Label Switched Path (LSP) Ping/Traceroute for Segment Routing (SR) Egress Peer Engineering Segment Identifiers (SIDs) with MPLS Data Plane
draft-ietf-mpls-sr-epe-oam-19

Routing area                                                    S. Hegde
Internet-Draft                                             M. Srivastava
Intended status: Standards Track                   Juniper Networks Inc.
Expires: 29 January 2025                                        K. Arora
                                                  Individual Contributor
                                                                S. Ninan
                                                                   Ciena
                                                                   X. Xu
                                                            China Mobile
                                                            28 July 2024

   Label Switched Path (LSP) Ping/Traceroute for Segment Routing (SR)
Egress Peer Engineering Segment Identifiers (SIDs) with MPLS Data Plane
                     draft-ietf-mpls-sr-epe-oam-19

Abstract

   Egress Peer Engineering (EPE) is an application of Segment Routing to
   solve the problem of egress peer selection.  The Segment Routing
   based BGP-EPE solution allows a centralized controller, e.g. a
   Software Defined Network (SDN) controller to program any egress peer.
   The EPE solution requires the node or the SDN controller to program
   the PeerNode Segment Identifier(SID) describing a session between two
   nodes, the PeerAdj SID describing the link (one or more) that is used
   by sessions between peer nodes, and the PeerSet SID describing any
   connected interface to any peer in the related group.  This document
   provides new sub-TLVs for EPE Segment Identifiers (SID) that would be
   used in the MPLS Target stack TLV (Type 1), in MPLS Ping and
   Traceroute procedures.

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 29 January 2025.

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Copyright Notice

   Copyright (c) 2024 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
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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .   4
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   4.  FEC Definitions . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  PeerNode SID Sub-TLV  . . . . . . . . . . . . . . . . . .   5
     4.2.  PeerAdj SID Sub-TLV . . . . . . . . . . . . . . . . . . .   7
     4.3.  PeerSet SID Sub-TLV . . . . . . . . . . . . . . . . . . .   9
   5.  EPE-SID FEC validation  . . . . . . . . . . . . . . . . . . .  11
     5.1.  EPE-SID FEC validiation . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Juniper Networks  . . . . . . . . . . . . . . . . . . . .  15
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  APPENDIX . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Egress Peer Engineering (EPE) as defined in [RFC9087] is an effective
   mechanism to select the egress peer link based on different criteria.
   In this scenario, egress peers may belong to a completely different
   ownership.  The EPE-SIDs provide means to represent egress peer
   nodes, links, sets of links and sets of nodes.  Many network
   deployments have built their networks consisting of multiple
   Autonomous Systems, either for the ease of operations or as a result
   of network mergers and acquisitions.  The inter-AS links connecting
   any two Autonomous Systems could be traffic-engineered using EPE-SIDs
   in this case, where there is single ownership but different AS

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   numbers.  It is important to validate the control plane to forwarding
   plane synchronization for these SIDs so that any anomaly can be
   detected easily by the network operator.  EPE-SIDs may also be used
   in ingress SR policy [RFC9256]to choose exit points where the remote
   AS belongs to completely different ownership.  This scenario is out
   of scope of this document.

      +---------+      +------+
      |         |      |      |
      |    H    B------D      G
      |         | +---/| AS 2 |\  +------+
      |         |/     +------+ \ |      |---L/8
      A   AS1   C---+            \|      |
      |         |\\  \  +------+ /| AS 4 |---M/8
      |         | \\  +-E      |/ +------+
      |    X    |  \\   |      K
      |         |   +===F AS 3 |
      +---------+       +------+

                        Figure 1: Reference Diagram

   In this reference diagram, EPE-SIDs are configured on AS1 towards AS2
   and AS3 and advertised in BGP-LS [RFC9086].  In certain cases the
   EPE-SIDs advertised by the control plane may not be in
   synchronization with the label programmed in the data plane.  For
   example, on C a PeerAdj SID could be advertised to indicate it is for
   the link C->D.  Due to some software anomaly, the actual data
   forwarding on this PeerAdj SID could be happening over the C->E link.
   If E had relevant data paths for further forwarding the packet, this
   kind of anomaly will go unnoticed by the network operator.  A
   detailed example of a correctly programmed state and an incorrectly
   programmed state along with a description of how the incorrect state
   can be detected is described in Appendix A.  A FEC definition for the
   EPE-SIDs will define the details of the control plane association of
   the SID.  The data plane validation of the SID will be done during
   the MPLS traceroute procedure.  When there is a multi-hop EBGP
   session between the ASBRs, PeerNode SID is advertised, and the
   traffic MAY be load-balanced between the interfaces connecting the
   two nodes.  In the reference diagram, C and F could have a PeerNode-
   SID advertised.  When the OAM packet is received on F, it needs to be
   validated that the packet came from one of the two interfaces
   connected to C.

