BGP Link State Extensions for SRv6
draft-ietf-idr-bgpls-srv6-ext-08

Document Type Active Internet-Draft (idr WG)
Authors Gaurav Dawra  , Clarence Filsfils  , Ketan Talaulikar  , Mach Chen  , Daniel Bernier  , Bruno Decraene 
Last updated 2021-06-08
Replaces draft-dawra-idr-bgpls-srv6-ext
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Inter-Domain Routing                                            G. Dawra
Internet-Draft                                                  LinkedIn
Intended status: Standards Track                             C. Filsfils
Expires: December 10, 2021                            K. Talaulikar, Ed.
                                                           Cisco Systems
                                                                 M. Chen
                                                                  Huawei
                                                              D. Bernier
                                                             Bell Canada
                                                             B. Decraene
                                                                  Orange
                                                            June 8, 2021

                   BGP Link State Extensions for SRv6
                    draft-ietf-idr-bgpls-srv6-ext-08

Abstract

   Segment Routing over IPv6 (SRv6) allows for a flexible definition of
   end-to-end paths within various topologies by encoding paths as
   sequences of topological or functional sub-paths, called "segments".
   These segments are advertised by various protocols such as BGP, IS-IS
   and OSPFv3.

   This document defines extensions to BGP Link-state (BGP-LS) to
   advertise SRv6 segments along with their behaviors and other
   attributes via BGP.  The BGP-LS address-family solution for SRv6
   described in this document is similar to BGP-LS for SR for the MPLS
   data-plane defined in a separate document.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 10, 2021.

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Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  BGP-LS Extensions for SRv6  . . . . . . . . . . . . . . . . .   4
   3.  SRv6 Node Attributes  . . . . . . . . . . . . . . . . . . . .   5
     3.1.  SRv6 Capabilities TLV . . . . . . . . . . . . . . . . . .   5
     3.2.  SRv6 Node MSD Types . . . . . . . . . . . . . . . . . . .   6
   4.  SRv6 Link Attributes  . . . . . . . . . . . . . . . . . . . .   6
     4.1.  SRv6 End.X SID TLV  . . . . . . . . . . . . . . . . . . .   7
     4.2.  SRv6 LAN End.X SID TLV  . . . . . . . . . . . . . . . . .   8
     4.3.  SRv6 Link MSD Types . . . . . . . . . . . . . . . . . . .  10
   5.  SRv6 Prefix Attributes  . . . . . . . . . . . . . . . . . . .  10
     5.1.  SRv6 Locator TLV  . . . . . . . . . . . . . . . . . . . .  10
   6.  SRv6 SID NLRI . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  SRv6 SID Information TLV  . . . . . . . . . . . . . . . .  13
   7.  SRv6 SID Attributes . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  SRv6 Endpoint Behavior TLV  . . . . . . . . . . . . . . .  14
     7.2.  SRv6 BGP Peer Node SID TLV  . . . . . . . . . . . . . . .  15
   8.  SRv6 SID Structure TLV  . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  BGP-LS NLRI-Types . . . . . . . . . . . . . . . . . . . .  17
     9.2.  BGP-LS TLVs . . . . . . . . . . . . . . . . . . . . . . .  17
     9.3.  SRv6 BGP EPE SID Flags  . . . . . . . . . . . . . . . . .  18
   10. Manageability Considerations  . . . . . . . . . . . . . . . .  18
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  19
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  20
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     14.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  Differences with BGP-EPE for SR-MPLS . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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1.  Introduction

   SRv6 refers to Segment Routing instantiated on the IPv6 data-plane
   [RFC8402].  An SRv6 Segment is often referred to by its SRv6 Segment
   Identifier (SID).

   The network programming paradigm [RFC8986] is central to SRv6.  It
   describes how different behaviors can be bound to SIDs and how a
   network program can be expressed as a combination of SIDs.

   An SRv6-capable node N maintains all the SRv6 segments explicitly
   instantiated at node N.

   The IS-IS and OSPFv3 link-state routing protocols have been extended
   to advertise some of these SRv6 SIDs and SRv6-related information
   [I-D.ietf-lsr-isis-srv6-extensions],
   [I-D.ietf-lsr-ospfv3-srv6-extensions].  Other SRv6 SIDs may be
   instantiated on a node via other mechanisms for topological or
   service functionalities.

   The advertisement of SR related information along with the topology
   for the MPLS data-plane instantiation (SR-MPLS) is specified in
   [I-D.ietf-idr-bgp-ls-segment-routing-ext] and for the BGP Egress Peer
   Engineering (EPE) is specified in
   [I-D.ietf-idr-bgpls-segment-routing-epe].  On similar lines,
   introducing the SRv6 related information in BGP-LS allows consumer
   applications that require topological visibility to also receive the
   SRv6 SIDs from nodes across an IGP domain or even across Autonomous
   Systems (AS), as required.  This allows applications to leverage the
   SRv6 capabilities for network programming.

   The identifying key of each Link-State object, namely a node, link,
   or prefix, is encoded in the NLRI and the properties of the object
   are encoded in the BGP-LS Attribute [RFC7752].

