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Path Segment Identifier in MPLS Based Segment Routing Network
draft-ietf-spring-mpls-path-segment-22

Document Type Active Internet-Draft (spring WG)
Authors Weiqiang Cheng , Han Li , Cheng Li , Rakesh Gandhi , Royi Zigler
Last updated 2024-02-21 (Latest revision 2023-11-30)
Replaces draft-cheng-spring-mpls-path-segment
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Details
draft-ietf-spring-mpls-path-segment-22
SPRING Working Group                                       W. Cheng, Ed.
Internet-Draft                                                     H. Li
Intended status: Standards Track                            China Mobile
Expires: 2 June 2024                                          C. Li, Ed.
                                                     Huawei Technologies
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                               R. Zigler
                                                                Broadcom
                                                        30 November 2023

     Path Segment Identifier in MPLS Based Segment Routing Network
                 draft-ietf-spring-mpls-path-segment-22

Abstract

   A Segment Routing (SR) path is identified by an SR segment list.  A
   sub-set of segments from the segment list cannot be leveraged to
   distinguish one SR path from another as they may be partially
   congruent.  SR path identification is a pre-requisite for various
   use-cases such as Performance Measurement, and end-to-end 1+1 path
   protection.

   In SR for MPLS data plane (SR-MPLS), an Egress node cannot determine
   on which SR path a packet traversed the network from the label stack
   because the segment identifiers are removed from the label stack as
   the packet transits the network.

   This document defines Path Segment Identifier(PSID) to identify an SR
   path on the egress node of the path.

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 2 June 2024.

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Copyright Notice

   Copyright (c) 2023 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.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Abbreviations and Terms . . . . . . . . . . . . . . . . .   3
   2.  Path Segment  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Equal-Cost Multipath(ECMP) Considerations . . . . . . . .   5
   3.  Use cases . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  PSID for Performance Measurement  . . . . . . . . . . . .   6
     3.2.  PSID for Bidirectional SR Path  . . . . . . . . . . . . .   7
     3.3.  PSID for End-to-end Path Protection . . . . . . . . . . .   7
     3.4.  Nesting of PSIDs  . . . . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Huawei Technologies . . . . . . . . . . . . . . . . . . .  10
     5.2.  ZTE Corp  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  New H3C Technologies  . . . . . . . . . . . . . . . . . .  11
     5.4.  Spirent Communications  . . . . . . . . . . . . . . . . .  11
     5.5.  Fiberhome . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.6.  Interoperability Test . . . . . . . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

   Segment Routing (SR) [RFC8402] leverages the source-routing paradigm
   to steer packets from a source node through a controlled set of
   instructions, called segments, by prepending the packet with an SR
   header.  In the MPLS data plane SR-MPLS [RFC8660] the SR header is
   instantiated through a label stack.

   In an SR-MPLS network, when a packet is transmitted along an SR path,
   the labels in the MPLS label stack will be swapped or popped.  The
   result of this is that no label or only the last label may be left in
   the MPLS label stack when the packet reaches the egress node.  Thus,
   the egress node cannot use the SR label stack to determine along
   which SR path the packet came.

   However, identifying a path on the egress node is a pre-requisite for
   various use-cases in SR-MPLS networks, such as Performance
   Measurement (Section 3.1), bidirectional path (Section 3.2), and end-
   to-end 1+1 path protection (Live-Live case) (Section 3.3).

   Therefore, this document defines a new segment type, referred to
   herein as a Path Segment.  A Path Segment is defined to uniquely
   identify an SR path on the egress node of the path.  It MAY be used
   by the egress node for path identification.  Note that, per-path
   state will be maintained in the egress node due to the requirements
   in the aforementioned use cases, though in normal cases that the per-
   path state will be maintained in the ingress node only.

1.1.  Requirements Language

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

1.2.  Abbreviations and Terms

   MPLS: Multiprotocol Label Switching.

   SR: Segment Routing.

   SID: Segment Identifier.

   SR-MPLS: Instantiation of SR on the MPLS data plane.

   SR path: A SR path is a path described by a Segment-List.

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   Sub-Path: A sub-path is a part of a path, which contains a sub-set of
   the nodes and links of the path.

   PSID: Path Segment Identifier.

