path: root/doc/rfc/rfc8664.txt

Internet Engineering Task Force (IETF)                      S. Sivabalan
Request for Comments: 8664                                   C. Filsfils
Updates: 8408                                        Cisco Systems, Inc.
Category: Standards Track                                    J. Tantsura
ISSN: 2070-1721                                             Apstra, Inc.
                                                           W. Henderickx
                                                                   Nokia
                                                             J. Hardwick
                                                     Metaswitch Networks
                                                           December 2019


 Path Computation Element Communication Protocol (PCEP) Extensions for
                            Segment Routing

Abstract

   Segment Routing (SR) enables any head-end node to select any path
   without relying on a hop-by-hop signaling technique (e.g., LDP or
   RSVP-TE).  It depends only on "segments" that are advertised by link-
   state Interior Gateway Protocols (IGPs).  An SR path can be derived
   from a variety of mechanisms, including an IGP Shortest Path Tree
   (SPT), an explicit configuration, or a Path Computation Element
   (PCE).  This document specifies extensions to the Path Computation
   Element Communication Protocol (PCEP) that allow a stateful PCE to
   compute and initiate Traffic-Engineering (TE) paths, as well as a
   Path Computation Client (PCC) to request a path subject to certain
   constraints and optimization criteria in SR networks.

   This document updates RFC 8408.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8664.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction
   2.  Terminology
     2.1.  Requirements Language
   3.  Overview of PCEP Operation in SR Networks
   4.  Object Formats
     4.1.  The OPEN Object
       4.1.1.  The Path Setup Type Capability TLV
       4.1.2.  The SR PCE Capability Sub-TLV
     4.2.  The RP/SRP Object
     4.3.  ERO
       4.3.1.  SR-ERO Subobject
       4.3.2.  NAI Associated with SID
     4.4.  RRO
     4.5.  METRIC Object
   5.  Procedures
     5.1.  Exchanging the SR PCE Capability
     5.2.  ERO Processing
       5.2.1.  SR-ERO Validation
       5.2.2.  Interpreting the SR-ERO
     5.3.  RRO Processing
   6.  Management Considerations
     6.1.  Controlling the Path Setup Type
     6.2.  Migrating a Network to Use PCEP Segment-Routed Paths
     6.3.  Verification of Network Operation
     6.4.  Relationship to Existing Management Models
   7.  Security Considerations
   8.  IANA Considerations
     8.1.  PCEP ERO and RRO Subobjects
     8.2.  New NAI Type Registry
     8.3.  New SR-ERO Flag Registry
     8.4.  PCEP-Error Object
     8.5.  PCEP TLV Type Indicators
     8.6.  PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators
     8.7.  New Path Setup Type
     8.8.  New Metric Type
     8.9.  SR PCE Capability Flags
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Compatibility with Early Implementations
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   Segment Routing (SR) leverages the source-routing paradigm.  Using
   SR, a source node steers a packet through a path without relying on
   hop-by-hop signaling protocols such as LDP or RSVP-TE.  Each path is
   specified as an ordered list of instructions called "segments".  Each
   segment is an instruction to route the packet to a specific place in
   the network or to perform a function on the packet.  A database of
   segments can be distributed through the network using a routing
   protocol (such as IS-IS or OSPF) or by any other means.  Several
   types of segments are defined.  A node segment uniquely identifies a
   specific node in the SR domain.  Each router in the SR domain
   associates a node segment with an ECMP-aware shortest path to the
   node that it identifies.  An adjacency segment represents a
   unidirectional adjacency.  An adjacency segment is local to the node
   that advertises it.  Both node segments and adjacency segments can be
   used for SR.

   [RFC8402] describes the SR architecture.  The corresponding IS-IS and
   OSPF extensions are specified in [RFC8667] and [RFC8665],
   respectively.

   The SR architecture can be implemented using either an MPLS
   forwarding plane [RFC8660] or an IPv6 forwarding plane [IPv6-SRH].
   The MPLS forwarding plane can be applied to SR without any change; in
   which case, an SR path corresponds to an MPLS Label Switching Path
   (LSP).  This document is relevant to the MPLS forwarding plane only.
   In this document, "Node-SID" and "Adj-SID" denote the Node Segment
   Identifier and Adjacency Segment Identifier, respectively.

   An SR path can be derived from an IGP Shortest Path Tree (SPT).
   Segment Routing Traffic-Engineering (SR-TE) paths may not follow an
   IGP SPT.  Such paths may be chosen by a suitable network planning
   tool and provisioned on the ingress node of the SR-TE path.

   [RFC5440] describes the Path Computation Element Communication
   Protocol (PCEP) for communication between a Path Computation Client
   (PCC) and a Path Computation Element (PCE) or between a pair of PCEs.
   A PCE computes paths for MPLS Traffic-Engineering (MPLS-TE) LSPs
   based on various constraints and optimization criteria.  [RFC8231]
   specifies extensions to PCEP that allow a stateful PCE to compute and
   recommend network paths in compliance with [RFC4657].  It also
   defines objects and TLVs for MPLS-TE LSPs.  Stateful PCEP extensions
   provide synchronization of LSP state between a PCC and a PCE or
   between a pair of PCEs, delegation of LSP control, reporting of LSP
   state from a PCC to a PCE, and control of the setup and path routing
   of an LSP from a PCE to a PCC.  Stateful PCEP extensions are intended
   for an operational model in which LSPs are configured on the PCC, and
   control over them is delegated to the PCE.

   A mechanism to dynamically initiate LSPs on a PCC based on the
   requests from a stateful PCE or a controller using stateful PCE is
   specified in [RFC8281].  This mechanism is useful in Software-Defined
   Networking (SDN) applications, such as on-demand engineering or
   bandwidth calendaring [RFC8413].

   It is possible to use a stateful PCE for computing one or more SR-TE
   paths, taking into account various constraints and objective
   functions.  Once a path is chosen, the stateful PCE can initiate an
   SR-TE path on a PCC using the PCEP extensions specified in [RFC8281]
   and the SR-specific PCEP extensions specified in this document.
   Additionally, using procedures described in this document, a PCC can
   request an SR path from either a stateful or a stateless PCE.

   This specification relies on the procedures specified in [RFC8408] to
   exchange the Segment Routing capability and to specify that the path
   setup type of an LSP is Segment Routing.  This specification also
   updates [RFC8408] to clarify the use of sub-TLVs in the PATH-SETUP-
   TYPE-CAPABILITY TLV.  See Section 4.1.1 for details.

   This specification provides a mechanism for a network controller
   (acting as a PCE) to instantiate candidate paths for an SR Policy
   onto a head-end node (acting as a PCC) using PCEP.  For more
   information on the SR Policy Architecture, see [SR-POLICY].

2.  Terminology

   The following terminology is used in this document:

   ERO:    Explicit Route Object

   IGP:    Interior Gateway Protocol

   IS-IS:  Intermediate System to Intermediate System

   LSR:    Label Switching Router

   MSD:    Base MPLS Imposition Maximum SID Depth, as defined in
           [RFC8491]

   NAI:    Node or Adjacency Identifier

   OSPF:   Open Shortest Path First

   PCC:    Path Computation Client

   PCE:    Path Computation Element

   PCEP:   Path Computation Element Communication Protocol

   RRO:    Record Route Object

   SID:    Segment Identifier

   SR:     Segment Routing

   SR-DB:  Segment Routing Database: the collection of SRGBs, SRLBs, and
           SIDs and the objects they map to, advertised by a link-state
           IGP

   SR-TE:  Segment Routing Traffic Engineering

   SRGB:   Segment Routing Global Block

   SRLB:   Segment Routing Local Block

2.1.  Requirements Language

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

3.  Overview of PCEP Operation in SR Networks

   In an SR network, the ingress node of an SR path prepends an SR
   header to all outgoing packets.  The SR header consists of a list of
   SIDs (or MPLS labels in the context of this document).  The header
   has all necessary information so that, in combination with the
   information distributed by the IGP, the packets can be guided from
   the ingress node to the egress node of the path; hence, there is no
   need for any signaling protocol.

