SPRING Working Group                                               C. Li
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                                W. Cheng
Expires: February 14, 2020                                  China Mobile
                                                                 M. Chen
                                                                D. Dhody
                                                                   Z. Li
                                                                 J. Dong
                                                     Huawei Technologies
                                                               R. Gandhi
                                                     Cisco Systems, Inc.
                                                         August 13, 2019

            Path Segment for SRv6 (Segment Routing in IPv6)


   Segment Routing (SR) allows for a flexible definition of end-to-end
   paths by encoding paths as sequences of sub-paths, called "segments".
   Segment routing architecture can be implemented over an MPLS data
   plane as well as an IPv6 data plane.

   Further, Path Segment has been defined in order to identify an SR
   path in SR-MPLS networks, and used for various use-cases such as end-
   to-end SR Path Protection and Performance Measurement (PM) of an SR
   path.  Similar to SR-MPLS, this document defines the Path Segment in
   SRv6 networks in order to identify an SRv6 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 February 14, 2020.

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Copyright Notice

   Copyright (c) 2019 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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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Use Cases of SRv6 Path Segment  . . . . . . . . . . . . . . .   4
   3.  SRv6 Path Segment . . . . . . . . . . . . . . . . . . . . . .   5
   4.  SRv6 Path Segment Allocation  . . . . . . . . . . . . . . . .   6
   5.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at the ingress
   node by inserting an ordered list of instructions, called segments.

   When segment routing is deployed on MPLS dataplane, called SR-MPLS
   [I-D.ietf-spring-segment-routing-mpls], a segment is an MPLS label.
   When segment routing is deployed on IPv6 dataplane, called SRv6
   [I-D.ietf-6man-segment-routing-header], a segment is a 128 bit value,
   and it can be an IPv6 address of a local interface but it does not
   have to.  For supporting SR, an extended header called Segment
   Routing Header (SRH), which contains a list of SIDs and several
   needed information such as Segments Left, has been defined in

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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 no
   label or only the last label may be left in the MPLS label stack when
   the packet reaches the egress node.  Thus, the egress node can not
   determine from which ingress node or SR path the packet came in.  For
   identifying an SR-MPLS path, Path Segment is defined in

   Likewise, a path needs to be identified in an SRv6 network for
   several use cases such as binding bidirectional paths
   [I-D.li-pce-sr-bidir-path] and end-to-end performance measurement
   [I-D.gandhi-spring-udp-pm].  An SRv6 path can be identified by the
   content of segment list (i.e., the several SRv6 segments that are in
   the segment list).  However, the segment list may not be a good key
   to identify an SRv6 path, since the the length of segment list is too
   long and flexible according to the number of SIDs.  For instance, if
   packet A uses the SRH with 3 SIDs while Packet B uses the SRH with 10
   SIDs, the key to identify these two paths which the Packet A and B
   belong to will be a 384-bits value and a 1280-bits value.

   This document defines a new SRv6 segment called "SRv6 Path Segment",
   which is a 128-bits value, to identify an SRv6 path.  Using the Path
   Segment as an SRv6 SID will improve performance and operations in
   both SR-MPLS and SRv6.  Also, when the SRv6 Path Segment is used in
   reduced SRH [I-D.ietf-6man-segment-routing-header], the entire path
   information is maintained in SRH and the performance will be better
   than using SID list as the path identifier, while the overhead equals
   to the normal SRH.  The Path Segment is inserted as the last segment
   in the segment list and will not affect the order of the original SID
   list and has the solely scope to give a unique identifier to the
   segment list.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   MPLS: Multiprotocol Label Switching.

   PM: Performance Measurement.

   SID: Segment ID.

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   SR: Segment Routing.

   SR-MPLS: Segment Routing with MPLS data plane.

   SRH: Segment Routing Header.

   PSID: Path Segment Identifier.

   PSP: Penultimate Segment Popping.

   Further, this document makes use of the terms defined in [RFC8402]
   and [I-D.ietf-spring-srv6-network-programming].

2.  Use Cases of SRv6 Path Segment

   Similar to SR-MPLS Path Segment [I-D.ietf-spring-mpls-path-segment],
   SRv6 Path Segment also can be used for identifying an SRv6 Path in
   some use cases:

   o  Performance Measurement: For Passive measurement [RFC7799], path
      identification at the measuring points is the pre-requisite
      [I-D.ietf-spring-mpls-path-segment].  SRv6 Path segment can be
      used by the measuring points (e.g., the ingress/egress nodes of an
      SRv6 path) or a centralized controller to correlate the packets
      counts/timestamps, then packet loss/delay can be calculated.

   o  Bi-directioinal SRv6 Path Association: In some scenarios, such as
      mobile backhaul transport network, there are requirements to
      support bidirectional path.  Similar to SR-MPLS
      [I-D.ietf-spring-mpls-path-segment], to support bidirectional SRv6
      path, a straightforward way is to bind two unidirectional SRv6
      paths to a single bidirectional path.  SRv6 Path segments can be
      used to correlate the two unidirectional SRv6 paths at both ends
      of the paths.  [I-D.li-pce-sr-bidir-path] defines how to use PCEP
      and Path segment to initiate a bidirectional SR path.

   o  End-to-end Path Protection: For end-to-end 1+1 path protection
      (i.e., Live-Live case), the egress node of an SRv6 path needs to
      know the set of paths that constitute the primary and the
      secondary(s), in order to select the primary packet for onward
      transmission, and to discard the packets from the secondary(s), so
      each SRv6 path needs a unique path identifier at the egress node,
      which can be an SRv6 Path Segment.

