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Path Segment in MPLS Based Segment Routing Network
draft-ietf-spring-mpls-path-segment-08

Document Type Active Internet-Draft (spring WG)
Authors Weiqiang Cheng , Han Li , Mach Chen , Rakesh Gandhi , Royi Zigler
Last updated 2022-11-20 (Latest revision 2022-09-28)
Replaces draft-cheng-spring-mpls-path-segment
Stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Reviews
SECDIR Last Call Review Incomplete, due 2022-11-20
OPSDIR Last Call Review Incomplete, due 2022-11-20
Stream WG state Submitted to IESG for Publication
Document shepherd Jim Guichard
Shepherd write-up Show Last changed 2021-12-15
IESG IESG state Waiting for Writeup
Action Holder
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Andrew Alston
Send notices to james.n.guichard@futurewei.com
IANA IANA review state IANA OK - No Actions Needed
draft-ietf-spring-mpls-path-segment-08
SPRING Working Group                                            W. Cheng
Internet-Draft                                                     H. Li
Intended status: Standards Track                            China Mobile
Expires: 1 April 2023                                            M. Chen
                                                                  Huawei
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                               R. Zigler
                                                                Broadcom
                                                       28 September 2022

           Path Segment in MPLS Based Segment Routing Network
                 draft-ietf-spring-mpls-path-segment-08

Abstract

   A Segment Routing (SR) path is identified by an SR segment list.
   Only the complete segment list can identify the end-to-end SR path,
   and a sub-set of segments from the segment list cannot 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 (PM), bidirectional paths correlation, and
   end-to-end 1+1 path protection.

   In SR for MPLS data plane (SR-MPLS), the segment identifiers are
   stripped from the packet through label popping as the packet transits
   the network.  This means that when a packet reaches the egress of the
   SR path, it is not possible to determine on which SR path it
   traversed the network.

   This document defines a new type of segment that is referred to as
   Path Segment, which is used to identify an SR path in an SR-MPLS
   network.  When used, it is inserted by the ingress node of the SR
   path and immediately follows the last segment identifier in the
   segment list of the SR path.  The Path Segment is preserved until it
   reaches the egress node for SR path identification and correlation.

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/.

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   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 1 April 2023.

Copyright Notice

   Copyright (c) 2022 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Path Segment  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Path Segment Allocation and Distribution  . . . . . . . . . .   6
   4.  Nesting of Path Segments  . . . . . . . . . . . . . . . . . .   7
   5.  Path Segment for Performance Measurement  . . . . . . . . . .   8
   6.  Path Segment for Bidirectional SR Path  . . . . . . . . . . .   9
   7.  Path Segment for End-to-end Path Protection . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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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] through a label stack
   or IPv6 data plane using an SRH header via SRv6 [RFC8986] to
   construct an SR path.

   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.  So
   that no label or only the last label (e.g.  Explicit-Null label) may
   be left in the MPLS label stack when the packet reaches the egress
   node.  Thus, the egress node cannot determine along which SR path the
   packet came.

   However, to support various use-cases in SR-MPLS networks, like end-
   to-end 1+1 path protection (Live-Live case) [RFC4426], bidirectional
   path [RFC5654], or Performance Measurement (PM) [RFC7799], the
   ability to implement path identification on the egress node is a pre-
   requisite.

   Therefore, this document introduces a new segment type that is
   referred to as the Path Segment.  A Path Segment is defined to
   uniquely identify an SR path in an SR-MPLS network.  It MAY be used
   by the egress nodes for path identification hence to support various
   use-cases including SR path PM, end-to-end 1+1 SR path protection,
   and bidirectional SR paths correlation.

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
   [RFC2119] [RFC8174] when, and only when, they appear in all capitals,
   as shown here.

1.2.  Abbreviations

   DM: Delay Measurement.

   LM: Loss Measurement.

   MPLS: Multiprotocol Label Switching.

   MSD: Maximum SID Depth.

   PM: Performance Measurement.

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   PSID: Path Segment ID.