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   This document provides Target Forwarding Equivalence Class (FEC)
   stack TLV definitions for EPE-SIDs.  This solution requires that the
   node constructing the target FEC stack can determine the type of the
   SIDs along the path of the LSP.  Other procedures for MPLS Ping and
   Traceroute as defined in [RFC8287] section 7 and clarified by
   [RFC8690] are applicable for EPE-SIDs as well.

2.  Theory of Operation

   [RFC9086] provides mechanisms to advertise the EPE-SIDs in BGP-LS.
   These EPE-SIDs may be used to build Segment Routing paths as
   described in [I-D.ietf-idr-segment-routing-te-policy] or using Path
   Computation Element Protocol (PCEP) extensions as defined in
   [RFC8664].  Data plane monitoring for such paths which consist of
   EPE-SIDs will use extensions defined in this document to build the
   Target FEC stack TLV.  The MPLS Ping and Traceroute procedures MAY be
   initiated by the head-end of the Segment Routing path or a
   centralized topology-aware data plane monitoring system as described
   in [RFC8403].  The extensions in
   [I-D.ietf-idr-segment-routing-te-policy] and [RFC8664] do not define
   how to carry the details of the SID that can be used to construct the
   FEC.  Such extensions are out of the scope for this document.  The
   node initiating the data plane monitoring may acquire the details of
   EPE-SIDs through BGP-LS advertisements as described in [RFC9086].
   There may be other possible mechanisms to learn the definition of the
   SID from controller.  Details of such mechanisms are out of scope for
   this document.

   The EPE-SIDs are advertised for inter-AS links which run EBGP
   sessions.  [RFC9086] does not define the detailed procedures to
   operate EBGP sessions in a scenario with unnumbered interfaces.
   Therefore, these scenarios are out of scope for this document.
   Anycast and multicast addresses are not in the scope of this
   document.  During AS migration scenario procedures described in
   [RFC7705] may be in force.  In these scenarios, if the local and
   remote AS fields in the FEC as described in Section 4 carries the
   globally configured ASN and not the "local AS" as defined in
   [RFC7705], the FEC validation procedures may fail.

   As described in Section 1, this document defines FEC stack TLVs for
   EPE-SIDs, that can be used in detecting MPLS data plane failures
   [RFC8029].  This mechanism applies to paths created across across
   ASes of co-operating administrations.  If the ping or traceroute
   packet enters a non co-operating AS domain, it might be dropped by
   the routers in the non co-operating domain.  Although complete path
   validation cannot be done across, non co-operating domains, it still
   provides useful information that the ping/traceroute packet entered a
   non co-operating domain.

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3.  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.

4.  FEC Definitions

   Three new sub-TLVs are defined for the Target FEC Stack TLV (Type 1),
   the Reverse-Path Target FEC Stack TLV (Type 16), and the Reply Path
   TLV (Type 21).

               Sub-Type    Sub-TLV Name
               --------  ---------------
                TBD1      PeerAdj SID Sub-TLV
                TBD2      PeerNode SID Sub-TLV
                TBD3      PeerSet SID Sub-TLV

                        Figure 2: New sub-TLV types

4.1.  PeerNode SID Sub-TLV

        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 = TBD2                    |          Length               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               Local AS Number (4  octets)                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Remote AS Number (4 octets)                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Local BGP router ID (4 octets)                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Remote BGP Router ID (4 octets)                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 3: PeerNode SID Sub-TLV

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   Type : 2 octets

   Value:TBD2

   Length : 2 octets

   Value: 16

   Local AS Number : 4 octets

   The unsigned integer representing the AS number [RFC6793] of the AS
   to which the PeerNode SID advertising node belongs.  If
   Confederations [RFC5065] are in use, and if the remote node is a
   member of a different Member-AS within the local Confederation, this
   is the Member-AS Number inside the Confederation and not the
   Confederation Identifier.

