



Internet Engineering Task Force (IETF)                        S. Previdi
Request for Comments: 9086                           Huawei Technologies
Category: Standards Track                             K. Talaulikar, Ed.
ISSN: 2070-1721                                              C. Filsfils
                                                     Cisco Systems, Inc.
                                                                K. Patel
                                                            Arrcus, Inc.
                                                                  S. Ray
                                                              Individual
                                                                 J. Dong
                                                     Huawei Technologies
                                                             August 2021


  Border Gateway Protocol - Link State (BGP-LS) Extensions for Segment
                  Routing BGP Egress Peer Engineering

Abstract

   A node steers a packet through a controlled set of instructions,
   called segments, by prepending the packet with a list of segment
   identifiers (SIDs).  A segment can represent any instruction,
   topological or service based.  SR segments allow steering a flow
   through any topological path and service chain while maintaining per-
   flow state only at the ingress node of the SR domain.

   This document describes an extension to Border Gateway Protocol -
   Link State (BGP-LS) for advertisement of BGP Peering Segments along
   with their BGP peering node information so that efficient BGP Egress
   Peer Engineering (EPE) policies and strategies can be computed based
   on Segment Routing.

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/rfc9086.

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
   2.  Requirements Language
   3.  BGP Peering Segments
   4.  BGP-LS NLRI Advertisement for BGP Protocol
     4.1.  BGP Router-ID and Member AS Number
     4.2.  Mandatory BGP Node Descriptors
     4.3.  Optional BGP Node Descriptors
   5.  BGP-LS Attributes for BGP Peering Segments
     5.1.  Advertisement of the PeerNode SID
     5.2.  Advertisement of the PeerAdj SID
     5.3.  Advertisement of the PeerSet SID
   6.  IANA Considerations
     6.1.  New BGP-LS Protocol-ID
     6.2.  Node Descriptors and Link Attribute TLVs
   7.  Manageability Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   Segment Routing (SR) leverages source routing.  A node steers a
   packet through a controlled set of instructions, called segments, by
   prepending the packet with a list of segment identifiers (SIDs).  A
   SID can represent any instruction, topological or service based.  SR
   segments allows to enforce a flow through any topological path or
   service function while maintaining per-flow state only at the ingress
   node of the SR domain.

   The SR architecture [RFC8402] defines three types of BGP Peering
   Segments that may be instantiated at a BGP node:

   *  Peer Node Segment (PeerNode SID) : instruction to steer to a
      specific peer node

   *  Peer Adjacency Segment (PeerAdj SID) : instruction to steer over a
      specific local interface towards a specific peer node

   *  Peer Set Segment (PeerSet SID) : instruction to load-balance to a
      set of specific peer nodes

   SR can be directly applied to either an MPLS data plane (SR-MPLS)
   with no change on the forwarding plane or to a modified IPv6
   forwarding plane (SRv6).

   This document describes extensions to the BGP - Link State Network
   Layer Reachability Information (BGP-LS NLRI) and the BGP-LS Attribute
   defined for BGP-LS [RFC7752] for advertising BGP peering segments
   from a BGP node along with its peering topology information (i.e.,
   its peers, interfaces, and peering Autonomous Systems (ASes)) to
   enable computation of efficient BGP Egress Peer Engineering (BGP-EPE)
   policies and strategies using the SR-MPLS data plane.  The
   corresponding extensions for SRv6 are specified in [BGPLS-SRV6].

   [RFC9087] illustrates a centralized controller-based BGP Egress Peer
   Engineering solution involving SR path computation using the BGP
   Peering Segments.  This use case comprises a centralized controller
   that learns the BGP Peering SIDs via BGP-LS and then uses this
   information to program a BGP-EPE policy at any node in the domain to
   perform traffic steering via a specific BGP egress node to specific
   External BGP (EBGP) peer(s) optionally also over a specific
   interface.  The BGP-EPE policy can be realized using the SR Policy
   framework [SR-POLICY].

