path: root/doc/rfc/rfc6830.txt

Internet Engineering Task Force (IETF)                      D. Farinacci
Request for Comments: 6830                                 Cisco Systems
Category: Experimental                                         V. Fuller
ISSN: 2070-1721
                                                                D. Meyer
                                                                D. Lewis
                                                           Cisco Systems
                                                            January 2013


               The Locator/ID Separation Protocol (LISP)

Abstract

   This document describes a network-layer-based protocol that enables
   separation of IP addresses into two new numbering spaces: Endpoint
   Identifiers (EIDs) and Routing Locators (RLOCs).  No changes are
   required to either host protocol stacks or to the "core" of the
   Internet infrastructure.  The Locator/ID Separation Protocol (LISP)
   can be incrementally deployed, without a "flag day", and offers
   Traffic Engineering, multihoming, and mobility benefits to early
   adopters, even when there are relatively few LISP-capable sites.

   Design and development of LISP was largely motivated by the problem
   statement produced by the October 2006 IAB Routing and Addressing
   Workshop.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6830.







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Copyright Notice

   Copyright (c) 2013 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
   (http://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 ....................................................3
   2. Requirements Notation ...........................................5
   3. Definition of Terms .............................................5
   4. Basic Overview .................................................10
      4.1. Packet Flow Sequence ......................................13
   5. LISP Encapsulation Details .....................................15
      5.1. LISP IPv4-in-IPv4 Header Format ...........................16
      5.2. LISP IPv6-in-IPv6 Header Format ...........................17
      5.3. Tunnel Header Field Descriptions ..........................18
      5.4. Dealing with Large Encapsulated Packets ...................22
           5.4.1. A Stateless Solution to MTU Handling ...............22
           5.4.2. A Stateful Solution to MTU Handling ................23
      5.5. Using Virtualization and Segmentation with LISP ...........24
   6. EID-to-RLOC Mapping ............................................25
      6.1. LISP IPv4 and IPv6 Control-Plane Packet Formats ...........25
           6.1.1. LISP Packet Type Allocations .......................27
           6.1.2. Map-Request Message Format .........................27
           6.1.3. EID-to-RLOC UDP Map-Request Message ................30
           6.1.4. Map-Reply Message Format ...........................31
           6.1.5. EID-to-RLOC UDP Map-Reply Message ..................35
           6.1.6. Map-Register Message Format ........................37
           6.1.7. Map-Notify Message Format ..........................39
           6.1.8. Encapsulated Control Message Format ................41
      6.2. Routing Locator Selection .................................42
      6.3. Routing Locator Reachability ..............................44
           6.3.1. Echo Nonce Algorithm ...............................46
           6.3.2. RLOC-Probing Algorithm .............................48
      6.4. EID Reachability within a LISP Site .......................49
      6.5. Routing Locator Hashing ...................................49





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      6.6. Changing the Contents of EID-to-RLOC Mappings .............50
           6.6.1. Clock Sweep ........................................51
           6.6.2. Solicit-Map-Request (SMR) ..........................52
           6.6.3. Database Map-Versioning ............................53
   7. Router Performance Considerations ..............................54
   8. Deployment Scenarios ...........................................55
      8.1. First-Hop/Last-Hop Tunnel Routers .........................56
      8.2. Border/Edge Tunnel Routers ................................56
      8.3. ISP Provider Edge (PE) Tunnel Routers .....................57
      8.4. LISP Functionality with Conventional NATs .................58
      8.5. Packets Egressing a LISP Site .............................58
   9. Traceroute Considerations ......................................58
      9.1. IPv6 Traceroute ...........................................59
      9.2. IPv4 Traceroute ...........................................60
      9.3. Traceroute Using Mixed Locators ...........................60
   10. Mobility Considerations .......................................61
      10.1. Site Mobility ............................................61
      10.2. Slow Endpoint Mobility ...................................61
      10.3. Fast Endpoint Mobility ...................................61
      10.4. Fast Network Mobility ....................................63
      10.5. LISP Mobile Node Mobility ................................64
   11. Multicast Considerations ......................................64
   12. Security Considerations .......................................65
   13. Network Management Considerations .............................67
   14. IANA Considerations ...........................................67
      14.1. LISP ACT and Flag Fields .................................67
      14.2. LISP Address Type Codes ..................................68
      14.3. LISP UDP Port Numbers ....................................68
      14.4. LISP Key ID Numbers ......................................68
   15. Known Open Issues and Areas of Future Work ....................68
   16. References ....................................................70
      16.1. Normative References .....................................70
      16.2. Informative References ...................................71
   Appendix A. Acknowledgments .......................................74

1.  Introduction

   This document describes the Locator/Identifier Separation Protocol
   (LISP), which provides a set of functions for routers to exchange
   information used to map from Endpoint Identifiers (EIDs) that are not
   globally routable to routable Routing Locators (RLOCs).  It also
   defines a mechanism for these LISP routers to encapsulate IP packets
   addressed with EIDs for transmission across a network infrastructure
   that uses RLOCs for routing and forwarding.







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   Creation of LISP was initially motivated by discussions during the
   IAB-sponsored Routing and Addressing Workshop held in Amsterdam in
   October 2006 (see [RFC4984]).  A key conclusion of the workshop was
   that the Internet routing and addressing system was not scaling well
   in the face of the explosive growth of new sites; one reason for this
   poor scaling is the increasing number of multihomed sites and other
   sites that cannot be addressed as part of topology-based or provider-
   based aggregated prefixes.  Additional work that more completely
   describes the problem statement may be found in [RADIR].

   A basic observation, made many years ago in early networking research
   such as that documented in [CHIAPPA] and [RFC4984], is that using a
   single address field for both identifying a device and for
   determining where it is topologically located in the network requires
   optimization along two conflicting axes: for routing to be efficient,
   the address must be assigned topologically; for collections of
   devices to be easily and effectively managed, without the need for
   renumbering in response to topological change (such as that caused by
   adding or removing attachment points to the network or by mobility
   events), the address must explicitly not be tied to the topology.

   The approach that LISP takes to solving the routing scalability
   problem is to replace IP addresses with two new types of numbers:
   Routing Locators (RLOCs), which are topologically assigned to network
   attachment points (and are therefore amenable to aggregation) and
   used for routing and forwarding of packets through the network; and
   Endpoint Identifiers (EIDs), which are assigned independently from
   the network topology, are used for numbering devices, and are
   aggregated along administrative boundaries.  LISP then defines
   functions for mapping between the two numbering spaces and for
   encapsulating traffic originated by devices using non-routable EIDs
   for transport across a network infrastructure that routes and
   forwards using RLOCs.  Both RLOCs and EIDs are syntactically
   identical to IP addresses; it is the semantics of how they are used
   that differs.

   This document describes the protocol that implements these functions.
   The database that stores the mappings between EIDs and RLOCs is
   explicitly a separate "module" to facilitate experimentation with a
   variety of approaches.  One database design that is being developed
   for experimentation as part of the LISP working group work is
   [RFC6836].  Others that have been described include [CONS], [EMACS],
   and [RFC6837].  Finally, [RFC6833] documents a general-purpose
   service interface for accessing a mapping database; this interface is
   intended to make the mapping database modular so that different
   approaches can be tried without the need to modify installed LISP-
   capable devices in LISP sites.




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   This experimental specification has areas that require additional
   experience and measurement.  It is NOT RECOMMENDED for deployment
   beyond experimental situations.  Results of experimentation may lead
   to modifications and enhancements of protocol mechanisms defined in
   this document.  See Section 15 for specific, known issues that are in
   need of further work during development, implementation, and
   experimentation.

   An examination of the implications of LISP on Internet traffic,
   applications, routers, and security is for future study.  This
   analysis will explain what role LISP can play in scalable routing and
   will also look at scalability and levels of state required for
   encapsulation, decapsulation, liveness, and so on.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Definition of Terms

   Provider-Independent (PI) Addresses:   PI addresses are an address
      block assigned from a pool where blocks are not associated with
      any particular location in the network (e.g., from a particular
      service provider) and are therefore not topologically aggregatable
      in the routing system.

   Provider-Assigned (PA) Addresses:   PA addresses are an address block
      assigned to a site by each service provider to which a site
      connects.  Typically, each block is a sub-block of a service
      provider Classless Inter-Domain Routing (CIDR) [RFC4632] block and
      is aggregated into the larger block before being advertised into
      the global Internet.  Traditionally, IP multihoming has been
      implemented by each multihomed site acquiring its own globally
      visible prefix.  LISP uses only topologically assigned and
      aggregatable address blocks for RLOCs, eliminating this
      demonstrably non-scalable practice.

   Routing Locator (RLOC):   An RLOC is an IPv4 [RFC0791] or IPv6
      [RFC2460] address of an Egress Tunnel Router (ETR).  An RLOC is
      the output of an EID-to-RLOC mapping lookup.  An EID maps to one
      or more RLOCs.  Typically, RLOCs are numbered from topologically
      aggregatable blocks that are assigned to a site at each point to
      which it attaches to the global Internet; where the topology is
      defined by the connectivity of provider networks, RLOCs can be
      thought of as PA addresses.  Multiple RLOCs can be assigned to the
      same ETR device or to multiple ETR devices at a site.



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   Endpoint ID (EID):   An EID is a 32-bit (for IPv4) or 128-bit (for
      IPv6) value used in the source and destination address fields of
      the first (most inner) LISP header of a packet.  The host obtains
      a destination EID the same way it obtains a destination address
      today, for example, through a Domain Name System (DNS) [RFC1034]
      lookup or Session Initiation Protocol (SIP) [RFC3261] exchange.
      The source EID is obtained via existing mechanisms used to set a
      host's "local" IP address.  An EID used on the public Internet
      must have the same properties as any other IP address used in that
      manner; this means, among other things, that it must be globally
      unique.  An EID is allocated to a host from an EID-Prefix block
      associated with the site where the host is located.  An EID can be
      used by a host to refer to other hosts.  EIDs MUST NOT be used as
      LISP RLOCs.  Note that EID blocks MAY be assigned in a
      hierarchical manner, independent of the network topology, to
      facilitate scaling of the mapping database.  In addition, an EID
      block assigned to a site may have site-local structure
      (subnetting) for routing within the site; this structure is not
      visible to the global routing system.  In theory, the bit string
      that represents an EID for one device can represent an RLOC for a
      different device.  As the architecture is realized, if a given bit
      string is both an RLOC and an EID, it must refer to the same
      entity in both cases.  When used in discussions with other
      Locator/ID separation proposals, a LISP EID will be called an
      "LEID".  Throughout this document, any references to "EID" refer
      to an LEID.

   EID-Prefix:   An EID-Prefix is a power-of-two block of EIDs that are
      allocated to a site by an address allocation authority.
      EID-Prefixes are associated with a set of RLOC addresses that make
      up a "database mapping".  EID-Prefix allocations can be broken up
      into smaller blocks when an RLOC set is to be associated with the
      larger EID-Prefix block.  A globally routed address block (whether
      PI or PA) is not inherently an EID-Prefix.  A globally routed
      address block MAY be used by its assignee as an EID block.  The
      converse is not supported.  That is, a site that receives an
      explicitly allocated EID-Prefix may not use that EID-Prefix as a
      globally routed prefix.  This would require coordination and
      cooperation with the entities managing the mapping infrastructure.
      Once this has been done, that block could be removed from the
      globally routed IP system, if other suitable transition and access
      mechanisms are in place.  Discussion of such transition and access
      mechanisms can be found in [RFC6832] and [LISP-DEPLOY].








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   End-system:   An end-system is an IPv4 or IPv6 device that originates
      packets with a single IPv4 or IPv6 header.  The end-system
      supplies an EID value for the destination address field of the IP
      header when communicating globally (i.e., outside of its routing
      domain).  An end-system can be a host computer, a switch or router
      device, or any network appliance.

   Ingress Tunnel Router (ITR):   An ITR is a router that resides in a
      LISP site.  Packets sent by sources inside of the LISP site to
      destinations outside of the site are candidates for encapsulation
      by the ITR.  The ITR treats the IP destination address as an EID
      and performs an EID-to-RLOC mapping lookup.  The router then
      prepends an "outer" IP header with one of its globally routable
      RLOCs in the source address field and the result of the mapping
      lookup in the destination address field.  Note that this
      destination RLOC MAY be an intermediate, proxy device that has
      better knowledge of the EID-to-RLOC mapping closer to the
      destination EID.  In general, an ITR receives IP packets from site
      end-systems on one side and sends LISP-encapsulated IP packets
      toward the Internet on the other side.

      Specifically, when a service provider prepends a LISP header for
      Traffic Engineering purposes, the router that does this is also
      regarded as an ITR.  The outer RLOC the ISP ITR uses can be based
      on the outer destination address (the originating ITR's supplied
      RLOC) or the inner destination address (the originating host's
      supplied EID).

   TE-ITR:   A TE-ITR is an ITR that is deployed in a service provider
      network that prepends an additional LISP header for Traffic
      Engineering purposes.

   Egress Tunnel Router (ETR):   An ETR is a router that accepts an IP
      packet where the destination address in the "outer" IP header is
      one of its own RLOCs.  The router strips the "outer" header and
      forwards the packet based on the next IP header found.  In
      general, an ETR receives LISP-encapsulated IP packets from the
      Internet on one side and sends decapsulated IP packets to site
      end-systems on the other side.  ETR functionality does not have to
      be limited to a router device.  A server host can be the endpoint
      of a LISP tunnel as well.

   TE-ETR:   A TE-ETR is an ETR that is deployed in a service provider
      network that strips an outer LISP header for Traffic Engineering
      purposes.






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   xTR:   An xTR is a reference to an ITR or ETR when direction of data
      flow is not part of the context description. "xTR" refers to the
      router that is the tunnel endpoint and is used synonymously with
      the term "Tunnel Router".  For example, "An xTR can be located at
      the Customer Edge (CE) router" indicates both ITR and ETR
      functionality at the CE router.

   LISP Router:   A LISP router is a router that performs the functions
      of any or all of the following: ITR, ETR, Proxy-ITR (PITR), or
      Proxy-ETR (PETR).

   EID-to-RLOC Cache:   The EID-to-RLOC Cache is a short-lived,
      on-demand table in an ITR that stores, tracks, and is responsible
      for timing out and otherwise validating EID-to-RLOC mappings.
      This cache is distinct from the full "database" of EID-to-RLOC
      mappings; it is dynamic, local to the ITR(s), and relatively
      small, while the database is distributed, relatively static, and
      much more global in scope.

   EID-to-RLOC Database:   The EID-to-RLOC Database is a global
      distributed database that contains all known EID-Prefix-to-RLOC
      mappings.  Each potential ETR typically contains a small piece of
      the database: the EID-to-RLOC mappings for the EID-Prefixes
      "behind" the router.  These map to one of the router's own
      globally visible IP addresses.  The same database mapping entries
      MUST be configured on all ETRs for a given site.  In a steady
      state, the EID-Prefixes for the site and the Locator-Set for each
      EID-Prefix MUST be the same on all ETRs.  Procedures to enforce
      and/or verify this are outside the scope of this document.  Note
      that there MAY be transient conditions when the EID-Prefix for the
      site and Locator-Set for each EID-Prefix may not be the same on
      all ETRs.  This has no negative implications, since a partial set
      of Locators can be used.

   Recursive Tunneling:   Recursive Tunneling occurs when a packet has
      more than one LISP IP header.  Additional layers of tunneling MAY
      be employed to implement Traffic Engineering or other re-routing
      as needed.  When this is done, an additional "outer" LISP header
      is added, and the original RLOCs are preserved in the "inner"
      header.  Any references to tunnels in this specification refer to
      dynamic encapsulating tunnels; they are never statically
      configured.

   Re-encapsulating Tunnels:   Re-encapsulating Tunneling occurs when an
      ETR removes a LISP header, then acts as an ITR to prepend another
      LISP header.  Doing this allows a packet to be re-routed by the
      re-encapsulating router without adding the overhead of additional
      tunnel headers.  Any references to tunnels in this specification



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      refer to dynamic encapsulating tunnels; they are never statically
      configured.  When using multiple mapping database systems, care
      must be taken to not create re-encapsulation loops through
      misconfiguration.

   LISP Header:   LISP header is a term used in this document to refer
      to the outer IPv4 or IPv6 header, a UDP header, and a LISP-
      specific 8-octet header that follow the UDP header and that an ITR
      prepends or an ETR strips.

   Address Family Identifier (AFI):   AFI is a term used to describe an
      address encoding in a packet.  An address family currently
      pertains to an IPv4 or IPv6 address.  See [AFI] and [RFC3232] for
      details.  An AFI value of 0 used in this specification indicates
      an unspecified encoded address where the length of the address is
      0 octets following the 16-bit AFI value of 0.

   Negative Mapping Entry:   A negative mapping entry, also known as a
      negative cache entry, is an EID-to-RLOC entry where an EID-Prefix
      is advertised or stored with no RLOCs.  That is, the Locator-Set
      for the EID-to-RLOC entry is empty or has an encoded Locator count
      of 0.  This type of entry could be used to describe a prefix from
      a non-LISP site, which is explicitly not in the mapping database.
      There are a set of well-defined actions that are encoded in a
      Negative Map-Reply (Section 6.1.5).