   This document describes extensions to BGP-LS to advertise the SRv6
   SIDs and other SRv6 information from all the SRv6 capable nodes in
   the IGP domain when sourced from link-state routing protocols and
   directly from individual SRv6 capable nodes (e.g. when sourced from
   BGP for EPE).

1.1.  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.

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2.  BGP-LS Extensions for SRv6

   BGP-LS [RFC7752] defines the Node, Link, and Prefix Link-State
   Network Layer Reachability Information (NLRI) types and the
   advertisement of their attributes via BGP.

   When the BGP-LS router is advertising topology information that it
   sources from the underlying link-state routing protocol (as described
   in [RFC7752]), then it maps the corresponding SRv6 information from
   the SRv6 extensions for IS-IS [I-D.ietf-lsr-isis-srv6-extensions] and
   OSPFv3 [I-D.ietf-lsr-ospfv3-srv6-extensions] protocols to their BGP-
   LS TLVs/sub-TLVs for all SRv6 capable nodes in that routing protocol
   domain.  When the BGP-LS router is advertising topology information
   from the BGP routing protocol (e.g., for EPE as described in
   [I-D.ietf-idr-bgpls-segment-routing-epe]), then it advertises the
   SRv6 information from the local node alone.

   The SRv6 information pertaining to a node is advertised via the BGP-
   LS Node NLRI and using the BGP-LS Attribute TLVs as follows:

   o  SRv6 Capabilities of the node are advertised via the SRv6
      Capabilities TLV (Section 3.1).

   o  Maximum SID Depth (MSD) types introduced for SRv6 are advertised
      (Section 3.2) using the Node MSD TLV specified in [RFC8814]

   o  Algorithm support for SRv6 is advertised via the SR Algorithm TLV
      specified in [I-D.ietf-idr-bgp-ls-segment-routing-ext].

   The SRv6 information pertaining to a link is advertised via the BGP-
   LS Link NLRI and using the BGP-LS Attribute TLVs as follows:

   o  SRv6 SID of the IGP Adjacency SID or the BGP EPE Peer Adjacency
      SID [RFC8402] is advertised via the SRv6 End.X SID TLV introduced
      in this document (Section 4.1).

   o  SRv6 SID of the IGP Adjacency SID to a non-Designated Router (DR)
      or non-Designated Intermediate-System (DIS) [RFC8402] is
      advertised via the SRv6 LAN End.X SID TLV introduced in this
      document (Section 4.2).

   o  MSD types introduced for SRv6 are advertised (Section 4.3) using
      the Link MSD TLV specified in [RFC8814].

   The SRv6 information pertaining to a prefix is advertised via the
   BGP-LS Prefix NLRI and using the BGP-LS Attribute TLVs as follows:

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   o  SRv6 Locator is advertised via the SRv6 Locator TLV introduced in
      this document (Section 5.1).

   o  The attributes of the SRv6 Locator are advertised via the Prefix
      Attribute Flags TLV specified in
      [I-D.ietf-idr-bgp-ls-segment-routing-ext].

   The SRv6 SIDs associated with the node are advertised using the BGP-
   LS SRv6 SID NLRI introduced in this document (Section 6).  This
   enables the BGP-LS encoding to scale to cover a potentially large set
   of SRv6 SIDs instantiated on a node with the granularity of
   individual SIDs and without affecting the size and scalability of the
   BGP-LS updates.  BGP-LS Attribute TLVs for the SRv6 SID NLRI are
   introduced in this document as follows:

   o  The endpoint behavior of the SRv6 SID is advertised via the SRv6
      Endpoint Behavior TLV (Section 7.1).

   o  The BGP EPE Peer Node context for a Peer Node, and the Peer Set
      context for a Peer Set SID [RFC8402] are advertised via the SRv6
      BGP EPE Peer Node SID TLV (Section 7.2),

   Subsequent sections of this document specify the encoding and usage
   of these extensions.

3.  SRv6 Node Attributes

   SRv6 attributes of a node are advertised using the BGP-LS Attribute
   TLVs defined in this section and associated with the BGP-LS Node
   NLRI.

3.1.  SRv6 Capabilities TLV

   This BGP-LS Attribute TLV is used to announce the SRv6 capabilities
   of the node along with the BGP-LS Node NLRI and indicates the SRv6
   support by the node.  A single instance of this TLV MUST be included
   in the BGP-LS attribute for each SRv6 capable node.  This TLV maps
   from the SRv6 Capabilities sub-TLV
   [I-D.ietf-lsr-isis-srv6-extensions] and the SRv6 Capabilities TLV
   [I-D.ietf-lsr-ospfv3-srv6-extensions] of the IS-IS and OSPFv3
   protocol SRv6 extensions respectively.  Any sub-TLVs introduced by
   future documents for IS-IS and OSPFv3 SRv6 Capabilities are REQUIRED
   to be introduced as top-level TLVs in BGP-LS in the future.