   Remote AS Number : 4 octets

   The unsigned integer representing the AS number [RFC6793] of the AS
   of the remote node for which the PeerNode SID is advertised.  If
   Confederations [RFC5065] are in use, and if the remote node is a
   member of a different Member-AS within the local Confederation, this
   is the Member-AS Number inside the Confederation and not the
   Confederation Identifier.

   Local BGP Router ID : 4 octets

   unsigned integer representing the BGP Identifier of the PeerNode SID
   advertising node as defined in [RFC4271] and [RFC6286].

   Remote BGP Router ID : 4 octets

   unsigned integer representing the BGP Identifier of the remote node
   as defined in [RFC4271] and [RFC6286].

   When there is a multi-hop EBGP session between two ASBRs, PeerNode
   SID is advertised for this session and traffic can be load balanced
   across these interfaces.  An EPE controller that does bandwidth
   management for these links should be aware of the links on which the
   traffic will be load-balanced.  As per [RFC8029], the node
   advertising the EPE SIDs will send Downstream Detailed Mapping TLV
   (DDMAP TLV) specifying the details of nexthop interfaces, the OAM
   packet will be sent out.  Based on this information controller MAY
   choose to verify the actual forwarding state with the topology
   information controller has.  On the router, the validation procedures
   will include, received DDMAP validation as specified in [RFC8029] to
   verify the control and forwarding state synchronization on the two

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   routers.  Any discrepancies between controller's state and forwarding
   state will not be detected by the procedures described in the
   document.

4.2.  PeerAdj SID Sub-TLV

        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 = TBD1                    |          Length               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Adj-Type      |            RESERVED                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               Local AS Number (4  octets)                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Remote AS Number (4 octets)                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Local BGP router ID (4 octets)                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Remote BGP Router ID (4 octets)                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Local Interface address (4/16 octets)            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Remote Interface address (4/16 octets)           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: PeerAdj SID Sub-TLV

   Type : 2 octets

   Value: TBD1

   Length : 2 octets

   Value: variable based on IPv4/IPv6 interface address.  Length
   excludes the length of Type and Length fields.For IPv4 interface
   addresses length will be 28 octets.  In case of IPv6 address length
   will be 52 octets.

   Adj-Type : 1 octet

   Value: Set to 1 when the Adjacency Segment is IPv4 Set to 2 when the
   Adjacency Segment is IPv6

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   RESERVED : 3 octets.  MUST be zero when sending, and ignored on
   receiving.

   Local AS Number : 4 octets

   The unsigned integer representing the AS number [RFC6793] of the AS
   to which the PeerAdj SID advertising node belongs.  If Confederations
   [RFC5065] are in use, and if the remote node is a member of a
   different Member-AS within the local Confederation, this is the
   Member-AS Number inside the Confederation and not the Confederation
   Identifier.

   Remote AS Number : 4 octets

   The unsigned integer representing the AS number[RFC6793] of the AS of
   the remote node for which the PeerAdj SID is advertised.  If
   Confederations [RFC5065] are in use, and if the remote node is a
   member of a different Member-AS within the local Confederation, this
   is the Member-AS Number inside the Confederation and not the
   Confederation Identifier.

   Local BGP Router ID : 4 octets

   unsigned integer representing the BGP Identifier of the PeerAdj SID
   advertising node as defined in [RFC4271] and [RFC6286].

   Remote BGP Router ID : 4 octets

   unsigned integer representing the BGP Identifier of the remote node
   as defined in [RFC4271] and [RFC6286].

   Local Interface Address :4 octets/16 octets

   In case of PeerAdj SID, Local interface address corresponding to the
   PeerAdj SID should be specified in this field.  For IPv4,this field
   is 4 octets; for IPv6, this field is 16 octets.  Link-local IPv6
   addresses are not in the scope of this document.

   Remote Interface Address :4 octets/16 octets

   In case of PeerAdj SID Remote interface address corresponding to the
   PeerAdj SID should be apecified in this field.  For IPv4, this field
   is 4 octets; for IPv6, this field is 16 octets.  Link-local IPv6
   addresses are not in the scope of this document..