   In PCEP messages, LSP route information is carried in the Explicit
   Route Object (ERO), which consists of a sequence of subobjects.  SR-
   TE paths computed by a PCE can be represented in an ERO in one of the
   following forms:

   *  An ordered set of IP addresses representing network nodes/links.

   *  An ordered set of SIDs, with or without the corresponding IP
      addresses.

   *  An ordered set of MPLS labels, with or without corresponding IP
      addresses.

   The PCC converts these into an MPLS label stack and next hop, as
   described in Section 5.2.2.

   This document defines a new ERO subobject denoted by "SR-ERO
   subobject" that is capable of carrying a SID as well as the identity
   of the node/adjacency represented by the SID.  SR-capable PCEP
   speakers should be able to generate and/or process such an ERO
   subobject.  An ERO containing SR-ERO subobjects can be included in
   the PCEP Path Computation Reply (PCRep) message defined in [RFC5440],
   the Path Computation LSP Initiate Request (PCInitiate) message
   defined in [RFC8281], and the Path Computation Update Request (PCUpd)
   and Path Computation State Report (PCRpt) messages for LSPs defined
   in [RFC8231].

   When a PCEP session between a PCC and a PCE is established, both PCEP
   speakers exchange their capabilities to indicate their ability to
   support SR-specific functionality.

   A PCE can update an LSP that is initially established via RSVP-TE
   signaling to use an SR-TE path by sending a PCUpd to the PCC that
   delegated the LSP to it [RFC8231].  A PCC can update an undelegated
   LSP that is initially established via RSVP-TE signaling to use an SR-
   TE path as follows.  First, it requests an SR-TE path from a PCE by
   sending a Path Computation Request (PCReq) message.  If it receives a
   suitable path, it establishes the path in the data plane and then
   tears down the original RSVP-TE path.  If the PCE is stateful, then
   the PCC sends PCRpt messages indicating that the new path is set up
   and the old path is torn down, per [RFC8231].

   Similarly, a PCE or PCC can update an LSP initially created with an
   SR-TE path to use RSVP-TE signaling, if necessary.  This capability
   is useful for rolling back a change when a network is migrated from
   RSVP-TE to SR-TE technology.

   A PCC MAY include a Record Route Object (RRO) containing the recorded
   LSP in PCReq and PCRpt messages as specified in [RFC5440] and
   [RFC8231], respectively.  This document defines a new RRO subobject
   for SR networks.  The methods used by a PCC to record the SR-TE LSP
   are outside the scope of this document.

   In summary, this document:

   *  Defines a new ERO subobject, a new RRO subobject, and new PCEP
      error codes.

   *  Specifies how two PCEP speakers can establish a PCEP session that
      can carry information about SR-TE paths.

   *  Specifies processing rules for the ERO subobject.

   *  Defines a new path setup type to be used in the PATH-SETUP-TYPE
      and PATH-SETUP-TYPE-CAPABILITY TLVs [RFC8408].

   *  Defines a new sub-TLV for the PATH-SETUP-TYPE-CAPABILITY TLV.

   The extensions specified in this document complement the existing
   PCEP specifications to support SR-TE paths.  As such, the PCEP
   messages (e.g., PCReq, PCRep, PCRpt, PCUpd, PCInitiate, etc.) are
   formatted according to [RFC5440], [RFC8231], [RFC8281], and any other
   applicable PCEP specifications.

4.  Object Formats

4.1.  The OPEN Object

4.1.1.  The Path Setup Type Capability TLV

   [RFC8408] defines the PATH-SETUP-TYPE-CAPABILITY TLV for use in the
   OPEN object.  The PATH-SETUP-TYPE-CAPABILITY TLV contains an optional
   list of sub-TLVs, which are intended to convey parameters that are
   associated with the path setup types supported by a PCEP speaker.

   This specification updates [RFC8408] as follows.  It creates a new
   registry that defines the valid type indicators of the sub-TLVs of
   the PATH-SETUP-TYPE-CAPABILITY TLV (see Section 8.6).  A PCEP speaker
   MUST NOT include a sub-TLV in the PATH-SETUP-TYPE-CAPABILITY TLV
   unless it appears in this registry.  If a PCEP speaker receives a
   sub-TLV whose type indicator does not match one of those from the
   registry or is not recognized by the speaker, then the speaker MUST
   ignore the sub-TLV.

4.1.2.  The SR PCE Capability Sub-TLV

   This document defines a new Path Setup Type (PST) for SR, as follows:

      PST = 1:  Traffic-engineering path is set up using Segment
         Routing.

   A PCEP speaker SHOULD indicate its support of the function described
   in this document by sending a PATH-SETUP-TYPE-CAPABILITY TLV in the
   OPEN object with this new PST included in the PST list.

   This document also defines the SR-PCE-CAPABILITY sub-TLV.  PCEP
   speakers use this sub-TLV to exchange information about their SR
   capability.  If a PCEP speaker includes PST=1 in the PST list of the
   PATH-SETUP-TYPE-CAPABILITY TLV, then it MUST also include the SR-PCE-
   CAPABILITY sub-TLV inside the PATH-SETUP-TYPE-CAPABILITY TLV.

   The format of the SR-PCE-CAPABILITY sub-TLV is shown in the following
   figure:

      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=26               |            Length=4           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Reserved              |   Flags   |N|X|      MSD      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: SR-PCE-CAPABILITY Sub-TLV Format

   The codepoint for the TLV type is 26.  The TLV length is 4 octets.

   The 32-bit value is formatted as follows.

   Reserved:  MUST be set to zero by the sender and MUST be ignored by
      the receiver.

   Flags:  This document defines the following flag bits.  The other
      bits MUST be set to zero by the sender and MUST be ignored by the
      receiver.

         N:   A PCC sets this flag bit to 1 to indicate that it is
              capable of resolving a Node or Adjacency Identifier (NAI)
              to a SID.

         X:   A PCC sets this flag bit to 1 to indicate that it does not
              impose any limit on the MSD.

   Maximum SID Depth (MSD):  specifies the maximum number of SIDs (MPLS
      label stack depth in the context of this document) that a PCC is
      capable of imposing on a packet.  Section 5.1 explains the
      relationship between this field and the X-Flag.

4.2.  The RP/SRP Object

   To set up an SR-TE LSP using SR, the Request Parameter (RP) or
   Stateful PCE Request Parameter (SRP) object MUST include the PATH-
   SETUP-TYPE TLV, specified in [RFC8408], with the PST set to 1 (and
   path setup using SR-TE).

   The LSP-IDENTIFIERS TLV MAY be present for the above PST type.

4.3.  ERO

   An SR-TE path consists of one or more SIDs where each SID MAY be
   associated with the identifier that represents the node or adjacency
   corresponding to the SID.  This identifier is referred to as the NAI.
   As described later, an NAI can be represented in various formats
   (e.g., IPv4 address, IPv6 address, etc).  Furthermore, an NAI is used
   for troubleshooting purposes and, if necessary, to derive a SID value
   as described below.