   This document introduces a new BGP-LS Protocol-ID for BGP and defines
   new BGP-LS Node and Link Descriptor TLVs to facilitate advertising
   BGP-LS Link NLRI to represent the BGP peering topology.  Further, it
   specifies the BGP-LS Attribute TLVs for advertisement of the BGP
   Peering Segments (i.e., PeerNode SID, PeerAdj SID, and PeerSet SID)
   to be advertised in the same BGP-LS Link NLRI.

2.  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.  BGP Peering Segments

   As described in [RFC8402], a BGP-EPE-enabled Egress Provider Edge
   (PE) node instantiates SR Segments corresponding to its attached
   peers.  These segments are called BGP Peering Segments or BGP Peering
   SIDs.  In the case of EBGP, they enable the expression of source-
   routed interdomain paths.

   An ingress border router of an AS may compose a list of SIDs to steer
   a flow along a selected path within the AS, towards a selected egress
   border router C of the AS, and to a specific EBGP peer.  At minimum,
   a BGP-EPE policy applied at an ingress PE involves two SIDs: the Node
   SID of the chosen egress PE and then the BGP Peering SID for the
   chosen egress PE peer or peering interface.

   Each BGP session MUST be described by a PeerNode SID.  The
   description of the BGP session MAY be augmented by additional PeerAdj
   SIDs.  Finally, multiple PeerNode SIDs or PeerAdj SIDs MAY be part of
   the same group/set in order to group EPE resources under a common
   PeerSet SID.  These BGP Peering SIDs and their encoding are described
   in detail in Section 5.

   The following BGP Peering SIDs need to be instantiated on a BGP
   router for each of its BGP peer sessions that are enabled for Egress
   Peer Engineering:

   *  One PeerNode SID MUST be instantiated to describe the BGP peer
      session.

   *  One or more PeerAdj SID MAY be instantiated corresponding to the
      underlying link(s) to the directly connected BGP peer session.

   *  A PeerSet SID MAY be instantiated and additionally associated and
      shared between one or more PeerNode SIDs or PeerAdj SIDs.

   While an egress point in a topology usually refers to EBGP sessions
   between external peers, there's nothing in the extensions defined in
   this document that would prevent the use of these extensions in the
   context of Internal BGP (IBGP) sessions.  However, unlike EBGP
   sessions, which are generally between directly connected BGP routers
   also along the traffic forwarding path, IBGP peer sessions may be set
   up to BGP routers that are not in the forwarding path.  As such, when
   the IBGP design includes sessions with route reflectors, a BGP router
   SHOULD NOT instantiate a BGP Peering SID for those sessions to peer
   nodes that are not in the forwarding path since the purpose of BGP
   Peering SID is to steer traffic to those specific peers.  Thus, the
   applicability for IBGP peering may be limited to only those
   deployments where the IBGP peer is also along the forwarding data
   path.

   Any BGP Peering SIDs instantiated on the node are advertised via BGP-
   LS Link NLRI type as described in the sections below.  An
   illustration of the BGP Peering SIDs' allocations in a reference BGP
   peering topology along with the information carried in the BGP-LS
   Link NLRI and its corresponding BGP-LS Attribute are described in
   [RFC9087].

4.  BGP-LS NLRI Advertisement for BGP Protocol

   This section describes the BGP-LS NLRI encodings that describe the
   BGP peering and link connectivity between BGP routers.

   This document specifies the advertisement of BGP peering topology
   information via BGP-LS Link NLRI type, which requires use of a new
   BGP-LS Protocol-ID.

            +=============+==================================+
            | Protocol-ID | NLRI Information Source Protocol |
            +=============+==================================+
            |      7      | BGP                              |
            +-------------+----------------------------------+

               Table 1: BGP-LS Protocol Identifier for BGP

   The use of a new Protocol-ID allows separation and differentiation
   between the BGP-LS NLRIs carrying BGP information from the BGP-LS
   NLRIs carrying IGP link-state information defined in [RFC7752].

