Network Working Group                                           W. Cheng
Internet-Draft                                                     H. Li
Intended status: Standards Track                            China Mobile
Expires: September 9, 2019 March 19, 2020                                          M. Chen
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                               R. Zigler
                                                          March 08,
                                                      September 16, 2019

           Path Segment in MPLS Based Segment Routing Network


   A Segment Routing (SR) path is identified by an SR segment list, list.
   Only the full segment list identifies the full end-to-end path, and
   one segment or a partial segments set of segments from the list cannot uniquely
   identify the SR path.
   Path path and distinguish it from other SR paths that may
   be partially congruent.  But path identification is a pre-requisite
   for various use-cases such as performance measurement (PM) of an SR

   In SR for MPLS (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

   This document defines a new type of segment that is referred to as
   Path Segment, which is used to identify an SR path. path in an SR-MPLS
   network.  When used, it is inserted at 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 will not be popped off
   until it reaches the egress node of the SR path.

   A Path Segment can be used by the egress node to implement path
   identification hence to support various use-cases including SR path
   PM, end-to-end 1+1 SR path protection protection, and bidirectional SR paths

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Path Segment  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Nesting of Path Segments  . . . . . . . . . . . . . . . . . .   5
   4.  Path Segment Allocation . . . . . . . . . . . . . . . . . . .   6
   5.  Path Segment for PM . . . . . . . . . . . . . . . . . . . . .   6   7
   6.  Path Segment for Bi-directional SR Path . . . . . . . . . . .   7
   7.  Path Segment for End-to-end Path Protection . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   8
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9   8
     12.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     12.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Segment Routing (SR) [RFC8402] is a source routed forwarding method
   that allows to directly encode forwarding instructions (called
   segments) in each packet, hence it enables to steer steering traffic through a
   network without the per-flow states maintained on the transit nodes.
   Segment Routing can be instantiated on an MPLS data plane or an IPv6
   data plane.  The former is called SR-MPLS, SR-MPLS
   [I-D.ietf-spring-segment-routing-mpls], the latter is called SRv6
   [RFC8402].  SR-MPLS leverages the MPLS label stack to construct SR
   path, and SRv6 uses the a new IPv6 Extension Header (EH) called the
   IPv6 Segment Routing Header (SRH)
   [I-D.ietf-6man-segment-routing-header] to construct 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 may be left in the MPLS label
   stack when the packet reaches the egress node.  Thus, the egress node
   cannot determine from along which SR path the packet comes. came.

   However, to support use cases like end-to-end 1+1 path protection
   (Live-Live case), bidirectional path correlation or performance
   measurement (PM), the ability to implement path identification is a
   pre-requisite.  More detail can be found in Section 5, 6, and 7.

   Therefore, this document introduces a new segment that is referred to
   as the Path Segment.  A Path Segment is defined to uniquely identify
   an SR path in an SR-MPLS network in the context of the egress node.
   It is normally used by egress nodes for path identification or

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP14 [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.

   PM: Performance Measurement.

   PSID: Path Segment ID.

   SID: Segment ID.

   SL: Segment List.

   SR: Segment Routing.

   SR-MPLS: Segment Routing instantiated on MPLS data plane.

   SRv6: Segment Routing instantiated on IPv6 data plane

2.  Path Segment

   A Path Segment is a single label that is assigned from the Segment
   Routing Local Block (SRLB) or Segment Routing Global Block (SRGB) of
   the egress node of an SR path.  It means that the Path Segment is
   unique in the context of the egress node of the SR path.  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.  The
   Path Segment may be used to identify an SR-MPLS Policy, its
   Candidate-Path (CP) (CP), or a SID List (SL)
   [I-D.ietf-spring-segment-routing-policy] terminating on an egress
   node depending on the use-case.

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

   Normally, the intermediate nodes will not see the Path Segment label
   and do not know how to process it.  A Path Segment presenting to an
   intermediate node is an error condition.

   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.

   The label stack with Path Segment is as below (Figure 1):

               |       ...          |
               |      Label 1       |
               |      Label 2       |
               |       ...          |
               |      Label n       |
               |     Path Segment   |
               |       ...          |

      Figure 1: Label Stack with Path Segment


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

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

3.  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.

            /--------\       /--------\       /--------\
          /            \   /            \   /            \
        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

4.  Path Segment Allocation

   Several ways can be used to allocate the Path Segment.

   One way is to set up a communication channel (e.g., MPLS Generic
   Associated Channel (G-ACh)) between the ingress node and the egress
   node, and the ingress node of the SR path can directly send a request
   to the egress node to ask for a Path Segment.

   Another way is to leverage a centralized controller (e.g., PCE, SDN
   controller) to assign the Path Segment.  PCEP based Path Segment
   allocation is defined in [], and SR-policy
   based path segment allocation is defined in

   A Path Segment can be used in the case of PHP, where some labels MAY
   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 LSR.
   In the case of a Path Segment that is assigned by a centralized
   controller, the controller must make sure (e.g., by some capability
   discovery mechanisms) that the egress LSR knows and can process the
   Path Segment.