   Data-Probe:   A Data-Probe is a LISP-encapsulated data packet where
      the inner-header destination address equals the outer-header
      destination address used to trigger a Map-Reply by a decapsulating
      ETR.  In addition, the original packet is decapsulated and
      delivered to the destination host if the destination EID is in the
      EID-Prefix range configured on the ETR.  Otherwise, the packet is
      discarded.  A Data-Probe is used in some of the mapping database
      designs to "probe" or request a Map-Reply from an ETR; in other
      cases, Map-Requests are used.  See each mapping database design
      for details.  When using Data-Probes, by sending Map-Requests on
      the underlying routing system, EID-Prefixes must be advertised.
      However, this is discouraged if the core is to scale by having
      less EID-Prefixes stored in the core router's routing tables.

   Proxy-ITR (PITR):   A PITR is defined and described in [RFC6832].  A
      PITR acts like an ITR but does so on behalf of non-LISP sites that
      send packets to destinations at LISP sites.

   Proxy-ETR (PETR):   A PETR is defined and described in [RFC6832].  A
      PETR acts like an ETR but does so on behalf of LISP sites that
      send packets to destinations at non-LISP sites.




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   Route-returnability:  Route-returnability is an assumption that the
      underlying routing system will deliver packets to the destination.
      When combined with a nonce that is provided by a sender and
      returned by a receiver, this limits off-path data insertion.  A
      route-returnability check is verified when a message is sent with
      a nonce, another message is returned with the same nonce, and the
      destination of the original message appears as the source of the
      returned message.

   LISP site:  LISP site is a set of routers in an edge network that are
      under a single technical administration.  LISP routers that reside
      in the edge network are the demarcation points to separate the
      edge network from the core network.

   Client-side:  Client-side is a term used in this document to indicate
      a connection initiation attempt by an EID.  The ITR(s) at the LISP
      site are the first to get involved in obtaining database Map-Cache
      entries by sending Map-Request messages.

   Server-side:  Server-side is a term used in this document to indicate
      that a connection initiation attempt is being accepted for a
      destination EID.  The ETR(s) at the destination LISP site are the
      first to send Map-Replies to the source site initiating the
      connection.  The ETR(s) at this destination site can obtain
      mappings by gleaning information from Map-Requests, Data-Probes,
      or encapsulated packets.

   Locator-Status-Bits (LSBs):  Locator-Status-Bits are present in the
      LISP header.  They are used by ITRs to inform ETRs about the up/
      down status of all ETRs at the local site.  These bits are used as
      a hint to convey up/down router status and not path reachability
      status.  The LSBs can be verified by use of one of the Locator
      reachability algorithms described in Section 6.3.

   Anycast Address:  Anycast Address is a term used in this document to
      refer to the same IPv4 or IPv6 address configured and used on
      multiple systems at the same time.  An EID or RLOC can be an
      anycast address in each of their own address spaces.

4.  Basic Overview

   One key concept of LISP is that end-systems (hosts) operate the same
   way they do today.  The IP addresses that hosts use for tracking
   sockets and connections, and for sending and receiving packets, do
   not change.  In LISP terminology, these IP addresses are called
   Endpoint Identifiers (EIDs).





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   Routers continue to forward packets based on IP destination
   addresses.  When a packet is LISP encapsulated, these addresses are
   referred to as Routing Locators (RLOCs).  Most routers along a path
   between two hosts will not change; they continue to perform routing/
   forwarding lookups on the destination addresses.  For routers between
   the source host and the ITR as well as routers from the ETR to the
   destination host, the destination address is an EID.  For the routers
   between the ITR and the ETR, the destination address is an RLOC.

   Another key LISP concept is the "Tunnel Router".  A Tunnel Router
   prepends LISP headers on host-originated packets and strips them
   prior to final delivery to their destination.  The IP addresses in
   this "outer header" are RLOCs.  During end-to-end packet exchange
   between two Internet hosts, an ITR prepends a new LISP header to each
   packet, and an ETR strips the new header.  The ITR performs
   EID-to-RLOC lookups to determine the routing path to the ETR, which
   has the RLOC as one of its IP addresses.

   Some basic rules governing LISP are:

   o  End-systems (hosts) only send to addresses that are EIDs.  They
      don't know that addresses are EIDs versus RLOCs but assume that
      packets get to their intended destinations.  In a system where
      LISP is deployed, LISP routers intercept EID-addressed packets and
      assist in delivering them across the network core where EIDs
      cannot be routed.  The procedure a host uses to send IP packets
      does not change.

   o  EIDs are always IP addresses assigned to hosts.

   o  LISP routers mostly deal with Routing Locator addresses.  See
      details in Section 4.1 to clarify what is meant by "mostly".

   o  RLOCs are always IP addresses assigned to routers, preferably
      topologically oriented addresses from provider CIDR (Classless
      Inter-Domain Routing) blocks.

   o  When a router originates packets, it may use as a source address
      either an EID or RLOC.  When acting as a host (e.g., when
      terminating a transport session such as Secure SHell (SSH),
      TELNET, or the Simple Network Management Protocol (SNMP)), it may
      use an EID that is explicitly assigned for that purpose.  An EID
      that identifies the router as a host MUST NOT be used as an RLOC;
      an EID is only routable within the scope of a site.  A typical BGP
      configuration might demonstrate this "hybrid" EID/RLOC usage where
      a router could use its "host-like" EID to terminate iBGP sessions
      to other routers in a site while at the same time using RLOCs to
      terminate eBGP sessions to routers outside the site.



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   o  Packets with EIDs in them are not expected to be delivered
      end-to-end in the absence of an EID-to-RLOC mapping operation.
      They are expected to be used locally for intra-site communication
      or to be encapsulated for inter-site communication.

   o  EID-Prefixes are likely to be hierarchically assigned in a manner
      that is optimized for administrative convenience and to facilitate
      scaling of the EID-to-RLOC mapping database.  The hierarchy is
      based on an address allocation hierarchy that is independent of
      the network topology.

   o  EIDs may also be structured (subnetted) in a manner suitable for
      local routing within an Autonomous System (AS).

   An additional LISP header MAY be prepended to packets by a TE-ITR
   when re-routing of the path for a packet is desired.  A potential
   use-case for this would be an ISP router that needs to perform
   Traffic Engineering for packets flowing through its network.  In such
   a situation, termed "Recursive Tunneling", an ISP transit acts as an
   additional ITR, and the RLOC it uses for the new prepended header
   would be either a TE-ETR within the ISP (along an intra-ISP traffic
   engineered path) or a TE-ETR within another ISP (an inter-ISP traffic
   engineered path, where an agreement to build such a path exists).

   In order to avoid excessive packet overhead as well as possible
   encapsulation loops, this document mandates that a maximum of two
   LISP headers can be prepended to a packet.  For initial LISP
   deployments, it is assumed that two headers is sufficient, where the
   first prepended header is used at a site for Location/Identity
   separation and the second prepended header is used inside a service
   provider for Traffic Engineering purposes.

   Tunnel Routers can be placed fairly flexibly in a multi-AS topology.
   For example, the ITR for a particular end-to-end packet exchange
   might be the first-hop or default router within a site for the source
   host.  Similarly, the ETR might be the last-hop router directly
   connected to the destination host.  Another example, perhaps for a
   VPN service outsourced to an ISP by a site, the ITR could be the
   site's border router at the service provider attachment point.
   Mixing and matching of site-operated, ISP-operated, and other Tunnel
   Routers is allowed for maximum flexibility.  See Section 8 for more
   details.









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4.1.  Packet Flow Sequence

   This section provides an example of the unicast packet flow with the
   following conditions:

   o  Source host "host1.abc.example.com" is sending a packet to
      "host2.xyz.example.com", exactly what host1 would do if the site
      was not using LISP.

   o  Each site is multihomed, so each Tunnel Router has an address
      (RLOC) assigned from the service provider address block for each
      provider to which that particular Tunnel Router is attached.

   o  The ITR(s) and ETR(s) are directly connected to the source and
      destination, respectively, but the source and destination can be
      located anywhere in the LISP site.

   o  Map-Requests can be sent on the underlying routing system
      topology, to a mapping database system, or directly over an
      Alternative Logical Topology [RFC6836].  A Map-Request is sent for
      an external destination when the destination is not found in the
      forwarding table or matches a default route.

   o  Map-Replies are sent on the underlying routing system topology.

   Client host1.abc.example.com wants to communicate with server
   host2.xyz.example.com:

   1.  host1.abc.example.com wants to open a TCP connection to
       host2.xyz.example.com.  It does a DNS lookup on
       host2.xyz.example.com.  An A/AAAA record is returned.  This
       address is the destination EID.  The locally assigned address of
       host1.abc.example.com is used as the source EID.  An IPv4 or IPv6
       packet is built and forwarded through the LISP site as a normal
       IP packet until it reaches a LISP ITR.

   2.  The LISP ITR must be able to map the destination EID to an RLOC
       of one of the ETRs at the destination site.  The specific method
       used to do this is not described in this example.  See [RFC6836]
       or [CONS] for possible solutions.

   3.  The ITR will send a LISP Map-Request.  Map-Requests SHOULD be
       rate-limited.








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   4.  When an alternate mapping system is not in use, the Map-Request
       packet is routed through the underlying routing system.
       Otherwise, the Map-Request packet is routed on an alternate
       logical topology, for example, the [RFC6836] database mapping
       system.  In either case, when the Map-Request arrives at one of
       the ETRs at the destination site, it will process the packet as a
       control message.

   5.  The ETR looks at the destination EID of the Map-Request and
       matches it against the prefixes in the ETR's configured
       EID-to-RLOC mapping database.  This is the list of EID-Prefixes
       the ETR is supporting for the site it resides in.  If there is no
       match, the Map-Request is dropped.  Otherwise, a LISP Map-Reply
       is returned to the ITR.

   6.  The ITR receives the Map-Reply message, parses the message (to
       check for format validity), and stores the mapping information
       from the packet.  This information is stored in the ITR's
       EID-to-RLOC mapping cache.  Note that the map-cache is an
       on-demand cache.  An ITR will manage its map-cache in such a way
       that optimizes for its resource constraints.

   7.  Subsequent packets from host1.abc.example.com to
       host2.xyz.example.com will have a LISP header prepended by the
       ITR using the appropriate RLOC as the LISP header destination
       address learned from the ETR.  Note that the packet MAY be sent
       to a different ETR than the one that returned the Map-Reply due
       to the source site's hashing policy or the destination site's
       Locator-Set policy.

   8.  The ETR receives these packets directly (since the destination
       address is one of its assigned IP addresses), checks the validity
       of the addresses, strips the LISP header, and forwards packets to
       the attached destination host.

   In order to defer the need for a mapping lookup in the reverse
   direction, an ETR MAY create a cache entry that maps the source EID
   (inner-header source IP address) to the source RLOC (outer-header
   source IP address) in a received LISP packet.  Such a cache entry is
   termed a "gleaned" mapping and only contains a single RLOC for the
   EID in question.  More complete information about additional RLOCs
   SHOULD be verified by sending a LISP Map-Request for that EID.  Both
   the ITR and the ETR may also influence the decision the other makes
   in selecting an RLOC.  See Section 6 for more details.







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5.  LISP Encapsulation Details

   Since additional tunnel headers are prepended, the packet becomes
   larger and can exceed the MTU of any link traversed from the ITR to
   the ETR.  It is RECOMMENDED in IPv4 that packets do not get
   fragmented as they are encapsulated by the ITR.  Instead, the packet
   is dropped and an ICMP Too Big message is returned to the source.

   This specification RECOMMENDS that implementations provide support
   for one of the proposed fragmentation and reassembly schemes.  Two
   existing schemes are detailed in Section 5.4.

   Since IPv4 or IPv6 addresses can be either EIDs or RLOCs, the LISP
   architecture supports IPv4 EIDs with IPv6 RLOCs (where the inner
   header is in IPv4 packet format and the outer header is in IPv6
   packet format) or IPv6 EIDs with IPv4 RLOCs (where the inner header
   is in IPv6 packet format and the outer header is in IPv4 packet
   format).  The next sub-sections illustrate packet formats for the
   homogeneous case (IPv4-in-IPv4 and IPv6-in-IPv6), but all 4
   combinations MUST be supported.































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5.1.  LISP IPv4-in-IPv4 Header Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |Version|  IHL  |Type of Service|          Total Length         |
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |         Identification        |Flags|      Fragment Offset    |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   OH  |  Time to Live | Protocol = 17 |         Header Checksum       |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |                    Source Routing Locator                     |
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |                 Destination Routing Locator                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |       Source Port = xxxx      |       Dest Port = 4341        |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   L   |N|L|E|V|I|flags|            Nonce/Map-Version                  |
   I \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   S / |                 Instance ID/Locator-Status-Bits               |
   P   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |Version|  IHL  |Type of Service|          Total Length         |
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |         Identification        |Flags|      Fragment Offset    |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   IH  |  Time to Live |    Protocol   |         Header Checksum       |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |                           Source EID                          |
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |                         Destination EID                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       IHL = IP-Header-Length
















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5.2.  LISP IPv6-in-IPv6 Header Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |Version| Traffic Class |           Flow Label                  |
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |         Payload Length        | Next Header=17|   Hop Limit   |
   v   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
   O   +                                                               +
   u   |                                                               |
   t   +                     Source Routing Locator                    +
   e   |                                                               |
   r   +                                                               +
       |                                                               |
   H   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   d   |                                                               |
   r   +                                                               +
       |                                                               |
   ^   +                  Destination Routing Locator                  +
   |   |                                                               |
    \  +                                                               +
     \ |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |       Source Port = xxxx      |       Dest Port = 4341        |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   L   |N|L|E|V|I|flags|            Nonce/Map-Version                  |
   I \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   S / |                 Instance ID/Locator-Status-Bits               |
   P   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |Version| Traffic Class |           Flow Label                  |
    /  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /   |         Payload Length        |  Next Header  |   Hop Limit   |
   v   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+














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       |                                                               |
   I   +                                                               +
   n   |                                                               |
   n   +                          Source EID                           +
   e   |                                                               |
   r   +                                                               +
       |                                                               |
   H   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   d   |                                                               |
   r   +                                                               +
       |                                                               |
   ^   +                        Destination EID                        +
   \   |                                                               |
    \  +                                                               +
     \ |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.3.  Tunnel Header Field Descriptions

   Inner Header (IH):  The inner header is the header on the datagram
      received from the originating host.  The source and destination IP
      addresses are EIDs [RFC0791] [RFC2460].

   Outer Header: (OH)  The outer header is a new header prepended by an
      ITR.  The address fields contain RLOCs obtained from the ingress
      router's EID-to-RLOC Cache.  The IP protocol number is "UDP (17)"
      from [RFC0768].  The setting of the Don't Fragment (DF) bit
      'Flags' field is according to rules listed in Sections 5.4.1 and
      5.4.2.

   UDP Header:  The UDP header contains an ITR selected source port when
      encapsulating a packet.  See Section 6.5 for details on the hash
      algorithm used to select a source port based on the 5-tuple of the
      inner header.  The destination port MUST be set to the well-known
      IANA-assigned port value 4341.

   UDP Checksum:  The 'UDP Checksum' field SHOULD be transmitted as zero
      by an ITR for either IPv4 [RFC0768] or IPv6 encapsulation
      [UDP-TUNNELS] [UDP-ZERO].  When a packet with a zero UDP checksum
      is received by an ETR, the ETR MUST accept the packet for
      decapsulation.  When an ITR transmits a non-zero value for the UDP
      checksum, it MUST send a correctly computed value in this field.
      When an ETR receives a packet with a non-zero UDP checksum, it MAY
      choose to verify the checksum value.  If it chooses to perform
      such verification, and the verification fails, the packet MUST be
      silently dropped.  If the ETR chooses not to perform the
      verification, or performs the verification successfully, the
      packet MUST be accepted for decapsulation.  The handling of UDP



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      checksums for all tunneling protocols, including LISP, is under
      active discussion within the IETF.  When that discussion
      concludes, any necessary changes will be made to align LISP with
      the outcome of the broader discussion.

   UDP Length:  The 'UDP Length' field is set for an IPv4-encapsulated
      packet to be the sum of the inner-header IPv4 Total Length plus
      the UDP and LISP header lengths.  For an IPv6-encapsulated packet,
      the 'UDP Length' field is the sum of the inner-header IPv6 Payload
      Length, the size of the IPv6 header (40 octets), and the size of
      the UDP and LISP headers.

   N: The N-bit is the nonce-present bit.  When this bit is set to 1,
      the low-order 24 bits of the first 32 bits of the LISP header
      contain a Nonce.  See Section 6.3.1 for details.  Both N- and
      V-bits MUST NOT be set in the same packet.  If they are, a
      decapsulating ETR MUST treat the 'Nonce/Map-Version' field as
      having a Nonce value present.

   L: The L-bit is the 'Locator-Status-Bits' field enabled bit.  When
      this bit is set to 1, the Locator-Status-Bits in the second
      32 bits of the LISP header are in use.

     x 1 x x 0 x x x
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |N|L|E|V|I|flags|            Nonce/Map-Version                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Locator-Status-Bits                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   E: The E-bit is the echo-nonce-request bit.  This bit MUST be ignored
      and has no meaning when the N-bit is set to 0.  When the N-bit is
      set to 1 and this bit is set to 1, an ITR is requesting that the
      nonce value in the 'Nonce' field be echoed back in LISP-
      encapsulated packets when the ITR is also an ETR.  See
      Section 6.3.1 for details.