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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Type            |          Length               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Flags             |         Reserved              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 1: SRv6 Capabilities TLV Format

   Where:

   o  Type: 1038

   o  Length : 4.

   o  Flags: 2 octet field.  The flags are derived from the SRv6
      Capabilities sub-TLV/TLV of IS-IS (section 2 of
      [I-D.ietf-lsr-isis-srv6-extensions]) and OSPFv3 (section 2 of
      [I-D.ietf-lsr-ospfv3-srv6-extensions]).

   o  Reserved: 2 octet that MUST be set to 0 and ignored on receipt.

3.2.  SRv6 Node MSD Types

   The Node MSD TLV [RFC8814] of the BGP-LS Attribute of the Node NLRI
   is also used to advertise the limits and the Segment Routing Header
   (SRH) [RFC8754] operations supported by the SRv6 capable node.  The
   SRv6 MSD Types specified in section 4 of
   [I-D.ietf-lsr-isis-srv6-extensions] are also used with the BGP-LS
   Node MSD TLV as these codepoints are shared between IS-IS, OSPF and
   BGP-LS protocols.  The description and semantics of these new MSD
   types for BGP-LS are identical as specified in
   [I-D.ietf-lsr-isis-srv6-extensions].

   Each MSD type is encoded as a one-octet type followed by a one-octet
   value as specified in [RFC8814].

4.  SRv6 Link Attributes

   SRv6 attributes and SIDs associated with a link or adjacency are
   advertised using the BGP-LS Attribute TLVs defined in this section
   and associated with the BGP-LS Link NLRI.

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4.1.  SRv6 End.X SID TLV

   The SRv6 End.X SID TLV is used to advertise the SRv6 SIDs associated
   with an IGP Adjacency SID behavior that correspond to a point-to-
   point or point-to-multipoint link or adjacency of the node running
   IS-IS and OSPFv3 protocols.  This TLV can also be used to advertise
   the SRv6 SID corresponding to the underlying layer-2 member links for
   a layer-3 bundle interface as a sub-TLV of the L2 Bundle Member
   Attribute TLV [I-D.ietf-idr-bgp-ls-segment-routing-ext].  The SRv6
   SID for the IGP adjacency using the End.X behaviors (viz.  End.X,
   End.X with PSP, End.X with USP, and End.X with PSP & USP) [RFC8986]
   are advertised using the SRv6 End.X SID TLV.

   This TLV is also used by BGP-LS to advertise the BGP EPE Peer
   Adjacency SID for SRv6 on the same lines as specified for SR-MPLS in
   [I-D.ietf-idr-bgpls-segment-routing-epe].  The SRv6 SID for the BGP
   Peer Adjacency using End.X behaviors (viz.  End.X, End.X with PSP,
   End.X with USP, and End.X with PSP & USP) [RFC8986] indicates the
   cross-connect to a specific layer-3 link to the specific BGP session
   peer (neighbor).

   The TLV has the following 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            |          Length               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        Endpoint Behavior      |      Flags    |   Algorithm   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Weight    |   Reserved    |  SID (16 octets) ...          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)             | Sub-TLVs (variable) . . .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: SRv6 End.X TLV Format

   Where:

      Type: 1106

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      Length: variable

      Endpoint Behavior: 2 octet field.  The Endpoint Behavior code
      point for this SRv6 SID as defined in section 10.2 of [RFC8986].

      Flags: 1 octet of flags.  The flags are derived from the SRv6
      End.X SID sub-TLV/TLV of IS-IS (section 8.1 of
      [I-D.ietf-lsr-isis-srv6-extensions]) and OSPFv3 (section 8.1 of
      [I-D.ietf-lsr-ospfv3-srv6-extensions]).  In the case of BGP EPE
      Peer Adjacency SID, the flags are as defined for the SRv6 BGP Peer
      Node SID TLV (Section 7.2).

      Algorithm: 1 octet field.  Algorithm associated with the SID.
      Algorithm values are defined in the IANA IGP Algorithm Type
      registry (https://www.iana.org/assignments/igp-parameters/igp-
      parameters.xhtml).

      Weight: 1 octet field.  The value represents the weight of the SID
      for the purpose of load balancing.  The use of the weight is
      defined in [RFC8402].

      Reserved: 1 octet field that MUST be set to 0 and ignored on
      receipt.

      SID: 16 octet field.  This field encodes the advertised SRv6 SID
      as 128 bit value.

      Sub-TLVs : They are allocated from the IANA "BGP-LS Node
      Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
      TLVs" registry and are used to advertise sub-TLVs that provide
      additional attributes for the specific SRv6 SID.  This document
      defines one inSection 8.

4.2.  SRv6 LAN End.X SID TLV

   For a LAN interface, an IGP node announces normally only its
   adjacency to the IS-IS pseudo-node (or the equivalent OSPF DR).  The
   SRv6 LAN End.X SID TLV allows a node to announce the SRv6 SID
   corresponding to its adjacencies to all other (i.e., non-DIS or non-
   DR) nodes attached to the LAN in a single instance of the BGP-LS Link
   NLRI.  Without this TLV, multiple BGP-LS Link NLRIs would need to be
   originated for each additional adjacency to advertise the SRv6 End.X
   SID TLVs for these neighbor adjacencies.  The SRv6 SID for these IGP
   adjacencies using the End.X behaviors (viz.  End.X, End.X with PSP,
   End.X with USP, and End.X with PSP & USP) [RFC8986] are advertised
   using the SRv6 LAN End.X SID TLV.