   The ERO specified in [RFC5440] is used to carry SR-TE path
   information.  In order to carry a SID and/or NAI, this document
   defines a new ERO subobject referred to as the "SR-ERO subobject",
   whose format is specified in the following section.  An ERO carrying
   an SR-TE path consists of one or more ERO subobjects, and it MUST
   carry only SR-ERO subobjects.  Note that an SR-ERO subobject does not
   need to have both the SID and NAI.  However, at least one of them
   MUST be present.

   When building the MPLS label stack from ERO, a PCC MUST assume that
   SR-ERO subobjects are organized as a last-in-first-out stack.  The
   first subobject relative to the beginning of ERO contains the
   information about the topmost label.  The last subobject contains
   information about the bottommost label.

4.3.1.  SR-ERO Subobject

   An SR-ERO subobject is formatted as shown in the following diagram.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |L|   Type=36   |     Length    |  NT   |     Flags     |F|S|C|M|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         SID (optional)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                   NAI (variable, optional)                  //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 2: SR-ERO Subobject Format

   The fields in the SR-ERO subobject are as follows:

   The L-Flag:  Indicates whether the subobject represents a loose hop
      in the LSP [RFC3209].  If this flag is set to zero, a PCC MUST NOT
      overwrite the SID value present in the SR-ERO subobject.
      Otherwise, a PCC MAY expand or replace one or more SID values in
      the received SR-ERO based on its local policy.

   Type:  Set to 36.

   Length:  Contains the total length of the subobject in octets.  The
      Length MUST be at least 8 and MUST be a multiple of 4.  An SR-ERO
      subobject MUST contain at least one SID or NAI.  The flags
      described below indicate whether the SID or NAI fields are absent.

   NAI Type (NT):  Indicates the type and format of the NAI contained in
      the object body, if any is present.  If the F bit is set to zero
      (see below), then the NT field has no meaning and MUST be ignored
      by the receiver.  This document describes the following NT values:

      NT=0  The NAI is absent.

      NT=1  The NAI is an IPv4 node ID.

      NT=2  The NAI is an IPv6 node ID.

      NT=3  The NAI is an IPv4 adjacency.

      NT=4  The NAI is an IPv6 adjacency with global IPv6 addresses.

      NT=5  The NAI is an unnumbered adjacency with IPv4 node IDs.

      NT=6  The NAI is an IPv6 adjacency with link-local IPv6 addresses.

   Flags:  Used to carry additional information pertaining to the SID.
      This document defines the following flag bits.  The other bits
      MUST be set to zero by the sender and MUST be ignored by the
      receiver.

      M:   If this bit is set to 1, the SID value represents an MPLS
           label stack entry as specified in [RFC3032].  Otherwise, the
           SID value is an administratively configured value that
           represents an index into an MPLS label space (either SRGB or
           SRLB) per [RFC8402].

      C:   If the M bit and the C bit are both set to 1, then the TC, S,
           and TTL fields in the MPLS label stack entry are specified by
           the PCE.  However, a PCC MAY choose to override these values
           according to its local policy and MPLS forwarding rules.  If
           the M bit is set to 1 but the C bit is set to zero, then the
           TC, S, and TTL fields MUST be ignored by the PCC.  The PCC
           MUST set these fields according to its local policy and MPLS
           forwarding rules.  If the M bit is set to zero, then the C
           bit MUST be set to zero.

      S:   When this bit is set to 1, the SID value in the subobject
           body is absent.  In this case, the PCC is responsible for
           choosing the SID value, e.g., by looking it up in the SR-DB
           using the NAI that, in this case, MUST be present in the
           subobject.  If the S bit is set to 1, then the M and C bits
           MUST be set to zero.

      F:   When this bit is set to 1, the NAI value in the subobject
           body is absent.  The F bit MUST be set to 1 if NT=0;
           otherwise, it MUST be set to zero.  The S and F bits MUST NOT
           both be set to 1.

   SID:  The Segment Identifier.  Depending on the M bit, it contains
      either:

      *  A 4-octet index defining the offset into an MPLS label space
         per [RFC8402] or

      *  A 4-octet MPLS label stack entry, where the 20 most significant
         bits encode the label value per [RFC3032].

   NAI:  The NAI associated with the SID.  The NAI's format depends on
      the value in the NT field and is described in the following
      section.

   At least one SID and NAI MUST be included in the SR-ERO subobject,
   and both MAY be included.

4.3.2.  NAI Associated with SID

   This document defines the following NAIs:

   IPv4 Node ID:  Specified as an IPv4 address.  In this case, the NT
      value is 1, and the NAI field length is 4 octets.

   IPv6 Node ID:  Specified as an IPv6 address.  In this case, the NT
      value is 2, and the NAI field length is 16 octets.

   IPv4 Adjacency:  Specified as a pair of IPv4 addresses.  In this
      case, the NT value is 3, and the NAI field length is 8 octets.
      The format of the NAI is shown in the following figure:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Local IPv4 address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Remote IPv4 address                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 3: NAI for IPv4 Adjacency

   IPv6 Global Adjacency:  Specified as a pair of global IPv6 addresses.
      It is used to describe an IPv6 adjacency for a link that uses
      global IPv6 addresses.  Each global IPv6 address is configured on
      a specific router interface, so together they identify an
      adjacency between a pair of routers.  In this case, the NT value
      is 4, and the NAI field length is 32 octets.  The format of the
      NAI is shown in the following figure:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //               Local IPv6 address (16 octets)                //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //               Remote IPv6 address (16 octets)               //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 4: NAI for IPv6 Global Adjacency

   Unnumbered Adjacency with IPv4 NodeIDs:  Specified as a pair of (node
      ID, interface ID) tuples.  In this case, the NT value is 5, and
      the NAI field length is 16 octets.  The format of the NAI is shown
      in the following figure:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Local Node ID                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    Local Interface ID                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Remote Node ID                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Remote Interface ID                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 5: NAI for Unnumbered Adjacency with IPv4 Node IDs

   IPv6 Link-Local Adjacency:  Specified as a pair of (global IPv6
      address, interface ID) tuples.  It is used to describe an IPv6
      adjacency for a link that uses only link-local IPv6 addresses.
      Each global IPv6 address is configured on a specific router, so
      together they identify a pair of adjacent routers.  The interface
      IDs identify the link that the adjacency is formed over.  In this
      case, the NT value is 6, and the NAI field length is 40 octets.
      The format of the NAI is shown in the following figure:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //               Local IPv6 address (16 octets)                //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    Local Interface ID                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //               Remote IPv6 address (16 octets)               //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Remote Interface ID                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: NAI for IPv6 Link-Local Adjacency

4.4.  RRO

   A PCC reports an SR-TE LSP to a PCE by sending a PCRpt message, per
   [RFC8231].  The RRO on this message represents the SID list that was
   applied by the PCC, that is, the actual path taken by the LSP.  The
   procedures of [RFC8231] with respect to the RRO apply equally to this
   specification without change.

   An RRO contains one or more subobjects called "SR-RRO subobjects",
   whose format is shown below:

     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=36    |     Length    |  NT   |     Flags     |F|S|C|M|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              SID                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                        NAI (variable)                       //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 7: SR-RRO Subobject Format

   The format of the SR-RRO subobject is the same as that of the SR-ERO
   subobject, but without the L-Flag.

   A PCC MUST order the SR-RRO subobjects such that the first subobject
   relative to the beginning of the RRO identifies the first segment
   visited by the SR-TE LSP, and the last subobject identifies the final
   segment of the SR-TE LSP, that is, its endpoint.