   SID: Segment ID.

   SL: Segment List.

   SR: Segment Routing.

   SRLB: SR Local Block

   SRGB: SR Global Block

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

   SRv6: Instantiation of SR on the IPv6 data plane.

2.  Path Segment

   A Path Segment is a single label that is assigned from the Segment
   Routing Local Block (SRLB) [RFC8402] or Segment Routing Global Block
   (SRGB) [RFC8402] or dynamic MPLS label pool of the egress node of an
   SR path.  Whether a Path Segment is allocated from the SRLB, SRGB, or
   a dynamic range depends on specific use cases.  If the Path Segment
   is only used by the egress node to identify a SR path, the SRLB, SRGB
   or dynamic MPLS label pool can be used.  If the Path Segment is used
   by an intermediate node to identify a SR path, the SRGB MUST be used.
   Three use cases are introduced in Section 5, 6, and 7 of this
   document.

   When a Path Segment is used, the Path Segment MUST 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.

   The term of SR path used in this document is a general term that can
   be used to describe a SR Policy, a Candidate-Path (CP), or a Segment-
   List (SL) [RFC9256].  Therefore, the Path Segment may be used to
   identify an SR Policy, its CP, or a SL terminating on an egress node
   depending on the use-case.

   The value of the TTL field in the MPLS label stack entry containing
   the Path Segment MUST be set to the same value as the TTL of the last
   label stack entry for the last segment in the SR path.  If the Path
   Segment is the bottom label, the S bit MUST be set.

   When Path Segment is not allocated from the SRGB pool, the
   intermediate nodes MUST not see the Path Segment label at the top of
   the label stack.  A Path Segment presenting to an intermediate node

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   in this case is an error condition.  When Path Segment is allocated
   from the SRGB pool, an intermediate node MAY see the Path Segment
   label at the top of the label stack.

   A Path Segment can be used in the case of Penultimate Hop Popping
   (PHP), where some labels are be popped off at the penultimate hop of
   an SR path, but the Path Segment MUST NOT be popped off until it
   reaches at the egress node.

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

   In some deployments, service labels may be added after the Path
   Segment label in the MPLS label stack.  In this case, the egress node
   MUST be capable of processing more than one label.  The additional
   processing required, may have an impact on forwarding performance.

   Generic Associated Label (GAL) is used for Operations, Administration
   and Maintenance (OAM) in MPLS networks [RFC5586].  When GAL is used,
   it MUST be added at the bottom of the label stack after the Path
   Segment label.

   Entropy label and Entropy Label Indicator (ELI) as described in
   [RFC8662] for SR-MPLS path, can be placed before or after the Path
   Segment label in the MPLS label stack.

   The SR path computation needs to know the Maximum SID Depth (MSD)
   that can be imposed at each node/link 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.  The MSD
   used for path computation MUST include the Path Segment label.

   The label stack with Path Segment is shown in Figure 1:

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               +--------------------+
               |       ...          |
               +--------------------+
               |      Label 1       |
               +--------------------+
               |      Label 2       |
               +--------------------+
               |       ...          |
               +--------------------+
               |      Label n       |
               +--------------------+
               |     Path Segment   |
               +--------------------+
               |       ...          |
               +--------------------+
               ~       Payload      ~
               +--------------------+

      Figure 1: Label Stack with Path Segment

   Where:

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

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

   There may be multiple paths (or sub-path(s)) carried in the label
   stack, for each path (or sub-path), there may be a corresponding Path
   Segment carried.  A use case can be found in Section 4.

   In addition, adding a Path Segment to a label stack will increase the
   depth of the label stack, the Path Segment should be accounted when
   considering Maximum SID Depth (MSD)[RFC8992].

