, Max Pritikin, Michael
   Richardson, and Ravi Kumar Vadapalli.

   Special thanks to Brian Carpenter, Elwyn Davies, Joel Halpern, and
   Sheng Jiang for their thorough reviews.

   Many thanks to Ben Kaduk, Roman Danyliw, and Eric Rescorla for their
   thorough SEC AD reviews, Russ Housley and Erik Kline for their
   reviews, and to Valery Smyslov, Tero Kivinen, Paul Wouters, and Yoav
   Nir for review of IPsec and IKEv2 parameters and helping to
   understand those and other security protocol details better.  Thanks
   to Carsten Bormann for CBOR/CDDL help.

   Further input, review, or suggestions were received from Rene Struik,
   Benoit Claise, William Atwood, and Yongkang Zhang.

Contributors

   For all things GRASP including validation code, ongoing document text
   support, and technical input:

   Brian Carpenter
   School of Computer Science
   University of Auckland
   PB 92019
   Auckland 1142
   New Zealand

   Email: brian.e.carpenter@gmail.com


   For RPL contributions and all things BRSKI/bootstrap including
   validation code, ongoing document text support, and technical input:

   Michael C. Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca
   URI:   http://www.sandelman.ca/mcr/


   For the RPL technology choices and text:

   Pascal Thubert
   Cisco Systems, Inc
   Building D
   45 Allee des Ormes - BP1200
   06254 Mougins - Sophia Antipolis
   France

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com


Authors' Addresses

   Toerless Eckert (editor)
   Futurewei Technologies Inc. USA
   2330 Central Expy
   Santa Clara, CA 95050
   United States of America

   Email: tte+ietf@cs.fau.de


   Michael H. Behringer (editor)

   Email: michael.h.behringer@gmail.com


   Steinthor Bjarnason
   Arbor Networks
   2727 South State Street, Suite 200
   Ann Arbor, MI 48104
   United States of America

   Email: sbjarnason@arbor.net