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Path Monitoring System/Head-end based MPLS Ping and Traceroute in Inter-domain Segment Routing Networks
draft-ietf-mpls-spring-inter-domain-oam-13

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Shraddha Hegde , Kapil Arora , Mukul Srivastava , Samson Ninan , Nagendra Kumar Nainar
Last updated 2024-05-03
Replaces draft-ninan-mpls-spring-inter-domain-oam
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draft-ietf-mpls-spring-inter-domain-oam-13
Routing area                                                    S. Hegde
Internet-Draft                                     Juniper Networks Inc.
Intended status: Standards Track                                K. Arora
Expires: 4 November 2024                          Individual Contributor
                                                           M. Srivastava
                                                   Juniper Networks Inc.
                                                                S. Ninan
                                                                   Ciena
                                                                N. Kumar
                                                     Cisco Systems, Inc.
                                                              3 May 2024

Path Monitoring System/Head-end based MPLS Ping and Traceroute in Inter-
                    domain Segment Routing Networks
               draft-ietf-mpls-spring-inter-domain-oam-13

Abstract

   The Segment Routing (SR) architecture leverages source routing and
   can be directly applied to the use of a MPLS data plane.  An SR-MPLS
   network may consist of multiple IGP domains or multiple Autonomous
   Systems(ASes) under the control of the same organization.  It is
   useful to have the Label Switched Path (LSP) ping and traceroute
   procedures when an SR end-to-end path traverses multiple ASes or
   domains.  This document describes mechanisms to facilitate LSP ping
   and traceroute in inter-AS and inter-domain SR-MPLS networks in an
   efficient manner with a simple Operations, Administration and
   Maintenance (OAM) protocol extension which uses data plane forwarding
   alone for forwarding echo replies on transit nodes.

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 4 November 2024.

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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
   and restrictions with respect to this document.  Code Components
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Definition of Domain  . . . . . . . . . . . . . . . . . .   5
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Inter-domain Networks with Multiple IGPs  . . . . . . . . . .   5
   3.  Reply Path TLV  . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Segment Sub-TLV . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Type-A: SID only, in the form of MPLS Label . . . . . . .   7
     4.2.  Type-C: IPv4 Node Address with Optional SID for
           SR-MPLS . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Type D: IPv6 Node Address with Optional SID for SR
           MPLS  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Segment Flags . . . . . . . . . . . . . . . . . . . . . .  11
   5.  SRv6 Dataplane  . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Detailed Procedures . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Sending an Echo Request . . . . . . . . . . . . . . . . .  12
     6.2.  Receiving an Echo Request . . . . . . . . . . . . . . . .  12
     6.3.  Sending an Echo Reply . . . . . . . . . . . . . . . . . .  12
     6.4.  Receiving an Echo Reply . . . . . . . . . . . . . . . . .  13
   7.  Detailed Example  . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Procedures for Segment Routing LSP ping . . . . . . . . .  14
     7.2.  Procedures for SR LSP traceroute  . . . . . . . . . . . .  14
       7.2.1.  Procedures for SR LSP traceroute with the Same SRGB on
               All Nodes . . . . . . . . . . . . . . . . . . . . . .  14
       7.2.2.  Procedures for SR LSP Traceroute with the Different
               SRGBs . . . . . . . . . . . . . . . . . . . . . . . .  16
   8.  Building Reply Path TLV Dynamically . . . . . . . . . . . . .  17
     8.1.  The procedures to Build the Return Path . . . . . . . . .  17
     8.2.  Details with Example  . . . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  Segment Sub-TLV  . . . . . . . . . . . . . . . . . . . .  21
     10.2.  New Registry for Segment Sub-TLV Flags . . . . . . . . .  21

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     10.3.  Reply Path Return Codes Registry . . . . . . . . . . . .  22
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  22
   12. Implementation Status . . . . . . . . . . . . . . . . . . . .  22
     12.1.  Juniper Networks . . . . . . . . . . . . . . . . . . . .  23
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     14.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   Many network deployments have built their networks consisting of
   multiple ASes either for the ease of operations or as a result of
   network mergers and acquisitions.  SR can be deployed in such
   scenarios to provide end-to-end paths, traversing multiple Autonomous
   systems(ASes).

   [RFC8660] specifies SR with an MPLS data plane.  [RFC8402] describes
   BGP peering SIDs, and [RFC9087] describes Egress Peer Engineering,
   which will help in steering packet from one AS to another.  Using the
   above SR capabilities, paths that span across multiple ASes can be
   created.

                     +----------------+
                     | Controller/PMS |
                     +----------------+

  |---AS1-----|                |------AS2------|            |----AS3---|

                 ASBR2----ASBR3                ASBR5------ASBR7
                 /             \               /            \
                /               \             /              \
  PE1----P1---P2                 P3---P4---PE4              P5---P6--PE5
                \               /            \               /
                 \             /              \             /
                  ASBR1----ASBR4              ASBR6------ASBR8

     Autonomous System: AS1, AS2, AS3
     Provider Edge: PE1, PE4, PE5
     Provider: P1, P2, P3, P4, P5, P6
     AS Boundary Router:ASBR1, ASBR2, ASBR3, ASBR4,
                        ASBR5, ASBR6, ASBR7, ASBR8