   The BGP Peering information along with their Peering Segments are
   advertised using BGP-LS Link NLRI type with the Protocol-ID set to
   BGP.  BGP-LS Link NLRI type uses the Descriptor TLVs and BGP-LS
   Attribute TLVs as defined in [RFC7752].  In order to correctly
   describe BGP nodes, new TLVs are defined in this section.

   [RFC7752] defines BGP-LS Link NLRI type as follows:

    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                                 //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //      Remote Node Descriptors                                //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //      Link Descriptors                                       //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 1: BGP-LS Link NLRI

      Node Descriptors and Link Descriptors are defined in [RFC7752].

4.1.  BGP Router-ID and Member AS Number

   Two new Node Descriptor TLVs are defined in this document:

   *  BGP Router Identifier (BGP Router-ID):

         Type: 516

         Length: 4 octets

         Value: 4-octet unsigned non-zero integer representing the BGP
         Identifier as defined in [RFC6286]

   *  Member-AS Number (Member-ASN)

         Type: 517

         Length: 4 octets

         Value: 4-octet unsigned non-zero integer representing the
         Member-AS Number [RFC5065]

4.2.  Mandatory BGP Node Descriptors

   The following Node Descriptor TLVs MUST be included in BGP-LS NLRI as
   Local Node Descriptors when distributing BGP information:

   *  BGP Router-ID (TLV 516), which contains a valid BGP Identifier of
      the local BGP node.

   *  Autonomous System Number (TLV 512) [RFC7752], which contains the
      Autonomous System Number (ASN) or AS Confederation Identifier (an
      ASN) [RFC5065], if confederations are used, of the local BGP node.

   Note that Section 2.1 of [RFC6286] requires the BGP identifier
   (Router-ID) to be unique within an Autonomous System and non-zero.
   Therefore, the <ASN, BGP Router-ID> tuple is globally unique.  Their
   use in the Node Descriptor helps map Link-State NLRIs with BGP
   protocol-ID to a unique BGP router in the administrative domain where
   BGP-LS is enabled.

   The following Node Descriptor TLVs MUST be included in BGP-LS Link
   NLRI as Remote Node Descriptors when distributing BGP information:

   *  BGP Router-ID (TLV 516), which contains the valid BGP Identifier
      of the peer BGP node.

   *  Autonomous System Number (TLV 512) [RFC7752], which contains the
      ASN or the AS Confederation Identifier (an ASN) [RFC5065], if
      confederations are used, of the peer BGP node.

4.3.  Optional BGP Node Descriptors

   The following Node Descriptor TLVs MAY be included in BGP-LS NLRI as
   Local Node Descriptors when distributing BGP information:

   *  Member-ASN (TLV 517), which contains the ASN of the confederation
      member (i.e., Member-AS Number), if BGP confederations are used,
      of the local BGP node.

   *  Node Descriptors as defined in [RFC7752].

   The following Node Descriptor TLVs MAY be included in BGP-LS Link
   NLRI as Remote Node Descriptors when distributing BGP information:

   *  Member-ASN (TLV 517), which contains the ASN of the confederation
      member (i.e., Member-AS Number), if BGP confederations are used,
      of the peer BGP node.

   *  Node Descriptors as defined in [RFC7752].