4.5.  METRIC Object

   A PCC MAY request that PCE optimizes an individual path computation
   request to minimize the SID depth of the computed path by using the
   METRIC object defined in [RFC5440].  This document defines a new type
   for the METRIC object to be used for this purpose, as follows:

      T = 11:  Maximum SID Depth of the requested path.

   If the PCC includes a METRIC object of this type on a path
   computation request, then the PCE minimizes the SID depth of the
   computed path.  If the B (bound) bit is set to 1 in the METRIC
   object, then the PCE MUST NOT return a path whose SID depth exceeds
   the given metric value.  If the PCC did not set the X-Flag in its SR-
   PCE-CAPABILITY TLV, then it MUST set the B bit to 1.  If the PCC set
   the X-Flag in its SR-PCE-CAPABILITY TLV, then it MAY set the B bit to
   1 or zero.

   If a PCEP session is established with a non-zero default MSD value,
   then the PCC MUST NOT send an MSD METRIC object with an MSD greater
   than the session's default MSD.  If the PCE receives a path
   computation request with an MSD METRIC object on such a session that
   is greater than the session's default MSD, then it MUST consider the
   request invalid and send a PCEP Error (PCErr) with Error-Type = 10
   ("Reception of an invalid object") and Error-value = 9 ("MSD exceeds
   the default for the PCEP session").

5.  Procedures

5.1.  Exchanging the SR PCE Capability

   A PCC indicates that it is capable of supporting the head-end
   functions for SR-TE LSP by including the SR-PCE-CAPABILITY sub-TLV in
   the Open message that it sends to a PCE.  A PCE indicates that it is
   capable of computing SR-TE paths by including the SR-PCE-CAPABILITY
   sub-TLV in the Open message that it sends to a PCC.

   If a PCEP speaker receives a PATH-SETUP-TYPE-CAPABILITY TLV with a
   PST list containing PST=1, and supports that path setup type, then it
   checks for the presence of the SR-PCE-CAPABILITY sub-TLV.  If that
   sub-TLV is absent, then the PCEP speaker MUST send a PCErr message
   with Error-Type = 10 ("Reception of an invalid object") and Error-
   value = 12 ("Missing PCE-SR-CAPABILITY sub-TLV") and MUST then close
   the PCEP session.  If a PCEP speaker receives a PATH-SETUP-TYPE-
   CAPABILITY TLV with a SR-PCE-CAPABILITY sub-TLV, but the PST list
   does not contain PST=1, then the PCEP speaker MUST ignore the SR-PCE-
   CAPABILITY sub-TLV.

   If a PCC sets the N-Flag to 1, then the PCE MAY send an SR-ERO
   subobject containing an NAI and no SID (see Section 5.2).  Otherwise,
   the PCE MUST NOT send an SR-ERO subobject containing an NAI and no
   SID.

   The number of SIDs that can be imposed on a packet depends on the
   PCC's data-plane capability.  If a PCC sets the X-Flag to 1, then the
   MSD is not used and MUST be set to zero.  If a PCE receives an SR-
   PCE-CAPABILITY sub-TLV with the X-Flag set to 1, then it MUST ignore
   the MSD field and assume that the sender can impose a SID stack of
   any depth.  If a PCC sets the X-Flag to zero, then it sets the MSD
   field to the maximum number of SIDs that it can impose on a packet.
   In this case, the PCC MUST set the MSD to a number greater than zero.
   If a PCE receives an SR-PCE-CAPABILITY sub-TLV with the X-Flag and
   MSD both set to zero, then it MUST send a PCErr message with Error-
   Type = 10 ("Reception of an invalid object") and Error-value = 21
   ("Maximum SID depth must be non-zero") and MUST then close the PCEP
   session.

   Note that the MSD value exchanged via the SR-PCE-CAPABILITY sub-TLV
   indicates the SID/label imposition limit for the PCC node.  It is
   anticipated that, in many deployments, the PCCs will have network
   interfaces that are homogeneous with respect to MSD (that is, each
   interface has the same MSD).  In such cases, having a per-node MSD on
   the PCEP session is sufficient; the PCE SHOULD interpret this to mean
   that all network interfaces on the PCC have the given MSD.  However,
   the PCE MAY also learn a per-node MSD and a per-interface MSD from
   the routing protocols, as specified in [RFC8491], [RFC8476], and
   [MSD-BGP].  If the PCE learns the per-node MSD of a PCC from a
   routing protocol, then it MUST ignore the per-node MSD value in the
   SR-PCE-CAPABILITY sub-TLV and use the per-node MSD learned from the
   routing protocol instead.  If the PCE learns the MSD of a network
   interface on a PCC from a routing protocol, then it MUST use the per-
   interface MSD instead of the MSD value in the SR-PCE-CAPABILITY sub-
   TLV when it computes a path that uses that interface.

   Once an SR-capable PCEP session is established with a non-zero MSD
   value, the corresponding PCE MUST NOT send SR-TE paths with a number
   of SIDs exceeding that MSD value.  If a PCC needs to modify the MSD
   value, it MUST close the PCEP session and re-establish it with the
   new MSD value.  If a PCEP session is established with a non-zero MSD
   value, and the PCC receives an SR-TE path containing more SIDs than
   specified in the MSD value, the PCC MUST send a PCErr message with
   Error-Type = 10 ("Reception of an invalid object") and Error-value =
   3 ("Unsupported number of SR-ERO subobjects").  If a PCEP session is
   established with an MSD value of zero, then the PCC MAY specify an
   MSD for each path computation request that it sends to the PCE, by
   including a "maximum SID depth" METRIC object on the request, as
   defined in Section 4.5.

   The N-Flag, X-Flag, and MSD value inside the SR-PCE-CAPABILITY sub-
   TLV are meaningful only in the Open message sent from a PCC to a PCE.
   As such, a PCE MUST set the N-Flag to zero, X-Flag to 1, and MSD
   value to zero in an outbound message to a PCC.  Similarly, a PCC MUST
   ignore any MSD value received from a PCE.  If a PCE receives multiple
   SR-PCE-CAPABILITY sub-TLVs in an Open message, it processes only the
   first sub-TLV received.

5.2.  ERO Processing

5.2.1.  SR-ERO Validation

   If a PCC does not support the SR PCE Capability and thus cannot
   recognize the SR-ERO or SR-RRO subobjects, it will respond according
   to the rules for a malformed object per [RFC5440].

   On receiving an SR-ERO, a PCC MUST validate that the Length field, S
   bit, F bit, and NT field are consistent, as follows.

   *  If NT=0, the F bit MUST be 1, the S bit MUST be zero, and the
      Length MUST be 8.

   *  If NT=1, the F bit MUST be zero.  If the S bit is 1, the Length
      MUST be 8; otherwise, the Length MUST be 12.

   *  If NT=2, the F bit MUST be zero.  If the S bit is 1, the Length
      MUST be 20; otherwise, the Length MUST be 24.

   *  If NT=3, the F bit MUST be zero.  If the S bit is 1, the Length
      MUST be 12; otherwise, the Length MUST be 16.

   *  If NT=4, the F bit MUST be zero.  If the S bit is 1, the Length
      MUST be 36; otherwise, the Length MUST be 40.

   *  If NT=5, the F bit MUST be zero.  If the S bit is 1, the Length
      MUST be 20; otherwise, the Length MUST be 24.

   *  If NT=6, the F bit MUST be zero.  If the S bit is 1, the Length
      MUST be 44; otherwise, the Length MUST be 48.