              Figure 1: Inter-AS Segment Routing Topology

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   For example, Figure 1 describes an inter-AS network scenario
   consisting of ASes AS1, AS2 and AS3.  AS1, AS2 and AS3 are SR enabled
   and the egress links have PeerNode SID/PeerAdj SID/ PeerSet SID
   configured and advertised via [RFC9086].  PeerNode SID/PeerAdj SID/
   PeerSet SID are referred to as Egress Peer Engineering SIDs (EPE-
   SIDs) in this document.  The controller or the head-end can build an
   end-to-end Traffic-Engineered path consisting of Node-SIDs,
   Adjacency-SIDs and EPE-SIDs.  It is useful for operators to be able
   to perform LSP ping and traceroute procedures on these inter-AS SR-
   MPLS paths, in order to detect and diagnose failed deliveries and to
   determine the actual path that traffic takes through the network.
   LSP ping/traceroute procedures use IP connectivity for echo reply to
   reach the head-end.  In inter-AS networks, IP connectivity may not be
   there from each router in the path.  For example, in Figure 1, P3 and
   P4 may not have IP connectivity for PE1.

   It is not possible to carry out LSP ping and traceroute functionality
   on these paths to verify basic connectivity and fault isolation using
   existing LSP ping and traceroute mechanism([RFC8287] and [RFC8029]).
   That is because there might not always be IP connectivity from a
   responding node back to the source address of the ping packet when
   the responding node is in a different AS from the source of the ping.

   [RFC8403] describes mechanisms to carry out MPLS ping/traceroute from
   a Path Monitoring System (PMS).  It is possible to build GRE tunnels
   or static routes to each router in the network to get IP connectivity
   for the reverse path.  This mechanism is operationally very heavy and
   requires the PMS to be capable of building a huge number of GRE
   tunnels or installing the necessary static routes, which may not be
   feasible.

   [RFC7743] describes an Echo-relay based solution based on advertising
   a new Relay Node Address Stack TLV containing a stack of Echo-relay
   IP addresses.  These mechanisms can be applied to SR networks as
   well.  The [RFC7743] mechanism requires the return ping packet to be
   processed on the slow path or as a bump-in-the-wire on every relay
   node.  The motivation of the current document is to provide an
   alternate mechanism for ping/traceroute in inter-domain SR networks.
   The definition of the term "domain" as applicable to this document is
   defined in Section 1.1.

   This document describes a new mechanism that is efficient and simple
   and can be easily deployed in SR-MPLS networks.  This mechanism uses
   MPLS paths and no changes are required in the forwarding path.  Any
   MPLS-capable node will be able to forward the echo-reply packet in
   the fast path.  The current document describes a mechanism that uses
   the Reply Path TLV [RFC7110] to convey the reverse path.  Three new
   sub-TLVs are defined for the Reply path TLV that faciliate encoding

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   SR label stack.  The return path can either be derived by a smart
   application or controller which has a full topology view.  This
   document also proposes mechanisms to derive the return path
   dynamically during traceroute procedures.

   The current document is focused on the inter-domain use case.
   However, the protocol extensions described in this document may be
   applied to indicate the return path for other use cases as well which
   are out-of-scope for this document and therefore not further
   described in this document.

1.1.  Definition of Domain

   The term domain used in this document implies an IGP domain where
   every node is visible to every other node for shortest path
   computation.  The domain implies an IGP area or level.  An AS
   consists of one or more IGP domains.  The procedures described in
   this document apply to paths built across multiple domains which
   include inter-area as well as inter-AS paths.  The procedures and
   deployment scenarios described in this document apply to inter-AS
   paths where the participating ASes belong to closely coordinating
   administrations or to a single ownership.  This document applies to
   SR-MPLS networks where all nodes in each of the domains are SR
   capable.  It also applies to SR-MPLS networks where SR acts as an
   overlay having SR incapable underlay nodes.  In such networks, the
   traceroute procedure is executed only on the overlay SR nodes.

1.2.  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.

2.  Inter-domain Networks with Multiple IGPs

   When the network consists of large number of nodes, the nodes are
   seggregated into multiple IGP domains as shown in Figure 2.  The
   connectivity to the remote PEs can be achieved using BGP-Labeled
   Unicast (BGP-LU) [RFC8277] or by stacking the labels for each domain
   as described in [RFC8604].

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    |-Domain 1|-------Domain 2-----|--Domain 3-|

    PE1------ABR1--------P--------ABR2------PE4
     \        / \                  /\        /
      --------   -----------------   -------
       BGP-LU         BGP-LU          BGP-LU

             Figure 2: Inter-domain Networks with Multiple IGPs

   It is useful to support MPLS ping and traceroute mechanisms for these
   networks.  The procedures described in this document for constructing
   Reply Path TLV and its use in an echo reply is equally applicable to
   networks consisting of multiple IGP domains that use BGP-LU or label
   stacking.

3.  Reply Path TLV

   Reply Path (RP) TLv is defined in [RFC7110].  SR networks statically
   assign the labels to nodes and a PMS/Head-end may know the entire
   database.  The reverse path can be built from the PMS/Head-end by
   stacking segments for the reverse path.  Reply Path TLV as defined in
   [RFC7110] is used to carry the return path.  Reply mode 5, Reply via
   Specified Path is defined in section 4.1 of [RFC7110].  While using
   the procedures described in this document, the reply mode is set to 5
   (Reply via Specified Path), and Reply Path TLV is included in the
   echo request message as described in [RFC7110].  The Reply Path TLV
   is constructed as per Section 4.2 of [RFC7110].  This document
   defines three new sub-TLVs to encode the SR path.