   V: The V-bit is the Map-Version present bit.  When this bit is set to
      1, the N-bit MUST be 0.  Refer to Section 6.6.3 for more details.
      This bit indicates that the LISP header is encoded in this
      case as:

     0 x 0 1 x x x x
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |N|L|E|V|I|flags|  Source Map-Version   |   Dest Map-Version    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Instance ID/Locator-Status-Bits               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   I: The I-bit is the Instance ID bit.  See Section 5.5 for more
      details.  When this bit is set to 1, the 'Locator-Status-Bits'
      field is reduced to 8 bits and the high-order 24 bits are used as
      an Instance ID.  If the L-bit is set to 0, then the low-order
      8 bits are transmitted as zero and ignored on receipt.  The format
      of the LISP header would look like this:

     x x x x 1 x x x
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |N|L|E|V|I|flags|            Nonce/Map-Version                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Instance ID                   |     LSBs      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   flags:  The 'flags' field is a 3-bit field reserved for future flag
      use.  It MUST be set to 0 on transmit and MUST be ignored on
      receipt.

   LISP Nonce:  The LISP 'Nonce' field is a 24-bit value that is
      randomly generated by an ITR when the N-bit is set to 1.  Nonce
      generation algorithms are an implementation matter but are
      required to generate different nonces when sending to different
      destinations.  However, the same nonce can be used for a period of
      time to the same destination.  The nonce is also used when the
      E-bit is set to request the nonce value to be echoed by the other
      side when packets are returned.  When the E-bit is clear but the
      N-bit is set, a remote ITR is either echoing a previously
      requested echo-nonce or providing a random nonce.  See
      Section 6.3.1 for more details.

   LISP Locator-Status-Bits (LSBs):  When the L-bit is also set, the
      'Locator-Status-Bits' field in the LISP header is set by an ITR to
      indicate to an ETR the up/down status of the Locators in the
      source site.  Each RLOC in a Map-Reply is assigned an ordinal
      value from 0 to n-1 (when there are n RLOCs in a mapping entry).
      The Locator-Status-Bits are numbered from 0 to n-1 from the least
      significant bit of the field.  The field is 32 bits when the I-bit
      is set to 0 and is 8 bits when the I-bit is set to 1.  When a
      Locator-Status-Bit is set to 1, the ITR is indicating to the ETR
      that the RLOC associated with the bit ordinal has up status.  See
      Section 6.3 for details on how an ITR can determine the status of
      the ETRs at the same site.  When a site has multiple EID-Prefixes
      that result in multiple mappings (where each could have a
      different Locator-Set), the Locator-Status-Bits setting in an
      encapsulated packet MUST reflect the mapping for the EID-Prefix
      that the inner-header source EID address matches.  If the LSB for
      an anycast Locator is set to 1, then there is at least one RLOC
      with that address, and the ETR is considered 'up'.



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   When doing ITR/PITR encapsulation:

   o  The outer-header 'Time to Live' field (or 'Hop Limit' field, in
      the case of IPv6) SHOULD be copied from the inner-header 'Time to
      Live' field.

   o  The outer-header 'Type of Service' field (or the 'Traffic Class'
      field, in the case of IPv6) SHOULD be copied from the inner-header
      'Type of Service' field (with one exception; see below).

   When doing ETR/PETR decapsulation:

   o  The inner-header 'Time to Live' field (or 'Hop Limit' field, in
      the case of IPv6) SHOULD be copied from the outer-header 'Time to
      Live' field, when the Time to Live value of the outer header is
      less than the Time to Live value of the inner header.  Failing to
      perform this check can cause the Time to Live of the inner header
      to increment across encapsulation/decapsulation cycles.  This
      check is also performed when doing initial encapsulation, when a
      packet comes to an ITR or PITR destined for a LISP site.

   o  The inner-header 'Type of Service' field (or the 'Traffic Class'
      field, in the case of IPv6) SHOULD be copied from the outer-header
      'Type of Service' field (with one exception; see below).

   Note that if an ETR/PETR is also an ITR/PITR and chooses to
   re-encapsulate after decapsulating, the net effect of this is that
   the new outer header will carry the same Time to Live as the old
   outer header minus 1.

   Copying the Time to Live (TTL) serves two purposes: first, it
   preserves the distance the host intended the packet to travel;
   second, and more importantly, it provides for suppression of looping
   packets in the event there is a loop of concatenated tunnels due to
   misconfiguration.  See Section 9.3 for TTL exception handling for
   traceroute packets.

   The Explicit Congestion Notification ('ECN') field occupies bits 6
   and 7 of both the IPv4 'Type of Service' field and the IPv6 'Traffic
   Class' field [RFC3168].  The 'ECN' field requires special treatment
   in order to avoid discarding indications of congestion [RFC3168].
   ITR encapsulation MUST copy the 2-bit 'ECN' field from the inner
   header to the outer header.  Re-encapsulation MUST copy the 2-bit
   'ECN' field from the stripped outer header to the new outer header.
   If the 'ECN' field contains a congestion indication codepoint (the
   value is '11', the Congestion Experienced (CE) codepoint), then ETR
   decapsulation MUST copy the 2-bit 'ECN' field from the stripped outer
   header to the surviving inner header that is used to forward the



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   packet beyond the ETR.  These requirements preserve CE indications
   when a packet that uses ECN traverses a LISP tunnel and becomes
   marked with a CE indication due to congestion between the tunnel
   endpoints.

5.4.  Dealing with Large Encapsulated Packets

   This section proposes two mechanisms to deal with packets that exceed
   the path MTU between the ITR and ETR.

   It is left to the implementor to decide if the stateless or stateful
   mechanism should be implemented.  Both or neither can be used, since
   it is a local decision in the ITR regarding how to deal with MTU
   issues, and sites can interoperate with differing mechanisms.

   Both stateless and stateful mechanisms also apply to Re-encapsulating
   and Recursive Tunneling, so any actions below referring to an ITR
   also apply to a TE-ITR.

5.4.1.  A Stateless Solution to MTU Handling

   An ITR stateless solution to handle MTU issues is described as
   follows:

   1.  Define H to be the size, in octets, of the outer header an ITR
       prepends to a packet.  This includes the UDP and LISP header
       lengths.

   2.  Define L to be the size, in octets, of the maximum-sized packet
       an ITR can send to an ETR without the need for the ITR or any
       intermediate routers to fragment the packet.

   3.  Define an architectural constant S for the maximum size of a
       packet, in octets, an ITR must receive so the effective MTU can
       be met.  That is, S = L - H.

   When an ITR receives a packet from a site-facing interface and adds H
   octets worth of encapsulation to yield a packet size greater than L
   octets, it resolves the MTU issue by first splitting the original
   packet into 2 equal-sized fragments.  A LISP header is then prepended
   to each fragment.  The size of the encapsulated fragments is then
   (S/2 + H), which is less than the ITR's estimate of the path MTU
   between the ITR and its correspondent ETR.

   When an ETR receives encapsulated fragments, it treats them as two
   individually encapsulated packets.  It strips the LISP headers and
   then forwards each fragment to the destination host of the
   destination site.  The two fragments are reassembled at the



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   destination host into the single IP datagram that was originated by
   the source host.  Note that reassembly can happen at the ETR if the
   encapsulated packet was fragmented at or after the ITR.

   This behavior is performed by the ITR when the source host originates
   a packet with the 'DF' field of the IP header set to 0.  When the
   'DF' field of the IP header is set to 1, or the packet is an IPv6
   packet originated by the source host, the ITR will drop the packet
   when the size is greater than L and send an ICMP Too Big message to
   the source with a value of S, where S is (L - H).

   When the outer-header encapsulation uses an IPv4 header, an
   implementation SHOULD set the DF bit to 1 so ETR fragment reassembly
   can be avoided.  An implementation MAY set the DF bit in such headers
   to 0 if it has good reason to believe there are unresolvable path MTU
   issues between the sending ITR and the receiving ETR.

   This specification RECOMMENDS that L be defined as 1500.

5.4.2.  A Stateful Solution to MTU Handling

   An ITR stateful solution to handle MTU issues is described as follows
   and was first introduced in [OPENLISP]:

   1.  The ITR will keep state of the effective MTU for each Locator per
       Map-Cache entry.  The effective MTU is what the core network can
       deliver along the path between the ITR and ETR.

   2.  When an IPv6-encapsulated packet, or an IPv4-encapsulated packet
       with the DF bit set to 1, exceeds what the core network can
       deliver, one of the intermediate routers on the path will send an
       ICMP Too Big message to the ITR.  The ITR will parse the ICMP
       message to determine which Locator is affected by the effective
       MTU change and then record the new effective MTU value in the
       Map-Cache entry.

   3.  When a packet is received by the ITR from a source inside of the
       site and the size of the packet is greater than the effective MTU
       stored with the Map-Cache entry associated with the destination
       EID the packet is for, the ITR will send an ICMP Too Big message
       back to the source.  The packet size advertised by the ITR in the
       ICMP Too Big message is the effective MTU minus the LISP
       encapsulation length.

   Even though this mechanism is stateful, it has advantages over the
   stateless IP fragmentation mechanism, by not involving the
   destination host with reassembly of ITR fragmented packets.




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5.5.  Using Virtualization and Segmentation with LISP

   When multiple organizations inside of a LISP site are using private
   addresses [RFC1918] as EID-Prefixes, their address spaces MUST remain
   segregated due to possible address duplication.  An Instance ID in
   the address encoding can aid in making the entire AFI-based address
   unique.  See IANA Considerations (Section 14.2) for details on
   possible address encodings.

   An Instance ID can be carried in a LISP-encapsulated packet.  An ITR
   that prepends a LISP header will copy a 24-bit value used by the LISP
   router to uniquely identify the address space.  The value is copied
   to the 'Instance ID' field of the LISP header, and the I-bit is set
   to 1.

   When an ETR decapsulates a packet, the Instance ID from the LISP
   header is used as a table identifier to locate the forwarding table
   to use for the inner destination EID lookup.

   For example, an 802.1Q VLAN tag or VPN identifier could be used as a
   24-bit Instance ID.






























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6.  EID-to-RLOC Mapping

6.1.  LISP IPv4 and IPv6 Control-Plane Packet Formats

   The following UDP packet formats are used by the LISP control plane.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Version|  IHL  |Type of Service|          Total Length         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Identification        |Flags|      Fragment Offset    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Time to Live | Protocol = 17 |         Header Checksum       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    Source Routing Locator                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Destination Routing Locator                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |           Source Port         |         Dest Port             |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                         LISP Message                          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
























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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Version| Traffic Class |           Flow Label                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Payload Length        | Next Header=17|   Hop Limit   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                     Source Routing Locator                    +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                  Destination Routing Locator                  +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |           Source Port         |         Dest Port             |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                         LISP Message                          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The LISP UDP-based messages are the Map-Request and Map-Reply
   messages.  When a UDP Map-Request is sent, the UDP source port is
   chosen by the sender and the destination UDP port number is set to
   4342.  When a UDP Map-Reply is sent, the source UDP port number is
   set to 4342 and the destination UDP port number is copied from the
   source port of either the Map-Request or the invoking data packet.
   Implementations MUST be prepared to accept packets when either the
   source port or destination UDP port is set to 4342 due to NATs
   changing port number values.

   The 'UDP Length' field will reflect the length of the UDP header and
   the LISP Message payload.







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   The UDP checksum is computed and set to non-zero for Map-Request,
   Map-Reply, Map-Register, and Encapsulated Control Message (ECM)
   control messages.  It MUST be checked on receipt, and if the checksum
   fails, the packet MUST be dropped.

   The format of control messages includes the UDP header so the
   checksum and length fields can be used to protect and delimit message
   boundaries.

6.1.1.  LISP Packet Type Allocations

   This section will be the authoritative source for allocating LISP
   Type values and for defining LISP control message formats.  Current
   allocations are:

       Reserved:                          0    b'0000'
       LISP Map-Request:                  1    b'0001'
       LISP Map-Reply:                    2    b'0010'
       LISP Map-Register:                 3    b'0011'
       LISP Map-Notify:                   4    b'0100'
       LISP Encapsulated Control Message: 8    b'1000'

6.1.2.  Map-Request Message Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Type=1 |A|M|P|S|p|s|    Reserved     |   IRC   | Record Count  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Nonce . . .                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         . . . Nonce                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Source-EID-AFI        |   Source EID Address  ...     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         ITR-RLOC-AFI 1        |    ITR-RLOC Address 1  ...    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         ITR-RLOC-AFI n        |    ITR-RLOC Address n  ...    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |   Reserved    | EID mask-len  |        EID-Prefix-AFI         |
   Rec +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |                       EID-Prefix  ...                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Map-Reply Record  ...                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Packet field descriptions:

   Type:   1 (Map-Request)

   A: This is an authoritative bit, which is set to 0 for UDP-based
      Map-Requests sent by an ITR.  It is set to 1 when an ITR wants the
      destination site to return the Map-Reply rather than the mapping
      database system.

   M: This is the map-data-present bit.  When set, it indicates that a
      Map-Reply Record segment is included in the Map-Request.

   P: This is the probe-bit, which indicates that a Map-Request SHOULD
      be treated as a Locator reachability probe.  The receiver SHOULD
      respond with a Map-Reply with the probe-bit set, indicating that
      the Map-Reply is a Locator reachability probe reply, with the
      nonce copied from the Map-Request.  See Section 6.3.2 for more
      details.

   S: This is the Solicit-Map-Request (SMR) bit.  See Section 6.6.2 for
      details.

   p: This is the PITR bit.  This bit is set to 1 when a PITR sends a
      Map-Request.

   s: This is the SMR-invoked bit.  This bit is set to 1 when an xTR is
      sending a Map-Request in response to a received SMR-based
      Map-Request.

   Reserved:  This field MUST be set to 0 on transmit and MUST be
      ignored on receipt.

   IRC:  This 5-bit field is the ITR-RLOC Count, which encodes the
      additional number of ('ITR-RLOC-AFI', 'ITR-RLOC Address') fields
      present in this message.  At least one (ITR-RLOC-AFI,
      ITR-RLOC-Address) pair MUST be encoded.  Multiple 'ITR-RLOC
      Address' fields are used, so a Map-Replier can select which
      destination address to use for a Map-Reply.  The IRC value ranges
      from 0 to 31.  For a value of 0, there is 1 ITR-RLOC address
      encoded; for a value of 1, there are 2 ITR-RLOC addresses encoded,
      and so on up to 31, which encodes a total of 32 ITR-RLOC
      addresses.

   Record Count:  This is the number of records in this Map-Request
      message.  A record is comprised of the portion of the packet that
      is labeled 'Rec' above and occurs the number of times equal to
      Record Count.  For this version of the protocol, a receiver MUST
      accept and process Map-Requests that contain one or more records,



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      but a sender MUST only send Map-Requests containing one record.
      Support for requesting multiple EIDs in a single Map-Request
      message will be specified in a future version of the protocol.

   Nonce:  This is an 8-octet random value created by the sender of the
      Map-Request.  This nonce will be returned in the Map-Reply.  The
      security of the LISP mapping protocol critically depends on the
      strength of the nonce in the Map-Request message.  The nonce
      SHOULD be generated by a properly seeded pseudo-random (or strong
      random) source.  See [RFC4086] for advice on generating security-
      sensitive random data.

   Source-EID-AFI:  This is the address family of the 'Source EID
      Address' field.

   Source EID Address:  This is the EID of the source host that
      originated the packet that caused the Map-Request.  When
      Map-Requests are used for refreshing a Map-Cache entry or for
      RLOC-Probing, an AFI value 0 is used and this field is of zero
      length.

   ITR-RLOC-AFI:  This is the address family of the 'ITR-RLOC Address'
      field that follows this field.

   ITR-RLOC Address:  This is used to give the ETR the option of
      selecting the destination address from any address family for the
      Map-Reply message.  This address MUST be a routable RLOC address
      of the sender of the Map-Request message.

   EID mask-len:  This is the mask length for the EID-Prefix.

   EID-Prefix-AFI:  This is the address family of the EID-Prefix
      according to [AFI].

   EID-Prefix:  This prefix is 4 octets for an IPv4 address family and
      16 octets for an IPv6 address family.  When a Map-Request is sent
      by an ITR because a data packet is received for a destination
      where there is no mapping entry, the EID-Prefix is set to the
      destination IP address of the data packet, and the 'EID mask-len'
      is set to 32 or 128 for IPv4 or IPv6, respectively.  When an xTR
      wants to query a site about the status of a mapping it already has
      cached, the EID-Prefix used in the Map-Request has the same mask
      length as the EID-Prefix returned from the site when it sent a
      Map-Reply message.







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   Map-Reply Record:  When the M-bit is set, this field is the size of a
      single "Record" in the Map-Reply format.  This Map-Reply record
      contains the EID-to-RLOC mapping entry associated with the Source
      EID.  This allows the ETR that will receive this Map-Request to
      cache the data if it chooses to do so.