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   The BGP-LS IS-IS SRv6 LAN End.X SID and BGP-LS OSPFv3 SRv6 LAN End.X
   SID TLVs have the following 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            |          Length               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |       Endpoint Behavior       |      Flags    |   Algorithm   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Weight    |   Reserved    |   Neighbor ID -               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    | IS-IS System-ID (6 octets) or OSPFv3 Router-ID (4 octets)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (16 octets) ...                                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Sub-TLVs (variable) . . .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3: SRv6 LAN End.X SID TLV Format

   Where:

   o  Type: 1107 in case of IS-IS and 1108 in case of OSPFv3

   o  Length: variable

   o  Endpoint Behavior: 2 octet field.  The Endpoint Behavior code
      point for this SRv6 SID as defined in section 10.2 of [RFC8986].

   o  Flags: 1 octet of flags.  The flags are derived from the SRv6 LAN
      End.X SID sub-TLV/TLV of IS-IS (section 8.2 of
      [I-D.ietf-lsr-isis-srv6-extensions]) and OSPFv3 (section 8.2 of
      [I-D.ietf-lsr-ospfv3-srv6-extensions]).

   o  Algorithm: 1 octet field.  Algorithm associated with the SID.
      Algorithm values are defined in the IANA IGP Algorithm Type
      registry (https://www.iana.org/assignments/igp-parameters/igp-
      parameters.xhtml).

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   o  Weight: 1 octet field.  The value represents the weight of the SID
      for the purpose of load balancing.  The use of the weight is
      defined in [RFC8402].

   o  Reserved: 1 octet field that MUST be set to 0 and ignored on
      receipt.

   o  Neighbor ID : 6 octets of IS-IS System ID of the neighbor for the
      IS-IS SRv6 LAN End.X SID TLV and 4 octets of OSPFv3 Router-id of
      the neighbor for the OSPFv3 SRv6 LAN End.X SID TLV.

   o  SID: 16 octet field.  This field encodes the advertised SRv6 SID
      as 128 bit value.

   o  Sub-TLVs : They are allocated from the IANA "BGP-LS Node
      Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
      TLVs" registry and are used to advertise sub-TLVs that provide
      additional attributes for the specific SRv6 SID.  This document
      defines one inSection 8.

4.3.  SRv6 Link MSD Types

   The Link MSD TLV [RFC8814] of the BGP-LS Attribute of the Link NLRI
   is also used to advertise the limits and the SRH operations supported
   on the specific link by the SRv6 capable node.  The SRv6 MSD Types
   specified in section 4 of[I-D.ietf-lsr-isis-srv6-extensions] are also
   used with the BGP-LS Link MSD TLV as these codepoints are shared
   between IS-IS, OSPF, and BGP-LS protocols.  The description and
   semantics of these new MSD types for BGP-LS are identical as
   specified in [I-D.ietf-lsr-isis-srv6-extensions].

   Each MSD type is encoded as a one-octet type followed by a one-octet
   value as specified in [RFC8814].

5.  SRv6 Prefix Attributes

   SRv6 attributes with an IPv6 prefix are advertised using the BGP-LS
   Attribute TLVs defined in this section and associated with the BGP-LS
   Prefix NLRI.

5.1.  SRv6 Locator TLV

   As specified in [RFC8986], an SRv6 SID comprises Locator, Function
   and Argument parts.

   A node is provisioned with one or more locators supported by that
   node.  Locators are covering prefixes for the set of SIDs provisioned

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   on that node.  These Locators are advertised as BGP-LS Prefix NLRI
   objects along with the SRv6 Locator TLV in its BGP-LS Attribute.

   The IPv6 Prefix matching the Locator MAY be also advertised as a
   prefix reachability by the underlying routing protocol.  In this
   case, the Prefix NLRI would be also associated with the Prefix Metric
   TLV that carries the routing metric for this prefix.  When the
   Locator prefix is not being advertised as a prefix reachability, then
   the Prefix NLRI would have the SRv6 Locator TLV associated with it
   but no Prefix Metric TLV.  In the absence of Prefix Metric TLV, the
   consumer of the BGP-LS topology information MUST NOT interpret the
   Locator prefix as a prefix reachability routing advertisement in the
   IGPs default SPF computation.

   The SRv6 Locator TLV has the following 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            |          Length               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Flags    |   Algorithm   |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Metric                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Sub-TLVs (variable) . . .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: SRv6 Locator TLV Format

   Where:

      Type: 1162

      Length: variable

      Flags: 1 octet of flags.  The flags are derived from the SRv6
      Locator TLV of IS-IS (section 7.1 of
      [I-D.ietf-lsr-isis-srv6-extensions]) and OSPFv3 (section 6.1 of
      [I-D.ietf-lsr-ospfv3-srv6-extensions]).

