[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-cheng-spring-mpls-path-segment) 00 01 02

SPRING Working Group                                            W. Cheng
Internet-Draft                                                     H. Li
Intended status: Standards Track                            China Mobile
Expires: August 29, 2020                                         M. Chen
                                                                  Huawei
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                               R. Zigler
                                                                Broadcom
                                                       February 26, 2020


           Path Segment in MPLS Based Segment Routing Network
                 draft-ietf-spring-mpls-path-segment-02

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 will not be popped off
   until it reaches the egress node of the SR path.  The Path Segment
   then can be used by the egress node to implement 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




Cheng, et al.            Expires August 29, 2020                [Page 1]


Internet-Draft           Path Segment in SR-MPLS           February 2020


   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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 August 29, 2020.

Copyright Notice

   Copyright (c) 2020 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/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified 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 . . . . . . . . . . . . . . . . . . .   6
   4.  Nesting of Path Segments  . . . . . . . . . . . . . . . . . .   6
   5.  Path Segment for Performance Measurement  . . . . . . . . . .   7
   6.  Path Segment for Bidirectional SR Path  . . . . . . . . . . .   8
   7.  Path Segment for End-to-end Path Protection . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12







Cheng, et al.            Expires August 29, 2020                [Page 2]


Internet-Draft           Path Segment in SR-MPLS           February 2020


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 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 [RFC8660], the latter is
   called SRv6 [RFC8402].  SR-MPLS leverages the MPLS 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.  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 in the context of
   the egress node.  It is normally 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.




Cheng, et al.            Expires August 29, 2020                [Page 3]


Internet-Draft           Path Segment in SR-MPLS           February 2020


   MSD: Maximum SID Depth.

   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.

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) or
   dynamic MPLS label pool 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, in other words, inserted after the routing
   segment (adjacency/node/prefix segment) pointing to the egress node.

   The Path Segment may be used to identify an SR-MPLS Policy, its
   Candidate-Path (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 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.

   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.

   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.



Cheng, et al.            Expires August 29, 2020                [Page 4]


Internet-Draft           Path Segment in SR-MPLS           February 2020


   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 can 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:

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

      Figure 1: Label Stack with Path Segment


   Where:

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




Cheng, et al.            Expires August 29, 2020                [Page 5]


Internet-Draft           Path Segment in SR-MPLS           February 2020


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

3.  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)) [RFC5586] 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 allocate a Path Segment.

   Another way is to leverage a centralized controller (e.g., SDN
   controller) to assign the Path Segment.  In this case, 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
   with any label allocation done by the egress node.

   Path Computation Element Protocol (PCEP) based Path Segment
   allocation for SR Policy is defined in
   [I-D.ietf-pce-sr-path-segment].  Also, BGP based Path Segment
   allocation for SR Policy is defined in
   [I-D.li-idr-sr-policy-path-segment].

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.



Cheng, et al.            Expires August 29, 2020                [Page 6]


Internet-Draft           Path Segment in SR-MPLS           February 2020


            /--------\       /--------\       /--------\
          /            \   /            \   /            \
        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.

   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.gandhi-mpls-rfc6374-sr] and [I-D.gandhi-spring-twamp-srpm].

   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-sr].






Cheng, et al.            Expires August 29, 2020                [Page 7]


Internet-Draft           Path Segment in SR-MPLS           February 2020


   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.cheng-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.li-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.








Cheng, et al.            Expires August 29, 2020                [Page 8]


Internet-Draft           Path Segment in SR-MPLS           February 2020


8.  Security Considerations

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

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

   [I-D.cheng-mpls-inband-pm-encapsulation]
              Cheng, W., Xiao, M., Zhou, T., Dong, X., and Y. Peleg,
              "Encapsulation For MPLS Performance Measurement with
              Alternate Marking Method", draft-cheng-mpls-inband-pm-
              encapsulation-02 (work in progress), November 2019.

   [I-D.gandhi-mpls-ioam-sr]
              Gandhi, R., Ali, Z., Filsfils, C., Brockners, F., Wen, B.,
              and V. Kozak, "Segment Routing with MPLS Data Plane
              Encapsulation for In-situ OAM Data", draft-gandhi-mpls-
              ioam-sr-01 (work in progress), December 2019.





Cheng, et al.            Expires August 29, 2020                [Page 9]


Internet-Draft           Path Segment in SR-MPLS           February 2020


   [I-D.gandhi-mpls-rfc6374-sr]
              Gandhi, R., Filsfils, C., Voyer, D., Salsano, S., and M.
              Chen, "Performance Measurement for Segment Routing
              Networks with MPLS Data Plane", draft-gandhi-mpls-
              rfc6374-sr-01 (work in progress), December 2019.

   [I-D.gandhi-spring-twamp-srpm]
              Gandhi, R., Filsfils, C., Voyer, D., Chen, M., and B.
              Janssens, "Performance Measurement Using TWAMP Light for
              Segment Routing Networks", draft-gandhi-spring-twamp-
              srpm-05 (work in progress), December 2019.

   [I-D.ietf-pce-sr-bidir-path]
              Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
              "PCEP Extensions for Associated Bidirectional Segment
              Routing (SR) Paths", draft-ietf-pce-sr-bidir-path-01 (work
              in progress), February 2020.

   [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)",
              draft-ietf-pce-sr-path-segment-00 (work in progress),
              October 2019.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-06 (work in progress),
              December 2019.

   [I-D.li-idr-sr-policy-path-segment]
              Li, C., Telecom, C., Chen, M., Dong, J., and Z. Li, "SR
              Policy Extensions for Path Segment and Bidirectional
              Path", draft-li-idr-sr-policy-path-segment-01 (work in
              progress), August 2019.

   [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>.




Cheng, et al.            Expires August 29, 2020               [Page 10]


Internet-Draft           Path Segment in SR-MPLS           February 2020


   [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>.

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:

















Cheng, et al.            Expires August 29, 2020               [Page 11]


Internet-Draft           Path Segment in SR-MPLS           February 2020


    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


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









Cheng, et al.            Expires August 29, 2020               [Page 12]


Internet-Draft           Path Segment in SR-MPLS           February 2020


   Rakesh Gandhi
   Cisco Systems, Inc.
   Canada

   Email: rgandhi@cisco.com


   Royi Zigler
   Broadcom

   Email: royi.zigler@broadcom.com








































Cheng, et al.            Expires August 29, 2020               [Page 13]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/