   The type of segment that the head-end chooses to send in the Reply
   Path TLV is governed by local policy.  Implementations MAY provide
   Command Line Interface (CLI) input parameters in the form of Labels/
   IPv4 addresses/IPv6 addresses or a combination of these which get
   encoded in the Reply Path TLV.  Implementations MAY also provide
   mechanisms to acquire the database of remote domains and compute the
   return path based on the acquired database.  For traceroute purposes,
   the return path will have to consider the reply being sent from every
   node along the path.  The return path changes when the traceroute
   progresses and crosses each domain.  One of the ways this can be
   implemented on the headend is to acquire the entire database (of all
   domains) and build a return path for every node along the SR-MPLS
   path based on the knowledge of the database.  Another mechanism is to
   use a dynamically computed return path as described in Section 8

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   Some networks may consist of pure IPv4 domains and pure IPv6 domains.
   Handling end-to-end MPLS OAM for such networks is out of the scope
   for this document.  It is recommended to use dual-stack in such cases
   and use end-to-end IPv6 addresses for MPLS ping and traceroute
   procedures.

4.  Segment Sub-TLV

   Section 4 of [RFC9256] defines various segments types.  The types of
   segments applicable to this document have been defined in this
   section for the use of MPLS OAM.  The intention was to keep the
   definitions as close to those in [RFC9256] as possible with
   modifications only when needed.  One or more Segment Sub-TLVs can be
   included in the Reply Path TLV.  The Segment Sub-TLVs included in a
   Reply Path TLV MAY be of different types.

   Below types of Segment Sub-TLVs are applicable to the Reply Path TLV.

   Type-A: SID only, in the form of MPLS Label

   Type-C: IPv4 Node Address with optional SID

   Type-D: IPv6 Node Address with optional SID for SR MPLS

4.1.  Type-A: SID only, in the form of MPLS Label

   The Type A Segment Sub-TLV encodes a single SID in the form of an
   MPLS label.  The format is 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type                      |   Length                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Flags       |   RESERVED                                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Label                        | TC  |S|       TTL     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3: Type-A Segment Sub-TLV

   where:

   Type: TBD1(to be assigned by IANA from the registry "Sub-TLVs for TLV
   Types 1, 16, and 21").

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   Length is 8 octets.  The length excludes the length of the the Type
   and Length Fields.

   Flags: 1 octet of flags as defined in Section 4.4.

   RESERVED: 3 octets of reserved bits.  MUST be set to zero when
   sending; MUST be ignored on receipt..

   Label: 20 bits of label value.

   TC: 3 bits of traffic class

   S: 1 bit Reserved

   TTL: 1 octet of TTL.

   The following applies to the Type-A Segment Sub-TLV:

   The S bit SHOULD be zero upon transmission, and MUST be ignored upon
   reception.

   If the originator wants the receiver to choose the TC value, it sets
   the Traffic Class(TC) field to zero.

   If the originator wants the receiver to choose the TTL value, it sets
   the TTL field to 255.

   If the originator wants to recommend a value for these fields, it
   puts those values in the TC and/or TTL fields.

   The receiver MAY override the originator's values for these fields.
   This would be determined by local policy at the receiver.  One
   possible policy would be to override the fields only if the fields
   have the default values specified above.

4.2.  Type-C: IPv4 Node Address with Optional SID for SR-MPLS

   The Type-C Segment Sub-TLV encodes an IPv4 node address, SR Algorithm
   and an optional SID in the form of an MPLS label.  The format is as
   follows:

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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 (MBZ)             | SR Algorithm    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 IPv4 Node Address (4 octets)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                SID (optional, 4 octets)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 4: Type-C Segment Sub-TLV

   where:

   Type: TBD2 (to be assigned by IANA from the registry "Sub-TLVs for
   TLV Types 1, 16, and 21").

   Length is 8 or 12.

   Flags: 1 octet of flags as defined in Section 4.4.

   SR Algorithm: 1 octet specifying SR Algorithm as described in section
   3.1.1 in [RFC8402], when A-Flag as defined in Section 4.4is present.
   SR Algorithm is used by the receiver to derive the Label.  When
   A-flag is unset, this field has no meaning and thus MUST be set to
   zero on transmission and ignored on receipt.

   RESERVED: 2 octets of reserved bits.  MUST be set to zero when
   sending; MUST be ignored on receipt.

   IPv4 Node Address: 4-octet IPv4 address representing a node.

   SID: optional: 4-octet field containing label, TC, S and TTL as
   defined in Section 4.1.  When the SID field is present, it MUST be
   used for constructing the Reply Path.

   The following applies to the Type-C Segment Sub-TLV:

   The IPv4 Node Address MUST be present.

   The SID is optional and specifies a 4-octet MPLS SID containing
   label, TC, S and TTL as defined in Section 4.1.

   If the length is 8, then only the IPv4 Node Address is present.

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   If the length is 12, then the IPv4 Node Address and the MPLS SID are
   present.

4.3.  Type D: IPv6 Node Address with Optional SID for SR MPLS

   The Type-D Segment Sub-TLV encodes an IPv6 node address, SR Algorithm
   and an optional SID in the form of an MPLS label.  The format is 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type                      |   Length                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Flags       |       RESERVED(MBZ)           | SR Algorithm  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //                IPv6 Node Address (16 octets)                //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                SID (optional, 4 octets)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 5: Type-D Segment Sub-TLV

   where:

   Type: TBD3 (to be assigned by IANA from the registry "Sub-TLVs for
   TLV Types 1, 16, and 21").

   Length is 20 or 24.

   Flags: 1 octet of flags as defined in Section 4.4.