5.  Path Segment for PM

   As defined in [RFC7799], performance measurement can be classified
   into Active, Passive and Hybrid measurement.  For Passive
   measurement, path identification at the measuring points is the pre-
   requisite.  Path segment can be used by the measuring points (e.g.,
   the ingress/egress nodes of an SR path) or a centralized controller
   to correlate the packets counts/timestamps that are from the ingress
   and egress nodes to a specific SR path, then packet loss/delay can be

   Performance Delay Measurement (DM) and Loss Measurements (LM) in SR
   networks with MPLS data plane can be found in
   [I-D.gandhi-spring-sr-mpls-pm] and [I-D.gandhi-spring-udp-pm].

6.  Path Segment for Bi-directional SR Path

   With the current SR architecture, an SR path is a unidirectional
   path.  In some scenarios, for example, mobile backhaul transport
   network, there are requirements to support bidirectional path, paths, and
   the path is normally treated as a single entity and both directions
   of the path have the same fate, for example, failure in one direction
   will result in switching at both directions.

   MPLS supports this by introducing the concepts of co-routed
   bidirectional LSP and associated bidirectional LSP.  With SR, to
   support bidirectional path, paths, a straightforward way is to bind two
   unidirectional SR paths to a single bidirectional path.  Path
   Segments can be used to correlate the two unidirectional SR paths at
   both ends of the paths.

   [] defines how to use PCEP and Path segment
   to initiate a bidirectional SR path, and
   [] defines how to use
   SR policy and Path segment 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 an SR path needs to know the set of paths that constitute the
   primary and the secondary(s), secondaries, in order to select the primary packet
   for onward transmission, and to discard the packets from the

   To do this, each path needs a path identifier that is unique at the
   egress node.  Depending on the design, this is a single unique path
   segment label chosen by the egress PE.

   There then needs to be a method of binding this path identifiers into
   equivalence groups such that the egress PE can determine the set of
   packets that represent a single path and its secondary.

   It is obvious that this group can be instantiated in the network by
   an SDN controller. controller and that the Path Segment achieves the desired

8.  IANA Considerations

   This document does not require any IANA actions.

9.  Security Considerations

   This document does not introduce additional security requirements and
   mechanisms other than the ones described in [RFC8402].

10.  Contributors

   Many thanks to

   The following contributors for their contribution individuals also contribute to this document.

       Greg Mirsky
       ZTE Corp


   o  Cheng Li
       Huawei Technologies Li, Huawei Campus, No. 156 Beiqing Rd.
       Beijing  100095


       Shuangping Zhan


   o  Lei Wang Wang, China Mobile


   o  Shuangping Zhan, ZTE

   o  Greg

11.  Acknowledgements

   The authors would like to thank Adrian Farrel, Stewart Bryant,
   Alexander Vainshtein, Andrew G.  Malis 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".

12.  References
12.1.  Normative References

              Bashandy, A., Filsfils, C., Previdi, S., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-18 draft-ietf-spring-segment-routing-mpls-22
              (work in progress), December 2018. May 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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, <>.

12.2.  Informative References

              Gandhi, R., Filsfils, C.,, d.,
              Salsano, S., Ventre, P., and M. Chen, "Performance
              Measurement in Segment Routing Networks with MPLS Data
              Plane", draft-gandhi-spring-sr-mpls-pm-03 (work in
              progress), September 2018.

              Gandhi, R., Filsfils, C.,, d.,
              Salsano, S., Ventre, P., and M. Chen, "UDP Path for In-
              band Performance Measurement for Segment Routing
              Networks", draft-gandhi-spring-udp-pm-02 (work in
              progress), September 2018.

              Filsfils, C., Dukes, D., Previdi, S., Leddy, J.,
              Matsushima, S., and d., "IPv6 Segment
              Routing Header (SRH)", draft-ietf-6man-segment-routing-header-16 draft-ietf-6man-segment-routing-
              header-22 (work in progress), February August 2019.

              Filsfils, C., Sivabalan, S.,, d.,
    , b., and P. Mattes, "Segment Routing
              Policy Architecture", draft-ietf-spring-segment-routing-
              policy-03 (work in progress), October 2018. May 2019.

              Li, C., Chen, M., Dong, J., and Z. Li, "Segment Routing
              Policies for Path Segment and Bidirectional Path", draft-
              li-idr-sr-policy-path-segment-distribution-01 (work in
              progress), October 2018.

              Li, C., Chen, M., Dhody, D., Cheng, W., Li, Z., Dong, J.,
              and R. Gandhi, R.,
              and Q. Xiong, "PCEP Extension Extensions for Associated
              Bidirectional Segment Routing (SR)
              Bidirectional Associated Paths", draft-li-pce-sr-bidir-
              path-02 draft-li-pce-
              sr-bidir-path-06 (work in progress), October 2018. August 2019.

              Li, C., Chen, M., Dhody, D., Cheng, W., Dong, J., Li, Z.,
              and R. Gandhi, R.,
              and Q. Xiong, "Path Computation Element Communication
              Protocol (PCEP) Extension for Path Segment in Segment
              Routing (SR)", draft-li-pce-sr-path-segment-03 draft-li-pce-sr-path-segment-08 (work in
              progress), October 2018. August 2019.

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

Authors' Addresses

   Weiqiang Cheng
   China Mobile


   Han Li
   China Mobile


   Mach(Guoyi) Chen

   Rakesh Gandhi
   Cisco Systems, Inc.


   Royi Zigler