   [RFC9086] mandates sending local interface ID and remote interface ID
   in the Link Descriptors and allows a value of 0 in the remote
   descriptors.  It is useful to validate the incoming interface for an

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   OAM packet and if the remote descriptor is 0 this validation is not
   possible.  [RFC9086] allows optional link descriptors of local and
   remote interface addresses as described in section 4.2.  This
   document RECOMMENDs sending these optional descriptors and using them
   to validate incoming interface.  When these local and remote
   interface addresses are not available, an ingress node can send 0 in
   the local and/or remote interface address field.  The receiver SHOULD
   skip the validation for the incoming interface if the address field
   contains 0.

4.3.  PeerSet SID Sub-TLV

        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 = TBD3                    |          Length               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Local AS Number (4  octets)                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Local BGP router ID (4 octets)                   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   No.of elements in set       |          Reserved             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Remote AS Number (4 octets)                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Remote BGP Router ID (4 octets)                  |
       ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++

        One element in set consists of below details
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Remote AS Number (4 octets)                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Remote BGP Router ID (4 octets)                  |
       ++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++

                    Figure 5: PeerSet SID Sub-TLV

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   Type : 2 octets

   Value: TBD3

   Length : 2 octets

   Value: Expressed in octets and variable based on the number of
   elements in the set.  The length field does not include the length of
   Type and Length fields.

   Local AS Number :4 octets

   The unsigned integer representing the AS number [RFC6793] of the AS
   to which the PeerSet SID advertising node belongs.  If Confederations
   [RFC5065] are in use, and if the remote node is a member of a
   different Member-AS within the local Confederation, this is the
   Member-AS Number inside the Confederation and not the Confederation
   Identifier.

   Local BGP Router ID : 4 octets

   unsigned integer representing the BGP Identifier of the PeerSet SID
   advertising node as defined in [RFC4271] and [RFC6286].

   No.of elements in set: 2 octets

   The number of remote ASes over which the set SID performs load
   balancing.

   Reserved : 2 octets.  MUST be zero when sent and ignored when
   received.

   Remote AS Number : 4 octets

   The unsigned integer representing the AS number [RFC6793] of the AS
   of the remote node for which the PeerSet SID is advertised.  If
   Confederations [RFC5065] are in use, and if the remote node is a
   member of a different Member-AS within the local Confederation, this
   is the Member-AS Number inside the Confederation and not the
   Confederation Identifier.

   Remote BGP Router ID : 4 octets

   unsigned integer representing the BGP Identifier of the remote node
   as defined in [RFC4271] and [RFC6286].

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   PeerSet SID may be associated with a number of PeerNode SIDs and
   PeerAdj SIDs.  The remote AS number and the Router ID of each of
   these PeerNode SIDs PeerAdj SIDs MUST be included in the FEC.

5.  EPE-SID FEC validation

   When a remote ASBR of the EPE-SID advertisement receives the MPLS OAM
   packet with top FEC being the EPE-SID, it MUST perform validity
   checks on the content of the EPE-SID FEC sub-TLV.  The basic length
   check should be performed on the received FEC.

    PeerAdj SID
    -----------
    if Adj type = 1 Length should be 28 octets
    If Adj type =2 Length should be 52 octets

    PeerNode SID
    -------------
    Length = ( 20 + No.of IPv4 interface pairs * 8  +
             No.of IPv6 interface pairs * 32 ) octets

    PeerSet SID
    -----------
    Length = (9 + No.of elements in the set *
             (8 + No.of IPv4 interface pairs * 8 +
              No.of IPv6 interface pairs * 32)) octets

                        Figure 6: Length Validation

   If a malformed FEC sub-TLV is received, then a return code of 1,
   "Malformed echo request received" as defined in [RFC8029] MUST be
   sent.  The below section is appended to the procedure given in
   Section 7.4 point 4a of [RFC8287].

5.1.  EPE-SID FEC validiation

   Segment Routing IGP-Prefix, IGP-Adjacency SID and EPE-SID Validation
   : Receiving node term used in this section implies the node that
   receives OAM message with the FEC stack TLV.

    Else, if the Label-stack-depth is 0 and the Target FEC Stack sub-TLV
    at FEC-stack-depth is TBD1 (PeerAdj SID sub-TLV), {

        Set the Best-return-code to 10, "Mapping for this FEC is not
        the given label at stack-depth  if any below
        conditions fail:

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               -  Validate that the receiving node's BGP Local AS matches
                  with the remote AS field in the received PeerAdj SID
                  FEC sub-TLV.