   SR Algorithm: 1 octet specifying SR Algorithm as described in section
   3.1.1 in [RFC8402], when A-Flag as defined in Section 4.4 is present.
   SR Algorithm is used by the receiver to derive the label.When A-flag
   is unset, this field has no meaning and thus MUST be set to zero
   (MBZ) on transmission and ignored on receipt.

   RESERVED: 2-octet of reserved bits.  MUST be set to zero when
   sending; MUST be ignored on receipt.

   IPv6 Node Address: 16-octet IPv6 address representing a node.

   SID: optional: 4-octet field containing label, TC, S and TTL as
   defined in Section 4.1

   The following applies to the Type-D Segment Sub-TLV:

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   The IPv6 Node Address MUST be present.

   The SID is optional and specifies a 4-octet MPLS SID containing
   label, TC, S and TTL as defined in Section 4.1.When the SID field is
   present, it MUST be used for constructing the Reply Path.

   If the length is 20, then only the IPv6 Node Address is present.

   If the length is 24, then the IPv6 Node Address and the MPLS SID are
   present.

4.4.  Segment Flags

   The Segment Types described above contain the following flags in the
   "Flags" field (codes to be assigned by IANA from the new registry
   "Segment Sub-TLV Flags" )

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      | |A|           |
      +-+-+-+-+-+-+-+-+

                              Figure 6: Flags

   where:

   A-Flag: This flag indicates the presence of SR Algorithm ID in the
   "SR-Algorithm" field applicable to various Segment Types.

   Unused bits in the Flag octet MUST be set to zero upon transmission
   and MUST be ignored upon receipt.

   The following applies to the Segment Flags:

   A-Flag applies to Segment Type-C and Type-D.  If A-Flag appears with
   Type-A Segment Type, it MUST be ignored.

5.  SRv6 Dataplane

   SRv6 dataplane is not in the scope of this document.

6.  Detailed Procedures

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6.1.  Sending an Echo Request

   In the inter-AS scenario, the procedures described in this document
   are used to specify the return path, if IP connectivity to the
   initiator is not available, and may be used in any case.  LSP ping
   initiator MUST set the Reply Mode of the echo request to 5 "Reply via
   Specified Path", and a Reply Path TLV MUST be carried in the echo
   request message correspondingly.  The Reply Path TLV MUST contain the
   Segment Routing Path in the reverse direction encoded as an ordered
   list of segments.  The first Segment MUST correspond to the top
   Segment in MPLS header that the responder MUST use while sending the
   echo reply.

6.2.  Receiving an Echo Request

   As described in [RFC7110], when Reply mode is set to 5 (Reply via
   Specified Path), the echo request must contain the Reply path TLV.
   Absence of Reply Path TLV is treated as a malformed echo request.
   When an echo request is received, if the egress LSR does not know the
   Reply Mode 5 defined in [RFC7110], an echo reply with the return code
   set to "Malformed echo request received" and the Subcode set to zero
   must be sent back to the ingress LSR according to the rules of
   [RFC8029].  When a Reply Path TLV is received, and the responder that
   supports processing it, it MUST use the segments in Reply Path TLV to
   build the echo reply.  The responder MUST follow the normal FEC
   validation procedures as described in [RFC8029] and [RFC8287] and
   this document does not suggest any change to those procedures.  When
   the echo reply has to be sent out the Reply Path TLV MUST be used to
   construct the MPLS packet to send out.

6.3.  Sending an Echo Reply

   The echo reply message is sent as an MPLS packet with an MPLS label
   stack.  The echo reply message MUST be constructed as described in
   the [RFC8029].  An MPLS packet is constructed with an echo reply in
   the payload.  The top label MUST be constructed from the first
   Segment from the Reply Path TLV.  The remaining labels MUST follow
   the order from the Reply Path TLV.  The responder MAY check the
   reachability of the top label in its own Label Forwarding Information
   Base (LFIB) before sending the echo reply and provide necessary log
   information in case of unreachabilty.  In certain scenarios, the
   head-end MAY choose to send Type-C/Type-D segments consisting of IPV4
   address or IPv6 address, when it is unable to derive the SID from
   available topology information.  Optionally SID may also be
   associated with the Type-C/Type-D segment, if such information is
   available from the controller or via operator input.  In such cases,
   the node sending the echo reply MUST derive the MPLS labels based on
   Node-SIDs associated with the IPv4 /IPv6 addresses or from the

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   optional MPLS SIDs in the Type-C/Type-D segments and encode the echo
   reply with MPLS labels.

   The reply path return code is set as described in section 7.4 of
   [RFC7110].According to section 5.3 of [RFC7110] The Reply Path TLV is
   included in an echo reply indicating the specified return path that
   the echo reply message is required to follow.

   When the node is configured to dynamically create a return path for
   the next echo request, the procedures described in Section 8 MUST be
   used.  The reply path return code MUST be set to TBA1 and the same
   Reply Path TLV or a new Reply Path TLV MUST be included in the echo
   reply.

6.4.  Receiving an Echo Reply

   The rules and process defined in Section 4.6 of [RFC8029] and section
   5.4 of [RFC7110] apply here.  In addition, if the Reply path return
   code is "Use Reply Path TLV in the echo reply for building the next
   echo request" (defined in this document), the Reply Path TLV from the
   echo Reply MUST be sent in the next echo request with TTL incremented
   by 1.  If the TTL is already 255, the traceroute procedure MUST be
   ended with an appropriate log message.

