Path Segment in MPLS Based Segment Routing Network
draft-ietf-spring-mpls-path-segment-11

SPRING Working Group                                            W. Cheng
Internet-Draft                                                     H. Li
Intended status: Standards Track                            China Mobile
Expires: 1 March 2024                                              C. Li
                                            Huawei Technologies Co., Ltd
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                               R. Zigler
                                                                Broadcom
                                                          29 August 2023

           Path Segment in MPLS Based Segment Routing Network
                 draft-ietf-spring-mpls-path-segment-11

Abstract

   A Segment Routing (SR) path is identified by an SR segment list.  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), and end-to-end 1+1 path protection.

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

   This document defines Path Segment 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
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   This Internet-Draft will expire on 1 March 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
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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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Abbreviations and Terms . . . . . . . . . . . . . . . . .   3
   2.  Path Segment  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Use cases . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Path Segment for Performance Measurement  . . . . . . . .   6
     3.2.  Path Segment for Bidirectional SR Path  . . . . . . . . .   7
     3.3.  Path Segment for End-to-end Path Protection . . . . . . .   7
     3.4.  Nesting of Path Segments  . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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
   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. a service label or an
   Explicit-Null 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.

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   However, to support various use-cases in SR-MPLS networks, like end-
   to-end 1+1 path protection (Live-Live case) Section 3.3,
   bidirectional path Section 3.2, or Performance Measurement (PM)
   Section 3.1, 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 on the egress node of the path.  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.  Note that, Per-
   path states will be maintained in the egress node due to the
   requirements in these use cases, though in normal cases that the per-
   path states will be maintained in the ingress node only in the SR
   architecture.

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

   DM: Delay Measurement.

   LM: Loss Measurement.

   MPLS: Multiprotocol Label Switching.

   MSD: Maximum SID Depth.

   PM: Performance Measurement.

   PSID: Path Segment ID.

   SID: Segment ID.

   SL: Segment List.

   SR: Segment Routing.

   SRLB: SR Local Block

   SRGB: SR Global Block

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

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

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.  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 path described by a
   Segment-List (SL).  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, all the Segment lists in a
   Candidate path can use a single PSID, and all the Segment Lists in an
   SR policy can share the same PSID, if customers would like to
   aggregate the data among the Segment Lists.  How to use the PSID to
   Segment Lists depends on the requirements of the use cases.

   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 can be set to any value including 0, or 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.

   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.  As per regular MPLS processing, the label below (including the
   PSID in this case) will not be popped by the penultimate 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.

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   The addition of the PSID will require the egress to read and process
   the PSID label in addition to the regular processing (such as a below
   MPLS label or the MPLS payload).  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.  As per
   [RFC5586], when GAL is used, the ACH appears immediately after the
   bottom of the label stack.

   If Entropy Label is also used on this 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.

   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.  This includes the PSID.

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

               +--------------------+
               |       ...          |
               +--------------------+
               |      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.

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   *  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 3.4.

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

3.  Use cases

   This section describes use cases which can leveage the Path Segment.

3.1.  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.

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3.2.  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.

3.3.  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.

3.4.  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.

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   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

4.  Security Considerations

   A Path Segment in SR-MPLS is a label similar to other labels/Segment,
   such as a VPN label or a Prefix SID, 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.

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   A Path Segment is used within an SR-MPLS domain [RFC8402] and should
   not leak outside the domain, therefore no new security threats are
   introduced comparing to current SR-MPLS.  The security consideration
   of SR-MPLS, such as boundary filtering described in Section 8.1 of
   [RFC8402] applies to this document.

   A PSID is allocated by an egress node and distributed to an ingress.
   The distribution is performed within an SR trusted domain.  However,
   the mechanism of distributing a PSID is out of the scope of this
   document, and its security consideration will be described in other
   documents.

5.  IANA Considerations

   This document does not require any IANA actions.

6.  References

6.1.  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>.

6.2.  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>.

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   [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>.

   [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>.

   [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>.

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

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   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

   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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