               -  Validate that the receiving node's BGP Router-ID
                  matches with the Remote Router ID field in the
                  received PeerAdj SID FEC.

               -  Validate that there is a EBGP session with a peer
                  having local AS number and BGP Router-ID as
                  specified in the Local AS number and Local Router-ID
                  field in the received PeerAdj SID FEC sub-TLV.

        If the Remote interface address is not zero, validate the
        incoming interface.
        Set the Best-return-code to 35 "Mapping for this FEC is not
        associated with the incoming interface"  [RFC8287] if any below
        conditions fail:

               -  Validate the incoming interface on which the OAM
                  packet was receieved, matches with the remote
                  interface specified in the PeerAdj SID FEC sub-TLV

        If all above validations have passed, set the return code to 3
        "Replying router is an egress for the FEC at stack-depth"
        }

    Else, if the Target FEC sub-TLV at FEC-stack-depth is TBD2
         (PeerNode SID sub-TLV), {

        Set the Best-return-code to 10, "Mapping for this FEC is not
        the given label at stack-depth  if any below
        conditions fail:

           -  Validate that the receiving node's BGP Local AS matches
              with the remote AS field in the
              received PeerNode SID FEC sub-TLV.

           -  Validate that the receiving node's BGP Router-ID matches
              with the Remote Router ID field in the received
              PeerNode SID FEC.

           -  Validate that there is a EBGP session with a peer
              having local AS number and BGP Router-ID as
              specified in the Local AS number and Local Router-ID
              field in the received PeerNode SID FEC sub-TLV.

        If all above validations have passed, set the return code to 3

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        "Replying router is an egress for the FEC at stack-depth".
    }
    Else, if the Target FEC sub-TLV at FEC-stack-depth is TBD3
         (PeerSet SID sub-TLV), {

        Set the Best-return-code to 10, "Mapping for this FEC is not
        the given label at stack-depth"  if any below
        conditions fail:

           -  Validate that the Receiving Node BGP Local AS matches
              with one of the remote AS field in the received PeerSet
              SID FEC sub-TLV.

           -  Validate that the Receiving Node BGP Router-ID matches
              with one of the  Remote Router ID field in the received
              PeerSet SID FEC sub-TLV.

           -  Validate that there is a EBGP session with a peer having
              local AS number and BGP Router-ID as
              specified in the Local AS number and Local Router-ID
              field in the received PeerSet SID FEC sub-TLV.

        If all above validations have passed, set the return code to 3
        "Replying router is an egress for the FEC at stack-depth"
        }

6.  IANA Considerations

   IANA is requested to allocate three new Target FEC stack sub-TLVs
   from the "Sub-TLVs for TLV types 1,16 and 21" subregistry in the
   "TLVs" registry of the "Multi-Protocol Label switching (MPLS) Label
   Switched Paths (LSPs) Ping parameters" namespace.

      PeerAdj SID Sub-TLV : TBD1

      PeerNode SID Sub-TLV: TBD2

      PeerSet SID Sub-TLV : TBD3

   The three lowest free values from the Standard Tracks range should be
   allocated if possible.

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7.  Security Considerations

   The EPE-SIDs are advertised for egress links for Egress Peer
   Engineering purposes or for inter-AS links between co-operating ASes.
   When co-operating domains are involved, they can allow the packets
   arriving on trusted interfaces to reach the control plane and get
   processed.

   When EPE-SIDs are created for egress TE links where the neighbor AS
   is an independent entity, it may not allow packets arriving from
   external world to reach the control plane.  In such deployments MPLS
   OAM packets will be dropped by the neighboring AS that receives the
   MPLS OAM packet.

   In MPLS traceroute applications, when the AS boundary is crossed with
   the EPE-SIDs, the FEC stack is changed.  [RFC8287] does not mandate
   that the initiator upon receiving an MPLS Echo Reply message that
   includes the FEC Stack Change TLV with one or more of the original
   segments being popped remove a corresponding FEC(s) from the Target
   FEC Stack TLV in the next (TTL+1) traceroute request.

   If an initiator does not remove the FECs belonging to the previous AS
   that has traversed, it may expose the internal AS information to the
   following AS being traversed in traceroute.

8.  Implementation Status

   This section is to be removed before publishing as an RFC.