7.  Detailed Example

   The example topology given in Figure 1 will be used in the below
   sections to explain LSP ping and traceroute procedures.  The PMS/
   Head-end has a complete view of topology.  PE1, P1, P2, ASBR1 and
   ASBR2 are in AS1.  Similarly ASBR3, ASBR4, P3, P4 and PE4 are in AS2.

   AS1 and AS2 have SR enabled.  IGPs like OSPF/ISIS are used to flood
   SIDs in each AS.  The ASBR1, ASBR2, ASBR3 and ASBR4 advertise BGP
   EPE-SIDs for the inter-AS links.  Topology of AS1 and AS2 are
   advertised via BGP-Link State (BGP-LS) to the controller/PMS or Head-
   end node.  The EPE-SIDs are also advertised via BGP-LS as described
   in [RFC9086].  The example uses EPE-SIDs for the inter-AS links but
   the same could be achieved using adjacency-SIDs advertised for a
   passive IGP link.

   The description in the document uses below notations for Segment
   Identifiers(SIDs).

   Node SIDs: N-PE1, N-P1, N-ASBR1 etc.

   Adjacency SIDs: Adj-PE1-P1, Adj-P1-P2 etc.

   EPE-SIDs: EPE-ASBR2-ASBR3, EPE-ASBR1-ASBR4, EPE-ASBR3-ASBR2 etc.

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7.1.  Procedures for Segment Routing LSP ping

   Consider an SR-MPLS path from PE1 to PE4 consisting of a label stack
   [N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4] from Figure 1.  In order to
   perform MPLS ping procedures on this path, the remote end (PE4) needs
   IP connectivity to head end PE1, for the echo reply to travel back to
   PE1.  In a deployment that uses controller-computed inter-domain
   path, there may be no IP connectivity from PE4 to PE1 as they lie in
   different ASes.

   PE1 sends an echo request message to the end-point PE4 along the path
   that consists of label stacks [N-P1, N-ASBR1, EPE-ASBR1-ASBR4,
   N-PE4].  PE1 adds the return path from PE4 to PE1 in the echo request
   message in the Reply Path TLV.  As an example, Reply Path TLV for PE1
   to PE4 for LSP ping is [N-ASBR4, EPE-ASBR4-ASBR1, N-PE1].  This
   example path provides the entire return path up to the head-end node
   PE1.  The mechanism used to construct the return path is
   implementation dependent.

   An implementation may also build a return Path consisting of labels
   to reach its own AS.  Once the label stack is popped off the echo
   reply message will be exposed.  The further packet forwarding will be
   based on IP lookup.  An example return Path for this case could be
   [N-ASBR4, EPE-ASBR4-ASBR1].

   On receiving MPLS echo request PE4 first validates FEC in the echo
   request.  PE4 then builds a label stack to send the response from PE4
   to PE1 by copying the labels from Reply Path TLV.  PE4 builds the
   echo reply packet with the MPLS label stack constructed and imposes
   MPLS headers on top of echo reply packet and sends out the packet
   towards PE1.  This Segment List stack can successfully steer reply
   back to the Head-end node (PE1).

7.2.  Procedures for SR LSP traceroute

7.2.1.  Procedures for SR LSP traceroute with the Same SRGB on All Nodes

   The traceroute procedure involves visiting every node on the path and
   echo replies sent from every node.  In this section, we describe the
   traceroute mechanims when the headend/PMS has complete visibility of
   the database.  Headend/PMS computes the return path from each node in
   the entire SR-MPLS path that is being tracerouted.  The return path
   computation is implementation dependent.  As the headend/PMS
   completely controls the return path, it can use proprietary
   computations to build the return path.

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   One of the ways the return path can be built, is to use the principle
   of building label stacks by adding each domain border node's Node SID
   on the return path label stack as the traceroute progresses.  For
   inter-AS networks, in addition to border node's Node-SID, EPE-SID in
   the reverse direction also needs to be added to the label stack.

   The Inter-domain/inter-as traceroute procedure uses the TTL expiry
   mechanism as specified in [RFC8029] and [RFC8287].  Every echo
   request packet Headend/PMS will include the appropriate return path
   in the Reply Path TLV.  The node that receives the echo request will
   follow procedures described in section Section 6.1 and section
   Section 6.2 to send out an echo reply.

   For Example:

   Let us consider a topology from Figure 1.  Let us consider a SR-MPLS
   path [N-P1, N-ASBR1, EPE-ASBR1-ASBR4, N-PE4].  The traceroute is
   being executed for this inter-AS path for destination PE4.  PE1 sends
   the first echo request with TTL set to 1 and includes Reply Path TLV
   consisting of Type-A Segment containing label derived from its own SR
   Global Block (SRGB).  Note that the type of segment used in
   constructing the return Path is local policy.  If the entire network
   has the same SRGB configured, Type-A segments can be used.  The TTL
   expires on P1 and P1 sends an echo reply using the return path.  Note
   that implementations may choose to exclude the Reply Path TLV until
   traceroute reaches the first domain border as the return IP path to
   PE1 is expected to be available inside the first domain.

   TTL is set to 2 and the next the echo request is sent out.  Until the
   traceroute procedure reaches the domain border node ASBR1, the same
   return path TLV consisting of single Label (PE1's node Label) is
   used.  When echo request reaches the border node ASBR1, and an echo
   reply is received from ASBR1, the next echo request needs to include
   an additional label as ASBR1 is a border node.  The head-end node has
   complete visibility of the network database learned via BGP-LS
   [RFC9552] and [RFC9086] and can derive the details of Autonomous
   System Boundary Router (ASBR) nodes.  The Reply Path TLV is built
   based on the forward path.  As the forward path consists of EPE-
   ASBR1-ASBR4, an EPE-SID in the reverse direction is included in the
   Reply Path TLV.  The return path now consists of two labels [EPE-
   ASBR4-ASBR1, N-PE1].  The echo reply from ASBR4 will use this return
   path to send the reply.