   If a PCC finds that the NT field, Length field, S bit, and F bit are
   not consistent, it MUST consider the entire ERO invalid and MUST send
   a PCErr message with Error-Type = 10 ("Reception of an invalid
   object") and Error-value = 11 ("Malformed object").

   If a PCC does not recognize or support the value in the NT field, it
   MUST consider the entire ERO invalid and MUST send a PCErr message
   with Error-Type = 10 ("Reception of an invalid object") and Error-
   value = 13 ("Unsupported NAI Type in the SR-ERO/SR-RRO subobject").

   If a PCC receives an SR-ERO subobject in which the S and F bits are
   both set to 1 (that is, both the SID and NAI are absent), it MUST
   consider the entire ERO invalid and send a PCErr message with Error-
   Type = 10 ("Reception of an invalid object") and Error-value = 6
   ("Both SID and NAI are absent in the SR-ERO subobject").

   If a PCC receives an SR-ERO subobject in which the S bit is set to 1
   and the F bit is set to zero (that is, the SID is absent and the NAI
   is present), but the PCC does not support NAI resolution, it MUST
   consider the entire ERO invalid and send a PCErr message with Error-
   Type = 4 ("Not supported object") and Error-value = 4 ("Unsupported
   parameter").

   If a PCC receives an SR-ERO subobject in which the S bit is set to 1
   and either (or both) the M bit or the C bit is set to 1, it MUST
   consider the entire ERO invalid and send a PCErr message with Error-
   Type = 10 ("Reception of an invalid object") and Error-value = 11
   ("Malformed object").

   If a PCC receives an SR-ERO subobject in which the S bit is set to
   zero and the M bit is set to 1, then the subobject contains an MPLS
   label.  The PCC MAY choose not to accept a label provided by the PCE,
   based on its local policy.  The PCC MUST NOT accept MPLS label value
   3 (Implicit NULL), but it MAY accept other special-purpose MPLS label
   values.  If the PCC decides not to accept an MPLS label value, it
   MUST send a PCErr message with Error-Type = 10 ("Reception of an
   invalid object") and Error-value = 2 ("Bad label value").

   If both the M and C bits of an SR-ERO subobject are set to 1, and if
   a PCC finds an erroneous setting in one or more of the TC, S, and TTL
   fields, it MAY overwrite those fields with values chosen according to
   its own policy.  If the PCC does not overwrite them, it MUST send a
   PCErr message with Error-Type = 10 ("Reception of an invalid object")
   and Error-value = 4 ("Bad label format").

   If the M bit of an SR-ERO subobject is set to zero but the C bit is
   set to 1, then the PCC MUST consider the entire ERO invalid and MUST
   send a PCErr message with Error-Type = 10 ("Reception of an invalid
   object") and Error-value = 11 ("Malformed object").

   If a PCC receives an SR-ERO subobject in which the S bit is set to
   zero and the M bit is set to zero, then the subobject contains a SID
   index value.  If the SID is an Adj-SID, then the L-Flag MUST NOT be
   set.  If the L-Flag is set for an Adj-SID, then the PCC MUST send a
   PCErr message with Error-Type = 10 ("Reception of an invalid object")
   and Error-value = 11 ("Malformed object").

   If a PCC detects that the subobjects of an ERO are a mixture of SR-
   ERO subobjects and subobjects of other types, then it MUST send a
   PCErr message with Error-Type = 10 ("Reception of an invalid object")
   and Error-value = 5 ("ERO mixes SR-ERO subobjects with other
   subobject types").

   The SR-ERO subobjects can be classified according to whether they
   contain a SID representing an MPLS label value or an index value, or
   no SID.  If a PCC detects that the SR-ERO subobjects are a mixture of
   more than one of these types, then it MUST send a PCErr message with
   Error-Type = 10 ("Reception of an invalid object") and Error-value =
   20 ("Inconsistent SIDs in SR-ERO/SR-RRO subobjects").

   If an ERO specifies a new SR-TE path for an existing LSP and the PCC
   determines that the ERO contains SR-ERO subobjects that are not
   valid, then the PCC MUST NOT update the LSP.

5.2.2.  Interpreting the SR-ERO

   The SR-ERO contains a sequence of subobjects.  Each SR-ERO subobject
   in the sequence identifies a segment that the traffic will be
   directed to, in the order given.  That is, the first subobject
   identifies the first segment the traffic will be directed to, the
   second subobject represents the second segment, and so on.

   The PCC interprets the SR-ERO by converting it to an MPLS label stack
   plus a next hop.  The PCC sends packets along the segment-routed path
   by prepending the MPLS label stack onto the packets and sending the
   resulting, modified packet to the next hop.

   The PCC uses a different procedure to do this conversion, depending
   on the information that the PCE has provided in the subobjects.

   *  If the subobjects contain SID index values, then the PCC converts
      them into the corresponding MPLS labels by following the procedure
      defined in [RFC8660].

   *  If the subobjects contain NAIs only, the PCC first converts each
      NAI into a SID index value and then proceeds as above.  To convert
      an NAI to a SID index, the PCC looks for a fully specified prefix
      or adjacency matching the fields in the NAI.  If the PCC finds a
      matching prefix/adjacency, and the matching prefix/adjacency has a
      SID associated with it, then the PCC uses that SID.  If the PCC
      cannot find a matching prefix/adjacency, or if the matching
      prefix/adjacency has no SID associated with it, the PCC behaves as
      specified in Section 5.2.2.1.

   *  If the subobjects contain MPLS labels, then the PCC looks up the
      offset of the first subobject's label in its SRGB or SRLB.  This
      gives the first SID.  The PCC pushes the labels in any remaining
      subobjects onto the packet (with the final subobject specifying
      the bottom-of-stack label).

   For all cases above, after the PCC has imposed the label stack on the
   packet, it sends the packet to the segment identified by the first
   SID.

5.2.2.1.  Handling Errors During SR-ERO Conversion

   There are several errors that can occur during the process of
   converting an SR-ERO sequence to an MPLS label stack and a next hop.
   The PCC deals with them as follows.

   *  If the PCC cannot find a SID index in the SR-DB, it MUST send a
      PCErr message with Error-Type = 10 ("Reception of an invalid
      object") and Error-value = 14 ("Unknown SID").

   *  If the PCC cannot find an NAI in the SR-DB, it MUST send a PCErr
      message with Error-Type = 10 ("Reception of an invalid object")
      and Error-value = 15 ("NAI cannot be resolved to a SID").

   *  If the PCC needs to convert a SID into an MPLS label value but
      cannot find the corresponding router's SRGB in the SR-DB, it MUST
      send a PCErr message with Error-Type = 10 ("Reception of an
      invalid object") and Error-value = 16 ("Could not find SRGB").

   *  If the PCC finds that a router's SRGB is not large enough for a
      SID index value, it MUST send a PCErr message with Error-Type = 10
      ("Reception of an invalid object") and Error-value = 17 ("SID
      index exceeds SRGB size").

   *  If the PCC needs to convert a SID into an MPLS label value but
      cannot find the corresponding router's SRLB in the SR-DB, it MUST
      send a PCErr message with Error-Type = 10 ("Reception of an
      invalid object") and Error-value = 18 ("Could not find SRLB").

   *  If the PCC finds that a router's SRLB is not large enough for a
      SID index value, it MUST send a PCErr message with Error-Type = 10
      ("Reception of an invalid object") and Error-value = 19 ("SID
      index exceeds SRLB size").