2.  Path Segment

   A Path Segment is a Local Segment [RFC8402] which uniquely identifies
   an SR path on the egress node.  A Path Segment Identifier(PSID) is a
   single label that is assigned from the Segment Routing Local Block
   (SRLB) [RFC8402] of the egress node of an SR path.

   A PSID is used to identify a Segment List.  However, one PSID can be
   used to identify multiple Segment Lists in some use cases if needed.
   For example, one single PSID MAY be used to identify some or all
   Segment lists in a Candidate path or an SR policy, if an operator
   would like to aggregate these Segment Lists in operation.

   When a PSID is used, the PSID can be inserted at the ingress node and
   MUST immediately follow the last label of the SR path, in other
   words, inserted after the routing segment (adjacency/node/prefix
   segment) pointing to the egress node of the SR path.  Therefore, a
   PSID will not be the top label in the label stack when received on an
   intermediate node of the associated path, but it can be the top label
   in the label stack on the penultimate node.

   The value of the TTL field in the MPLS label stack entry containing a
   PSID can be set to any value except 0.  If a PSID is the bottom
   label, the S bit MUST be set, and if the PSID is NOT the bottom
   label, the S bit MUST be 0.

   The egress node MUST pop the PSID.  The egress node MAY use the PSID
   for further processing.  For example, when performance measurement is
   enabled on the SR path, it can trigger packet counting or
   timestamping.

   The addition of the PSID will require the egress to read and process
   the PSID label in addition to the regular processing.  This
   additional processing may have an impact on forwarding performance.
   Behavior relating to the use of explicit null directly preceding the
   PSID is undefined in this document.

   Generic Associated Channel Label (GAL) MAY be used for Operations,
   Administration and Maintenance (OAM) in MPLS networks.  As per
   [RFC5586], when GAL is used, the ACH appears immediately after the
   bottom of the label stack.

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   The SR path computation needs to know the Maximum SID Depth (MSD)
   that can be imposed at the ingress node of a given SR path [RFC8664].
   This ensures that the SID stack depth of a computed path does not
   exceed the number of SIDs the node is capable of imposing.  As per
   [RFC8491] the MSD signals the total number of MPLS labels that can be
   imposed, where the total number of MPLS labels includes the PSID.

   An example label stack with PSID is shown in Figure 1:

               +--------------------+
               |       ...          |
               +--------------------+
               |      Label 1       |
               +--------------------+
               |      Label 2       |
               +--------------------+
               |       ...          |
               +--------------------+
               |      Label n       |
               +--------------------+
               |        PSID        |
               +--------------------+
               ~       Payload      ~
               +--------------------+

                      Figure 1: Label Stack with PSID

   Where:

   *  The Labels 1 to n are the segment label stack used to direct how
      to steer the packets along the SR path.

   *  The PSID identifies the SR path in the context of the egress node
      of the SR path.

   Signaling of the PSID between the egress node, the ingress node and
   possibly a centralized controller is out of the scope of this
   document.

2.1.  Equal-Cost Multipath(ECMP) Considerations

   If Entropy Label(EL) is also used on the egress node, as per
   [RFC6790] the Entropy label Indicator (ELI) and Entropy Label (EL)
   would be placed before the tunnel label and hence does not interfere
   with the PSID which is placed below.

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   It is worthy to note that in case of ECMP, with or without the use of
   EL, the SR packets may be forwarded over multiple paths.  In this
   case, the SID list cannot directly reflect the actual forwarding path
   and the PSID can only identify the SID list rather than the actual
   forwarding path.

   Also, similar to Synonymous Flow Labels(SFL) [RFC8957], the
   introduction of an PSID to an existing flow may cause that flow to
   take a different path through the network under conditions of Equal-
   Cost Multipath.  This, in turn, may invalidate certain uses of the
   PSID, such as performance measurement applications.  Therefore, the
   considerations as per section 5 in [RFC8957] of SFL also apply to
   PSID in implementation.

3.  Use cases

   This section describes use cases which can leverage the PSID.  The
   content is for informative purpose, and the detailed solutions might
   be defined in other documents in the future.

3.1.  PSID for Performance Measurement

   As defined in [RFC7799], performance measurement can be classified
   into Passive, Active, and Hybrid measurement.  Since a PSID is
   encoded in the SR-MPLS Label Stack as shown in Figure 1, existing
   implementation on the egress node can leverage PSID for measuring
   packet counts.