3.  Path Segment Allocation and Distribution

   There are two ways that leverage a centralized controller (e.g., SDN
   controller) to assign and distribute the Path Segment.  One is the
   Path Computation Element Communication Protocol (PCEP) based Path
   Segment allocation for SR Policy defined in
   [I-D.ietf-pce-sr-path-segment], the other is BGP based Path Segment
   allocation for SR Policy defined in
   [I-D.ietf-idr-sr-policy-path-segment].  For both ways, the controller
   MUST make sure (e.g., by some capability discovery mechanisms outside
   the scope of this document) that the egress node knows the Path
   Segment and it can process it, as well as the label does not collide

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   with any label allocation done by the egress node.

4.  Nesting of Path Segments

   Binding SID (BSID) [RFC8402] can be used for SID list compression.
   With BSID, an end-to-end SR path 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 Path SID (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) that spans three
   domains (Access, Aggregation and Core domain) and consists of three
   sub-paths, one in each sub-domain (sub-path (A->B), sub-path (B->C)
   and sub-path (C->D)).  Each sub-path is allocated a BSID.  For
   nesting the sub-paths, each sub-path is allocated a PSID.  Then, 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.  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.

   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.

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            /--------\       /--------\       /--------\
          /            \   /            \   /            \
        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)|
    +------------+  +------------+  +------------+
    | BSID(C->D) |  | BSID(C->D) |  ~C->D SubPath~
    +------------+  +------------+  +------------+  +------------+
    |e-PSID(A->D)|  |e-PSID(A->D)|  |e-PSID(A->D)|  |e-PSID(A->D)|
    +------------+  +------------+  +------------+  +------------+

                Figure 2: Nesting of Path Segments

5.  Path Segment for Performance Measurement

   As defined in [RFC7799], performance measurement can be classified
   into Passive, Active, and Hybrid measurement.  Since Path Segment is
   encoded in the SR-MPLS Label Stack as shown in Figure 1, existing
   implementation on the egress node can be leveraged for measuring
   packet counts using the incoming SID (the Path Segment).  A different
   scheme such as carrying such identifier after the bottom of the label
   stack may require new implementation.

   For Passive performance measurement, path identification at the
   measuring points is the pre-requisite.  Path Segment 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.

   Path Segment 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
   [I-D.ietf-mpls-rfc6374-sr] and [I-D.ietf-spring-stamp-srpm].

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   Path Segment can also be used for In-situ OAM for SR-MPLS to identify
   the SR Path associated with the in-situ data fields in the data
   packets on the egress node [I-D.gandhi-mpls-ioam].

   Path Segment can also be used for In-band PM for SR-MPLS to identify
   the SR Path associated with the collected performance metrics
   [I-D.ietf-mpls-inband-pm-encapsulation].

6.  Path Segment for Bidirectional SR Path

   In some scenarios, for example, mobile backhaul transport networks,
   there are requirements to support bidirectional paths, and the path
   is normally treated as a single entity.  The both 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.  Path Segments can then be used to
   identify and correlate the traffic for the two unidirectional SR
   paths at both ends of the bidirectional path.

   [I-D.ietf-pce-sr-bidir-path] defines procedures on how to use PCEP
   for SR Policy to initiate a bidirectional SR path.  Also,
   [I-D.ietf-idr-sr-policy-path-segment] defines procedures on how to
   use BGP for SR Policy to initiate a bidirectional SR path.

7.  Path Segment 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.

   There then needs to be a method of binding this SR path identifiers
   into equivalence groups such that the egress node can determine for
   example, the set of packets that represent a single primary path.  It
   is obvious that this equivalence group can be instantiated in the
   network by an SDN controller using the Path Segments of the SR paths.

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8.  Security Considerations

   Path Segment in SR-MPLS does not introduce any new behavior or any
   change in the way the MPLS data plane works.  Section 8.1 of
   [RFC8402] describe the security consideration for SR-MPLS.  Path
   segment is additional metadata that is added to the packet consisting
   of the SR path.

   An attacker could exploit path segment to manipulate the accounting
   of SR traffic at the egress.  Path segment could also be used to
   monitor traffic patterns for the E2E paths.  The control protocols
   used to allocate path segments could also be exploited to disseminate
   incorrect path segment information.  Note that, the path segment is
   imposed at the ingress and removed at the egress boundary and is not
   leaked out of the administered domain.