6.1.3.  EID-to-RLOC UDP Map-Request Message

   A Map-Request is sent from an ITR when it needs a mapping for an EID,
   wants to test an RLOC for reachability, or wants to refresh a mapping
   before TTL expiration.  For the initial case, the destination IP
   address used for the Map-Request is the data packet's destination
   address (i.e., the destination EID) that had a mapping cache lookup
   failure.  For the latter two cases, the destination IP address used
   for the Map-Request is one of the RLOC addresses from the Locator-Set
   of the Map-Cache entry.  The source address is either an IPv4 or IPv6
   RLOC address, depending on whether the Map-Request is using an IPv4
   or IPv6 header, respectively.  In all cases, the UDP source port
   number for the Map-Request message is a 16-bit value selected by the
   ITR/PITR, and the UDP destination port number is set to the well-
   known destination port number 4342.  A successful Map-Reply, which is
   one that has a nonce that matches an outstanding Map-Request nonce,
   will update the cached set of RLOCs associated with the EID-Prefix
   range.

   One or more Map-Request ('ITR-RLOC-AFI', 'ITR-RLOC-Address') fields
   MUST be filled in by the ITR.  The number of fields (minus 1) encoded
   MUST be placed in the 'IRC' field.  The ITR MAY include all locally
   configured Locators in this list or just provide one locator address
   from each address family it supports.  If the ITR erroneously
   provides no ITR-RLOC addresses, the Map-Replier MUST drop the
   Map-Request.

   Map-Requests can also be LISP encapsulated using UDP destination
   port 4342 with a LISP Type value set to "Encapsulated Control
   Message", when sent from an ITR to a Map-Resolver.  Likewise,
   Map-Requests are LISP encapsulated the same way from a Map-Server to
   an ETR.  Details on Encapsulated Map-Requests and Map-Resolvers can
   be found in [RFC6833].

   Map-Requests MUST be rate-limited.  It is RECOMMENDED that a
   Map-Request for the same EID-Prefix be sent no more than once per
   second.

   An ITR that is configured with mapping database information (i.e., it
   is also an ETR) MAY optionally include those mappings in a
   Map-Request.  When an ETR configured to accept and verify such
   "piggybacked" mapping data receives such a Map-Request and it does



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   not have this mapping in the map-cache, it MAY originate a "verifying
   Map-Request", addressed to the map-requesting ITR and the ETR MAY add
   a Map-Cache entry.  If the ETR has a Map-Cache entry that matches the
   "piggybacked" EID and the RLOC is in the Locator-Set for the entry,
   then it may send the "verifying Map-Request" directly to the
   originating Map-Request source.  If the RLOC is not in the
   Locator-Set, then the ETR MUST send the "verifying Map-Request" to
   the "piggybacked" EID.  Doing this forces the "verifying Map-Request"
   to go through the mapping database system to reach the authoritative
   source of information about that EID, guarding against RLOC-spoofing
   in the "piggybacked" mapping data.

6.1.4.  Map-Reply Message Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Type=2 |P|E|S|          Reserved               | Record Count  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Nonce . . .                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         . . . Nonce                           |
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |                          Record TTL                           |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   R   | Locator Count | EID mask-len  | ACT |A|      Reserved         |
   e   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   c   | Rsvd  |  Map-Version Number   |       EID-Prefix-AFI          |
   o   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   r   |                          EID-Prefix                           |
   d   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  /|    Priority   |    Weight     |  M Priority   |   M Weight    |
   | L +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | o |        Unused Flags     |L|p|R|           Loc-AFI             |
   | c +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  \|                             Locator                           |
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+














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   Packet field descriptions:

   Type:   2 (Map-Reply)

   P: This is the probe-bit, which indicates that the Map-Reply is in
      response to a Locator reachability probe Map-Request.  The 'Nonce'
      field MUST contain a copy of the nonce value from the original
      Map-Request.  See Section 6.3.2 for more details.

   E: This bit indicates that the ETR that sends this Map-Reply message
      is advertising that the site is enabled for the Echo-Nonce Locator
      reachability algorithm.  See Section 6.3.1 for more details.

   S: This is the Security bit.  When set to 1, the following
      authentication information will be appended to the end of the
      Map-Reply.  The detailed format of the Authentication Data Content
      is for further study.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    AD Type    |       Authentication Data Content . . .       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Reserved:  This field MUST be set to 0 on transmit and MUST be
      ignored on receipt.

   Record Count:  This is the number of records in this reply message.
      A record is comprised of that portion of the packet labeled
      'Record' above and occurs the number of times equal to Record
      Count.

   Nonce:  This is a 24-bit value set in a Data-Probe packet, or a
      64-bit value from the Map-Request is echoed in this 'Nonce' field
      of the Map-Reply.  When a 24-bit value is supplied, it resides in
      the low-order 64 bits of the 'Nonce' field.

   Record TTL:  This is the time in minutes the recipient of the
      Map-Reply will store the mapping.  If the TTL is 0, the entry
      SHOULD be removed from the cache immediately.  If the value is
      0xffffffff, the recipient can decide locally how long to store the
      mapping.

   Locator Count:  This is the number of Locator entries.  A Locator
      entry comprises what is labeled above as 'Loc'.  The Locator count
      can be 0, indicating that there are no Locators for the
      EID-Prefix.




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   EID mask-len:  This is the mask length for the EID-Prefix.

   ACT:  This 3-bit field describes Negative Map-Reply actions.  In any
      other message type, these bits are set to 0 and ignored on
      receipt.  These bits are used only when the 'Locator Count' field
      is set to 0.  The action bits are encoded only in Map-Reply
      messages.  The actions defined are used by an ITR or PITR when a
      destination EID matches a negative Map-Cache entry.  Unassigned
      values should cause a Map-Cache entry to be created, and when
      packets match this negative cache entry, they will be dropped.
      The current assigned values are:

      (0) No-Action:  The map-cache is kept alive, and no packet
                      encapsulation occurs.

      (1) Natively-Forward:  The packet is not encapsulated or dropped
                             but natively forwarded.

      (2) Send-Map-Request:  The packet invokes sending a Map-Request.

      (3) Drop:  A packet that matches this map-cache entry is dropped.
                 An ICMP Destination Unreachable message SHOULD be sent.

   A: The Authoritative bit, when sent, is always set to 1 by an ETR.
      When a Map-Server is proxy Map-Replying [RFC6833] for a LISP site,
      the Authoritative bit is set to 0.  This indicates to requesting
      ITRs that the Map-Reply was not originated by a LISP node managed
      at the site that owns the EID-Prefix.

   Map-Version Number:  When this 12-bit value is non-zero, the
      Map-Reply sender is informing the ITR what the version number is
      for the EID record contained in the Map-Reply.  The ETR can
      allocate this number internally but MUST coordinate this value
      with other ETRs for the site.  When this value is 0, there is no
      versioning information conveyed.  The Map-Version Number can be
      included in Map-Request and Map-Register messages.  See
      Section 6.6.3 for more details.

   EID-Prefix-AFI:  Address family of the EID-Prefix according to [AFI].

   EID-Prefix:  This prefix is 4 octets for an IPv4 address family and
      16 octets for an IPv6 address family.

   Priority:  Each RLOC is assigned a unicast Priority.  Lower values
      are more preferable.  When multiple RLOCs have the same Priority,
      they MAY be used in a load-split fashion.  A value of 255 means
      the RLOC MUST NOT be used for unicast forwarding.




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   Weight:  When priorities are the same for multiple RLOCs, the Weight
      indicates how to balance unicast traffic between them.  Weight is
      encoded as a relative weight of total unicast packets that match
      the mapping entry.  For example, if there are 4 Locators in a
      Locator-Set, where the Weights assigned are 30, 20, 20, and 10,
      the first Locator will get 37.5% of the traffic, the 2nd and 3rd
      Locators will get 25% of the traffic, and the 4th Locator will get
      12.5% of the traffic.  If all Weights for a Locator-Set are equal,
      the receiver of the Map-Reply will decide how to load-split the
      traffic.  See Section 6.5 for a suggested hash algorithm to
      distribute the load across Locators with the same Priority and
      equal Weight values.

   M Priority:  Each RLOC is assigned a multicast Priority used by an
      ETR in a receiver multicast site to select an ITR in a source
      multicast site for building multicast distribution trees.  A value
      of 255 means the RLOC MUST NOT be used for joining a multicast
      distribution tree.  For more details, see [RFC6831].

   M Weight:  When priorities are the same for multiple RLOCs, the
      Weight indicates how to balance building multicast distribution
      trees across multiple ITRs.  The Weight is encoded as a relative
      weight (similar to the unicast Weights) of the total number of
      trees built to the source site identified by the EID-Prefix.  If
      all Weights for a Locator-Set are equal, the receiver of the
      Map-Reply will decide how to distribute multicast state across
      ITRs.  For more details, see [RFC6831].

   Unused Flags:  These are set to 0 when sending and ignored on
      receipt.

   L: When this bit is set, the Locator is flagged as a local Locator to
      the ETR that is sending the Map-Reply.  When a Map-Server is doing
      proxy Map-Replying [RFC6833] for a LISP site, the L-bit is set to
      0 for all Locators in this Locator-Set.

   p: When this bit is set, an ETR informs the RLOC-Probing ITR that the
      locator address for which this bit is set is the one being
      RLOC-probed and MAY be different from the source address of the
      Map-Reply.  An ITR that RLOC-probes a particular Locator MUST use
      this Locator for retrieving the data structure used to store the
      fact that the Locator is reachable.  The p-bit is set for a single
      Locator in the same Locator-Set.  If an implementation sets more
      than one p-bit erroneously, the receiver of the Map-Reply MUST
      select the first Locator.  The p-bit MUST NOT be set for
      Locator-Set records sent in Map-Request and Map-Register messages.





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   R: This is set when the sender of a Map-Reply has a route to the
      Locator in the Locator data record.  This receiver may find this
      useful to know if the Locator is up but not necessarily reachable
      from the receiver's point of view.  See also Section 6.4 for
      another way the R-bit may be used.

   Locator:  This is an IPv4 or IPv6 address (as encoded by the
      'Loc-AFI' field) assigned to an ETR.  Note that the destination
      RLOC address MAY be an anycast address.  A source RLOC can be an
      anycast address as well.  The source or destination RLOC MUST NOT
      be the broadcast address (255.255.255.255 or any subnet broadcast
      address known to the router) and MUST NOT be a link-local
      multicast address.  The source RLOC MUST NOT be a multicast
      address.  The destination RLOC SHOULD be a multicast address if it
      is being mapped from a multicast destination EID.

6.1.5.  EID-to-RLOC UDP Map-Reply Message

   A Map-Reply returns an EID-Prefix with a prefix length that is less
   than or equal to the EID being requested.  The EID being requested is
   either from the destination field of an IP header of a Data-Probe or
   the EID record of a Map-Request.  The RLOCs in the Map-Reply are
   globally routable IP addresses of all ETRs for the LISP site.  Each
   RLOC conveys status reachability but does not convey path
   reachability from a requester's perspective.  Separate testing of
   path reachability is required.  See Section 6.3 for details.

   Note that a Map-Reply may contain different EID-Prefix granularity
   (prefix + length) than the Map-Request that triggers it.  This might
   occur if a Map-Request were for a prefix that had been returned by an
   earlier Map-Reply.  In such a case, the requester updates its cache
   with the new prefix information and granularity.  For example, a
   requester with two cached EID-Prefixes that are covered by a
   Map-Reply containing one less-specific prefix replaces the entry with
   the less-specific EID-Prefix.  Note that the reverse, replacement of
   one less-specific prefix with multiple more-specific prefixes, can
   also occur, not by removing the less-specific prefix but rather by
   adding the more-specific prefixes that, during a lookup, will
   override the less-specific prefix.












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   When an ETR is configured with overlapping EID-Prefixes, a
   Map-Request with an EID that best matches any EID-Prefix MUST be
   returned in a single Map-Reply message.  For instance, if an ETR had
   database mapping entries for EID-Prefixes:

     10.0.0.0/8
     10.1.0.0/16
     10.1.1.0/24
     10.1.2.0/24

   A Map-Request for EID 10.1.1.1 would cause a Map-Reply with a record
   count of 1 to be returned with a mapping record EID-Prefix of
   10.1.1.0/24.

   A Map-Request for EID 10.1.5.5 would cause a Map-Reply with a record
   count of 3 to be returned with mapping records for EID-Prefixes
   10.1.0.0/16, 10.1.1.0/24, and 10.1.2.0/24.

   Note that not all overlapping EID-Prefixes need to be returned but
   only the more-specific entries (note that in the second example above
   10.0.0.0/8 was not returned for requesting EID 10.1.5.5) for the
   matching EID-Prefix of the requesting EID.  When more than one
   EID-Prefix is returned, all SHOULD use the same Time to Live value so
   they can all time out at the same time.  When a more-specific
   EID-Prefix is received later, its Time to Live value in the Map-Reply
   record can be stored even when other less-specific entries exist.
   When a less-specific EID-Prefix is received later, its map-cache
   expiration time SHOULD be set to the minimum expiration time of any
   more-specific EID-Prefix in the map-cache.  This is done so the
   integrity of the EID-Prefix set is wholly maintained and so no more-
   specific entries are removed from the map-cache while keeping less-
   specific entries.

   Map-Replies SHOULD be sent for an EID-Prefix no more often than once
   per second to the same requesting router.  For scalability, it is
   expected that aggregation of EID addresses into EID-Prefixes will
   allow one Map-Reply to satisfy a mapping for the EID addresses in the
   prefix range, thereby reducing the number of Map-Request messages.

   Map-Reply records can have an empty Locator-Set.  A Negative
   Map-Reply is a Map-Reply with an empty Locator-Set.  Negative
   Map-Replies convey special actions by the sender to the ITR or PITR
   that have solicited the Map-Reply.  There are two primary
   applications for Negative Map-Replies.  The first is for a
   Map-Resolver to instruct an ITR or PITR when a destination is for a
   LISP site versus a non-LISP site, and the other is to source quench
   Map-Requests that are sent for non-allocated EIDs.




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   For each Map-Reply record, the list of Locators in a Locator-Set MUST
   appear in the same order for each ETR that originates a Map-Reply
   message.  The Locator-Set MUST be sorted in order of ascending IP
   address where an IPv4 locator address is considered numerically 'less
   than' an IPv6 locator address.

   When sending a Map-Reply message, the destination address is copied
   from one of the 'ITR-RLOC' fields from the Map-Request.  The ETR can
   choose a locator address from one of the address families it
   supports.  For Data-Probes, the destination address of the Map-Reply
   is copied from the source address of the Data-Probe message that is
   invoking the reply.  The source address of the Map-Reply is one of
   the local IP addresses chosen to allow Unicast Reverse Path
   Forwarding (uRPF) checks to succeed in the upstream service provider.
   The destination port of a Map-Reply message is copied from the source
   port of the Map-Request or Data-Probe, and the source port of the
   Map-Reply message is set to the well-known UDP port 4342.

6.1.5.1.  Traffic Redirection with Coarse EID-Prefixes

   When an ETR is misconfigured or compromised, it could return coarse
   EID-Prefixes in Map-Reply messages it sends.  The EID-Prefix could
   cover EID-Prefixes that are allocated to other sites, redirecting
   their traffic to the Locators of the compromised site.

   To solve this problem, there are two basic solutions that could be
   used.  The first is to have Map-Servers proxy Map-Reply on behalf of
   ETRs so their registered EID-Prefixes are the ones returned in
   Map-Replies.  Since the interaction between an ETR and Map-Server is
   secured with shared keys, it is easier for an ETR to detect
   misbehavior.  The second solution is to have ITRs and PITRs cache
   EID-Prefixes with mask lengths that are greater than or equal to a
   configured prefix length.  This limits the damage to a specific width
   of any EID-Prefix advertised but needs to be coordinated with the
   allocation of site prefixes.  These solutions can be used
   independently or at the same time.

   At the time of this writing, other approaches are being considered
   and researched.

6.1.6.  Map-Register Message Format

   The usage details of the Map-Register message can be found in
   specification [RFC6833].  This section solely defines the message
   format.

   The message is sent in UDP with a destination UDP port of 4342 and a
   randomly selected UDP source port number.



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   The Map-Register message format is:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Type=3 |P|            Reserved               |M| Record Count  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Nonce . . .                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         . . . Nonce                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Key ID             |  Authentication Data Length   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Authentication Data                       ~
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |                          Record TTL                           |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   R   | Locator Count | EID mask-len  | ACT |A|      Reserved         |
   e   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   c   | Rsvd  |  Map-Version Number   |        EID-Prefix-AFI         |
   o   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   r   |                          EID-Prefix                           |
   d   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  /|    Priority   |    Weight     |  M Priority   |   M Weight    |
   | L +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | o |        Unused Flags     |L|p|R|           Loc-AFI             |
   | c +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  \|                             Locator                           |
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Packet field descriptions:

   Type:   3 (Map-Register)

   P: This is the proxy Map-Reply bit.  When set to 1, an ETR sends a
      Map-Register message requesting the Map-Server to proxy a
      Map-Reply.  The Map-Server will send non-authoritative Map-Replies
      on behalf of the ETR.  Details on this usage can be found in
      [RFC6833].