   For Passive performance measurement, path identification at the
   measuring points is the pre-requisite.  PSID can be used by the
   measuring points (e.g., the ingress and egress nodes of the SR path
   or a centralized controller) to correlate the packet counts and
   timestamps from the ingress and egress nodes for a specific SR path,
   then packet loss and delay can be calculated for the end-to-end path,
   respectively.

   Furthermore, PSID can also be used for:

   *  Active Performance Measurement for an SR path in SR-MPLS networks
      for collecting packet counters and timestamps from the egress node
      using probe messages.

   *  In-situ OAM[RFC9197] for SR-MPLS to identify the SR Path
      associated with the in-situ data fields in the data packets on the
      egress node.

   *  In-band Performance Measurement for SR-MPLS to identify the SR
      Path associated with the collected performance metrics.

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3.2.  PSID for Bidirectional SR Path

   In some scenarios, for example, mobile backhaul transport networks,
   there are requirements to support bidirectional paths[RFC6965], and
   the path is normally treated as a single entity.  Forward and reverse
   directions of the path have the same fate, for example, failure in
   one direction will result in switching traffic at both directions.
   MPLS supports this by introducing the concepts of co-routed
   bidirectional LSP and associated bidirectional LSP [RFC5654].

   In the current SR architecture, an SR path is a unidirectional path
   [RFC8402].  In order to support bidirectional SR paths, a
   straightforward way is to bind two unidirectional SR paths to a
   single bidirectional SR path.  PSIDs can be used to identify and
   correlate the traffic for the two unidirectional SR paths at both
   ends of the bidirectional path.

   The mechanism of constructing bidirectional path using PSID is out of
   the scope of this document and has been described in several
   documents, such as [I-D.ietf-pce-sr-bidir-path] and
   [I-D.ietf-idr-sr-policy-path-segment].

3.3.  PSID for End-to-end Path Protection

   For end-to-end 1+1 path protection (i.e., Live-Live case), the egress
   node of the path needs to know the set of paths that constitute the
   primary and the secondaries, in order to select the primary path
   packets for onward transmission, and to discard the packets from the
   secondaries [RFC4426].

   To do this in Segment Routing, each SR path needs a path identifier
   that is unique at the egress node.  For SR-MPLS, this can be the Path
   Segment label allocated by the egress node.

   The detailed solution of using PSID in end-to-end 1+1 path protection
   is out of the scope of this document.

3.4.  Nesting of PSIDs

   Binding SID (BSID) [RFC8402] can be used for SID list compression.
   With BSID, an end-to-end SR path in a trusted domain can be split
   into several sub-paths, each sub-path is identified by a BSID.  Then
   an end-to-end SR path can be identified by a list of BSIDs,
   therefore, it can provide better scalability.

   BSID and PSID can be combined to achieve both sub-path and end-to-end
   path monitoring.  A reference model for such a combination in
   (Figure 2) shows an end-to-end path (A->D) in a trusted domain that

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   spans three subdomains (Access, Aggregation and Core domain) and
   consists of three sub-paths, one in each subdomain (sub-path (A->B),
   sub-path (B->C) and sub-path (C->D)).

   The SID list of a sub-path can be expressed as <SID1, SID2, ...SIDn,
   s-PSID>, where the s-PSID is the PSID of the sub-path.  Each sub-path
   is associated with a BSID and an s-PSID.

   The SID list of the end-to-end path can be expressed as <BSID1,
   BSID2, ..., BSIDn, e-PSID>, where the e-PSID is the PSID of the end-
   to-end path.

   Figure 2 shows the details of the label stacks when PSID and BSID are
   used to support both sub-path and end-to-end path monitoring in a
   multi-domain scenario.