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   The next echo request after visiting the border node ASBR4 will
   update the return path with the Node-SID label of ASBR4.  The return
   path beyond ASBR4 will be [N-ASBR4, EPE-ASBR4-ASBR1, N-PE1].  This
   same return path is used until the traceroute procedure reaches the
   next set of border nodes.  When there are multiple ASes the
   traceroute procedure will continue by adding a set of Node-SIDs and
   EPE-SIDs as the border nodes are visited.

   Note that the above return path-building procedure requires the
   database of all the domains to be available at the headend/PMS.

7.2.2.  Procedures for SR LSP Traceroute with the Different SRGBs

   The Section 7.2.1 assumes the same SRGB is configured on all nodes
   along the path.  The SRGB may differ from one node to another node
   and the SR architecture [RFC8402] allows the nodes to use different
   SRGB.  In such scenarios, PE1 sends Type-C (or Type-D in case of IPv6
   networks) segment with the Node address of PE1 and with optional MPLS
   SID associated with the Node address.  The receiving node derives the
   label for the return path based on its own SRGB.  When the traceroute
   procedure crosses the border ASBR1, headend PE1 should send a Type-A
   segment for N-PE1 based on the label derived from ASBR1's SRGB.  This
   is required because, ASBR4, P3, P4 etc may not have the topology
   information to derive SRGB for PE1.  After the traceroute procedure
   reaches ASBR4 the return path will be [N-PE1 (Type-A with label based
   on ASBR1's SRGB), EPE-ASBR4-ASBR1, N-ASBR4 (Type-C)].

   To extend the example to multiple ASes consisting of 3 or more ASes,
   let us consider a traceroute from PE1 to PE5 in Figure 1.  In this
   example, the PE1 to PE5 path has to cross 3 domains AS1, AS2 and AS3.
   Let us consider a path from PE1 to PE5 that goes through [PE1, ASBR1,
   ASBR4, ASBR6, ASBR8,PE5].  When the traceroute procedure is visiting
   the nodes in AS1, the Reply Path TLV sent from the headend consists
   of [N-PE1].  When the traceroute procedure reaches the ASBR4, the
   return Path consists of [N-PE1, EPE-ASBR4-ASBR1].  While visiting
   nodes in AS2, the traceroute procedure consists of Reply Path TLV
   [N-PE1, EPE-ASBR4-ASBR1, N-ASBR4]. similarly, while visiting the
   ASBR8 Reply Path TLV adds the EPE-SID from ASBR8 to ASBR6.  While
   visiting nodes in AS3 Node-SId of ASBR8 would also be added which
   makes the return Path [N-PE1, EPE-ASBR4-ASBR1, N-ASBR4, EPE-
   ASBR8-ASBR6, N-ASBR8]

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   Let us consider another example from topology Figure 2.  This
   topology consists of multi-domain IGP with a common border node
   between the domains.  This could be achieved with multi-area or
   multi-level IGP or multiple instances of IGP deployed on the same
   node.  The return path computation for this topology is similar to
   the multi-AS computation except that the return path consists of a
   single border node label.  When the traceroute procedure visits node
   P, the return path consists of [N-PE1, N-ABR1].

8.  Building Reply Path TLV Dynamically

   In some cases, the head-end may not have complete visibility of
   Inter-AS/Inter-domain topology.  In such cases, it can rely on
   downstream routers to build the reverse path for MPLS traceroute
   procedures.  For this purpose, Reply Path TLV in the echo reply
   corresponds to the return path to be used in building the next echo
   request.

      Value         Meaning
      ------        ----------------------
      TBA1        Use Reply Path TLV in the echo reply
                  for building the next echo request.

8.1.  The procedures to Build the Return Path

   To dynamically build the return Path for the traceroute procedures,
   the domain border nodes along the path being traced should support
   the procedures described in this section.  Local policy on the domain
   border nodes should determine whether the domain border node
   participates in building the return path dynamically during
   traceroute.

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   The headend/PMS node may include its node label while initiating
   traceroute procedure.  When an Area Border Router (ABR) receives the
   echo request, if the local policy implies building a dynamic return
   path, ABR should include its Node label in the reply path TLV and
   send it in the echo reply.  If there is a Reply Path TLV included in
   the received echo request message, the ABR's node label is added
   before the existing segments.  The type of segment added is based on
   local policy.  In cases when SRGB is not uniform across the network,
   it is RECOMMENDED to add a Type-C or a Type-D segment, but
   implementations MAY safely use other approaches if they see benefits
   in doing so.  If the existing segment in the Reply Path TLV is a
   Type-C/Type-D segment, that segment should be converted to a Type-A
   segment based on ABR's own SRGB.This is because downstream nodes will
   not know what SRGB to use to translate the IP address to a label.  As
   the ABR added its own Node label, it is guaranteed that this ABR will
   be in the return path and will be forwarding the traffic based on the
   next label after its label.