   RFC-Editor: Please clean up the references cited by this section
   before publication.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

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8.1.  Juniper Networks

   Juniper networks reported a prototype implementation of this draft.

9.  Acknowledgments

   Thanks to Loa Andersson, Dhruv Dhody, Ketan Talaulikar, Italo Busi
   and Alexander Vainshtein, Deepti Rathi for careful review and
   comments.  Thanks to Tarek Saad for providing the example described
   in Appendix section.

10.  References

10.1.  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>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <https://www.rfc-editor.org/info/rfc6793>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [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>.

   [RFC8287]  Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya,
              N., Kini, S., and M. Chen, "Label Switched Path (LSP)
              Ping/Traceroute for Segment Routing (SR) IGP-Prefix and
              IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data
              Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,
              <https://www.rfc-editor.org/info/rfc8287>.

   [RFC8690]  Nainar, N., Pignataro, C., Iqbal, F., and A. Vainshtein,
              "Clarification of Segment ID Sub-TLV Length for RFC 8287",
              RFC 8690, DOI 10.17487/RFC8690, December 2019,
              <https://www.rfc-editor.org/info/rfc8690>.

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   [RFC9086]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
              Ray, S., and J. Dong, "Border Gateway Protocol - Link
              State (BGP-LS) Extensions for Segment Routing BGP Egress
              Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
              2021, <https://www.rfc-editor.org/info/rfc9086>.

10.2.  Informative References

   [I-D.ietf-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
              D. Jain, "Advertising Segment Routing Policies in BGP",
              Work in Progress, Internet-Draft, draft-ietf-idr-segment-
              routing-te-policy-26, 23 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-
              segment-routing-te-policy-26>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [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>.

   [RFC7705]  George, W. and S. Amante, "Autonomous System Migration
              Mechanisms and Their Effects on the BGP AS_PATH
              Attribute", RFC 7705, DOI 10.17487/RFC7705, November 2015,
              <https://www.rfc-editor.org/info/rfc7705>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [RFC8403]  Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
              Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
              Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
              2018, <https://www.rfc-editor.org/info/rfc8403>.

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   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [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>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
              A., and P. Mattes, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2022,
              <https://www.rfc-editor.org/info/rfc9256>.

Appendix A.  APPENDIX

   This section describes an example of correctly programmed state and
   incorrectly programmed state and provides details on how the new sub-
   TLVs described in this document can be used to validate the
   correctness.  Consider the diagram from Figure 1,

   Correctly programed state:

      • C assigns label 16001 and binds it to adjacency C->E

      • C signals label 16001 is bound to adjacency C->E (e.g. via BGP-
      LS)

      • Controller/Ingress programs an SR path that has SID/label 16001
      to steer packet on the exit point from C onto adjacency C->E

      • Using MPLS trace procedures defined in this document, the
      PeerAdj SID Sub-TLV is populates with entities to be validated by
      C when OAM packet reaches it.

      • C receives the OAM packet, it validates the top label (16001) is
      indeed corresponding to the entities populated in the PeerAdj SID
      Sub-TLV

   Incorrectly programed state:

      • C assigns label 16001 and binds it to adjacency C->D

      • The controller learns of PeerAdj SID label 16001 is bound to
      adjacency C->E (e.g. via BGP-LS) – this could be a software bug on
      C or on controller

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      • Controller/Ingress programs an SR path that has SID/label 16001
      to steer packet on the exit point from C onto adjacency C->E

      • Using MPLS trace procedures defined in this document, the
      PeerAdj SID Sub-TLV is populates with entities to be validated by
      C (including local/remote interface address of C->E) when OAM
      packet reaches it.

      • C receives the OAM packet, it validates the top label (16001) is
      NOT bound to C->E as populated in the PeerAdj SID Sub-TLV and can
      respond with the respective error code

Authors' Addresses

   Shraddha Hegde
   Juniper Networks Inc.
   Exora Business Park
   Bangalore 560103
   KA
   India
   Email: shraddha@juniper.net

   Mukul Srivastava
   Juniper Networks Inc.
   Email: msri@juniper.net

   Kapil Arora
   Individual Contributor
   Email: kapil.it@gmail.com

   Samson Ninan
   Ciena
   Email: samson.cse@gmail.com

   Xiaohu Xu
   China Mobile
   Beijing
   China
   Email: xuxiaohu_ietf@hotmail.com

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