SR Policy Extensions for Path Segment and Bidirectional Path
draft-ietf-idr-sr-policy-path-segment-00

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Interdomain Routing Working Group                                  C. Li
Internet-Draft                                                     Z. Li
Intended status: Standards Track                     Huawei Technologies
Expires: May 1, 2020                                             H. Chen
                                                           China Telecom
                                                                W. Cheng
                                                            China Mobile
                                                          K.  Talaulikar
                                                           Cisco Systems
                                                        October 29, 2019

      SR Policy Extensions for Path Segment and Bidirectional Path
                draft-ietf-idr-sr-policy-path-segment-00

Abstract

   A Segment Routing (SR) policy is a set of candidate SR paths
   consisting of one or more segment lists with necessary path
   attributes.  For each SR path, it may also have its own path
   attributes, and Path Segment is one of them.  A Path Segment is
   defined to identify an SR path, which can be used for performance
   measurement, path correlation, and end-2-end path protection.  Path
   Segment can be also used to correlate two unidirctional SR paths into
   a bidirectional SR path which is required in some scenarios, for
   example, mobile backhaul transport network.

   This document defines extensions to BGP to distribute SR policies
   carrying Path Segment and bidirectional path information.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on May 1, 2020.

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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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Path Segment in SR Policy . . . . . . . . . . . . . . . . . .   3
     3.1.  SR Path Segment Sub-TLV . . . . . . . . . . . . . . . . .   4
   4.  SR Policy for Bidirectional Path  . . . . . . . . . . . . . .   6
     4.1.  SR Bidirectional Path Sub-TLV . . . . . . . . . . . . . .   6
     4.2.  SR Reverse Path Segment List Sub-TLV  . . . . . . . . . .   7
   5.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Existing Registry: BGP Tunnel Encapsulation Attribute
           sub-TLVs  . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Segment routing (SR) [RFC8402] is a source routing paradigm that
   explicitly indicates the forwarding path for packets at the ingress
   node.  The ingress node steers packets into a specific path according
   to the Segment Routing Policy ( SR Policy) as defined in
   [I-D.ietf-spring-segment-routing-policy].  For distributing SR

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   policies to the headend, [I-D.ietf-idr-segment-routing-te-policy]
   specifies a mechanism by using BGP, and new sub-TLVs are defined for
   SR Policies in BGP UPDATE message.

   In many use cases such as performance measurement, the path to which
   the packets belong is required to be identified.  Futhermore, in some
   scenarios, for example, mobile backhaul transport network, there are
   requirements to support bidirectional path.  However, there is no
   path identification information for each Segment List in the SR
   Policies defined in [I-D.ietf-spring-segment-routing-policy].  Also,
   the SR Policies defined in [I-D.ietf-spring-segment-routing-policy]
   only supports unidirectional SR paths.

   Therefore, this document defines the extension to SR policies that
   carry Path Segment in the Segment List and support bidirectional
   path.  The Path Segment can be a Path Segment in SR-MPLS
   [I-D.ietf-spring-mpls-path-segment] , or other IDs that can identify
   a path.  Also, this document defines extensions to BGP to distribute
   SR policies carriying Path Segment and bidirectional path
   information.

2.  Terminology

   This memo makes use of the terms defined in [RFC8402] and
   [I-D.ietf-idr-segment-routing-te-policy].

3.  Path Segment in SR Policy

   As defined in [I-D.ietf-idr-segment-routing-te-policy] , the SR
   Policy encoding structure is as follows:

      SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
      Attributes:
         Tunnel Encaps Attribute (23)
            Tunnel Type: SR Policy
                Binding SID
                Preference
                Priority
                Policy Name
                Explicit NULL Label Policy (ENLP)
                Segment List
                    Weight
                    Segment
                    Segment
                    ...
                ...

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   An SR path can be specified by an Segment List sub-TLV that contains
   a set of segment sub-TLVs and other sub-TLVs as shown above.  As
   defined in [I-D.ietf-spring-segment-routing-policy], a candidate path
   includes multiple SR paths specified by SID list.  The Path Segment
   can be used for idendifying an SR path(specified by SID list).  Also,
   it can be used for identifying an SR candidate path or an SR Policy
   in some use cases if needed.  New SR Policy encoding structure is
   expressed as below:

      SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
      Attributes:
         Tunnel Encaps Attribute (23)
            Tunnel Type: SR Policy
                Binding SID
                Preference
                Priority
                Policy Name
                Explicit NULL Label Policy (ENLP)
                Path Segment
                Segment List
                    Weight
                    Path Segment
                    Segment
                    Segment
                    ...
                Segment List
                    Weight
                    Path Segment
                    Segment
                    Segment
                    ...
                ...