5.  BGP-LS Attributes for BGP Peering Segments

   This section defines the BGP-LS Attributes corresponding to the
   following BGP Peer Segment SIDs:

   *  Peer Node Segment Identifier (PeerNode SID)

   *  Peer Adjacency Segment Identifier (PeerAdj SID)

   *  Peer Set Segment Identifier (PeerSet SID)

   The following new BGP-LS Link Attribute TLVs are defined for use with
   BGP-LS Link NLRI for advertising BGP Peering SIDs:

                     +================+==============+
                     | TLV Code Point | Description  |
                     +================+==============+
                     | 1101           | PeerNode SID |
                     +----------------+--------------+
                     | 1102           | PeerAdj SID  |
                     +----------------+--------------+
                     | 1103           | PeerSet SID  |
                     +----------------+--------------+

                          Table 2: BGP-LS TLV Code
                             Points for BGP-EPE


   PeerNode SID, PeerAdj SID, and PeerSet SID all have the same format
   as defined 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            |              Length           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Flags         |     Weight    |             Reserved          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   SID/Label/Index (variable)                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 2: BGP Peering SIDs TLV Format

   *  Type: 1101, 1102, or 1103 as listed in Table 2

   *  Length: variable.  Valid values are either 7 or 8 based on whether
      the encoding is done as a SID Index or a label.

   *  Flags: one octet of flags with the following definition:

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |V|L|B|P| Rsvd  |
      +-+-+-+-+-+-+-+-+

                    Figure 3: Peering SID TLV Flags Format

      -  V-Flag: Value Flag.  If set, then the SID carries a label
         value.  By default, the flag is SET.

      -  L-Flag: Local Flag.  If set, then the value/index carried by
         the SID has local significance.  By default, the flag is SET.

      -  B-Flag: Backup Flag.  If set, the SID refers to a path that is
         eligible for protection using fast reroute (FRR).  The
         computation of the backup forwarding path and its association
         with the BGP Peering SID forwarding entry is implementation
         specific.  Section 3.6 of [RFC9087] discusses some of the
         possible ways of identifying backup paths for BGP Peering SIDs.

      -  P-Flag: Persistent Flag: If set, the SID is persistently
         allocated, i.e., the SID value remains consistent across router
         restart and session/interface flap.

      -  Rsvd bits: Reserved for future use and MUST be zero when
         originated and ignored when received.

   *  Weight: 1 octet.  The value represents the weight of the SID for
      the purpose of load balancing.  An example use of the weight is
      described in [RFC8402].

   *  SID/Index/Label.  According to the TLV length and the V- and
      L-Flag settings, it contains either:

      -  A 3-octet local label where the 20 rightmost bits are used for
         encoding the label value.  In this case, the V- and L-Flags
         MUST be SET.

      -  A 4-octet index defining the offset in the Segment Routing
         Global Block (SRGB) [RFC8402] advertised by this router.  In
         this case, the SRGB MUST be advertised using the extensions
         defined in [RFC9085].

   The values of the PeerNode SID, PeerAdj SID, and PeerSet SID Sub-TLVs
   SHOULD be persistent across router restart.

   When enabled for Egress Peer Engineering, the BGP router MUST include
   the PeerNode SID TLV in the BGP-LS Attribute for the BGP-LS Link NLRI
   corresponding to its BGP peering sessions.  The PeerAdj SID and
   PeerSet SID TLVs MAY be included in the BGP-LS Attribute for the BGP-
   LS Link NLRI.

   Additional BGP-LS Link Attribute TLVs as defined in [RFC7752] MAY be
   included with the BGP-LS Link NLRI in order to advertise the
   characteristics of the peering link, e.g., one or more interface
   addresses (TLV 259 or TLV 261) of the underlying link(s) over which a
   multi-hop BGP peering session is set up may be included in the BGP-LS
   Attribute along with the PeerNode SID TLV.

5.1.  Advertisement of the PeerNode SID

   The PeerNode SID TLV includes a SID associated with the BGP peer node
   that is described by a BGP-LS Link NLRI as specified in Section 4.