            /--------\       /--------\       /--------\
          /            \   /            \   /            \
        A{    Access    }B{  Aggregation }C{     Core     }D
          \            /   \            /   \            /
            \--------/       \--------/       \--------/
          Sub-path(A->B)    Sub-path(B->C)   Sub-path(C->D)
       |<--------------->|<-------------->|<-------------->|
                             E2E Path(A->D)
       |<------------------------------------------------->|

    +------------+
    ~A->B SubPath~
    +------------+  +------------+
    |s-PSID(A->B)|  ~B->C SubPath~
    +------------+  +------------+  +------------+
    | BSID(B->C) |  |s-PSID(B->C)|  ~C->D SubPath~
    +------------+  +------------+  +------------+
    | BSID(C->D) |  | BSID(C->D) |  |s-PSID(C->D)|
    +------------+  +------------+  +------------+  +------------+
    |e-PSID(A->D)|  |e-PSID(A->D)|  |e-PSID(A->D)|  |e-PSID(A->D)|
    +------------+  +------------+  +------------+  +------------+

                         Figure 2: Nesting of PSIDs

4.  Security Considerations

   A PSID in SR-MPLS is a local label similar to other labels/Segment,
   such as a VPN label, defined in MPLS and SR-MPLS.  The data plane
   processing of a PSID is a local implementation of an ingress node, or
   an egress node, which follows the same logic of existing MPLS
   dataplane.  As per definition of PSID, only the egress node and the
   ingress node of the associated path will learn the information of

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   PSID.  The intermediate nodes of this path will not learn it.

   A PSID may be used on an ingress node that is not the ingress of the
   associated path, if the associated label stack with PSID is part of a
   deeper label stack which represents a longer path.  For example the
   case described in Section 3.4 and the related BSID is not used while
   the original label stack of sub-path is inserted as a part of the
   whole label stack.  In this case, the PSID must be distributed in a
   trusted domain under the considerations defined in Section 8.1 of
   [RFC8402].

   A PSID is used within an SR-MPLS trusted domain [RFC8402] and must
   not leak outside the domain, therefore no new security threats are
   introduced comparing to current SR-MPLS.  As per [RFC8402], SR domain
   boundary routers MUST filter any external traffic destined to a label
   associated with a segment within the trusted domain, this applies to
   PSID as well.  Other security considerations of SR-MPLS, described in
   Section 8.1 of [RFC8402] applies to this document.

   The distribution of a PSID from an egress node to an ingress nodes is
   performed within an SR trusted domain, and it is out of the scope of
   this document.  The details of the mechanism and related security
   considerations will be described in other documents.

5.  Implementation Status

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to [RFC7942].

   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.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

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5.1.  Huawei Technologies

   *  Organization: Huawei Technologies.

   *  Implementation: Huawei PTN7900 Series Routers implementation of
      SR-TP[HW-IMP].

   *  Description: SR-TP is a feature of Huawei PTN7900 series Routers,
      which uses PSIDs to associate with paths and build up
      bidirectional paths.  Huawei PTN7900 Series Routers with version
      V100R018C00 and above have commercially implemented the definition
      of PSID and use cases which is defined in section 2 and
      Section 3.2 in this document, including all the "MUST" and
      "SHOULD" clauses, while other use cases for PSID in section 3 are
      not yet implemented.  For control plane, PTN7900 Series Routers
      support configuring PSID using NETCONF.

   *  Maturity Level: Product

   *  Coverage: Partial, section 2 and use case section 3.2.

   *  Version: Draft-12

   *  Licensing: N/A

   *  Implementation experience: Nothing specific.

   *  Contact: li.fan@huawei.com

   *  Last updated: September 14, 2023

5.2.  ZTE Corp

   *  Organization: ZTE Corporation.

   *  Implementation: ZTE's SPN implementation of PSID[ZTE-IMP].

   *  Description: The feature of SR-MPLS PSID has been implemented in
      ZTE SPN products and follows the definition and mechanism as
      defined in section 2 and Section 3.2 including all the "MUST" and
      "SHOULD" clauses while other use cases for PSID in section 3 are
      not yet implemented.

   *  Maturity Level: Product

   *  Coverage: Partial,section 2 and use case section 3.2.

   *  Version: Draft-12

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   *  Licensing: N/A

   *  Implementation experience: Nothing specific.

   *  Contact: liu.aihua@zte.com.cn

   *  Last updated: September 21, 2023

5.3.  New H3C Technologies

   *  Organization: New H3C Technologies.

   *  Implementation: H3C CR16000, CR19000 series routers implementation
      of PSID.

   *  Description: Section 2 and Section 3.2 including all the "MUST"
      and "SHOULD" clauses have been implemented in above-mentioned New
      H3C Products(running Version 7.1.086 and above) for testing, while
      other use cases for PSID in section 3 are not yet implemented.

