Path Segment in MPLS Based Segment Routing Network
draft-ietf-spring-mpls-path-segment-12

SPRING Working Group                                            W. Cheng
Internet-Draft                                                     H. Li
Intended status: Standards Track                            China Mobile
Expires: 16 March 2024                                             C. Li
                                            Huawei Technologies Co., Ltd
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                               R. Zigler
                                                                Broadcom
                                                       13 September 2023

           Path Segment in MPLS Based Segment Routing Network
                 draft-ietf-spring-mpls-path-segment-12

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
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 16 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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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  . . . . . . . . .   6
     3.3.  Path Segment for End-to-end Path Protection . . . . . . .   7
     3.4.  Nesting of Path Segments  . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Huawei Technologies . . . . . . . . . . . . . . . . . . .   9
     5.2.  ZTE Corp  . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.3.  New H3C Technologies  . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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.

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   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.

   However, to support various use-cases in SR-MPLS networks, such as
   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, 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.

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   SR: Segment Routing.

   SRLB: SR Local Block

   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 is a Local Segment which uniquely identify 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.

   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, 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.  How to use the PSID to Segment Lists depends on the
   requirements of the use cases.

   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 associated to
   it, 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 receiving on an intermidate node of the associated path, but it
   can be the top label in the label stack on the penultimate node after
   the last forwarding label with Penultimate Hop Popping (PHP) is
   popped off.  Otherwise, the PSID may be processed by an intermediate
   node, which may cause error in forwarding because of mis-matching.

   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.

   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).  This additional processing 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, where the total number of MPLS labels 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.

   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.

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].

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   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 splitted 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.

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   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 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.

   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.

   The distribution of a PSID from an egress nodes 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.

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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".

5.1.  Huawei Technologies

   *  Organization: Huawei Technologies.

   *  Implementation: Huawei Routers and NCE controller

   *  Description: The implementation is under development and follows
      the defination and mechanism as defined in section 2 and
      Section 3.2.

   *  Maturity Level: Product

   *  Coverage: Partial

   *  Contact: tanren@huawei.com

5.2.  ZTE Corp

   The feature of SR-MPLS Path Segment has been developed.

   *  Organization: ZTE

   *  Implementation: ZTE's Commercial Delivery implementation

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   *  Description: The implementation is under development and follows
      the defination and mechanism as defined in section 2.

   *  Maturity Level: Product

   *  Coverage: Partial

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

5.3.  New H3C Technologies

   *  Organization: New H3C Technologies

   *  Implementation: H3C CR16000, CR19000 series routers running
      Version 7.1.086 or above

   *  Description: Path Segment for Bidirectional SR Path (Section 2 &
      Section 3.2)

   *  Maturity Level: Demo

   *  Coverage: Partial

   *  Contact: linchangwang.04414@h3c.com

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>.

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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>.

7.2.  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>.

   [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>.

   [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>.

   [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>.

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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, 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

Authors' Addresses

   Weiqiang Cheng
   China Mobile
   Email: chengweiqiang@chinamobile.com

   Han Li
   China Mobile

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   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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