      Algorithm: 1 octet field.  Algorithm associated with the SID.
      Algorithm values are defined in the IANA IGP Algorithm Type
      registry (https://www.iana.org/assignments/igp-parameters/igp-
      parameters.xhtml).

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      Reserved: 2 octet field.  The value MUST be set to 0 and ignored
      on receipt.

      Metric: 4 octet field.  The value of the metric for the Locator.

      Sub-TLVs : They are allocated from the IANA "BGP-LS Node
      Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
      TLVs" registry and are used to advertise sub-TLVs that provide
      additional attributes for the given SRv6 Locator.  Currently none
      are defined.

6.  SRv6 SID NLRI

   SRv6 SID information is advertised in BGP UPDATE messages using the
   MP_REACH_NLRI and MP_UNREACH_NLRI attributes [RFC4760].  The "Link-
   State NLRI" defined in [RFC7752] is extended to carry the SRv6 SID
   information.

   A new "Link-State NLRI Type" is defined for SRv6 SID information as
   following:

   o  Link-State NLRI Type: SRv6 SID NLRI (value 6).

   The format of this new NLRI type is as 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
    +-+-+-+-+-+-+-+-+
    |  Protocol-ID  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Identifier                             |
    |                        (64 bits)                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Local Node Descriptors (variable)              //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               SRv6 SID Descriptors (variable)                //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 5: SRv6 SID NLRI Format

   Where:

   o  Protocol-ID: 1 octet field that specifies the protocol component
      through which BGP-LS learns the SRv6 SIDs of the node.  The BGP-LS
      Protocol-ID registry was created by [RFC7752] and then additional
      assignments were made for other BGP-LS extensions.

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   o  Identifier: 8 octet value as defined in [RFC7752].

   o  Local Node Descriptors TLV: as defined in [RFC7752] for IGPs,
      local and static configuration and as defined in
      [I-D.ietf-idr-bgpls-segment-routing-epe] for BGP protocol.

   o  SRv6 SID Descriptors: MUST include a single SRv6 SID Information
      TLV defined in Section 6.1 and optionally MAY include the Multi-
      Topology Identifier TLV as defined in [RFC7752].

   New TLVs for advertisement within the BGP Link State Attribute
   [RFC7752] are defined in Section 7 to carry the attributes of an SRv6
   SID.

6.1.  SRv6 SID Information TLV

   An SRv6 SID is encoded using the SRv6 SID Information TLV.

   The TLV has the following 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            |          Length               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (16 octets) ...                                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    SID (cont ...)                                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: SRv6 SID Information TLV Format

   Where:

      Type: 518

      Length: 16.

      SID: 16 octet field.  This field encodes the advertised SRv6 SID
      as 128 bit value.

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7.  SRv6 SID Attributes

   This section specifies the TLVs to be carried in the BGP Link State
   Attribute associated with the BGP-LS SRv6 SID NLRI.

7.1.  SRv6 Endpoint Behavior TLV

   Each SRv6 SID instantiated on an SRv6 capable node has specific
   instructions (called behavior) bound to it.  [RFC8986] describes how
   behaviors are bound to a SID and also defines the initial set of
   well-known behaviors.

   The SRv6 Endpoint Behavior TLV is a mandatory TLV that MUST be
   included in the BGP-LS Attribute associated with the BGP-LS SRv6 SID
   NLRI.  The TLV has the following 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            |          Length               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |        Endpoint Behavior      |      Flags    |   Algorithm   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 7: SRv6 Endpoint Behavior TLV

   Where:

      Type: 1250

      Length: 4.

      Endpoint Behavior: 2 octet field.  The Endpoint Behavior code
      point for this SRv6 SID as defined in section 10.2 of [RFC8986].

      Flags: 1 octet of flags with the none defined currently.  Reserved
      for future use and MUST be set to 0 and ignored on receipt.

      Algorithm: 1 octet field.  Algorithm associated with the SID.
      Algorithm values are defined in the IGP Algorithm Type registry
      (https://www.iana.org/assignments/igp-parameters/igp-
      parameters.xhtml).

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7.2.  SRv6 BGP Peer Node SID TLV

   The BGP Peer Node SID and Peer Set SID for SR-MPLS are specified in
   [I-D.ietf-idr-bgpls-segment-routing-epe].  Similar Peer Node and Peer
   Set functionality can be realized with SRv6 using SIDs with END.X
   behavior.  The SRv6 BGP Peer Node SID TLV is a mandatory TLV for use
   in the BGP-LS Attribute for an SRv6 SID NLRI advertised by BGP for
   the EPE functionality.  This TLV MUST be included along with SRv6
   SIDs that are associated with the BGP Peer Node or Peer Set
   functionality.