   *  Maturity Level: Beta

   *  Coverage: Partial, section 2 and use case section 3.2.

   *  Version: Draft-12

   *  Licensing: N/A

   *  Implementation experience: Nothing specific.

   *  Contact: linchangwang.04414@h3c.com

   *  Last updated: September 13, 2023

5.4.  Spirent Communications

   *  Organization: Spirent Communications

   *  Implementation: Spirent Testcenter Product Family implementation
      of SR-TP test capability[SP-IMP].

   *  Description: Spirent Testcenter product family implements SR-MPLS
      PSID test capabilities on the versions above Spirent Testcenter
      4.85.  Spirent Testcenter fully support testing all clauses
      defined in section 2 and Section 3.1,3.2,3.4 , including all the
      "MUST" and "SHOULD" clauses, and partially support the test of
      clauses in section 3.3.

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   *  Maturity Level: Production

   *  Coverage: fully cover section 2 and use case section 3.1,3.2, 3.4,
      partially cover section 3.3

   *  Version: Draft-12

   *  Licensing: N/A

   *  Implementation experience: Nothing specific.

   *  Contact: junqi.zhao@spirent.com

   *  Last updated: September 21, 2023

5.5.  Fiberhome

   *  Organization: Fiberhome Corporation.

   *  Implementation: Fiberhome SPN series of products (Citrans
      650/690E) implementation of PSID[FH-IMP].

   *  Description: SR-TP is a feature of SPN products, which realizes a
      controllable L3 tunnel, builds the end-to-end L3 deployment
      business model.  The PSID follows the definition and mechanism as
      defined in section 2 and Section 3.2 including all the "MUST" and
      "SHOULD" clauses had been implemented, while other use cases for
      PSID in section 3 are not yet implemented.

   *  Maturity Level: Product

   *  Coverage: Partial,section 2 and use case section 3.2.

   *  Version: Draft-12

   *  Licensing: N/A

   *  Implementation experience: Nothing specific.

   *  Contact: zhhan@fiberhome.com

   *  Last updated: September 21, 2023

5.6.  Interoperability Test

   [Note to the RFC Editor - remove this section before publication, as
   well as remove the reference to [RFC7942].

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   The Interoperability test of PSID had been done among products from
   several vendors, including Huawei(PTN7900, V100R018C00), ZTE(ZXCTN
   6180, Ver 4.00.00), FiberHome(Citrans 650/690E) , Spirent (Chassis:
   SPT-N4U-220.Test.  Module: PX3-QSFP28-12-225A.  Version: 4.86) and
   Nokia in 2018[INTEROP-TEST].  Note that PSID is a key feature of
   Layer3 in SPN architecture [SPN-L3].  This is reported by Weiqiang
   Cheng from China Mobile at September, 21, 2023.

6.  IANA Considerations

   This document does not require any IANA actions.

7.  References

7.1.  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/rfc/rfc2119>.

   [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/rfc/rfc8174>.

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

   [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/rfc/rfc8660>.

7.2.  Informative References

   [FH-IMP]   "Fiberhome Routers", 21 September 2021,
              <https://www.fiberhome.com/operator/product/
              products/294.aspx.html>.

   [HW-IMP]   "Huawei PTN7900 Routers", 21 September 2021,
              <https://carrier.huawei.com/en/products/fixed-network/
              carrier-ip/router/ptn/ptn7900>.

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   [I-D.ietf-idr-sr-policy-path-segment]
              Li, C., Li, Z., Yin, Y., Cheng, W., and K. Talaulikar, "SR
              Policy Extensions for Path Segment and Bidirectional
              Path", Work in Progress, Internet-Draft, draft-ietf-idr-
              sr-policy-path-segment-08, 16 August 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
              policy-path-segment-08>.

   [I-D.ietf-pce-sr-bidir-path]
              Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
              "Path Computation Element Communication Protocol (PCEP)
              Extensions for Associated Bidirectional Segment Routing
              (SR) Paths", Work in Progress, Internet-Draft, draft-ietf-
              pce-sr-bidir-path-12, 9 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pce-sr-
              bidir-path-12>.