   Reserved:  This field MUST be set to 0 on transmit and MUST be
      ignored on receipt.

   M: This is the want-map-notify bit.  When set to 1, an ETR is
      requesting a Map-Notify message to be returned in response to
      sending a Map-Register message.  The Map-Notify message sent by a
      Map-Server is used to acknowledge receipt of a Map-Register
      message.



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   Record Count:  This is the number of records in this Map-Register
      message.  A record is comprised of that portion of the packet
      labeled 'Record' above and occurs the number of times equal to
      Record Count.

   Nonce:  This 8-octet 'Nonce' field is set to 0 in Map-Register
      messages.  Since the Map-Register message is authenticated, the
      'Nonce' field is not currently used for any security function but
      may be in the future as part of an anti-replay solution.

   Key ID:  This is a configured ID to find the configured Message
      Authentication Code (MAC) algorithm and key value used for the
      authentication function.  See Section 14.4 for codepoint
      assignments.

   Authentication Data Length:  This is the length in octets of the
      'Authentication Data' field that follows this field.  The length
      of the 'Authentication Data' field is dependent on the MAC
      algorithm used.  The length field allows a device that doesn't
      know the MAC algorithm to correctly parse the packet.

   Authentication Data:  This is the message digest used from the output
      of the MAC algorithm.  The entire Map-Register payload is
      authenticated with this field preset to 0.  After the MAC is
      computed, it is placed in this field.  Implementations of this
      specification MUST include support for HMAC-SHA-1-96 [RFC2404],
      and support for HMAC-SHA-256-128 [RFC4868] is RECOMMENDED.

   The definition of the rest of the Map-Register can be found in
   Section 6.1.4.

6.1.7.  Map-Notify Message Format

   The usage details of the Map-Notify message can be found in
   specification [RFC6833].  This section solely defines the message
   format.

   The message is sent inside a UDP packet with source and destination
   UDP ports equal to 4342.












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   The Map-Notify message format is:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Type=4 |              Reserved                 | Record Count  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Nonce . . .                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         . . . Nonce                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            Key ID             |  Authentication Data Length   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                     Authentication Data                       ~
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |                          Record TTL                           |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   R   | Locator Count | EID mask-len  | ACT |A|      Reserved         |
   e   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   c   | Rsvd  |  Map-Version Number   |         EID-Prefix-AFI        |
   o   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   r   |                          EID-Prefix                           |
   d   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  /|    Priority   |    Weight     |  M Priority   |   M Weight    |
   | L +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | o |        Unused Flags     |L|p|R|           Loc-AFI             |
   | c +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  \|                             Locator                           |
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Packet field descriptions:

   Type:   4 (Map-Notify)

   The Map-Notify message has the same contents as a Map-Register
   message.  See the Map-Register section for field descriptions.















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6.1.8.  Encapsulated Control Message Format

   An Encapsulated Control Message (ECM) is used to encapsulate control
   packets sent between xTRs and the mapping database system described
   in [RFC6833].

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |                       IPv4 or IPv6 Header                     |
   OH  |                      (uses RLOC addresses)                    |
     \ |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |       Source Port = xxxx      |       Dest Port = 4342        |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   LH  |Type=8 |S|                  Reserved                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |                       IPv4 or IPv6 Header                     |
   IH  |                  (uses RLOC or EID addresses)                 |
     \ |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |       Source Port = xxxx      |       Dest Port = yyyy        |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   LCM |                      LISP Control Message                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Packet header descriptions:

   OH:   The outer IPv4 or IPv6 header, which uses RLOC addresses in the
         source and destination header address fields.

   UDP:  The outer UDP header with destination port 4342.  The source
         port is randomly allocated.  The checksum field MUST be
         non-zero.

   LH:   Type 8 is defined to be a "LISP Encapsulated Control Message",
         and what follows is either an IPv4 or IPv6 header as encoded by
         the first 4 bits after the 'Reserved' field.

   S:    This is the Security bit.  When set to 1, the field following
         the 'Reserved' field will have the following format.  The
         detailed format of the Authentication Data Content is for
         further study.




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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    AD Type    |       Authentication Data Content . . .       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IH:   The inner IPv4 or IPv6 header, which can use either RLOC or EID
         addresses in the header address fields.  When a Map-Request is
         encapsulated in this packet format, the destination address in
         this header is an EID.

   UDP:  The inner UDP header, where the port assignments depend on the
         control packet being encapsulated.  When the control packet is
         a Map-Request or Map-Register, the source port is selected by
         the ITR/PITR and the destination port is 4342.  When the
         control packet is a Map-Reply, the source port is 4342 and the
         destination port is assigned from the source port of the
         invoking Map-Request.  Port number 4341 MUST NOT be assigned to
         either port.  The checksum field MUST be non-zero.

   LCM:  The format is one of the control message formats described in
         this section.  At this time, only Map-Request messages are
         allowed to be encapsulated.  In the future, PIM Join/Prune
         messages [RFC6831] might be allowed.  Encapsulating other types
         of LISP control messages is for further study.  When
         Map-Requests are sent for RLOC-Probing purposes (i.e., the
         probe-bit is set), they MUST NOT be sent inside Encapsulated
         Control Messages.

6.2.  Routing Locator Selection

   Both the client-side and server-side may need control over the
   selection of RLOCs for conversations between them.  This control is
   achieved by manipulating the 'Priority' and 'Weight' fields in
   EID-to-RLOC Map-Reply messages.  Alternatively, RLOC information MAY
   be gleaned from received tunneled packets or EID-to-RLOC Map-Request
   messages.

   The following are different scenarios for choosing RLOCs and the
   controls that are available:

   o  The server-side returns one RLOC.  The client-side can only use
      one RLOC.  The server-side has complete control of the selection.

   o  The server-side returns a list of RLOCs where a subset of the list
      has the same best Priority.  The client can only use the subset
      list according to the weighting assigned by the server-side.  In
      this case, the server-side controls both the subset list and



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      load-splitting across its members.  The client-side can use RLOCs
      outside of the subset list if it determines that the subset list
      is unreachable (unless RLOCs are set to a Priority of 255).  Some
      sharing of control exists: the server-side determines the
      destination RLOC list and load distribution while the client-side
      has the option of using alternatives to this list if RLOCs in the
      list are unreachable.

   o  The server-side sets a Weight of 0 for the RLOC subset list.  In
      this case, the client-side can choose how the traffic load is
      spread across the subset list.  Control is shared by the server-
      side determining the list and the client determining load
      distribution.  Again, the client can use alternative RLOCs if the
      server-provided list of RLOCs is unreachable.

   o  Either side (more likely the server-side ETR) decides not to send
      a Map-Request.  For example, if the server-side ETR does not send
      Map-Requests, it gleans RLOCs from the client-side ITR, giving the
      client-side ITR responsibility for bidirectional RLOC reachability
      and preferability.  Server-side ETR gleaning of the client-side
      ITR RLOC is done by caching the inner-header source EID and the
      outer-header source RLOC of received packets.  The client-side ITR
      controls how traffic is returned and can alternate using an outer-
      header source RLOC, which then can be added to the list the
      server-side ETR uses to return traffic.  Since no Priority or
      Weights are provided using this method, the server-side ETR MUST
      assume that each client-side ITR RLOC uses the same best Priority
      with a Weight of zero.  In addition, since EID-Prefix encoding
      cannot be conveyed in data packets, the EID-to-RLOC Cache on
      Tunnel Routers can grow to be very large.

   o  A "gleaned" Map-Cache entry, one learned from the source RLOC of a
      received encapsulated packet, is only stored and used for a few
      seconds, pending verification.  Verification is performed by
      sending a Map-Request to the source EID (the inner-header IP
      source address) of the received encapsulated packet.  A reply to
      this "verifying Map-Request" is used to fully populate the
      Map-Cache entry for the "gleaned" EID and is stored and used for
      the time indicated from the 'TTL' field of a received Map-Reply.
      When a verified Map-Cache entry is stored, data gleaning no longer
      occurs for subsequent packets that have a source EID that matches
      the EID-Prefix of the verified entry.

   RLOCs that appear in EID-to-RLOC Map-Reply messages are assumed to be
   reachable when the R-bit for the Locator record is set to 1.  When
   the R-bit is set to 0, an ITR or PITR MUST NOT encapsulate to the
   RLOC.  Neither the information contained in a Map-Reply nor that
   stored in the mapping database system provides reachability



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   information for RLOCs.  Note that reachability is not part of the
   mapping system and is determined using one or more of the Routing
   Locator reachability algorithms described in the next section.

6.3.  Routing Locator Reachability

   Several mechanisms for determining RLOC reachability are currently
   defined:

   1.  An ETR may examine the Locator-Status-Bits in the LISP header of
       an encapsulated data packet received from an ITR.  If the ETR is
       also acting as an ITR and has traffic to return to the original
       ITR site, it can use this status information to help select an
       RLOC.

   2.  An ITR may receive an ICMP Network Unreachable or Host
       Unreachable message for an RLOC it is using.  This indicates that
       the RLOC is likely down.  Note that trusting ICMP messages may
       not be desirable, but neither is ignoring them completely.
       Implementations are encouraged to follow current best practices
       in treating these conditions.

   3.  An ITR that participates in the global routing system can
       determine that an RLOC is down if no BGP Routing Information Base
       (RIB) route exists that matches the RLOC IP address.

   4.  An ITR may receive an ICMP Port Unreachable message from a
       destination host.  This occurs if an ITR attempts to use
       interworking [RFC6832] and LISP-encapsulated data is sent to a
       non-LISP-capable site.

   5.  An ITR may receive a Map-Reply from an ETR in response to a
       previously sent Map-Request.  The RLOC source of the Map-Reply is
       likely up, since the ETR was able to send the Map-Reply to the
       ITR.

   6.  When an ETR receives an encapsulated packet from an ITR, the
       source RLOC from the outer header of the packet is likely up.

   7.  An ITR/ETR pair can use the Locator reachability algorithms
       described in this section, namely Echo-Noncing or RLOC-Probing.










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   When determining Locator up/down reachability by examining the
   Locator-Status-Bits from the LISP-encapsulated data packet, an ETR
   will receive up-to-date status from an encapsulating ITR about
   reachability for all ETRs at the site.  CE-based ITRs at the source
   site can determine reachability relative to each other using the site
   IGP as follows:

   o  Under normal circumstances, each ITR will advertise a default
      route into the site IGP.

   o  If an ITR fails or if the upstream link to its PE fails, its
      default route will either time out or be withdrawn.

   Each ITR can thus observe the presence or lack of a default route
   originated by the others to determine the Locator-Status-Bits it sets
   for them.

   RLOCs listed in a Map-Reply are numbered with ordinals 0 to n-1.  The
   Locator-Status-Bits in a LISP-encapsulated packet are numbered from 0
   to n-1 starting with the least significant bit.  For example, if an
   RLOC listed in the 3rd position of the Map-Reply goes down (ordinal
   value 2), then all ITRs at the site will clear the 3rd least
   significant bit (xxxx x0xx) of the 'Locator-Status-Bits' field for
   the packets they encapsulate.

   When an ETR decapsulates a packet, it will check for any change in
   the 'Locator-Status-Bits' field.  When a bit goes from 1 to 0, the
   ETR, if acting also as an ITR, will refrain from encapsulating
   packets to an RLOC that is indicated as down.  It will only resume
   using that RLOC if the corresponding Locator-Status-Bit returns to a
   value of 1.  Locator-Status-Bits are associated with a Locator-Set
   per EID-Prefix.  Therefore, when a Locator becomes unreachable, the
   Locator-Status-Bit that corresponds to that Locator's position in the
   list returned by the last Map-Reply will be set to zero for that
   particular EID-Prefix.

   When ITRs at the site are not deployed in CE routers, the IGP can
   still be used to determine the reachability of Locators, provided
   they are injected into the IGP.  This is typically done when a /32
   address is configured on a loopback interface.

   When ITRs receive ICMP Network Unreachable or Host Unreachable
   messages as a method to determine unreachability, they will refrain
   from using Locators that are described in Locator lists of
   Map-Replies.  However, using this approach is unreliable because many
   network operators turn off generation of ICMP Destination Unreachable
   messages.




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   If an ITR does receive an ICMP Network Unreachable or Host
   Unreachable message, it MAY originate its own ICMP Destination
   Unreachable message destined for the host that originated the data
   packet the ITR encapsulated.

   Also, BGP-enabled ITRs can unilaterally examine the RIB to see if a
   locator address from a Locator-Set in a mapping entry matches a
   prefix.  If it does not find one and BGP is running in the Default-
   Free Zone (DFZ), it can decide to not use the Locator even though the
   Locator-Status-Bits indicate that the Locator is up.  In this case,
   the path from the ITR to the ETR that is assigned the Locator is not
   available.  More details are in [LOC-ID-ARCH].

   Optionally, an ITR can send a Map-Request to a Locator, and if a
   Map-Reply is returned, reachability of the Locator has been
   determined.  Obviously, sending such probes increases the number of
   control messages originated by Tunnel Routers for active flows, so
   Locators are assumed to be reachable when they are advertised.

   This assumption does create a dependency: Locator unreachability is
   detected by the receipt of ICMP Host Unreachable messages.  When a
   Locator has been determined to be unreachable, it is not used for
   active traffic; this is the same as if it were listed in a Map-Reply
   with Priority 255.

   The ITR can test the reachability of the unreachable Locator by
   sending periodic Requests.  Both Requests and Replies MUST be rate-
   limited.  Locator reachability testing is never done with data
   packets, since that increases the risk of packet loss for end-to-end
   sessions.

   When an ETR decapsulates a packet, it knows that it is reachable from
   the encapsulating ITR because that is how the packet arrived.  In
   most cases, the ETR can also reach the ITR but cannot assume this to
   be true, due to the possibility of path asymmetry.  In the presence
   of unidirectional traffic flow from an ITR to an ETR, the ITR SHOULD
   NOT use the lack of return traffic as an indication that the ETR is
   unreachable.  Instead, it MUST use an alternate mechanism to
   determine reachability.

6.3.1.  Echo Nonce Algorithm

   When data flows bidirectionally between Locators from different
   sites, a data-plane mechanism called "nonce echoing" can be used to
   determine reachability between an ITR and ETR.  When an ITR wants to
   solicit a nonce echo, it sets the N- and E-bits and places a 24-bit
   nonce [RFC4086] in the LISP header of the next encapsulated data
   packet.



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   When this packet is received by the ETR, the encapsulated packet is
   forwarded as normal.  When the ETR next sends a data packet to the
   ITR, it includes the nonce received earlier with the N-bit set and
   E-bit cleared.  The ITR sees this "echoed nonce" and knows that the
   path to and from the ETR is up.

   The ITR will set the E-bit and N-bit for every packet it sends while
   in the echo-nonce-request state.  The time the ITR waits to process
   the echoed nonce before it determines the path is unreachable is
   variable and is a choice left for the implementation.

   If the ITR is receiving packets from the ETR but does not see the
   nonce echoed while being in the echo-nonce-request state, then the
   path to the ETR is unreachable.  This decision may be overridden by
   other Locator reachability algorithms.  Once the ITR determines that
   the path to the ETR is down, it can switch to another Locator for
   that EID-Prefix.

   Note that "ITR" and "ETR" are relative terms here.  Both devices MUST
   be implementing both ITR and ETR functionality for the echo nonce
   mechanism to operate.

   The ITR and ETR may both go into the echo-nonce-request state at the
   same time.  The number of packets sent or the time during which echo
   nonce requests are sent is an implementation-specific setting.
   However, when an ITR is in the echo-nonce-request state, it can echo
   the ETR's nonce in the next set of packets that it encapsulates and
   subsequently continue sending echo-nonce-request packets.

   This mechanism does not completely solve the forward path
   reachability problem, as traffic may be unidirectional.  That is, the
   ETR receiving traffic at a site may not be the same device as an ITR
   that transmits traffic from that site, or the site-to-site traffic is
   unidirectional so there is no ITR returning traffic.

   The echo-nonce algorithm is bilateral.  That is, if one side sets the
   E-bit and the other side is not enabled for echo-noncing, then the
   echoing of the nonce does not occur and the requesting side may
   erroneously consider the Locator unreachable.  An ITR SHOULD only set
   the E-bit in an encapsulated data packet when it knows the ETR is
   enabled for echo-noncing.  This is conveyed by the E-bit in the
   Map-Reply message.

   Note that other Locator reachability mechanisms are being researched
   and can be used to compliment or even override the echo nonce
   algorithm.  See the next section for an example of control-plane
   probing.




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6.3.2.  RLOC-Probing Algorithm

   RLOC-Probing is a method that an ITR or PITR can use to determine the
   reachability status of one or more Locators that it has cached in a
   Map-Cache entry.  The probe-bit of the Map-Request and Map-Reply
   messages is used for RLOC-Probing.

   RLOC-Probing is done in the control plane on a timer basis, where an
   ITR or PITR will originate a Map-Request destined to a locator
   address from one of its own locator addresses.  A Map-Request used as
   an RLOC-probe is NOT encapsulated and NOT sent to a Map-Server or to
   the mapping database system as one would when soliciting mapping
   data.  The EID record encoded in the Map-Request is the EID-Prefix of
   the Map-Cache entry cached by the ITR or PITR.  The ITR may include a
   mapping data record for its own database mapping information that
   contains the local EID-Prefixes and RLOCs for its site.  RLOC-probes
   are sent periodically using a jittered timer interval.