   The Path Segment can appear at both segment-list level and candidate
   path level, and generally it SHOULD also appear only at one level
   depending upon use case.  Path segment at segment list level and at
   candidate path level may be same or may be different based on usecase
   and the ID allocation scope.  When multiple Path Segments appear in
   both levels, it means the the Path Segment associated with candidate
   path and segment list SHOULD both be inserted into the SID list.

3.1.  SR Path Segment Sub-TLV

   This section defines an SR Path Segment sub-TLV.

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   An SR Path Segment sub-TLV can be included in the segment list sub-
   TLV to identify an SID list, and it MUST appear only once within a
   Segment List sub-TLV.  It has the following format:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Type     |    Length     |    Flag       |      ST       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Path Segment (Variable depends on ST)            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         Figure 1. Path Segment sub-TLV

   Where:

   Type: to be assigned by IANA (suggested value 10).

   Length: the total length of the value field not including Type and
   Length fields.

   Flag: 8 bits of flags.  Following flags are defined:

     0  1  2  3  4  5  6  7
   +--+--+--+--+--+--+--+--+
   |    Reserved        |G |
   +--+--+--+--+--+--+--+--+

   G-Flag: Global flag.  Set when the Path Segment is global within an
   SR domain.

   Reserved: 5 bits reserved and MUST be set to 0 on transmission and
   MUST be ignored on receipt.

   ST: Segment type, specifies the type of the Path Segment, and it has
   following types:

   o  0: SR-MPLS Path Segment

   o  1-255:Reserved

   Path Segment: The Path Segment of an SR path.  The Path Segment type
   is indicated by the Segment Type(ST) field.  It can be a Path Segment
   in SR-MPLS [I-D.ietf-spring-mpls-path-segment], which is 32-bits
   value, which is a 128-bits value, or other IDs that can identify a
   path.

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4.  SR Policy for Bidirectional Path

   In some scenarios, for example, mobile backhaul transport network,
   there are requirements to support bidirectional path.  In SR, a
   bidirectional path can be represented as a binding of two
   unidirectional SR paths.  This document also defines new sub-TLVs to
   describe an SR bidirectional path.  An SR policy carrying SR
   bidirectional path information is expressed as below:

       SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
           Attributes: Tunnel Encaps Attribute (23)
           Tunnel Type: SR Policy
               Binding SID
               Preference
               Priority
               Policy Name
               Explicit NULL Label Policy (ENLP)
               Bidirectioanl Path
                   Segment List
                       Weight
                       Path Segment
                       Segment
                       Segment
                       ...
                   Reverse Segment List
                       Weight
                       Path Segment
                       Segment
                       Segment
                       ...

4.1.  SR Bidirectional Path Sub-TLV

   This section defines an SR bidirectional path sub-TLV to specify a
   bidirectional path, which contains a Segment List sub-TLV
   [I-D.ietf-idr-segment-routing-te-policy] and an associated Reverse
   Path Segment List as defined at section 4.2.  The SR bidirectional
   path sub-TLV has the following format:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Type       |             Length            |   RESERVED    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sub-TLVs (Variable)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 2. SR Bidirectional path sub-TLV

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   Where:

   Type: TBA, and the suggest value is 14.

   Length: the total length of the sub-TLVs encoded within the SR
   Bidirectional Path Sub-TLV not including Type and Length fields.

   RESERVED: 1 octet of reserved bits.  SHOULD be unset on transmission
   and MUST be ignored on receipt.

   Sub-TLVs:

   o  An Segment List sub-TLV

   o  An associated Reverse Path Segment List sub-TLV

4.2.  SR Reverse Path Segment List Sub-TLV

   An SR Reverse Path Segment List sub-TLV is defined to specify an SR
   reverse path associated with the path specified by the Segment List
   in the same SR Bidirectional Path Sub-TLV, and it has the following
   format:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Type       |             Length            |   RESERVED    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sub-TLVs (Variable)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 2. SR Reverse Path Segment List Sub-TLV

   where:

   Type: TBA, and suggest value is 127.

   Length: the total length of the sub-TLVs encoded within the SR
   Reverse Path Segment List Sub-TLV not including the Type and Length
   fields.