   [INTEROP-TEST]
              China Mobile, "Adhering to Innovation-Driven Development
              and Focusing on Technological Breakthroughs--China Mobile
              Research Institute Accelerates 5G R&D and Tests", 30 May
              2019, <http://www.cww.net.cn/web/news/channel/
              articleinfo.action?id=452789>.

   [RFC4426]  Lang, J., Ed., Rajagopalan, B., Ed., and D. Papadimitriou,
              Ed., "Generalized Multi-Protocol Label Switching (GMPLS)
              Recovery Functional Specification", RFC 4426,
              DOI 10.17487/RFC4426, March 2006,
              <https://www.rfc-editor.org/rfc/rfc4426>.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <https://www.rfc-editor.org/rfc/rfc5586>.

   [RFC5654]  Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,
              Sprecher, N., and S. Ueno, "Requirements of an MPLS
              Transport Profile", RFC 5654, DOI 10.17487/RFC5654,
              September 2009, <https://www.rfc-editor.org/rfc/rfc5654>.

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,
              <https://www.rfc-editor.org/rfc/rfc6790>.

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   [RFC6965]  Fang, L., Ed., Bitar, N., Zhang, R., Daikoku, M., and P.
              Pan, "MPLS Transport Profile (MPLS-TP) Applicability: Use
              Cases and Design", RFC 6965, DOI 10.17487/RFC6965, August
              2013, <https://www.rfc-editor.org/rfc/rfc6965>.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/rfc/rfc7799>.

   [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/rfc/rfc7942>.

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

   [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/rfc/rfc8664>.

   [RFC8957]  Bryant, S., Chen, M., Swallow, G., Sivabalan, S., and G.
              Mirsky, "Synonymous Flow Label Framework", RFC 8957,
              DOI 10.17487/RFC8957, January 2021,
              <https://www.rfc-editor.org/rfc/rfc8957>.

   [RFC9197]  Brockners, F., Ed., Bhandari, S., Ed., and T. Mizrahi,
              Ed., "Data Fields for In Situ Operations, Administration,
              and Maintenance (IOAM)", RFC 9197, DOI 10.17487/RFC9197,
              May 2022, <https://www.rfc-editor.org/rfc/rfc9197>.

   [SP-IMP]   "Spirent Devices", 21 September 2021,
              <https://www.spirent.com/assets/u/flexe-test-solution-for-
              5g-backhaul>.

   [SPN-L3]   China Mobile, "The-transport-network-consi-deration-for-
              5G-in-CMCC", 1 December 2018, <https://opennetworking.org/
              wp-content/uploads/2018/12/The-transport-network-consi-
              deration-for-5G-in-CMCC.pdf>.

   [ZTE-IMP]  "ZTE ZXCTN-6700 Routers", 21 September 2021,
              <https://www.zte.com.cn/china/product_index/ip_network/
              item01/zxctn-6700/zxctn_6700.html>.

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Acknowledgements

   The authors would like to thank Adrian Farrel, Stewart Bryant,
   Shuangping Zhan, Alexander Vainshtein, Andrew G.  Malis, Ketan
   Talaulikar, Shraddha Hegde, Xinyue Zhang, Loa Andersson and Bruno
   Decraene for their review, suggestions, comments and contributions to
   this document.

   The authors would like to acknowledge the contribution from Alexander
   Vainshtein on "Nesting of PSIDs".

Contributors

   The following people have substantially contributed to this document.

   Mach(Guoyi) Chen
   Huawei Technologies Co., Ltd
   Email: mach.chen@huawei.com

   Lei Wang
   China Mobile
   Email: wangleiyj@chinamobile.com

   Aihua Liu
   ZTE Corp
   Email: liu.aihua@zte.com.cn

   Greg Mirsky
   ZTE Corp
   Email: gregimirsky@gmail.com

   Gyan S. Mishra
   Verizon Inc.
   Email: gyan.s.mishra@verizon.com

Authors' Addresses

   Weiqiang Cheng (editor)
   China Mobile
   Email: chengweiqiang@chinamobile.com

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   Han Li
   China Mobile
   Email: lihan@chinamobile.com

   Cheng Li (editor)
   Huawei Technologies
   China
   Email: c.l@huawei.com

   Rakesh Gandhi
   Cisco Systems, Inc.
   Canada
   Email: rgandhi@cisco.com

   Royi Zigler
   Broadcom
   Email: royi.zigler@broadcom.com

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