   The TLV has the following 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            |          Length               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Flags    |     Weight    |          Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Peer AS Number                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Peer BGP Identifier                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 8: SRv6 BGP Peer Node SID TLV Format

   Where:

   o  Type: 1251

   o  Length: 12

   o  Flags: 1 octet of flags with the following definition (also refer
      to Section 9.3 for IANA registry):

     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |B|S|P| Reserved|
    +-+-+-+-+-+-+-+-+

                  Figure 9: SRv6 BGP EPE SID Flags Format

      *  B-Flag: Backup Flag.  If set, the SID is eligible for
         protection (e.g. using IPFRR) as described in [RFC8355].

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      *  S-Flag: Set Flag.  When set, the S-Flag indicates that the SID
         refers to a set of BGP peering sessions (i.e.  BGP Peer Set SID
         functionality) and therefore MAY be assigned to one or more
         End.X SIDs associated with BGP peer sessions.

      *  P-Flag: Persistent Flag: When set, the P-Flag indicates that
         the SID is persistently allocated, i.e., the value remains
         consistent across router restart and/or session flap.

      *  Other bits are reserved for future use and MUST be set to 0 and
         ignored on receipt.

   o  Weight: 1 octet field.  The value represents the weight of the SID
      for the purpose of load balancing.  The use of the weight is
      defined in [RFC8402].

   o  Reserved: 2 octet field.  The value MUST be set to 0 and ignored
      on receipt.

   o  Peer AS Number : 4 octets of BGP AS number of the peer router.

   o  Peer BGP Identifier : 4 octets of the BGP Identifier (BGP Router-
      ID) of the peer router.

   For an SRv6 BGP EPE Peer Node SID, one instance of this TLV is
   associated with the SRv6 SID.  For SRv6 BGP EPE Peer Set SID,
   multiple instances of this TLV (one for each peer in the "peer set")
   are associated with the SRv6 SID and the S (set/group) flag is SET.

8.  SRv6 SID Structure TLV

   The SRv6 SID Structure TLV is used to advertise the length of each
   individual part of the SRv6 SID as defined in [RFC8986].  It is an
   optional TLV for use in the BGP-LS Attribute for an SRv6 SID NLRI and
   as a sub-TLV of the SRv6 End.X, IS-IS SRv6 LAN End.X and OSPFv3 SRv6
   LAN End.X TLVs.  The TLV has the following 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            |          Length               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    LB Length  |  LN Length    | Fun. Length   |  Arg. Length  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 10: SRv6 SID Structure TLV

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   Where:

      Type: 1252

      Length: 4

      Locator Block Length: 1 octet field.  SRv6 SID Locator Block
      length in bits.

      Locator Node Length: 1 octet field.  SRv6 SID Locator Node length
      in bits.

      Function Length: 1 octet field.  SRv6 SID Function length in bits.

      Argument Length: 1 octet field.  SRv6 SID Argument length in bits.

   The total of the locator block, locator node, function, and argument
   lengths MUST be less than or equal to 128.

9.  IANA Considerations

   This document requests assigning code-points from the IANA "Border
   Gateway Protocol - Link State (BGP-LS) Parameters" registry as
   described in the sub-sections below.

9.1.  BGP-LS NLRI-Types

   The following codepoints are assigned by IANA via the early
   allocation process from within the sub-registry called "BGP-LS NLRI-
   Types":

    +------+----------------------------+---------------+
    | Type | NLRI Type                  |   Reference   |
    +------+----------------------------+---------------+
    |  6   | SRv6 SID                   | this document |
    +------+----------------------------+---------------+

                  Figure 11: SRv6 SID NLRI Type Codepoint

9.2.  BGP-LS TLVs

   The following TLV codepoints are assigned by IANA via the early
   allocation process from within the sub-registry called "BGP-LS Node
   Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs":

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   +----------+----------------------------------------+---------------+
   | TLV Code |             Description                | Value defined |
   |  Point   |                                        |       in      |
   +----------+----------------------------------------+---------------+
   |   518    |   SRv6 SID Information TLV             | this document |
   |  1038    |   SRv6 Capabilities TLV                | this document |
   |  1106    |   SRv6 End.X SID TLV                   | this document |
   |  1107    |   IS-IS SRv6 LAN End.X SID TLV         | this document |
   |  1108    |   OSPFv3 SRv6 LAN End.X SID TLV        | this document |
   |  1162    |   SRv6 Locator TLV                     | this document |
   |  1250    |   SRv6 Endpoint Behavior TLV           | this document |
   |  1251    |   SRv6 BGP Peer Node SID TLV           | this document |
   |  1252    |   SRv6 SID Structure TLV               | this document |
   +----------+----------------------------------------+---------------+

              Figure 12: SRv6 BGP-LS Attribute TLV Codepoints

9.3.  SRv6 BGP EPE SID Flags

   This document requests the creation of a new registry called "SRv6
   BGP EPE SID Flags" under the "Border Gateway Protocol - Link State
   (BGP-LS) Parameters" registry.  The allocation policy of this
   registry is "Standards Action" according to [RFC8126].