   *  If the number of labels in the computed label stack exceeds the
      maximum number of SIDs that the PCC can impose on the packet, it
      MUST send a PCErr message with Error-Type = 10 ("Reception of an
      invalid object") and Error-value = 3 ("Unsupported number of SR-
      ERO subobjects").

   If an ERO specifies a new SR-TE path for an existing LSP and the PCC
   encounters an error while processing the ERO, then the PCC MUST NOT
   update the LSP.

5.3.  RRO Processing

   The syntax-checking rules that apply to the SR-RRO subobject are
   identical to those of the SR-ERO subobject, except as noted below.

   If a PCEP speaker receives an SR-RRO subobject in which both SID and
   NAI are absent, it MUST consider the entire RRO invalid and send a
   PCErr message with Error-Type = 10 ("Reception of an invalid object")
   and Error-value = 7 ("Both SID and NAI are absent in the SR-RRO
   subobject").

   If a PCE detects that the subobjects of an RRO are a mixture of SR-
   RRO subobjects and subobjects of other types, then it MUST send a
   PCErr message with Error-Type = 10 ("Reception of an invalid object")
   and Error-value = 10 ("RRO mixes SR-RRO subobjects with other
   subobject types").

   The SR-RRO subobjects can be classified according to whether they
   contain a SID representing an MPLS label value or an index value, or
   no SID.  If a PCE detects that the SR-RRO subobjects are a mixture of
   more than one of these types, then it MUST send a PCErr message with
   Error-Type = 10 ("Reception of an invalid object") and Error-value =
   20 ("Inconsistent SIDs in SR-ERO / SR-RRO subobjects").

6.  Management Considerations

   This document adds a new path setup type to PCEP to allow LSPs to be
   set up using Segment Routing techniques.  This path setup type may be
   used with PCEP alongside other path setup types, such as RSVP-TE, or
   it may be used exclusively.

6.1.  Controlling the Path Setup Type

   The following factors control which path setup type is used for a
   given LSP.

   *  The available path setup types are constrained to those that are
      supported by, or enabled on, the PCEP speakers.  The PATH-SETUP-
      TYPE-CAPABILITY TLV indicates which path setup types a PCEP
      speaker supports.  To use Segment Routing as a path setup type, it
      is a prerequisite that the PCC and PCE both include PST=1 in the
      list of supported path setup types in this TLV and also include
      the SR-PCE-CAPABILITY sub-TLV.

   *  When a PCE initiates an LSP, it proposes which path setup type to
      use by including it in the PATH-SETUP-TYPE TLV in the SRP object
      of the PCInitiate message.  The PCE chooses the path setup type
      based on the capabilities of the network nodes on the path and on
      its local policy.  The PCC MAY choose to accept the proposed path
      setup type or to reject the PCInitiate request, based on its local
      policy.

   *  When a PCC requests a path for an LSP, it can nominate a preferred
      path setup type by including it in the PATH-SETUP-TYPE TLV in the
      RP object of the PCReq message.  The PCE MAY choose to reply with
      a path of the requested type, reply with a path of a different
      type, or reject the request, based on the capabilities of the
      network nodes on the path and on its local policy.

   The operator can influence the path setup type as follows.

   *  Implementations MUST allow the operator to enable and disable the
      Segment Routing path setup type on a PCEP-speaking device.
      Implementations MAY also allow the operator to enable and disable
      the RSVP-TE path setup type.

   *  PCE implementations MUST allow the operator to specify that an LSP
      should be instantiated using Segment Routing or RSVP-TE as the
      proposed path setup type.

   *  PCE implementations MAY allow the operator to configure a
      preference for the PCE to propose paths using Segment Routing or
      RSVP-TE in the absence of a specified path setup type.

   *  PCC implementations MUST allow the operator to specify that a path
      requested for an LSP nominates Segment Routing or RSVP-TE as the
      path setup type.

   *  PCC implementations MAY allow the operator to configure a
      preference for the PCC to nominate Segment Routing or RSVP-TE as
      the path setup type if none is specified for an LSP.

   *  PCC implementations SHOULD allow the operator to configure a PCC
      to refuse to set up an LSP using an undesired path setup type.

6.2.  Migrating a Network to Use PCEP Segment-Routed Paths

   This section discusses the steps that the operator takes when
   migrating a network to enable PCEP to set up paths using Segment
   Routing as the path setup type.

   *  The operator enables the Segment Routing PST on the PCE servers.

   *  The operator enables the Segment Routing PST on the PCCs.

   *  The operator resets each PCEP session.  The PCEP sessions come
      back up with Segment Routing enabled.

   *  If the operator detects a problem, they can roll the network back
      to its initial state by disabling the Segment Routing PST on the
      PCEP speakers and resetting the PCEP sessions.

   Note that the data plane is unaffected if a PCEP session is reset.
   Any LSPs that were set up before the session reset will remain in
   place and will still be present after the session comes back up.

   An implementation SHOULD allow the operator to manually trigger a
   PCEP session to be reset.

   An implementation MAY automatically reset a PCEP session when an
   operator reconfigures the PCEP speaker's capabilities.  However, note
   that if the capabilities at both ends of the PCEP session are not
   reconfigured simultaneously, then the session could be reset twice,
   which could lead to unnecessary network traffic.  Therefore, such
   implementations SHOULD allow the operator to override this behavior
   and wait instead for a manual reset.

   Once Segment Routing is enabled on a PCEP session, it can be used as
   the path setup type for future LSPs.

   User traffic is not automatically migrated from existing LSPs onto
   segment-routed LSPs just by enabling the Segment Routing PST in PCEP.
   The migration of user traffic from existing LSPs onto Segment Routing
   LSPs is beyond the scope of this document.

6.3.  Verification of Network Operation

   The operator needs the following information to verify that PCEP is
   operating correctly with respect to the Segment Routing path setup
   type.

   *  An implementation SHOULD allow the operator to view whether the
      PCEP speaker sent the Segment Routing PST capability to its peer.
      If the PCEP speaker is a PCC, then the implementation SHOULD also
      allow the operator to view the values of the L-Flag and N-Flag
      that were sent and the value of the MSD field that was sent.

   *  An implementation SHOULD allow the operator to view whether the
      peer sent the Segment Routing PST capability.  If the peer is a
      PCC, then the implementation SHOULD also allow the operator to
      view the values of the L-Flag and N-Flag and MSD fields that the
      peer sent.

   *  An implementation SHOULD allow the operator to view whether the
      Segment Routing PST is enabled on the PCEP session.

   *  If one PCEP speaker advertises the Segment Routing PST capability,
      but the other does not, then the implementation SHOULD create a
      log to inform the operator of the capability mismatch.

   *  An implementation SHOULD allow the operator to view the PST that
      was proposed, or requested, for an LSP and the PST that was
      actually used.

   *  If a PCEP speaker decides to use a different PST to the one that
      was proposed, or requested, for an LSP, then the implementation
      SHOULD create a log to inform the operator that the expected PST
      has not been used.  The log SHOULD give the reason for this choice
      (local policy, equipment capability, etc.).

   *  If a PCEP speaker rejects a Segment Routing path, then it SHOULD
      create a log to inform the operator, giving the reason for the
      decision (local policy, MSD exceeded, etc.).

6.4.  Relationship to Existing Management Models

   The PCEP YANG module is defined in [PCE-PCEP-YANG].  In the future,
   this YANG module should be extended or augmented to provide the
   following additional information relating to Segment Routing:

   *  The advertised PST capabilities and MSD per PCEP session.

   *  The PST configured for, and used by, each LSP.

   The PCEP MIB [RFC7420] could also be updated to include this
   information.

