Path Segment in MPLS Based Segment Routing Network
draft-ietf-spring-mpls-path-segment-09

SPRING Working Group                                            W. Cheng
Internet-Draft                                                     H. Li
Intended status: Standards Track                            China Mobile
Expires: 28 December 2023                                          C. Li
                                            Huawei Technologies Co., Ltd
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                               R. Zigler
                                                                Broadcom
                                                            26 June 2023

           Path Segment in MPLS Based Segment Routing Network
                 draft-ietf-spring-mpls-path-segment-09

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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Path Segment  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  PSID Allocation and Distribution  . . . . . . . . . . . . . .   6
   4.  Nesting of Path Segments  . . . . . . . . . . . . . . . . . .   7
   5.  Path Segment for Performance Measurement  . . . . . . . . . .   8
   6.  Path Segment for Bidirectional SR Path  . . . . . . . . . . .   8
   7.  Path Segment for End-to-end Path Protection . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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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
   BCP 14 [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 Identifier(PSID) 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 PSID is allocated from the
   SRLB, SRGB, or a dynamic range depends on specific use cases.  If the
   PSID is only used by the egress node to identify an 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 an SR path, the
   SRGB MUST be used.  Three use cases are introduced in Section 5, 6,
   and 7 of this document.

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

   When a PSID is used, the PSID 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 of the SR path.  Otherwise, the
   PSID may be processed by an intermediate node, which may cause error
   in forwarding because of mis-matching if the PSID is allocated from a
   SRLB.

   The value of the TTL field in the MPLS label stack entry containing
   the PSID 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.

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   Normally, an intermediate node will not process the PSID in the label
   stack because the PSID is inserted after the routing segment pointing
   to the egress node.  But in some use cases, an intermediate node MAY
   process the PSID in the label stack by scanning the label stack or
   other means.  In these cases, the PSID MUST be learned before
   processing.  The detailed use cases and processing is out of the
   scope of this document.

   A PSID 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 PSID MUST NOT be popped off until it reaches at the
   egress node.

   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.

   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) MAY be 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 PSID.

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

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

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               +--------------------+
               |       ...          |
               +--------------------+
               |      Label 1       |
               +--------------------+
               |      Label 2       |
               +--------------------+
               |       ...          |
               +--------------------+
               |      Label n       |
               +--------------------+
               |        PSID        |
               +--------------------+
               |       ...          |
               +--------------------+
               ~       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 PSID 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 PSID to a label stack will increase the depth
   of the label stack, the PSID should be accounted when considering
   Maximum SID Depth (MSD)[RFC8992].

3.  PSID Allocation and Distribution

   There are some ways to assign and distribute the PSID.  The PSID can
   be configured locally or allocated by a centralized controller or by
   other means, this is out of the scope of this document.  If an egress
   cannot support the use of the PSID, it MUST reject the attempt to
   configure the label.

   If an egress cannot support the use of the PSID, it MUST reject the
   attemption of configuration.

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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 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 associated with a BSID and a
   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.  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

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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 PSID).

   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.

   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.

   Path Segment can also be used for In-band PM for SR-MPLS to identify
   the SR Path associated with the collected performance metrics.

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.  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.  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.

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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.
   This equivalence group can be instantiated in the network by an SDN
   controller using the Path Segments of the SR paths.

8.  Security Considerations

   Path Segment in SR-MPLS is used within the SR domain, and no new
   security threats are introduced comparing to current SR-MPLS.  The
   security consideration of SR-MPLS is described in Section 8.1 of
   [RFC8402] applies to this document.

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/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>.

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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/rfc/rfc8660>.

10.2.  Informative References

   [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>.

   [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>.

   [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/rfc/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/rfc/rfc8664>.

   [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/rfc/rfc8986>.

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   [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/rfc/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/rfc/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:

   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

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Authors' Addresses

   Weiqiang Cheng
   China Mobile
   Email: chengweiqiang@chinamobile.com

   Han Li
   China Mobile
   Email: lihan@chinamobile.com

   Cheng Li
   Huawei Technologies Co., Ltd
   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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