   When an ETR receives a Map-Request message with the probe-bit set, it
   returns a Map-Reply with the probe-bit set.  The source address of
   the Map-Reply is set according to the procedure described in
   Section 6.1.5.  The Map-Reply SHOULD contain mapping data for the
   EID-Prefix contained in the Map-Request.  This provides the
   opportunity for the ITR or PITR that sent the RLOC-probe to get
   mapping updates if there were changes to the ETR's database mapping
   entries.

   There are advantages and disadvantages of RLOC-Probing.  The greatest
   benefit of RLOC-Probing is that it can handle many failure scenarios
   allowing the ITR to determine when the path to a specific Locator is
   reachable or has become unreachable, thus providing a robust
   mechanism for switching to using another Locator from the cached
   Locator.  RLOC-Probing can also provide rough Round-Trip Time (RTT)
   estimates between a pair of Locators, which can be useful for network
   management purposes as well as for selecting low delay paths.  The
   major disadvantage of RLOC-Probing is in the number of control
   messages required and the amount of bandwidth used to obtain those
   benefits, especially if the requirement for failure detection times
   is very small.

   Continued research and testing will attempt to characterize the
   tradeoffs of failure detection times versus message overhead.









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6.4.  EID Reachability within a LISP Site

   A site may be multihomed using two or more ETRs.  The hosts and
   infrastructure within a site will be addressed using one or more
   EID-Prefixes that are mapped to the RLOCs of the relevant ETRs in the
   mapping system.  One possible failure mode is for an ETR to lose
   reachability to one or more of the EID-Prefixes within its own site.
   When this occurs when the ETR sends Map-Replies, it can clear the
   R-bit associated with its own Locator.  And when the ETR is also an
   ITR, it can clear its Locator-Status-Bit in the encapsulation data
   header.

   It is recognized that there are no simple solutions to the site
   partitioning problem because it is hard to know which part of the
   EID-Prefix range is partitioned and which Locators can reach any
   sub-ranges of the EID-Prefixes.  This problem is under investigation
   with the expectation that experiments will tell us more.  Note that
   this is not a new problem introduced by the LISP architecture.  The
   problem exists today when a multihomed site uses BGP to advertise its
   reachability upstream.

6.5.  Routing Locator Hashing

   When an ETR provides an EID-to-RLOC mapping in a Map-Reply message to
   a requesting ITR, the Locator-Set for the EID-Prefix may contain
   different Priority values for each locator address.  When more than
   one best Priority Locator exists, the ITR can decide how to load-
   share traffic against the corresponding Locators.

   The following hash algorithm may be used by an ITR to select a
   Locator for a packet destined to an EID for the EID-to-RLOC mapping:

   1.  Either a source and destination address hash or the traditional
       5-tuple hash can be used.  The traditional 5-tuple hash includes
       the source and destination addresses; source and destination TCP,
       UDP, or Stream Control Transmission Protocol (SCTP) port numbers;
       and the IP protocol number field or IPv6 next-protocol fields of
       a packet that a host originates from within a LISP site.  When a
       packet is not a TCP, UDP, or SCTP packet, the source and
       destination addresses only from the header are used to compute
       the hash.

   2.  Take the hash value and divide it by the number of Locators
       stored in the Locator-Set for the EID-to-RLOC mapping.

   3.  The remainder will yield a value of 0 to "number of Locators
       minus 1".  Use the remainder to select the Locator in the
       Locator-Set.



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   Note that when a packet is LISP encapsulated, the source port number
   in the outer UDP header needs to be set.  Selecting a hashed value
   allows core routers that are attached to Link Aggregation Groups
   (LAGs) to load-split the encapsulated packets across member links of
   such LAGs.  Otherwise, core routers would see a single flow, since
   packets have a source address of the ITR, for packets that are
   originated by different EIDs at the source site.  A suggested setting
   for the source port number computed by an ITR is a 5-tuple hash
   function on the inner header, as described above.

   Many core router implementations use a 5-tuple hash to decide how to
   balance packet load across members of a LAG.  The 5-tuple hash
   includes the source and destination addresses of the packet and the
   source and destination ports when the protocol number in the packet
   is TCP or UDP.  For this reason, UDP encoding is used for LISP
   encapsulation.

6.6.  Changing the Contents of EID-to-RLOC Mappings

   Since the LISP architecture uses a caching scheme to retrieve and
   store EID-to-RLOC mappings, the only way an ITR can get a more up-to-
   date mapping is to re-request the mapping.  However, the ITRs do not
   know when the mappings change, and the ETRs do not keep track of
   which ITRs requested its mappings.  For scalability reasons, we want
   to maintain this approach but need to provide a way for ETRs to
   change their mappings and inform the sites that are currently
   communicating with the ETR site using such mappings.

   When adding a new Locator record in lexicographic order to the end of
   a Locator-Set, it is easy to update mappings.  We assume that new
   mappings will maintain the same Locator ordering as the old mapping
   but will just have new Locators appended to the end of the list.  So,
   some ITRs can have a new mapping while other ITRs have only an old
   mapping that is used until they time out.  When an ITR has only an
   old mapping but detects bits set in the Locator-Status-Bits that
   correspond to Locators beyond the list it has cached, it simply
   ignores them.  However, this can only happen for locator addresses
   that are lexicographically greater than the locator addresses in the
   existing Locator-Set.

   When a Locator record is inserted in the middle of a Locator-Set, to
   maintain lexicographic order, the SMR procedure in Section 6.6.2 is
   used to inform ITRs and PITRs of the new Locator-Status-Bit mappings.

   When a Locator record is removed from a Locator-Set, ITRs that have
   the mapping cached will not use the removed Locator because the xTRs
   will set the Locator-Status-Bit to 0.  So, even if the Locator is in
   the list, it will not be used.  For new mapping requests, the xTRs



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   can set the Locator AFI to 0 (indicating an unspecified address), as
   well as setting the corresponding Locator-Status-Bit to 0.  This
   forces ITRs with old or new mappings to avoid using the removed
   Locator.

   If many changes occur to a mapping over a long period of time, one
   will find empty record slots in the middle of the Locator-Set and new
   records appended to the Locator-Set.  At some point, it would be
   useful to compact the Locator-Set so the Locator-Status-Bit settings
   can be efficiently packed.

   We propose here three approaches for Locator-Set compaction: one
   operational mechanism and two protocol mechanisms.  The operational
   approach uses a clock sweep method.  The protocol approaches use the
   concept of Solicit-Map-Requests and Map-Versioning.

6.6.1.  Clock Sweep

   The clock sweep approach uses planning in advance and the use of
   count-down TTLs to time out mappings that have already been cached.
   The default setting for an EID-to-RLOC mapping TTL is 24 hours.  So,
   there is a 24-hour window to time out old mappings.  The following
   clock sweep procedure is used:

   1.  24 hours before a mapping change is to take effect, a network
       administrator configures the ETRs at a site to start the clock
       sweep window.

   2.  During the clock sweep window, ETRs continue to send Map-Reply
       messages with the current (unchanged) mapping records.  The TTL
       for these mappings is set to 1 hour.

   3.  24 hours later, all previous cache entries will have timed out,
       and any active cache entries will time out within 1 hour.  During
       this 1-hour window, the ETRs continue to send Map-Reply messages
       with the current (unchanged) mapping records with the TTL set to
       1 minute.

   4.  At the end of the 1-hour window, the ETRs will send Map-Reply
       messages with the new (changed) mapping records.  So, any active
       caches can get the new mapping contents right away if not cached,
       or in 1 minute if they had the mapping cached.  The new mappings
       are cached with a TTL equal to the TTL in the Map-Reply.








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6.6.2.  Solicit-Map-Request (SMR)

   Soliciting a Map-Request is a selective way for ETRs, at the site
   where mappings change, to control the rate they receive requests for
   Map-Reply messages.  SMRs are also used to tell remote ITRs to update
   the mappings they have cached.

   Since the ETRs don't keep track of remote ITRs that have cached their
   mappings, they do not know which ITRs need to have their mappings
   updated.  As a result, an ETR will solicit Map-Requests (called an
   SMR message) from those sites to which it has been sending
   encapsulated data for the last minute.  In particular, an ETR will
   send an SMR to an ITR to which it has recently sent encapsulated
   data.

   An SMR message is simply a bit set in a Map-Request message.  An ITR
   or PITR will send a Map-Request when they receive an SMR message.
   Both the SMR sender and the Map-Request responder MUST rate-limit
   these messages.  Rate-limiting can be implemented as a global rate-
   limiter or one rate-limiter per SMR destination.

   The following procedure shows how an SMR exchange occurs when a site
   is doing Locator-Set compaction for an EID-to-RLOC mapping:

   1.  When the database mappings in an ETR change, the ETRs at the site
       begin to send Map-Requests with the SMR bit set for each Locator
       in each Map-Cache entry the ETR caches.

   2.  A remote ITR that receives the SMR message will schedule sending
       a Map-Request message to the source locator address of the SMR
       message or to the mapping database system.  A newly allocated
       random nonce is selected, and the EID-Prefix used is the one
       copied from the SMR message.  If the source Locator is the only
       Locator in the cached Locator-Set, the remote ITR SHOULD send a
       Map-Request to the database mapping system just in case the
       single Locator has changed and may no longer be reachable to
       accept the Map-Request.

   3.  The remote ITR MUST rate-limit the Map-Request until it gets a
       Map-Reply while continuing to use the cached mapping.  When
       Map-Versioning as described in Section 6.6.3 is used, an SMR
       sender can detect if an ITR is using the most up-to-date database
       mapping.

   4.  The ETRs at the site with the changed mapping will reply to the
       Map-Request with a Map-Reply message that has a nonce from the
       SMR-invoked Map-Request.  The Map-Reply messages SHOULD be rate-
       limited.  This is important to avoid Map-Reply implosion.



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   5.  The ETRs at the site with the changed mapping record the fact
       that the site that sent the Map-Request has received the new
       mapping data in the Map-Cache entry for the remote site so the
       Locator-Status-Bits are reflective of the new mapping for packets
       going to the remote site.  The ETR then stops sending SMR
       messages.

   Experimentation is in progress to determine the appropriate rate-
   limit parameters.

   For security reasons, an ITR MUST NOT process unsolicited
   Map-Replies.  To avoid Map-Cache entry corruption by a third party, a
   sender of an SMR-based Map-Request MUST be verified.  If an ITR
   receives an SMR-based Map-Request and the source is not in the
   Locator-Set for the stored Map-Cache entry, then the responding
   Map-Request MUST be sent with an EID destination to the mapping
   database system.  Since the mapping database system is a more secure
   way to reach an authoritative ETR, it will deliver the Map-Request to
   the authoritative source of the mapping data.

   When an ITR receives an SMR-based Map-Request for which it does not
   have a cached mapping for the EID in the SMR message, it MAY not send
   an SMR-invoked Map-Request.  This scenario can occur when an ETR
   sends SMR messages to all Locators in the Locator-Set it has stored
   in its map-cache but the remote ITRs that receive the SMR may not be
   sending packets to the site.  There is no point in updating the ITRs
   until they need to send, in which case they will send Map-Requests to
   obtain a Map-Cache entry.

6.6.3.  Database Map-Versioning

   When there is unidirectional packet flow between an ITR and ETR, and
   the EID-to-RLOC mappings change on the ETR, it needs to inform the
   ITR so encapsulation to a removed Locator can stop and can instead be
   started to a new Locator in the Locator-Set.

   An ETR, when it sends Map-Reply messages, conveys its own Map-Version
   Number.  This is known as the Destination Map-Version Number.  ITRs
   include the Destination Map-Version Number in packets they
   encapsulate to the site.  When an ETR decapsulates a packet and
   detects that the Destination Map-Version Number is less than the
   current version for its mapping, the SMR procedure described in
   Section 6.6.2 occurs.








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   An ITR, when it encapsulates packets to ETRs, can convey its own
   Map-Version Number.  This is known as the Source Map-Version Number.
   When an ETR decapsulates a packet and detects that the Source
   Map-Version Number is greater than the last Map-Version Number sent
   in a Map-Reply from the ITR's site, the ETR will send a Map-Request
   to one of the ETRs for the source site.

   A Map-Version Number is used as a sequence number per EID-Prefix, so
   values that are greater are considered to be more recent.  A value of
   0 for the Source Map-Version Number or the Destination Map-Version
   Number conveys no versioning information, and an ITR does no
   comparison with previously received Map-Version Numbers.

   A Map-Version Number can be included in Map-Register messages as
   well.  This is a good way for the Map-Server to assure that all ETRs
   for a site registering to it will be synchronized according to
   Map-Version Number.

   See [RFC6834] for a more detailed analysis and description of
   Database Map-Versioning.

7.  Router Performance Considerations

   LISP is designed to be very "hardware-based forwarding friendly".  A
   few implementation techniques can be used to incrementally implement
   LISP:

   o  When a tunnel-encapsulated packet is received by an ETR, the outer
      destination address may not be the address of the router.  This
      makes it challenging for the control plane to get packets from the
      hardware.  This may be mitigated by creating special Forwarding
      Information Base (FIB) entries for the EID-Prefixes of EIDs served
      by the ETR (those for which the router provides an RLOC
      translation).  These FIB entries are marked with a flag indicating
      that control-plane processing should be performed.  The forwarding
      logic of testing for particular IP protocol number values is not
      necessary.  There are a few proven cases where no changes to
      existing deployed hardware were needed to support the LISP data-
      plane.

   o  On an ITR, prepending a new IP header consists of adding more
      octets to a MAC rewrite string and prepending the string as part
      of the outgoing encapsulation procedure.  Routers that support
      Generic Routing Encapsulation (GRE) tunneling [RFC2784] or 6to4
      tunneling [RFC3056] may already support this action.






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   o  A packet's source address or interface the packet was received on
      can be used to select VRF (Virtual Routing/Forwarding).  The VRF's
      routing table can be used to find EID-to-RLOC mappings.

   For performance issues related to map-cache management, see
   Section 12.

8.  Deployment Scenarios

   This section will explore how and where ITRs and ETRs can be deployed
   and will discuss the pros and cons of each deployment scenario.  For
   a more detailed deployment recommendation, refer to [LISP-DEPLOY].

   There are two basic deployment tradeoffs to consider: centralized
   versus distributed caches; and flat, Recursive, or Re-encapsulating
   Tunneling.  When deciding on centralized versus distributed caching,
   the following issues should be considered:

   o  Are the Tunnel Routers spread out so that the caches are spread
      across all the memories of each router?  A centralized cache is
      when an ITR keeps a cache for all the EIDs it is encapsulating to.
      The packet takes a direct path to the destination Locator.  A
      distributed cache is when an ITR needs help from other
      re-encapsulating routers because it does not store all the cache
      entries for the EIDs it is encapsulating to.  So, the packet takes
      a path through re-encapsulating routers that have a different set
      of cache entries.

   o  Should management "touch points" be minimized by only choosing a
      few Tunnel Routers, just enough for redundancy?

   o  In general, using more ITRs doesn't increase management load,
      since caches are built and stored dynamically.  On the other hand,
      using more ETRs does require more management, since EID-Prefix-to-
      RLOC mappings need to be explicitly configured.

   When deciding on flat, Recursive, or Re-encapsulating Tunneling, the
   following issues should be considered:

   o  Flat tunneling implements a single tunnel between the source site
      and destination site.  This generally offers better paths between
      sources and destinations with a single tunnel path.

   o  Recursive Tunneling is when tunneled traffic is again further
      encapsulated in another tunnel, either to implement VPNs or to
      perform Traffic Engineering.  When doing VPN-based tunneling, the
      site has some control, since the site is prepending a new tunnel
      header.  In the case of TE-based tunneling, the site may have



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      control if it is prepending a new tunnel header, but if the site's
      ISP is doing the TE, then the site has no control.  Recursive
      Tunneling generally will result in suboptimal paths but with the
      benefit of steering traffic to parts of the network that have more
      resources available.

   o  The technique of re-encapsulation ensures that packets only
      require one tunnel header.  So, if a packet needs to be re-routed,
      it is first decapsulated by the ETR and then re-encapsulated with
      a new tunnel header using a new RLOC.

   The next sub-sections will examine where Tunnel Routers can reside in
   the network.

8.1.  First-Hop/Last-Hop Tunnel Routers

   By locating Tunnel Routers close to hosts, the EID-Prefix set is at
   the granularity of an IP subnet.  So, at the expense of more
   EID-Prefix-to-RLOC sets for the site, the caches in each Tunnel
   Router can remain relatively small.  But caches always depend on the
   number of non-aggregated EID destination flows active through these
   Tunnel Routers.

   With more Tunnel Routers doing encapsulation, the increase in control
   traffic grows as well: since the EID granularity is greater, more
   Map-Requests and Map-Replies are traveling between more routers.

   The advantage of placing the caches and databases at these stub
   routers is that the products deployed in this part of the network
   have better price-memory ratios than their core router counterparts.
   Memory is typically less expensive in these devices, and fewer routes
   are stored (only IGP routes).  These devices tend to have excess
   capacity, both for forwarding and routing states.