   RESERVED: 1 octet of reserved bits.  SHOULD be unset on transmission
   and MUST be ignored on receipt.

   sub-TLVs, reuse the sub-TLVs in Segment List defined in
   [I-D.ietf-idr-segment-routing-te-policy].

   o  An optional single Weight sub-TLV.

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   o  An mandatory SR Path Segment sub-TLV that contains the Path
      Segment of the reverse SR path.

   o  Zero or more Segment sub-TLVs to specify the reverse SR path.

   The Segment sub-TLVs in the Reverse Path Segment List sub-TLV
   provides the information of the reverse SR path, which can be used
   for directing egress BFD peer to use specific path for the reverse
   direction of the BFD session [I-D.ietf-mpls-bfd-directed] or other
   applications.

5.  Operations

   The document does not bring new operation beyong the description of
   operations defined in [I-D.ietf-idr-segment-routing-te-policy].  The
   existing operations defined in
   [I-D.ietf-idr-segment-routing-te-policy] can apply to this document
   directly.

   Typically but not limit to, the unidirectional or bidirectional SR
   policies carrying path identification infomation are configured by a
   controller.

   After configuration, the unidirectional or bidirectional SR policies
   carrying path identification infomation will be advertised by BGP
   update messages.  The operation of advertisement is the same as
   defined in [I-D.ietf-idr-segment-routing-te-policy], as well as the
   receiption.

   The consumer of the unidirectional or bidirectional SR policies is
   not the BGP process, it can be any applications, such as performance
   measurement [I-D.gandhi-spring-udp-pm].  The operation of sending
   information to consumers is out of scope of this document.

6.  IANA Considerations

   This document defines new Sub-TLVs in following registries:

6.1.  Existing Registry: BGP Tunnel Encapsulation Attribute sub-TLVs

   This document defines new sub-TLVs in the registry "BGP Tunnel
   Encapsulation Attribute sub-TLVs" to be assigned by IANA:

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          Codepoint   Description                           Reference
          -------------------------------------------------------------
          14          Path Segment sub-TLV                  This document
          15          SR Bidirectional Path sub-TLV         This document
          127         Reverse Segment List sub-TLV          This document

   This document defines new sub-TLVs in the registry "SR Policy List
   Sub-TLVs" [I-D.ietf-idr-segment-routing-te-policy] to be assigned by
   IANA:

          Codepoint   Description                           Reference
          -------------------------------------------------------------
          14          Path Segment sub-TLV                  This document

7.  Security Considerations

   TBA

8.  Contributors

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      Mach(Guoyi) Chen
      Huawei Technologies
      Huawei Campus, No. 156 Beiqing Rd.
      Beijing  100095
      China

      Email: Mach.chen@huawei.com

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

      Email: jie.dong@huawei.com

      James N Guichard
      Futurewei Technologies
      2330 Central Express Way
      Santa Clara
      USA

      Email: james.n.guichard@futurewei.com

9.  Acknowledgements

   Many thanks to Shraddha Hedge for her detailed review and
   professional comments.

10.  References

10.1.  Normative References

   [I-D.ietf-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Mattes, P., Rosen, E., Jain,
              D., and S. Lin, "Advertising Segment Routing Policies in
              BGP", draft-ietf-idr-segment-routing-te-policy-07 (work in
              progress), July 2019.

   [I-D.ietf-spring-mpls-path-segment]
              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-01 (work in progress),
              September 2019.

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   [I-D.ietf-spring-segment-routing-policy]
              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.

   [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>.

   [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>.

10.2.  Informative References

   [I-D.gandhi-spring-udp-pm]
              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.

   [I-D.ietf-mpls-bfd-directed]
              Mirsky, G., Tantsura, J., Varlashkin, I., and M. Chen,
              "Bidirectional Forwarding Detection (BFD) Directed Return
              Path", draft-ietf-mpls-bfd-directed-12 (work in progress),
              August 2019.

Authors' Addresses

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

   Email: chengli13@huawei.com

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

   Email: lizhenbin@huawei.com

   Huanan Chen
   China Telecom
   109 West Zhongshan Ave
   Guangzhou
   China

   Email: chenhn8.gd@chinatelecom.cn

   Weiqiang Cheng
   China Mobile
   Beijing
   China

   Email: chengweiqiang@chinamobile.com

   Ketan Talaulikar
   Cisco Systems

   Email: ketant@cisco.com

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