   The following flags are defined:

             Bit     Description                   Reference
            ---------------------------------------------------
               0     Backup Flag (B-Flag)        This document
               1     Set Flag (S-Flag)           This document
               2     Persistent Flag (P-Flag)    This document
             3-7     Unassigned

10.  Manageability Considerations

   This section is structured as recommended in [RFC5706].

   The new protocol extensions introduced in this document augment the
   existing IGP topology information that is distributed via [RFC7752].
   Procedures and protocol extensions defined in this document do not
   affect the BGP protocol operations and management other than as
   discussed in the Manageability Considerations section of [RFC7752].
   Specifically, the malformed attribute tests for syntactic checks in
   the Fault Management section of [RFC7752] now encompass the new BGP-
   LS extensions defined in this document.  The semantic or content
   checking for the TLVs specified in this document and their
   association with the BGP-LS NLRI types or their BGP-LS Attribute is

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   left to the consumer of the BGP-LS information (e.g., an application
   or a controller) and not the BGP protocol.

   A consumer of the BGP-LS information retrieves this information over
   a BGP-LS session (refer Section 1 and 2 of [RFC7752]).  The handling
   of semantic or content errors by the consumer would be dictated by
   the nature of its application usage and hence is beyond the scope of
   this document.

   The SR information introduced in BGP-LS by this specification may be
   used by BGP-LS consumer applications like an SR path computation
   engine (PCE) to learn the SRv6 capabilities of the nodes in the
   topology and the mapping of SRv6 segments to those nodes.  This can
   enable the SR PCE to perform path computations based on SR for
   traffic engineering use-cases and to steer traffic on paths different
   from the underlying IGP based distributed best path computation.
   Errors in the encoding or decoding of the SRv6 information may result
   in the unavailability of such information to the SR PCE or incorrect
   information being made available to it.  This may result in the SR
   PCE not being able to perform the desired SR-based optimization
   functionality or to perform it in an unexpected or inconsistent
   manner.  The handling of such errors by applications like SR PCE may
   be implementation-specific and out of the scope of this document.

   The manageability considerations related to BGP EPE functionality are
   discussed in [I-D.ietf-idr-bgpls-segment-routing-epe] in the context
   of SR-MPLS and they also apply to this document in the context of
   SRv6.

   The extensions, specified in this document, do not introduce any new
   configuration or monitoring aspects in BGP or BGP-LS other than as
   discussed in [RFC7752].  The manageability aspects of the underlying
   SRv6 features are covered by [I-D.ietf-spring-srv6-yang].

11.  Security Considerations

   The new protocol extensions introduced in this document augment the
   existing IGP topology information that is distributed via [RFC7752].
   The advertisement of the SRv6 link-state information defined in this
   document presents a similar risk as associated with the existing set
   of link-state information as described in [RFC7752].  The Security
   Considerations section of [RFC7752] also applies to these extensions.
   The procedures and new TLVs defined in this document, by themselves,
   do not affect the BGP-LS security model discussed in [RFC7752].

   The extensions introduced in this document are used to propagate IGP
   defined information ([I-D.ietf-lsr-isis-srv6-extensions] and
   [I-D.ietf-lsr-ospfv3-srv6-extensions]).  These extensions represent

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   the advertisement of SRv6 information associated with the IGP node,
   link, and prefix.  The IGP instances originating these TLVs are
   assumed to support all the required security and authentication
   mechanisms (as described in [I-D.ietf-lsr-isis-srv6-extensions] and
   [I-D.ietf-lsr-ospfv3-srv6-extensions]) in order to prevent any
   security issue when propagating the information into BGP-LS.

   The security considerations related to BGP EPE functionality are
   discussed in [I-D.ietf-idr-bgpls-segment-routing-epe] in the context
   of SR-MPLS and they also apply to this document in the context of
   SRv6.

   BGP-LS SRv6 extensions enable traffic engineering use-cases within
   the Segment Routing domain.  SR operates within a trusted domain
   [RFC8402] and its security considerations also apply to BGP-LS
   sessions when carrying SR information.  The SR traffic engineering
   policies using the SIDs advertised via BGP-LS are expected to be used
   entirely within this trusted SR domain (e.g., between multiple AS/
   domains within a single provider network).  Therefore, precaution is
   necessary to ensure that the link-state information (including SRv6
   information) advertised via BGP-LS sessions is limited to consumers
   in a secure manner within this trusted SR domain.  BGP peering
   sessions for address-families other than Link-State may be set up to
   routers outside the SR domain.  The isolation of BGP-LS peering
   sessions is recommended to ensure that BGP-LS topology information
   (including the newly added SR information) is not advertised to an
   external BGP peering session outside the SR domain.

12.  Contributors

   James Uttaro
   AT&T
   USA
   Email: ju1738@att.com

   Hani Elmalky
   Ericsson
   USA
   Email: hani.elmalky@gmail.com

   Arjun Sreekantiah
   Individual
   USA
   Email: arjunhrs@gmail.com

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   Les Ginsberg
   Cisco Systems
   USA
   Email: ginsberg@cisco.com

   Shunwan Zhuang
   Huawei
   China
   Email: zhuangshunwan@huawei.com

13.  Acknowledgements

   The authors would like to thank Peter Psenak, Arun Babu, Pablo
   Camarillo, Francois Clad, Peng Shaofu, Cheng Li, and Dhruv Dhody for
   their review of this document and their comments.  The authors would
   also like to thanks Susan Hares for her shepherd review and Adrian
   Farrel for his detailed Routing Directorate review.