   LISP functionality can also be deployed in edge switches.  These
   devices generally have layer-2 ports facing hosts and layer-3 ports
   facing the Internet.  Spare capacity is also often available in these
   devices.

8.2.  Border/Edge Tunnel Routers

   Using Customer Edge (CE) routers for tunnel endpoints allows the EID
   space associated with a site to be reachable via a small set of RLOCs
   assigned to the CE routers for that site.  This is the default
   behavior envisioned in the rest of this specification.






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   This offers the opposite benefit of the first-hop/last-hop Tunnel
   Router scenario: the number of mapping entries and network management
   touch points is reduced, allowing better scaling.

   One disadvantage is that fewer network resources are used to reach
   host endpoints, thereby centralizing the point-of-failure domain and
   creating network choke points at the CE router.

   Note that more than one CE router at a site can be configured with
   the same IP address.  In this case, an RLOC is an anycast address.
   This allows resilience between the CE routers.  That is, if a CE
   router fails, traffic is automatically routed to the other routers
   using the same anycast address.  However, this comes with the
   disadvantage where the site cannot control the entrance point when
   the anycast route is advertised out from all border routers.  Another
   disadvantage of using anycast Locators is the limited advertisement
   scope of /32 (or /128 for IPv6) routes.

8.3.  ISP Provider Edge (PE) Tunnel Routers

   The use of ISP PE routers as tunnel endpoint routers is not the
   typical deployment scenario envisioned in this specification.  This
   section attempts to capture some of the reasoning behind this
   preference for implementing LISP on CE routers.

   The use of ISP PE routers as tunnel endpoint routers gives an ISP,
   rather than a site, control over the location of the egress tunnel
   endpoints.  That is, the ISP can decide whether the tunnel endpoints
   are in the destination site (in either CE routers or last-hop routers
   within a site) or at other PE edges.  The advantage of this case is
   that two tunnel headers can be avoided.  By having the PE be the
   first router on the path to encapsulate, it can choose a TE path
   first, and the ETR can decapsulate and re-encapsulate for a tunnel to
   the destination end site.

   An obvious disadvantage is that the end site has no control over
   where its packets flow or over the RLOCs used.  Other disadvantages
   include difficulty in synchronizing path liveness updates between CE
   and PE routers.

   As mentioned in earlier sections, a combination of these scenarios is
   possible at the expense of extra packet header overhead; if both site
   and provider want control, then Recursive or Re-encapsulating Tunnels
   are used.







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8.4.  LISP Functionality with Conventional NATs

   LISP routers can be deployed behind Network Address Translator (NAT)
   devices to provide the same set of packet services hosts have today
   when they are addressed out of private address space.

   It is important to note that a locator address in any LISP control
   message MUST be a globally routable address and therefore SHOULD NOT
   contain [RFC1918] addresses.  If a LISP router is configured with
   private addresses, they MUST be used only in the outer IP header so
   the NAT device can translate properly.  Otherwise, EID addresses MUST
   be translated before encapsulation is performed.  Both NAT
   translation and LISP encapsulation functions could be co-located in
   the same device.

   More details on LISP address translation can be found in [RFC6832].

8.5.  Packets Egressing a LISP Site

   When a LISP site is using two ITRs for redundancy, the failure of one
   ITR will likely shift outbound traffic to the second.  This second
   ITR's cache may not be populated with the same EID-to-RLOC mapping
   entries as the first.  If this second ITR does not have these
   mappings, traffic will be dropped while the mappings are retrieved
   from the mapping system.  The retrieval of these messages may
   increase the load of requests being sent into the mapping system.
   Deployment and experimentation will determine whether this issue
   requires more attention.

9.  Traceroute Considerations

   When a source host in a LISP site initiates a traceroute to a
   destination host in another LISP site, it is highly desirable for it
   to see the entire path.  Since packets are encapsulated from the ITR
   to the ETR, the hop across the tunnel could be viewed as a single
   hop.  However, LISP traceroute will provide the entire path so the
   user can see 3 distinct segments of the path from a source LISP host
   to a destination LISP host:













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      Segment 1 (in source LISP site based on EIDs):

          source host ---> first hop ... next hop ---> ITR

      Segment 2 (in the core network based on RLOCs):

          ITR ---> next hop ... next hop ---> ETR

      Segment 3 (in the destination LISP site based on EIDs):

          ETR ---> next hop ... last hop ---> destination host

   For segment 1 of the path, ICMP Time Exceeded messages are returned
   in the normal manner as they are today.  The ITR performs a TTL
   decrement and tests for 0 before encapsulating.  Therefore, the ITR's
   hop is seen by the traceroute source as having an EID address (the
   address of the site-facing interface).

   For segment 2 of the path, ICMP Time Exceeded messages are returned
   to the ITR because the TTL decrement to 0 is done on the outer
   header, so the destinations of the ICMP messages are the ITR RLOC
   address and the source RLOC address of the encapsulated traceroute
   packet.  The ITR looks inside of the ICMP payload to inspect the
   traceroute source so it can return the ICMP message to the address of
   the traceroute client and also retain the core router IP address in
   the ICMP message.  This is so the traceroute client can display the
   core router address (the RLOC address) in the traceroute output.  The
   ETR returns its RLOC address and responds to the TTL decrement to 0,
   as the previous core routers did.

   For segment 3, the next-hop router downstream from the ETR will be
   decrementing the TTL for the packet that was encapsulated, sent into
   the core, decapsulated by the ETR, and forwarded because it isn't the
   final destination.  If the TTL is decremented to 0, any router on the
   path to the destination of the traceroute, including the next-hop
   router or destination, will send an ICMP Time Exceeded message to the
   source EID of the traceroute client.  The ICMP message will be
   encapsulated by the local ITR and sent back to the ETR in the
   originated traceroute source site, where the packet will be delivered
   to the host.

9.1.  IPv6 Traceroute

   IPv6 traceroute follows the procedure described above, since the
   entire traceroute data packet is included in the ICMP Time Exceeded
   message payload.  Therefore, only the ITR needs to pay special
   attention to forwarding ICMP messages back to the traceroute source.




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9.2.  IPv4 Traceroute

   For IPv4 traceroute, we cannot follow the above procedure, since IPv4
   ICMP Time Exceeded messages only include the invoking IP header and
   8 octets that follow the IP header.  Therefore, when a core router
   sends an IPv4 Time Exceeded message to an ITR, all the ITR has in the
   ICMP payload is the encapsulated header it prepended, followed by a
   UDP header.  The original invoking IP header, and therefore the
   identity of the traceroute source, is lost.

   The solution we propose to solve this problem is to cache traceroute
   IPv4 headers in the ITR and to match them up with corresponding IPv4
   Time Exceeded messages received from core routers and the ETR.  The
   ITR will use a circular buffer for caching the IPv4 and UDP headers
   of traceroute packets.  It will select a 16-bit number as a key to
   find them later when the IPv4 Time Exceeded messages are received.
   When an ITR encapsulates an IPv4 traceroute packet, it will use the
   16-bit number as the UDP source port in the encapsulating header.
   When the ICMP Time Exceeded message is returned to the ITR, the UDP
   header of the encapsulating header is present in the ICMP payload,
   thereby allowing the ITR to find the cached headers for the
   traceroute source.  The ITR puts the cached headers in the payload
   and sends the ICMP Time Exceeded message to the traceroute source
   retaining the source address of the original ICMP Time Exceeded
   message (a core router or the ETR of the site of the traceroute
   destination).

   The signature of a traceroute packet comes in two forms.  The first
   form is encoded as a UDP message where the destination port is
   inspected for a range of values.  The second form is encoded as an
   ICMP message where the IP identification field is inspected for a
   well-known value.

9.3.  Traceroute Using Mixed Locators

   When either an IPv4 traceroute or IPv6 traceroute is originated and
   the ITR encapsulates it in the other address family header, one
   cannot get all 3 segments of the traceroute.  Segment 2 of the
   traceroute cannot be conveyed to the traceroute source, since it is
   expecting addresses from intermediate hops in the same address format
   for the type of traceroute it originated.  Therefore, in this case,
   segment 2 will make the tunnel look like one hop.  All the ITR has to
   do to make this work is to not copy the inner TTL to the outer,
   encapsulating header's TTL when a traceroute packet is encapsulated
   using an RLOC from a different address family.  This will cause no
   TTL decrement to 0 to occur in core routers between the ITR and ETR.





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10.  Mobility Considerations

   There are several kinds of mobility, of which only some might be of
   concern to LISP.  Essentially, they are as follows.

10.1.  Site Mobility

   A site wishes to change its attachment points to the Internet, and
   its LISP Tunnel Routers will have new RLOCs when it changes upstream
   providers.  Changes in EID-to-RLOC mappings for sites are expected to
   be handled by configuration, outside of LISP.

10.2.  Slow Endpoint Mobility

   An individual endpoint wishes to move but is not concerned about
   maintaining session continuity.  Renumbering is involved.  LISP can
   help with the issues surrounding renumbering [RFC4192] [LISA96] by
   decoupling the address space used by a site from the address spaces
   used by its ISPs [RFC4984].

10.3.  Fast Endpoint Mobility

   Fast endpoint mobility occurs when an endpoint moves relatively
   rapidly, changing its IP-layer network attachment point.  Maintenance
   of session continuity is a goal.  This is where the Mobile IPv4
   [RFC5944] and Mobile IPv6 [RFC6275] [RFC4866] mechanisms are used and
   primarily where interactions with LISP need to be explored.

   The problem is that as an endpoint moves, it may require changes to
   the mapping between its EID and a set of RLOCs for its new network
   location.  When this is added to the overhead of Mobile IP binding
   updates, some packets might be delayed or dropped.

   In IPv4 mobility, when an endpoint is away from home, packets to it
   are encapsulated and forwarded via a home agent that resides in the
   home area the endpoint's address belongs to.  The home agent will
   encapsulate and forward packets either directly to the endpoint or to
   a foreign agent that resides where the endpoint has moved to.
   Packets from the endpoint may be sent directly to the correspondent
   node, may be sent via the foreign agent, or may be reverse-tunneled
   back to the home agent for delivery to the mobile node.  As the
   mobile node's EID or available RLOC changes, LISP EID-to-RLOC









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   mappings are required for communication between the mobile node and
   the home agent, whether via the foreign agent or not.  As a mobile
   endpoint changes networks, up to three LISP mapping changes may be
   required:

   o  The mobile node moves from an old location to a new visited
      network location and notifies its home agent that it has done so.
      The Mobile IPv4 control packets the mobile node sends pass through
      one of the new visited network's ITRs, which needs an EID-to-RLOC
      mapping for the home agent.

   o  The home agent might not have the EID-to-RLOC mappings for the
      mobile node's "care-of" address or its foreign agent in the new
      visited network, in which case it will need to acquire them.

   o  When packets are sent directly to the correspondent node, it may
      be that no traffic has been sent from the new visited network to
      the correspondent node's network, and the new visited network's
      ITR will need to obtain an EID-to-RLOC mapping for the
      correspondent node's site.

   In addition, if the IPv4 endpoint is sending packets from the new
   visited network using its original EID, then LISP will need to
   perform a route-returnability check on the new EID-to-RLOC mapping
   for that EID.

   In IPv6 mobility, packets can flow directly between the mobile node
   and the correspondent node in either direction.  The mobile node uses
   its "care-of" address (EID).  In this case, the route-returnability
   check would not be needed but one more LISP mapping lookup may be
   required instead:

   o  As above, three mapping changes may be needed for the mobile node
      to communicate with its home agent and to send packets to the
      correspondent node.

   o  In addition, another mapping will be needed in the correspondent
      node's ITR, in order for the correspondent node to send packets to
      the mobile node's "care-of" address (EID) at the new network
      location.

   When both endpoints are mobile, the number of potential mapping
   lookups increases accordingly.

   As a mobile node moves, there are not only mobility state changes in
   the mobile node, correspondent node, and home agent, but also state
   changes in the ITRs and ETRs for at least some EID-Prefixes.




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   The goal is to support rapid adaptation, with little delay or packet
   loss for the entire system.  Also, IP mobility can be modified to
   require fewer mapping changes.  In order to increase overall system
   performance, there may be a need to reduce the optimization of one
   area in order to place fewer demands on another.

   In LISP, one possibility is to "glean" information.  When a packet
   arrives, the ETR could examine the EID-to-RLOC mapping and use that
   mapping for all outgoing traffic to that EID.  It can do this after
   performing a route-returnability check, to ensure that the new
   network location does have an internal route to that endpoint.
   However, this does not cover the case where an ITR (the node assigned
   the RLOC) at the mobile-node location has been compromised.

   Mobile IP packet exchange is designed for an environment in which all
   routing information is disseminated before packets can be forwarded.
   In order to allow the Internet to grow to support expected future
   use, we are moving to an environment where some information may have
   to be obtained after packets are in flight.  Modifications to IP
   mobility should be considered in order to optimize the behavior of
   the overall system.  Anything that decreases the number of new
   EID-to-RLOC mappings needed when a node moves, or maintains the
   validity of an EID-to-RLOC mapping for a longer time, is useful.

10.4.  Fast Network Mobility

   In addition to endpoints, a network can be mobile, possibly changing
   xTRs.  A "network" can be as small as a single router and as large as
   a whole site.  This is different from site mobility in that it is
   fast and possibly short-lived, but different from endpoint mobility
   in that a whole prefix is changing RLOCs.  However, the mechanisms
   are the same, and there is no new overhead in LISP.  A map request
   for any endpoint will return a binding for the entire mobile prefix.

   If mobile networks become a more common occurrence, it may be useful
   to revisit the design of the mapping service and allow for dynamic
   updates of the database.

   The issue of interactions between mobility and LISP needs to be
   explored further.  Specific improvements to the entire system will
   depend on the details of mapping mechanisms.  Mapping mechanisms
   should be evaluated on how well they support session continuity for
   mobile nodes.








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10.5.  LISP Mobile Node Mobility

   A mobile device can use the LISP infrastructure to achieve mobility
   by implementing the LISP encapsulation and decapsulation functions
   and acting as a simple ITR/ETR.  By doing this, such a "LISP mobile
   node" can use topologically independent EID IP addresses that are not
   advertised into and do not impose a cost on the global routing
   system.  These EIDs are maintained at the edges of the mapping system
   (in LISP Map-Servers and Map-Resolvers) and are provided on demand to
   only the correspondents of the LISP mobile node.

   Refer to [LISP-MN] for more details.

11.  Multicast Considerations

   A multicast group address, as defined in the original Internet
   architecture, is an identifier of a grouping of topologically
   independent receiver host locations.  The address encoding itself
   does not determine the location of the receiver(s).  The multicast
   routing protocol, and the network-based state the protocol creates,
   determine where the receivers are located.

   In the context of LISP, a multicast group address is both an EID and
   a Routing Locator.  Therefore, no specific semantic or action needs
   to be taken for a destination address, as it would appear in an IP
   header.  Therefore, a group address that appears in an inner IP
   header built by a source host will be used as the destination EID.
   The outer IP header (the destination Routing Locator address),
   prepended by a LISP router, will use the same group address as the
   destination Routing Locator.

   Having said that, only the source EID and source Routing Locator need
   to be dealt with.  Therefore, an ITR merely needs to put its own IP
   address in the source 'Routing Locator' field when prepending the
   outer IP header.  This source Routing Locator address, like any other
   Routing Locator address, MUST be globally routable.

   Therefore, an EID-to-RLOC mapping does not need to be performed by an
   ITR when a received data packet is a multicast data packet or when
   processing a source-specific Join (either by IGMPv3 or PIM).  But the
   source Routing Locator is decided by the multicast routing protocol
   in a receiver site.  That is, an EID-to-RLOC translation is done at
   control time.

   Another approach is to have the ITR not encapsulate a multicast
   packet and allow the packet built by the host to flow into the core
   even if the source address is allocated out of the EID namespace.  If
   the RPF-Vector TLV [RFC5496] is used by PIM in the core, then core



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   routers can RPF to the ITR (the locator address, which is injected
   into core routing) rather than the host source address (the EID
   address, which is not injected into core routing).

   To avoid any EID-based multicast state in the network core, the first
   approach is chosen for LISP-Multicast.  Details for LISP-Multicast
   and interworking with non-LISP sites are described in [RFC6831] and
   [RFC6832].

12.  Security Considerations

   It is believed that most of the security mechanisms will be part of
   the mapping database service when using control-plane procedures for
   obtaining EID-to-RLOC mappings.  For data-plane-triggered mappings,
   as described in this specification, protection is provided against
   ETR spoofing by using route-returnability (see Section 3) mechanisms
   evidenced by the use of a 24-bit 'Nonce' field in the LISP
   encapsulation header and a 64-bit 'Nonce' field in the LISP control
   message.

   The nonce, coupled with the ITR accepting only solicited Map-Replies,
   provides a basic level of security, in many ways similar to the
   security experienced in the current Internet routing system.  It is
   hard for off-path attackers to launch attacks against these LISP
   mechanisms, as they do not have the nonce values.  Sending a large
   number of packets to accidentally find the right nonce value is
   possible but would already by itself be a denial-of-service (DoS)
   attack.  On-path attackers can perform far more serious attacks, but
   on-path attackers can launch serious attacks in the current Internet
   as well, including eavesdropping, blocking, or redirecting traffic.
   See more discussion on this topic in Section 6.1.5.1.