   The PeerNode SID, at the BGP node advertising it, has the following
   semantics (as defined in [RFC8402]):

   *  SR operation: NEXT

   *  Next-Hop: the connected peering node to which the segment is
      associated

   The PeerNode SID is advertised with a BGP-LS Link NLRI, where:

   *  Local Node Descriptors include:

      -  Local BGP Router-ID (TLV 516) of the BGP-EPE-enabled Egress PE

      -  Local ASN (TLV 512)

   *  Remote Node Descriptors include:

      -  Peer BGP Router-ID (TLV 516) (i.e., the peer BGP ID used in the
         BGP session)

      -  Peer ASN (TLV 512)

   *  Link Descriptors include the addresses used by the BGP session
      encoded using TLVs as defined in [RFC7752]:

      -  IPv4 Interface Address (TLV 259) contains the BGP session IPv4
         local address.

      -  IPv4 Neighbor Address (TLV 260) contains the BGP session IPv4
         peer address.

      -  IPv6 Interface Address (TLV 261) contains the BGP session IPv6
         local address.

      -  IPv6 Neighbor Address (TLV 262) contains the BGP session IPv6
         peer address.

   *  Link Attribute TLVs include the PeerNode SID TLV as defined in
      Figure 2.

5.2.  Advertisement of the PeerAdj SID

   The PeerAdj SID TLV includes a SID associated with the underlying
   link to the BGP peer node that is described by a BGP-LS Link NLRI as
   specified in Section 4.

   The PeerAdj SID, at the BGP node advertising it, has the following
   semantics (as defined in [RFC8402]):

   *  SR operation: NEXT

   *  Next-Hop: the interface peer address

   The PeerAdj SID is advertised with a BGP-LS Link NLRI, where:

   *  Local Node Descriptors include:

      -  Local BGP Router-ID (TLV 516) of the BGP-EPE-enabled Egress PE

      -  Local ASN (TLV 512)

   *  Remote Node Descriptors include:

      -  Peer BGP Router-ID (TLV 516) (i.e., the peer BGP ID used in the
         BGP session)

      -  Peer ASN (TLV 512)

   *  Link Descriptors MUST include the following TLV, as defined in
      [RFC7752]:

      -  Link Local/Remote Identifiers (TLV 258) contains the 4-octet
         Link Local Identifier followed by the 4-octet Link Remote
         Identifier.  The value 0 is used by default when the link
         remote identifier is unknown.

   *  Additional Link Descriptors TLVs, as defined in [RFC7752], MAY
      also be included to describe the addresses corresponding to the
      link between the BGP routers:

      -  IPv4 Interface Address (Sub-TLV 259) contains the address of
         the local interface through which the BGP session is
         established.

      -  IPv6 Interface Address (Sub-TLV 261) contains the address of
         the local interface through which the BGP session is
         established.

      -  IPv4 Neighbor Address (Sub-TLV 260) contains the IPv4 address
         of the peer interface used by the BGP session.

      -  IPv6 Neighbor Address (Sub-TLV 262) contains the IPv6 address
         of the peer interface used by the BGP session.

   *  Link Attribute TLVs include the PeerAdj SID TLV as defined in
      Figure 2.

5.3.  Advertisement of the PeerSet SID

   The PeerSet SID TLV includes a SID that is shared amongst BGP peer
   nodes or the underlying links that are described by BGP-LS Link NLRI
   as specified in Section 4.

   The PeerSet SID, at the BGP node advertising it, has the following
   semantics (as defined in [RFC8402]):

   *  SR operation: NEXT

   *  Next-Hop: load-balance across any connected interface to any peer
      in the associated peer set

   The PeerSet SID TLV containing the same SID value (encoded as defined
   in Figure 2) is included in the BGP-LS Attribute for all of the BGP-
   LS Link NLRI corresponding to the PeerNode or PeerAdj segments
   associated with the peer set.

6.  IANA Considerations

   This document defines:

   *  A new Protocol-ID: BGP.  The code point is from the "BGP-LS
      Protocol-IDs" registry.

   *  Two new TLVs: BGP-Router-ID and BGP Confederation Member.  The
      code points are in the "BGP-LS Node Descriptor, Link Descriptor,
      Prefix Descriptor, and Attribute TLVs" registry.