14.  References

14.1.  Normative References

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Talaulikar, K., Filsfils, C., Gredler, H.,
              and M. Chen, "BGP Link-State extensions for Segment
              Routing", draft-ietf-idr-bgp-ls-segment-routing-ext-18
              (work in progress), April 2021.

   [I-D.ietf-idr-bgpls-segment-routing-epe]
              Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
              S., and J. Dong, "BGP-LS extensions for Segment Routing
              BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
              segment-routing-epe-19 (work in progress), May 2019.

   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extension to Support Segment Routing over
              IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-15
              (work in progress), May 2021.

   [I-D.ietf-lsr-ospfv3-srv6-extensions]
              Li, Z., Hu, Z., Cheng, D., Talaulikar, K., and P. Psenak,
              "OSPFv3 Extensions for SRv6", draft-ietf-lsr-
              ospfv3-srv6-extensions-02 (work in progress), February
              2021.

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   [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>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [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>.

   [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>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [RFC8814]  Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
              and N. Triantafillis, "Signaling Maximum SID Depth (MSD)
              Using the Border Gateway Protocol - Link State", RFC 8814,
              DOI 10.17487/RFC8814, August 2020,
              <https://www.rfc-editor.org/info/rfc8814>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

14.2.  Informative References

   [I-D.ietf-spring-srv6-yang]
              Raza, K., Agarwal, S., Liu, X., Hu, Z., Hussain, I., Shah,
              H., Voyer, D., Matsushima, S., Horiba, K., AbdelSalam, A.,
              and J. Rajamanickam, "YANG Data Model for SRv6 Base and
              Static", draft-ietf-spring-srv6-yang-00 (work in
              progress), September 2020.

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   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC5706]  Harrington, D., "Guidelines for Considering Operations and
              Management of New Protocols and Protocol Extensions",
              RFC 5706, DOI 10.17487/RFC5706, November 2009,
              <https://www.rfc-editor.org/info/rfc5706>.

   [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>.

   [RFC8355]  Filsfils, C., Ed., Previdi, S., Ed., Decraene, B., and R.
              Shakir, "Resiliency Use Cases in Source Packet Routing in
              Networking (SPRING) Networks", RFC 8355,
              DOI 10.17487/RFC8355, March 2018,
              <https://www.rfc-editor.org/info/rfc8355>.

Appendix A.  Differences with BGP-EPE for SR-MPLS

   The signaling of SRv6 SIDs corresponding to BGP-EPE functionality as
   defined in this document differ from the signaling of SR-MPLS BGP-EPE
   SIDs as specified in [I-D.ietf-idr-bgpls-segment-routing-epe].  This
   section provides a high-level overview of the same.

   There is no difference in the advertisement of the BGP Peer Adjacency
   SID in both SR-MPLS and SRv6 and it is advertised as an attribute of
   the Link NLRI which identifies a specific Layer 3 interface on the
   BGP Speaker.  The difference is in the advertisement of the BGP Peer
   Node and Peer Set SIDs.

   In case of SR-MPLS, an additional Link NLRI is required to be
   advertised corresponding to each BGP Peering session on the node.
   Note that, this is not the same Link NLRI associated with the actual
   layer 3 interface even when the peering is setup using the interface
   IP addresses.  These BGP-LS Link NLRIs are not really links in the
   traditional link-state routing data model but instead identify BGP
   peering sessions.  The BGP Peer Node and/or Peer Set SIDs associated
   with that peering session are advertised as attributes associated
   with this peering Link NLRI.  In the case of SRv6, each BGP Peer Node
   or Peer Set SID is considered to be associated with the BGP Speaker
   node and is advertised using the BGP-LS SRv6 SID NLRI while the
   peering session information is advertised as attributes associated
   with it.

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   The advertisement of the BGP Peer Set SID for SR-MPLS is done by
   including that SID as an attribute in all the Link NLRIs
   corresponding to the peering sessions that are part of the "set".
   The advertisement of the BGP Peer Set SID for SRv6 is advertised
   using a single SRv6 SID NLRI and all the peers associated with that
   "set" are indicated as attributes associated with the NLRI.

Authors' Addresses

   Gaurav Dawra
   LinkedIn
   USA

   Email: gdawra.ietf@gmail.com

   Clarence Filsfils
   Cisco Systems
   Belgium

   Email: cfilsfil@cisco.com

   Ketan Talaulikar (editor)
   Cisco Systems
   India

   Email: ketant@cisco.com

   Mach Chen
   Huawei
   China

   Email: mach.chen@huawei.com

   Daniel Bernier
   Bell Canada
   Canada

   Email: daniel.bernier@bell.ca

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   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com

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