7.  Security Considerations

   The security considerations described in [RFC5440], [RFC8231],
   [RFC8281], and [RFC8408] are applicable to this specification.  No
   additional security measures are required.

   Note that this specification enables a network controller to
   instantiate a path in the network without the use of a hop-by-hop
   signaling protocol (such as RSVP-TE).  This creates an additional
   vulnerability if the security mechanisms of [RFC5440], [RFC8231], and
   [RFC8281] are not used.  If there is no integrity protection on the
   session, then an attacker could create a path that is not subjected
   to the further verification checks that would be performed by the
   signaling protocol.

   Note that this specification adds the MSD field to the Open message
   (see Section 4.1.2), which discloses how many MPLS labels the sender
   can push onto packets that it forwards into the network.  If the
   security mechanisms of [RFC8231] and [RFC8281] are not used with
   strong encryption, then an attacker could use this new field to gain
   intelligence about the capabilities of the edge devices in the
   network.

8.  IANA Considerations

8.1.  PCEP ERO and RRO Subobjects

   This document defines a new subobject type for the PCEP ERO and a new
   subobject type for the PCEP RRO.  The codepoints for subobject types
   of these objects are maintained in the "Resource Reservation Protocol
   (RSVP) Parameters" registry, under the EXPLICIT_ROUTE and
   ROUTE_RECORD objects, respectively.

       +----------------+------------------------+----------------+
       | Object         | Subobject              | Subobject Type |
       +================+========================+================+
       | EXPLICIT_ROUTE | SR-ERO (PCEP specific) | 36             |
       +----------------+------------------------+----------------+
       | ROUTE_RECORD   | SR-RRO (PCEP specific) | 36             |
       +----------------+------------------------+----------------+

                                 Table 1

8.2.  New NAI Type Registry

   IANA has created a new sub-registry within the "Path Computation
   Element Protocol (PCEP) Numbers" registry called "PCEP SR-ERO NAI
   Types".  The allocation policy for this new registry is by IETF
   Review [RFC8126].  The new registry contains the following values:

         +-------+-------------------------------+---------------+
         | Value | Description                   | Reference     |
         +=======+===============================+===============+
         | 0     | NAI is absent.                | This document |
         +-------+-------------------------------+---------------+
         | 1     | NAI is an IPv4 node ID.       | This document |
         +-------+-------------------------------+---------------+
         | 2     | NAI is an IPv6 node ID.       | This document |
         +-------+-------------------------------+---------------+
         | 3     | NAI is an IPv4 adjacency.     | This document |
         +-------+-------------------------------+---------------+
         | 4     | NAI is an IPv6 adjacency with | This document |
         |       | global IPv6 addresses.        |               |
         +-------+-------------------------------+---------------+
         | 5     | NAI is an unnumbered          | This document |
         |       | adjacency with IPv4 node IDs. |               |
         +-------+-------------------------------+---------------+
         | 6     | NAI is an IPv6 adjacency with | This document |
         |       | link-local IPv6 addresses.    |               |
         +-------+-------------------------------+---------------+
         | 7-15  | Unassigned                    |               |
         +-------+-------------------------------+---------------+

                                  Table 2

8.3.  New SR-ERO Flag Registry

   IANA has created a new sub-registry, named "SR-ERO Flag Field",
   within the "Path Computation Element Protocol (PCEP) Numbers"
   registry to manage the Flag field of the SR-ERO subobject.  New
   values are to be assigned by Standards Action [RFC8126].  Each bit
   should be tracked with the following qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Defining RFC

   The following values are defined in this document:

         +-----+---------------------------------+---------------+
         | Bit | Description                     | Reference     |
         +=====+=================================+===============+
         | 0-7 | Unassigned                      |               |
         +-----+---------------------------------+---------------+
         |  8  | NAI is absent (F)               | This document |
         +-----+---------------------------------+---------------+
         |  9  | SID is absent (S)               | This document |
         +-----+---------------------------------+---------------+
         |  10 | SID specifies TC, S, and TTL in | This document |
         |     | addition to an MPLS label (C)   |               |
         +-----+---------------------------------+---------------+
         |  11 | SID specifies an MPLS label (M) | This document |
         +-----+---------------------------------+---------------+

                                  Table 3

8.4.  PCEP-Error Object

   IANA has allocated the following codepoints in the "PCEP-ERROR Object
   Error Types and Values" registry for the following new Error-values:

    +------------+-----------------+---------------------------------+
    | Error-Type | Meaning         | Error-value                     |
    +============+=================+=================================+
    | 10         | Reception of an |                                 |
    |            | invalid object  |                                 |
    +------------+-----------------+---------------------------------+
    |            |                 | 2: Bad label value              |
    +------------+-----------------+---------------------------------+
    |            |                 | 3: Unsupported number of SR-ERO |
    |            |                 | subobjects                      |
    +------------+-----------------+---------------------------------+
    |            |                 | 4: Bad label format             |
    +------------+-----------------+---------------------------------+
    |            |                 | 5: ERO mixes SR-ERO subobjects  |
    |            |                 | with other subobject types      |
    +------------+-----------------+---------------------------------+
    |            |                 | 6: Both SID and NAI are absent  |
    |            |                 | in the SR-ERO subobject         |
    +------------+-----------------+---------------------------------+
    |            |                 | 7: Both SID and NAI are absent  |
    |            |                 | in the SR-RRO subobject         |
    +------------+-----------------+---------------------------------+
    |            |                 | 9: MSD exceeds the default for  |
    |            |                 | the PCEP session                |
    +------------+-----------------+---------------------------------+
    |            |                 | 10: RRO mixes SR-RRO subobjects |
    |            |                 | with other subobject types      |
    +------------+-----------------+---------------------------------+
    |            |                 | 12: Missing PCE-SR-CAPABILITY   |
    |            |                 | sub-TLV                         |
    +------------+-----------------+---------------------------------+
    |            |                 | 13: Unsupported NAI Type in the |
    |            |                 | SR-ERO/SR-RRO subobject         |
    +------------+-----------------+---------------------------------+
    |            |                 | 14: Unknown SID                 |
    +------------+-----------------+---------------------------------+
    |            |                 | 15: NAI cannot be resolved to a |
    |            |                 | SID                             |
    +------------+-----------------+---------------------------------+
    |            |                 | 16: Could not find SRGB         |
    +------------+-----------------+---------------------------------+
    |            |                 | 17: SID index exceeds SRGB size |
    +------------+-----------------+---------------------------------+
    |            |                 | 18: Could not find SRLB         |
    +------------+-----------------+---------------------------------+
    |            |                 | 19: SID index exceeds SRLB size |
    +------------+-----------------+---------------------------------+
    |            |                 | 20: Inconsistent SIDs in SR-ERO |
    |            |                 | / SR-RRO subobjects             |
    +------------+-----------------+---------------------------------+
    |            |                 | 21: MSD must be non-zero        |
    +------------+-----------------+---------------------------------+

                                 Table 4

8.5.  PCEP TLV Type Indicators

   IANA has allocated the following codepoint in the "PCEP TLV Type
   Indicators" registry.  Note that this TLV type indicator is
   deprecated but retained in the registry to ensure compatibility with
   early implementations of this specification.  See Appendix A for
   details.