9.  IANA Considerations

   This document does not require any IANA actions.

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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

   [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>.

10.2.  Informative References

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   [I-D.gandhi-mpls-ioam]
              Gandhi, R., Ali, Z., Brockners, F., Wen, B., Decraene, B.,
              and V. Kozak, "MPLS Data Plane Encapsulation for In-situ
              OAM Data", Work in Progress, Internet-Draft, draft-gandhi-
              mpls-ioam-05, 6 July 2022,
              <https://www.ietf.org/archive/id/draft-gandhi-mpls-ioam-
              05.txt>.

   [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-06, 7 August 2022,
              <https://www.ietf.org/archive/id/draft-ietf-idr-sr-policy-
              path-segment-06.txt>.

   [I-D.ietf-mpls-inband-pm-encapsulation]
              Cheng, W., Min, X., Zhou, T., Dong, X., and Y. Peleg,
              "Encapsulation For MPLS Performance Measurement with
              Alternate Marking Method", Work in Progress, Internet-
              Draft, draft-ietf-mpls-inband-pm-encapsulation-03, 1 July
              2022, <https://www.ietf.org/archive/id/draft-ietf-mpls-
              inband-pm-encapsulation-03.txt>.

   [I-D.ietf-mpls-rfc6374-sr]
              Gandhi, R., Filsfils, C., Voyer, D., Salsano, S., and M.
              Chen, "Performance Measurement Using RFC 6374 for Segment
              Routing Networks with MPLS Data Plane", Work in Progress,
              Internet-Draft, draft-ietf-mpls-rfc6374-sr-06, 23 August
              2022, <https://www.ietf.org/archive/id/draft-ietf-mpls-
              rfc6374-sr-06.txt>.

   [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-10, 31 August 2022,
              <https://www.ietf.org/archive/id/draft-ietf-pce-sr-bidir-
              path-10.txt>.

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   [I-D.ietf-pce-sr-path-segment]
              Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
              "Path Computation Element Communication Protocol (PCEP)
              Extension for Path Segment in Segment Routing (SR)", Work
              in Progress, Internet-Draft, draft-ietf-pce-sr-path-
              segment-06, 19 August 2022,
              <https://www.ietf.org/archive/id/draft-ietf-pce-sr-path-
              segment-06.txt>.

   [I-D.ietf-spring-stamp-srpm]
              Gandhi, R., Filsfils, C., Voyer, D., Chen, M., Janssens,
              B., and R. Foote, "Performance Measurement Using Simple
              TWAMP (STAMP) for Segment Routing Networks", Work in
              Progress, Internet-Draft, draft-ietf-spring-stamp-srpm-05,
              25 August 2022, <https://www.ietf.org/archive/id/draft-
              ietf-spring-stamp-srpm-05.txt>.

   [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/info/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/info/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/info/rfc5654>.

   [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/info/rfc7799>.

   [RFC8662]  Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
              Shakir, R., and J. Tantsura, "Entropy Label for Source
              Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
              DOI 10.17487/RFC8662, December 2019,
              <https://www.rfc-editor.org/info/rfc8662>.

   [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>.

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

   [RFC8992]  Jiang, S., Ed., Du, Z., Carpenter, B., and Q. Sun,
              "Autonomic IPv6 Edge Prefix Management in Large-Scale
              Networks", RFC 8992, DOI 10.17487/RFC8992, May 2021,
              <https://www.rfc-editor.org/info/rfc8992>.

   [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>.

Acknowledgements

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

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

Contributors

   The following people have substantially contributed to this document:

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      Cheng Li
      Huawei Technologies

      EMail: chengli13@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
   China Mobile
   Email: chengweiqiang@chinamobile.com

   Han Li
   China Mobile
   Email: lihan@chinamobile.com

   Mach(Guoyi) Chen
   Huawei
   Email: mach.chen@huawei.com

   Rakesh Gandhi
   Cisco Systems, Inc.
   Canada

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   Email: rgandhi@cisco.com

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

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