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3.  SRv6 Path Segment

   As defined in [I-D.ietf-spring-srv6-network-programming], an SRv6
   segment is a 128-bit value, which can be represented as LOC:FUNCT.

   In order to identify an SRv6 path, this document defines a new
   segment called SRv6 Path Segment.  A Path Segment is solely used in
   order to identify an SRv6 path.

   The Path Segment MUST appear only once in a SID list, and it MUST
   appear at the last entry.

       |                                                               |
       |            Segment List[0] (128 bits IPv6 address)            |
       |                                                               |
       |                                                               |
       |                                                               |
       |                                                               |
       |                              ...                              |
       |                                                               |
       |                                                               |
       |                                                               |
       |            Segment List[n-1] (128 bits IPv6 address)          |
       |                                                               |
       |                                                               |
       |                                                               |
       |          Path Segment(Segment List[n], 128 bits value)        |
       |                                                               |
       |                                                               |

                 Figure 1.  SRv6 Path Segment in SID List

   The detailed encoding of SRv6 Path Segment is out of scope of this
   document, and it is defined in [I-D.li-6man-srv6-path-segment-encap].

   Depending on the use case, a Path Segment identifies:

   o  an SRv6 path within an SRv6 domain

   o  an SRv6 Policy

   o  a Candidate-paths or a SID-List in a SRv6 Policy

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   Note that, based on the use-case, the different SID-Lists of SR
   Policies may use the same SRv6 Path Segment.

4.  SRv6 Path Segment Allocation

   A Path Segment is a local segment allocated by an egress node.  A
   Path Segment can be allocated through several ways, such as CLI, BGP
   [I-D.li-idr-sr-policy-path-segment-distribution], PCEP
   [I-D.li-pce-sr-path-segment] or other ways.  The mechanisms through
   which a Path Segment is allocated is out of scope of this document.

   When the Path Segment is allocated by the egress, it MUST be
   distributed to the ingress node.  In this case, only the egress will
   process the Path Segment, and other nodes specified by SIDs in the
   SID list do not know how to process the Path Segment.

   Depending on the use case, a Path Segment may be distributed to the
   SRv6 nodes along the SRv6 path.  In this case, the SRv6 nodes that
   learned Path Segment may process the Path Segment depending on the
   use case.

5.  Operations

   An egress node or other SRv6 nodes along the SRv6 path supporting the
   Path Segment processing will inspect the last entry of the segment
   list (giving the the node will inspect the last entry in the SID list
   and obtain the Path Segment.  The processing of the Path Segment is
   described in [I-D.li-6man-srv6-path-segment-encap].

6.  IANA Considerations

   This document does not require any IANA actions.

7.  Security Considerations

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

8.  Acknowledgements

   The authors would like to thank Stefano Previdi and Zafar Ali for
   their valuable comments and suggestions.

9.  References

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9.1.  Normative References

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

              Filsfils, C., Camarillo, P., Leddy, J.,
              daniel.voyer@bell.ca, d., Matsushima, S., and Z. Li, "SRv6
              Network Programming", draft-ietf-spring-srv6-network-
              programming-01 (work in progress), July 2019.

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

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

9.2.  Informative References

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

              Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler,
              "Path Segment in MPLS Based Segment Routing Network",
              draft-ietf-spring-mpls-path-segment-00 (work in progress),
              March 2019.

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

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              Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
              bogdanov@google.com, b., and P. Mattes, "Segment Routing
              Policy Architecture", draft-ietf-spring-segment-routing-
              policy-03 (work in progress), May 2019.

              Li, C., Cheng, W., Li, Z., and D. Dhody, "Encapsulation of
              Path Segment in SRv6", draft-li-6man-srv6-path-segment-
              encap-00 (work in progress), July 2019.

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

              Li, C., Chen, M., Cheng, W., Li, Z., Dong, J., Gandhi, R.,
              and Q. Xiong, "PCEP Extensions for Associated
              Bidirectional Segment Routing (SR) Paths", draft-li-pce-
              sr-bidir-path-05 (work in progress), March 2019.

              Li, C., Chen, M., Cheng, W., Dong, J., Li, Z., Gandhi, R.,
              and Q. Xiong, "Path Computation Element Communication
              Protocol (PCEP) Extension for Path Segment in Segment
              Routing (SR)", draft-li-pce-sr-path-segment-07 (work in
              progress), July 2019.

   [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>.

Authors' Addresses

   Cheng Li
   Huawei Technologies

   Email: chengli13@huawei.com

   Weiqiang Cheng
   China Mobile

   Email: chengweiqiang@chinamobile.com

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   Mach(Guoyi) Chen
   Huawei Technologies

   Email: mach.chen@huawei.com

   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560066

   Email: dhruv.ietf@gmail.com

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

   Email: lizhenbin@huawei.com

   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing  100095

   Email: jie.dong@huawei.com

   Rakesh Gandhi
   Cisco Systems, Inc.

   Email: rgandhi@cisco.com

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