   *  Three new BGP-LS Attribute TLVs: PeerNode SID, PeerAdj SID, and
      PeerSet SID.  The code points are in the "BGP-LS Node Descriptor,
      Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry.

6.1.  New BGP-LS Protocol-ID

   This document defines a new value in the registry "BGP-LS Protocol-
   IDs":

      +=============+==================================+===========+
      | Protocol-ID | NLRI information source protocol | Reference |
      +=============+==================================+===========+
      | 7           | BGP                              | RFC 9086  |
      +-------------+----------------------------------+-----------+

                       Table 3: BGP-LS Protocol-ID

6.2.  Node Descriptors and Link Attribute TLVs

   This document defines five new TLVs in the registry "BGP-LS Node
   Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs":

   *  Two new Node Descriptor TLVs

   *  Three new Link Attribute TLVs

   All five of the new code points are in the same registry: "BGP-LS
   Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
   TLVs".

   The following new Node Descriptor TLVs are defined:

         +================+==========================+===========+
         | TLV Code Point | Description              | Reference |
         +================+==========================+===========+
         | 516            | BGP Router-ID            | RFC 9086  |
         +----------------+--------------------------+-----------+
         | 517            | BGP Confederation Member | RFC 9086  |
         +----------------+--------------------------+-----------+

                 Table 4: BGP-LS Descriptor TLV Code Points

   The following new Link Attribute TLVs are defined:

               +================+==============+===========+
               | TLV Code Point | Description  | Reference |
               +================+==============+===========+
               | 1101           | PeerNode SID | RFC 9086  |
               +----------------+--------------+-----------+
               | 1102           | PeerAdj SID  | RFC 9086  |
               +----------------+--------------+-----------+
               | 1103           | PeerSet SID  | RFC 9086  |
               +----------------+--------------+-----------+

                 Table 5: BGP-LS Attribute TLV Code Points

7.  Manageability Considerations

   The new protocol extensions introduced in this document augment the
   existing IGP topology information BGP-LS distribution [RFC7752] by
   adding support for distribution of BGP peering topology information.
   As such, Section 6 of [RFC7752] (Manageability Considerations)
   applies to these new extensions as well.

   Specifically, the malformed Link-State NLRI and BGP-LS Attribute
   tests for syntactic checks in Section 6.2.2 of [RFC7752] (Fault
   Management) now apply to the TLVs 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 associated
   BGP-LS Attributes is 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 from
   a BGP Speaker, over a BGP-LS session (refer to Sections 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
   is hence beyond the scope of this document.  It may be expected that
   an error detected in the NLRI Descriptor TLVs would result in that
   specific NLRI update being unusable and hence its update to be
   discarded along with an error log, whereas an error in Attribute TLVs
   would result in only that specific attribute being discarded with an
   error log.

   The operator MUST be provided with the options of configuring,
   enabling, and disabling the advertisement of each of the PeerNode
   SID, PeerAdj SID, and PeerSet SID as well as control of which
   information is advertised to which internal or external peer.  This
   is not different from what is required by a BGP speaker in terms of
   information origination and advertisement.

   BGP Peering Segments are associated with the normal BGP routing
   peering sessions.  However, the BGP peering information along with
   these Peering Segments themselves are advertised via a distinct BGP-
   LS peering session.  It is expected that this isolation as described
   in [RFC7752] is followed when advertising BGP peering topology
   information via BGP-LS.

   BGP-EPE functionality enables the capability for instantiation of an
   SR path for traffic engineering a flow via an egress BGP router to a
   specific peer, bypassing the normal BGP best-path routing for that
   flow and any routing policies implemented in BGP on that egress BGP
   router.  As with any traffic-engineering solution, the controller or
   application implementing the policy needs to ensure that there is no
   looping or misrouting of traffic.  Traffic counters corresponding to
   the MPLS label of the BGP Peering SID on the router would indicate
   the traffic being forwarded based on the specific EPE path.
   Monitoring these counters and the flows hitting the corresponding
   MPLS forwarding entry would help identify issues, if any, with
   traffic engineering over the EPE paths.  Errors in the encoding or
   decoding of the SR information in the TLVs defined in this document
   may result in the unavailability of such information to a Centralized
   EPE Controller or incorrect information being made available to it.
   This may result in the controller not being able to perform the
   desired SR-based optimization functionality or performing it in an
   unexpected or inconsistent manner.  The handling of such errors by
   applications like such a controller may be implementation specific
   and out of scope of this document.