   When an ASBR receives an echo request from another AS, and ASBR is
   configured to build the return path dynamically, ASBR should build a
   Reply Path TLV and include it in the echo reply.  The Reply Path TLV
   should consist of its own node label and an EPE-SID to the AS from
   where the traceroute message was received.  A Reply path return code
   of TBA1 should be set in the echo reply to indicate that the next
   echo request should use the return Path from the Reply Path TLV in
   the echo reply.  ASBR should locally decide the outgoing interface
   for the echo reply packet.  Generally, remote ASBR will choose the
   interface on which the incoming OAM packet was received to send the
   echo reply out.  Reply Path TLV is built by adding two segment sub
   TLVs.  The top segment sub TLV consists of the ASBR's Node SID and
   the second segment consists of the EPE-SID in the reverse direction
   to reach the AS from which the OAM packet was received.  The type of
   segment chosen to build Reply Path TLV is a local policy.  It is
   recommended to use the Type-C/Type-D segment for the top segment when
   the SRGB is not guaranteed to be uniform in the domain.

   Irrespective of which type of segment is included in the Reply Path
   TLV, the responder of echo requests should always translate the Reply
   Path TLV to a label stack and build an MPLS header for the the echo
   reply packet.  This procedure can be applied to an end-to-end path
   consisting of multiple ASes.  Each ASBR that receives an echo request
   from another AS adds its Node-SID and EPE-SID on top of existing
   segments in the Reply Path TLV.

   An ASBR that receives the echo request from a neighbor belonging to
   the same AS, MUST look at the Reply Path TLV received in the echo
   request.  If the Reply Path TLV consists of a Type-C/Type-D segment,
   it MUST convert the Type-C/Type-D segment to a Type-A segment by

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   deriving a label from its own SRGB.  The ASBR MUST set the reply path
   return code to TBA1 and send the newly constructed Reply Path TLV in
   the echo reply.

   Internal nodes or nondomain border nodes MAY not set the Reply Path
   TLV return code to TBA1 in the echo reply message as there is no
   change in the return Path.  In these cases, the headend node/PMS that
   initiates the traceroute procedure MUST continue to send previously
   sent Reply Path TLV in the echo request message in every next echo
   request.

   Note that an ASBR's local policy may prohibit it from participating
   in the dynamic traceroute procedures.  If such an ASBR is encountered
   in the forward path, dynamic return path-building procedures will
   fail.  In such cases, ASBR that supports this document MUST set the
   return code TBA2 to indicate local policies do not allow the dynamic
   return path building.

      Value         Meaning
      ------        ---------------------------------------------------
       TBA2        Local policy does not allow dynamic return Path
                   building.

8.2.  Details with Example

   Let us consider a topology from Figure 1.  Let us consider an SR
   policy path built from PE1 to PE4 with a label stack as below.  N-P1,
   N-ASBR1, EPE-ASBR1-ASBR4, N-PE4.  PE1 begins traceroute with TTL set
   to 1 and includes [N-PE1] in the Reply Path TLV.  The traceroute
   packet TTL expires on P1 and P1 processes the traceroute as per the
   procedures described in [RFC8029] and [RFC8287].  P1 sends echo reply
   with the same Reply Path TLV with reply path return code set to 6.
   The return code of the echo reply itself is set to the return code as
   per [RFC8029] and [RFC8287].  This traceroute doesn't need any
   changes to the Reply Path TLV till it leaves AS1.  The same Reply
   Path TLV that is received may be included in the echo reply by P1 and
   P2 or no Reply Path TLV included so that headend continues to use the
   same return path in echo request that it used to send previous echo
   request.

   When ASBR1 receives the echo request, in case it received Type-C/
   Type-D segment in the Reply Path TLV in the echo request, converts
   that Type-C/Type-D segment to Type-A based on its own SRGB.  When
   ASBR4 receives the echo request, it should form this Reply Path TLV
   using its own Node SID (N-ASBR4) and EPE-SID (EPE-ASRB4-ASBR1) labels
   and set the reply path return code to TBA1.  Then PE1 should use this
   Reply Path TLV in subsequent echo requests.  In this example, when
   the subsequent echo request reaches P3, it should use this Reply Path

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   TLV for sending the echo reply.  The same Reply Path TLV is
   sufficient for any router in AS2 to send the reply.  Because the
   first label(N-ASBR4) can direct echo reply to ASBR4 and the second
   one (EPE-ASBR4-ASBR1) to direct echo reply to AS1.  Once the echo
   reply reaches AS1, normal IP forwarding or the N-PE1 helps it to
   reach PE1.

   The example described in the above paragraphs can be extended to
   multiple ASes by following the same procedure of each ASBR adding
   Node-SID and EPE-SID on receiving echo request from neighboring AS.

   Let us consider a topology from Figure 2.  It consists of multiple
   IGP domains with multiple areas/levels or separate IGP instances.
   There is a single border node that separates the two domains.  In
   this case, PE1 sends a traceroute packet with TTL set to 1 and
   includes N-PE1 in the Reply Path TLV.  ABR1 receives the echo request
   and while sending the echo reply adds its node Label to the Reply
   Path TLV and sets the Reply path return code to TBA1.  The Reply Path
   TLV in the echo reply from ABR1 consists of [N-ABR1, N-PE1].  Next
   echo request with TTL 2 reaches the P node.  It is an internal node
   so it does not change the return Path.  Echo request with TTL 3
   reaches ABR2 and it adds its own Node label so the Reply Path TLV
   sent in echo reply will be [N-ABR2, N-ABR1, N-PE1]. echo request with
   TTL 4 reaches PE4 and it sends an echo reply return code as Egress.
   PE4 does not include any Reply Path TLV in the echo reply.  The above
   example assumes uniform SRGB throughout the domain.  In the case of
   different SRGBs, the top segment will be a Type-C/Type-D segment and
   all other segments will be Type-A.  Each border node converts the
   Type-C/Type-D segment to Type-A before adding its own segment to the
   Reply Path TLV.