   LISP does not rely on a PKI or a more heavyweight authentication
   system.  These systems challenge one of the primary design goals of
   LISP -- scalability.

   DoS attack prevention will depend on implementations rate-limiting
   Map-Requests and Map-Replies to the control plane as well as
   rate-limiting the number of data-triggered Map-Replies.

   An incorrectly implemented or malicious ITR might choose to ignore
   the Priority and Weights provided by the ETR in its Map-Reply.  This
   traffic-steering would be limited to the traffic that is sent by this
   ITR's site and no more severe than if the site initiated a bandwidth
   DoS attack on (one of) the ETR's ingress links.  The ITR's site would
   typically gain no benefit from not respecting the Weights and would
   likely receive better service by abiding by them.




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   To deal with map-cache exhaustion attempts in an ITR/PITR, the
   implementation should consider putting a maximum cap on the number of
   entries stored with a reserve list for special or frequently accessed
   sites.  This should be a configuration policy control set by the
   network administrator who manages ITRs and PITRs.  When overlapping
   EID-Prefixes occur across multiple Map-Cache entries, the integrity
   of the set must be wholly maintained.  So, if a more-specific entry
   cannot be added due to reaching the maximum cap, then none of the
   less-specific entries should be stored in the map-cache.

   Given that the ITR/PITR maintains a cache of EID-to-RLOC mappings,
   cache sizing and maintenance are issues to be kept in mind during
   implementation.  It is a good idea to have instrumentation in place
   to detect thrashing of the cache.  Implementation experimentation
   will be used to determine which cache management strategies work
   best.  In general, it is difficult to defend against cache-thrashing
   attacks.  It should be noted that an undersized cache in an ITR/PITR
   not only causes adverse effects on the site or region it supports but
   may also cause increased Map-Request loads on the mapping system.

   "Piggybacked" mapping data as discussed in Section 6.1.3 specifies
   how to handle such mappings and includes the possibility for an ETR
   to temporarily accept such a mapping before verification when running
   in "trusted" environments.  In such cases, there is a potential
   threat that a fake mapping could be inserted (even if only for a
   short period) into a map-cache.  As noted in Section 6.1.3, an ETR
   MUST be specifically configured to run in such a mode and might
   usefully only consider some specific ITRs as also running in that
   same trusted environment.

   There is a security risk implicit in the fact that ETRs generate the
   EID-Prefix to which they are responding.  An ETR can claim a shorter
   prefix than it is actually responsible for.  Various mechanisms to
   ameliorate or resolve this issue will be examined in the future
   [LISP-SEC].

   Spoofing of inner-header addresses of LISP-encapsulated packets is
   possible, as with any tunneling mechanism.  ITRs MUST verify the
   source address of a packet to be an EID that belongs to the site's
   EID-Prefix range prior to encapsulation.  An ETR must only
   decapsulate and forward datagrams with an inner-header destination
   that matches one of its EID-Prefix ranges.  If, upon receipt and
   decapsulation, the destination EID of a datagram does not match one
   of the ETR's configured EID-Prefixes, the ETR MUST drop the datagram.
   If a LISP-encapsulated packet arrives at an ETR, it SHOULD compare
   the inner-header source EID address and the outer-header source RLOC
   address with the mapping that exists in the mapping database.  Then,




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   when spoofing attacks occur, the outer-header source RLOC address can
   be used to trace back the attack to the source site, using existing
   operational tools.

   This experimental specification does not address automated key
   management (AKM).  BCP 107 [RFC4107] provides guidance in this area.
   In addition, at the time of this writing, substantial work is being
   undertaken to improve security of the routing system [RFC6518]
   [RFC6480] [BGP-SEC] [LISP-SEC].  Future work on LISP should address
   the issues discussed in BCP 107 as well as other open security
   considerations, which may require changes to this specification.

13.  Network Management Considerations

   Considerations for network management tools exist so the LISP
   protocol suite can be operationally managed.  These mechanisms can be
   found in [LISP-MIB] and [RFC6835].

14.  IANA Considerations

   This section provides guidance to the Internet Assigned Numbers
   Authority (IANA) regarding registration of values related to the LISP
   specification, in accordance with BCP 26 [RFC5226].

   There are four namespaces (listed in the sub-sections below) in LISP
   that have been registered.

   o  LISP IANA registry allocations should not be made for purposes
      unrelated to LISP routing or transport protocols.

   o  The following policies are used here with the meanings defined in
      BCP 26: "Specification Required", "IETF Review", "Experimental
      Use", and "First Come First Served".

14.1.  LISP ACT and Flag Fields

   New ACT values (Section 6.1.4) can be allocated through IETF review
   or IESG approval.  Four values have already been allocated by this
   specification (Section 6.1.4).

   In addition, LISP has a number of flag fields and reserved fields,
   such as the LISP header flags field (Section 5.3).  New bits for
   flags in these fields can be implemented after IETF review or IESG
   approval, but these need not be managed by IANA.







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14.2.  LISP Address Type Codes

   LISP Address [LCAF] type codes have a range from 0 to 255.  New type
   codes MUST be allocated consecutively, starting at 0.  Type Codes
   0-127 are to be assigned by IETF review or IESG approval.

   Type Codes 128-255 are available according to the [RFC5226] First
   Come First Served policy.

   This registry, initially empty, is constructed for future use in
   experimental work related to LISP Canonical Address Format (LCAF)
   values.  See [LCAF] for details of other possible unapproved address
   encodings.  The unapproved LCAF encodings are an area for further
   study and experimentation.

14.3.  LISP UDP Port Numbers

   The IANA registry has allocated UDP port numbers 4341 and 4342 for
   lisp-data and lisp-control operation, respectively.  IANA has updated
   the description for UDP ports 4341 and 4342 as follows:

       lisp-data      4341 udp    LISP Data Packets
       lisp-control   4342 udp    LISP Control Packets

14.4.  LISP Key ID Numbers

   The following Key ID values are defined by this specification as used
   in any packet type that references a 'Key ID' field:

       Name                 Number          Defined in
       -----------------------------------------------
       None                 0               n/a
       HMAC-SHA-1-96        1               [RFC2404]
       HMAC-SHA-256-128     2               [RFC4868]

   Number values are in the range of 0 to 65535.  The allocation of
   values is on a first come first served basis.

15.  Known Open Issues and Areas of Future Work

   As an experimental specification, this work is, by definition,
   incomplete.  Specific areas where additional experience and work are
   needed include the following:

   o  At present, only [RFC6836] is defined for implementing a database
      of EID-to-RLOC mapping information.  Additional research on other
      mapping database systems is strongly encouraged.




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   o  Failure and recovery of LISP site partitioning (see Section 6.4)
      in the presence of redundant configuration (see Section 8.5) needs
      further research and experimentation.

   o  The characteristics of map-cache management under exceptional
      conditions, such as denial-of-service attacks, are not fully
      understood.  Further experience is needed to determine whether
      current caching methods are practical or in need of further
      development.  In particular, the performance, scaling, and
      security characteristics of the map-cache will be discovered as
      part of this experiment.  Performance metrics to be observed are
      packet reordering associated with the LISP Data-Probe and loss of
      the first packet in a flow associated with map-caching.  The
      impact of these upon TCP will be observed.  See Section 12 for
      additional thoughts and considerations.

   o  Preliminary work has been done to ensure that sites employing LISP
      can interconnect with the rest of the Internet.  This work is
      documented in [RFC6832], but further experimentation and
      experience are needed.

   o  At present, no mechanism for automated key management for message
      authentication is defined.  Addressing automated key management is
      necessary before this specification can be developed into a
      Standards Track RFC.  See Section 12 for further details regarding
      security considerations.

   o  In order to maintain security and stability, Internet protocols
      typically isolate the control and data planes.  Therefore, user
      activity cannot cause control-plane state to be created or
      destroyed.  LISP does not maintain this separation.  The degree to
      which the loss of separation impacts security and stability is a
      topic for experimental observation.

   o  LISP allows for the use of different mapping database systems.
      While only one [RFC6836] is currently well defined, each mapping
      database will likely have some impact on the security of the
      EID-to-RLOC mappings.  How each mapping database system's security
      properties impact LISP overall is for further study.

   o  An examination of the implications of LISP on Internet traffic,
      applications, routers, and security is needed.  This will help
      implementors understand the consequences for network stability,
      routing protocol function, routing scalability, migration and
      backward compatibility, and implementation scalability (as
      influenced by additional protocol components; additional state;
      and additional processing for encapsulation, decapsulation, and
      liveness).



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   o  Experiments need to verify that LISP produces no significant
      change in the behavior of protocols run between end-systems over a
      LISP infrastructure versus being run directly between those same
      end-systems.

   o  Experiments need to verify that the issues raised in the Critique
      section of [RFC6115] are either insignificant or have been
      addressed by updates to LISP.

   Other LISP documents may also include open issues and areas for
   future work.

16.  References

16.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2404]  Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96 within
              ESP and AH", RFC 2404, November 1998.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC3232]  Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
              an On-line Database", RFC 3232, January 2002.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, August 2006.



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   [RFC4868]  Kelly, S. and S. Frankel, "Using HMAC-SHA-256,
              HMAC-SHA-384, and HMAC-SHA-512 with IPsec", RFC 4868,
              May 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5496]  Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
              Forwarding (RPF) Vector TLV", RFC 5496, March 2009.

   [RFC5944]  Perkins, C., "IP Mobility Support for IPv4, Revised",
              RFC 5944, November 2010.

   [RFC6115]  Li, T., "Recommendation for a Routing Architecture",
              RFC 6115, February 2011.

   [RFC6275]  Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
              in IPv6", RFC 6275, July 2011.

   [RFC6833]  Farinacci, D. and V. Fuller, "Locator/ID Separation
              Protocol (LISP) Map-Server Interface", RFC 6833,
              January 2013.

   [RFC6834]  Iannone, L., Saucez, D., and O. Bonaventure, "Locator/ID
              Separation Protocol (LISP) Map-Versioning", RFC 6834,
              January 2013.

   [RFC6836]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol Alternative Logical
              Topology (LISP+ALT)", RFC 6836, January 2013.

16.2.  Informative References

   [AFI]      IANA, "Address Family Numbers",
              <http://www.iana.org/assignments/address-family-numbers>.

   [BGP-SEC]  Lepinski, M. and S. Turner, "An Overview of BGPSEC", Work
              in Progress, May 2012.

   [CHIAPPA]  Chiappa, J., "Endpoints and Endpoint names: A Proposed
              Enhancement to the Internet Architecture", 1999,
              <http://mercury.lcs.mit.edu/~jnc/tech/endpoints.txt>.

   [CONS]     Brim, S., Chiappa, N., Farinacci, D., Fuller, V., Lewis,
              D., and D. Meyer, "LISP-CONS: A Content distribution
              Overlay Network Service for LISP", Work in Progress,
              April 2008.



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   [EMACS]    Brim, S., Farinacci, D., Meyer, D., and J. Curran, "EID
              Mappings Multicast Across Cooperating Systems for LISP",
              Work in Progress, November 2007.

   [LCAF]     Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format (LCAF)", Work in Progress, January 2013.

   [LISA96]   Lear, E., Tharp, D., Katinsky, J., and J. Coffin,
              "Renumbering: Threat or Menace?", Usenix Tenth System
              Administration Conference (LISA 96), October 1996.

   [LISP-DEPLOY]
              Jakab, L., Cabellos-Aparicio, A., Coras, F.,
              Domingo-Pascual, J., and D. Lewis, "LISP Network Element
              Deployment Considerations", Work in Progress,
              October 2012.

   [LISP-MIB] Schudel, G., Jain, A., and V. Moreno, "LISP MIB", Work
              in Progress, January 2013.

   [LISP-MN]  Farinacci, D., Lewis, D., Meyer, D., and C. White, "LISP
              Mobile Node", Work in Progress, October 2012.

   [LISP-SEC] Maino, F., Ermagan, V., Cabellos, A., Saucez, D., and O.
              Bonaventure, "LISP-Security (LISP-SEC)", Work in Progress,
              October 2012.

   [LOC-ID-ARCH]
              Meyer, D. and D. Lewis, "Architectural Implications of
              Locator/ID Separation", Work in Progress, January 2009.

   [OPENLISP] Iannone, L., Saucez, D., and O. Bonaventure, "OpenLISP
              Implementation Report", Work in Progress, July 2008.

   [RADIR]    Narten, T., "On the Scalability of Internet Routing", Work
              in Progress, February 2010.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              March 2000.

   [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.





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RFC 6830                          LISP                      January 2013


   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, June 2005.

   [RFC4192]  Baker, F., Lear, E., and R. Droms, "Procedures for
              Renumbering an IPv6 Network without a Flag Day", RFC 4192,
              September 2005.

   [RFC4866]  Arkko, J., Vogt, C., and W. Haddad, "Enhanced Route
              Optimization for Mobile IPv6", RFC 4866, May 2007.

   [RFC4984]  Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
              Workshop on Routing and Addressing", RFC 4984,
              September 2007.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, February 2012.

   [RFC6518]  Lebovitz, G. and M. Bhatia, "Keying and Authentication for
              Routing Protocols (KARP) Design Guidelines", RFC 6518,
              February 2012.

   [RFC6831]  Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas, "The
              Locator/ID Separation Protocol (LISP) for Multicast
              Environments", RFC 6831, January 2013.

   [RFC6832]  Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
              "Interworking between Locator/ID Separation Protocol
              (LISP) and Non-LISP Sites", RFC 6832, January 2013.

   [RFC6835]  Farinacci, D. and D. Meyer, "The Locator/ID Separation
              Protocol Internet Groper (LIG)", RFC 6835, January 2013.

   [RFC6837]  Lear, E., "NERD: A Not-so-novel Endpoint ID (EID) to
              Routing Locator (RLOC) Database", RFC 6837, January 2013.

   [UDP-TUNNELS]
              Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", Work in Progress,
              January 2013.

   [UDP-ZERO] Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the use of IPv6 UDP Datagrams with Zero Checksums",
              Work in Progress, December 2012.



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RFC 6830                          LISP                      January 2013


Appendix A.  Acknowledgments

   An initial thank you goes to Dave Oran for planting the seeds for the
   initial ideas for LISP.  His consultation continues to provide value
   to the LISP authors.

   A special and appreciative thank you goes to Noel Chiappa for
   providing architectural impetus over the past decades on separation
   of location and identity, as well as detailed reviews of the LISP
   architecture and documents, coupled with enthusiasm for making LISP a
   practical and incremental transition for the Internet.

   The authors would like to gratefully acknowledge many people who have
   contributed discussions and ideas to the making of this proposal.
   They include Scott Brim, Andrew Partan, John Zwiebel, Jason Schiller,
   Lixia Zhang, Dorian Kim, Peter Schoenmaker, Vijay Gill, Geoff Huston,
   David Conrad, Mark Handley, Ron Bonica, Ted Seely, Mark Townsley,
   Chris Morrow, Brian Weis, Dave McGrew, Peter Lothberg, Dave Thaler,
   Eliot Lear, Shane Amante, Ved Kafle, Olivier Bonaventure, Luigi
   Iannone, Robin Whittle, Brian Carpenter, Joel Halpern, Terry
   Manderson, Roger Jorgensen, Ran Atkinson, Stig Venaas, Iljitsch van
   Beijnum, Roland Bless, Dana Blair, Bill Lynch, Marc Woolward, Damien
   Saucez, Damian Lezama, Attilla De Groot, Parantap Lahiri, David
   Black, Roque Gagliano, Isidor Kouvelas, Jesper Skriver, Fred Templin,
   Margaret Wasserman, Sam Hartman, Michael Hofling, Pedro Marques, Jari
   Arkko, Gregg Schudel, Srinivas Subramanian, Amit Jain, Xu Xiaohu,
   Dhirendra Trivedi, Yakov Rekhter, John Scudder, John Drake, Dimitri
   Papadimitriou, Ross Callon, Selina Heimlich, Job Snijders, Vina
   Ermagan, Albert Cabellos, Fabio Maino, Victor Moreno, Chris White,
   Clarence Filsfils, and Alia Atlas.

   This work originated in the Routing Research Group (RRG) of the IRTF.
   An individual submission was converted into the IETF LISP working
   group document that became this RFC.

   The LISP working group would like to give a special thanks to Jari
   Arkko, the Internet Area AD at the time that the set of LISP
   documents were being prepared for IESG last call, and for his
   meticulous reviews and detailed commentaries on the 7 working group
   last call documents progressing toward experimental RFCs.











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RFC 6830                          LISP                      January 2013


Authors' Addresses

   Dino Farinacci
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: farinacci@gmail.com


   Vince Fuller

   EMail: vaf@vaf.net


   Dave Meyer
   Cisco Systems
   170 Tasman Drive
   San Jose, CA
   USA

   EMail: dmm@1-4-5.net


   Darrel Lewis
   Cisco Systems
   170 Tasman Drive
   San Jose, CA
   USA

   EMail: darlewis@cisco.com



















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