8.  Security Considerations

   [RFC7752] defines BGP-LS NLRI to which the extensions defined in this
   document apply.  Section 8 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].

   BGP-EPE enables engineering of traffic when leaving the
   administrative domain via an egress BGP router.  Therefore,
   precaution is necessary to ensure that the BGP peering information
   collected via BGP-LS is limited to specific consumers in a secure
   manner.  Segment Routing operates within a trusted domain [RFC8402],
   and its security considerations also apply to BGP Peering Segments.
   The BGP-EPE policies are expected to be used entirely within this
   trusted SR domain (e.g., between multiple AS/domains within a single
   provider network).

   The isolation of BGP-LS peering sessions is also required to ensure
   that BGP-LS topology information (including the newly added BGP
   peering topology) is not advertised to an external BGP peering
   session outside an administrative domain.

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>.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065,
              DOI 10.17487/RFC5065, August 2007,
              <https://www.rfc-editor.org/info/rfc5065>.

   [RFC6286]  Chen, E. and J. Yuan, "Autonomous-System-Wide Unique BGP
              Identifier for BGP-4", RFC 6286, DOI 10.17487/RFC6286,
              June 2011, <https://www.rfc-editor.org/info/rfc6286>.

   [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>.

   [RFC9085]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
              H., and M. Chen, "Border Gateway Protocol - Link State
              (BGP-LS) Extensions for Segment Routing", RFC 9085,
              DOI 10.17487/RFC9085, August 2021,
              <https://www.rfc-editor.org/info/rfc9085>.

9.2.  Informative References

   [BGPLS-SRV6]
              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
              Bernier, D., and B. Decraene, "BGP Link State Extensions
              for SRv6", Work in Progress, Internet-Draft, draft-ietf-
              idr-bgpls-srv6-ext-08, 8 June 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-
              bgpls-srv6-ext-08>.

   [RFC9087]  Filsfils, C., Ed., Previdi, S., Dawra, G., Ed., Aries, E.,
              and D. Afanasiev, "Segment Routing Centralized BGP Egress
              Peer Engineering", RFC 9087, DOI 10.17487/RFC9087, August
              2021, <https://www.rfc-editor.org/info/rfc9087>.

   [SR-POLICY]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in
              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-13, 28 May 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              segment-routing-policy-13>.

Acknowledgements

   The authors would like to thank Jakob Heitz, Howard Yang, Hannes
   Gredler, Peter Psenak, Arjun Sreekantiah, and Bruno Decraene for
   their feedback and comments.  Susan Hares helped in improving the
   clarity of the document with her substantial contributions during her
   shepherd's review.  The authors would also like to thank Alvaro
   Retana for his extensive review and comments, which helped correct
   issues and improve the document.

Contributors

   Mach(Guoyi) Chen
   Huawei Technologies
   China

   Email: mach.chen@huawei.com


   Acee Lindem
   Cisco Systems Inc.
   United States of America

   Email: acee@cisco.com


Authors' Addresses

   Stefano Previdi
   Huawei Technologies

   Email: stefano@previdi.net


   Ketan Talaulikar (editor)
   Cisco Systems, Inc.
   India

   Email: ketant@cisco.com


   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com


   Keyur Patel
   Arrcus, Inc.

   Email: Keyur@arrcus.com


   Saikat Ray
   Individual

   Email: raysaikat@gmail.com


   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China

   Email: jie.dong@huawei.com