9.  Security Considerations

   The procedures described in this document enable LSP ping and
   traceroute to be executed across multiple IGP domains or multiple
   ASes that belong to same administration or closely co-operating
   administration.  It is assumed that sharing domain internal
   information across such domains does not pose a security risk.
   However, procedures described in this document may be used by an
   attacker to extract the domain's internal information.  An operator
   MUST deploy appropriate filter policies as described in [RFC8029] to
   restrict the LSP ping/traceroute packets based on origin.  It is also
   suggested that an operator SHOULD deploy security mechanisms such as
   MACsec on inter-domain links or security-vulnerable links to prevent
   spoofing attacks.

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   Appropriate filter policies SHOULD be applied at the edges to prevent
   attackers from getting into the network.  In the event of such a
   security breach, the network devices MUST have mechanisms to prevent
   of Denial-of-service attacks as described in [RFC8029].

10.  IANA Considerations

10.1.  Segment Sub-TLV

   IANA should assign three new sub-TLVs from the "sub-TLVs for TLV
   Types 1, 16, and 21" subregistry of the "Multi-Protocol Label
   Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
   registry.

      Sub-Type    Sub-TLV Name                  Reference
      --------    -----------------            ------------
    TBD1          SID only in the form of MPLS   Section 4.1
                  label                          of this document
    TBD2          IPv4 Node Address with         Section 4.2
                  optional SID for SR-MPLS       of this document
    TBD3          IPv6 Node Address with         Section 4.3
                  optional SID for SR-MPLS       of this document

   The allocation of code points for the Segment Sub-TLVs should be done
   from the Standards Action range (0-16383)

10.2.  New Registry for Segment Sub-TLV Flags

   IANA should create a new "Segment ID sub-TLV flags" (see
   Section Section 4.4) registry under the "Multi-Protocol Label
   Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
   registry.

   This registry tracks the assignment of 8 flags in the Segment ID sub-
   TLV flags field.  The flags are numbered from 0 (most significant
   bit, transmitted first) to 8.

   New entries are assigned by Standards Action.  Initial entries in the
   registry are as follows:

         Bit number  |  Name                      | Reference
         ------------+----------------------------+--------------
           1         |  A Flag                    | Section 4.4
                     |                            | of this document

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10.3.  Reply Path Return Codes Registry

   IANA should assign new return codes in the "Reply path return code"
   registry under the "Multi-Protocol Label Switching (MPLS) Label
   Switched Paths (LSPs) Ping Parameters" registry.

       Value            Meaning                  Reference
      --------         -----------------        ------------
    TBA1                Use Reply Path TLV       This document
                        from this echo reply
                        for building next
                        echo request.

    TBA2                Local policy does        This document
                        not allow dynamic
                        return Path building.

   The return codes should be assigned from the Standards Action range
   (0x0000-0xFFFB).

11.  Contributors

   1.Carlos Pignataro

   NC State University

   cpignata@gmail.com

   2.  Zafar Ali

   Cisco Systems, Inc.

   zali@cisco.com

12.  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

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   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.

12.1.  Juniper Networks

   Organization: Juniper Networks

   Implementation: JUNOS.

   Description: Implementation for sending Return path TLV with Type-A
   segment subTLV

   Maturity Level: Prototype

   Coverage: Partial.  Type-A SIDs in Return Path TLV

   Contact: shraddha@juniper.net

13.  Acknowledgments

   Thanks to Bruno Decreane for suggesting the use of generic Segment
   Sub-TLV.  Thanks to Adrian Farrel, Huub van Helvoort, Dhruv Dhody,
   Dongjie, for careful review and comments.  Thanks to Mach Chen for
   suggesting to use Reply Path TLV.  Thanks to Gregory Mirsky for the
   detailed review which helped improve the readability of the document
   to a great extent.

14.  References

14.1.  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>.

   [RFC7110]  Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
              "Return Path Specified Label Switched Path (LSP) Ping",
              RFC 7110, DOI 10.17487/RFC7110, January 2014,
              <https://www.rfc-editor.org/info/rfc7110>.

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   [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>.

14.2.  Informative References

   [RFC7743]  Luo, J., Ed., Jin, L., Ed., Nadeau, T., Ed., and G.
              Swallow, Ed., "Relayed Echo Reply Mechanism for Label
              Switched Path (LSP) Ping", RFC 7743, DOI 10.17487/RFC7743,
              January 2016, <https://www.rfc-editor.org/info/rfc7743>.

   [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>.

   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
              <https://www.rfc-editor.org/info/rfc8277>.

   [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>.

   [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>.

   [RFC8604]  Filsfils, C., Ed., Previdi, S., Dawra, G., Ed.,
              Henderickx, W., and D. Cooper, "Interconnecting Millions
              of Endpoints with Segment Routing", RFC 8604,
              DOI 10.17487/RFC8604, June 2019,
              <https://www.rfc-editor.org/info/rfc8604>.

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

   [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>.

   [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>.

   [RFC9552]  Talaulikar, K., Ed., "Distribution of Link-State and
              Traffic Engineering Information Using BGP", RFC 9552,
              DOI 10.17487/RFC9552, December 2023,
              <https://www.rfc-editor.org/info/rfc9552>.

Authors' Addresses

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

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

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

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

   Nagendra Kumar
   Cisco Systems, Inc.
   Email: naikumar@cisco.com

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