        +-------+--------------------------------+---------------+
        | Value | Meaning                        | Reference     |
        +=======+================================+===============+
        | 26    | SR-PCE-CAPABILITY (deprecated) | This document |
        +-------+--------------------------------+---------------+

                                 Table 5

8.6.  PATH-SETUP-TYPE-CAPABILITY Sub-TLV Type Indicators

   IANA has created a new sub-registry, named "PATH-SETUP-TYPE-
   CAPABILITY Sub-TLV Type Indicators", within the "Path Computation
   Element Protocol (PCEP) Numbers" registry to manage the type
   indicator space for sub-TLVs of the PATH-SETUP-TYPE-CAPABILITY TLV.
   New values are to be assigned by Standards Action [RFC8126].  The
   valid range of values in the registry is 0-65535.  IANA has
   initialized the registry with the following values.  All other values
   in the registry should be marked as "Unassigned".

               +-------+-------------------+---------------+
               | Value | Meaning           | Reference     |
               +=======+===================+===============+
               | 0     | Reserved          | This document |
               +-------+-------------------+---------------+
               | 26    | SR-PCE-CAPABILITY | This document |
               +-------+-------------------+---------------+

                                  Table 6

8.7.  New Path Setup Type

   A sub-registry within the "Path Computation Element Protocol (PCEP)
   Numbers" registry called "PCEP Path Setup Types" was created in
   [RFC8408].  IANA has allocated a new codepoint within this registry,
   as follows:

           +-------+-------------------------------+-----------+
           | Value | Description                   | Reference |
           +=======+===============================+===========+
           | 1     | Traffic-engineering path is   | This      |
           |       | set up using Segment Routing. | document  |
           +-------+-------------------------------+-----------+

                                  Table 7

8.8.  New Metric Type

   IANA has allocated the following codepoint in the PCEP "METRIC Object
   T Field" registry:

            +-------+-------------------------+---------------+
            | Value | Description             | Reference     |
            +=======+=========================+===============+
            | 11    | Segment-ID (SID) Depth. | This document |
            +-------+-------------------------+---------------+

                                  Table 8

8.9.  SR PCE Capability Flags

   IANA has created a new sub-registry, named "SR Capability Flag
   Field", within the "Path Computation Element Protocol (PCEP) Numbers"
   registry to manage the Flag field of the SR-PCE-CAPABILITY TLV.  New
   values are to be assigned by Standards Action [RFC8126].  Each bit
   should be tracked with the following qualities:

   *  Bit number (counting from bit 0 as the most significant bit)

   *  Capability description

   *  Defining RFC

   The following values are defined in this document:

            +-----+------------------------------+-----------+
            | Bit | Description                  | Reference |
            +=====+==============================+===========+
            | 0-5 | Unassigned                   |           |
            +-----+------------------------------+-----------+
            |  6  | Node or Adjacency Identifier | This      |
            |     | (NAI) is supported (N)       | document  |
            +-----+------------------------------+-----------+
            |  7  | Unlimited Maximum SID Depth  | This      |
            |     | (X)                          | document  |
            +-----+------------------------------+-----------+

                                 Table 9

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
              <https://www.rfc-editor.org/info/rfc3032>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8408]  Sivabalan, S., Tantsura, J., Minei, I., Varga, R., and J.
              Hardwick, "Conveying Path Setup Type in PCE Communication
              Protocol (PCEP) Messages", RFC 8408, DOI 10.17487/RFC8408,
              July 2018, <https://www.rfc-editor.org/info/rfc8408>.

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,
              <https://www.rfc-editor.org/info/rfc8491>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

9.2.  Informative References

   [IPv6-SRH] Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", Work in Progress, Internet-Draft, draft-ietf-6man-
              segment-routing-header-26, 22 October 2019,
              <https://tools.ietf.org/html/draft-ietf-6man-segment-
              routing-header-26>.

   [MSD-BGP]  Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
              and N. Triantafillis, "Signaling MSD (Maximum SID Depth)
              using Border Gateway Protocol Link-State", Work in
              Progress, Internet-Draft, draft-ietf-idr-bgp-ls-segment-
              routing-msd-09, 15 October 2019,
              <https://tools.ietf.org/html/draft-ietf-idr-bgp-ls-
              segment-routing-msd-09>.

   [PCE-PCEP-YANG]
              Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
              YANG Data Model for Path Computation Element
              Communications Protocol (PCEP)", Work in Progress,
              Internet-Draft, draft-ietf-pce-pcep-yang-13, 31 October
              2019,
              <https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-13>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC4657]  Ash, J., Ed. and J.L. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol Generic
              Requirements", RFC 4657, DOI 10.17487/RFC4657, September
              2006, <https://www.rfc-editor.org/info/rfc4657>.

   [RFC7420]  Koushik, A., Stephan, E., Zhao, Q., King, D., and J.
              Hardwick, "Path Computation Element Communication Protocol
              (PCEP) Management Information Base (MIB) Module",
              RFC 7420, DOI 10.17487/RFC7420, December 2014,
              <https://www.rfc-editor.org/info/rfc7420>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8413]  Zhuang, Y., Wu, Q., Chen, H., and A. Farrel, "Framework
              for Scheduled Use of Resources", RFC 8413,
              DOI 10.17487/RFC8413, July 2018,
              <https://www.rfc-editor.org/info/rfc8413>.

   [RFC8476]  Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
              "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
              DOI 10.17487/RFC8476, December 2018,
              <https://www.rfc-editor.org/info/rfc8476>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [SR-POLICY]
              Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in
              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-05, 17 November 2019,
              <https://tools.ietf.org/html/draft-ietf-spring-segment-
              routing-policy-05>.

Appendix A.  Compatibility with Early Implementations

   An early implementation of this specification will send the SR-
   CAPABILITY-TLV as a top-level TLV in the OPEN object instead of
   sending the PATH-SETUP-TYPE-CAPABILITY TLV in the OPEN object.
   Implementations that wish to interoperate with such early
   implementations should also send the SR-CAPABILITY-TLV as a top-level
   TLV in their OPEN object and should interpret receiving this top-
   level TLV as though the sender had sent a PATH-SETUP-TYPE-CAPABILITY
   TLV with a PST list of (0, 1) (that is, both RSVP-TE and SR-TE PSTs
   are supported) with the SR-CAPABILITY-TLV as a sub-TLV.  If a PCEP
   speaker receives an OPEN object in which both the SR-CAPABILITY-TLV
   and PATH-SETUP-TYPE-CAPABILITY TLV appear as top-level TLVs, then it
   should ignore the top-level SR-CAPABILITY-TLV and process only the
   PATH-SETUP-TYPE-CAPABILITY TLV.

Acknowledgements

   We thank Ina Minei, George Swallow, Marek Zavodsky, Dhruv Dhody, Ing-
   Wher Chen, and Tomas Janciga for the valuable comments.

Contributors

   The following people contributed to this document:

   *  Lakshmi Sharma
   *  Jan Medved
   *  Edward Crabbe
   *  Robert Raszuk
   *  Victor Lopez

Authors' Addresses

   Siva Sivabalan
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata Ontario K2K 3E8
   Canada

   Email: msiva@cisco.com


   Clarence Filsfils
   Cisco Systems, Inc.
   Pegasus Parc
   Brabant 1831 De kleetlaan 6a
   Belgium

   Email: cfilsfil@cisco.com


   Jeff Tantsura
   Apstra, Inc.
   333 Middlefield Rd #200
   Menlo Park, CA 94025
   United States of America

   Email: jefftant.ietf@gmail.com


   Wim Henderickx
   Nokia
   Copernicuslaan 50
   95134 Antwerp 2018
   Belgium

   Email: wim.henderickx@nokia.com


   Jon Hardwick
   Metaswitch Networks
   100 Church Street
   Enfield
   United Kingdom

   Email: jonathan.hardwick@metaswitch.com