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Network Working Group                                              H. Li
Internet-Draft                                                   M. Chen
Intended status: Standards Track                                  C. Lin
Expires: August 6, 2021                                              H3C
                                                        February 2, 2021


        BGP Extensions of SR Policy for Composite Candidate Path
                draft-li-idr-sr-policy-composite-path-00

Abstract

   Segment Routing is a source routing paradigm that explicitly
   indicates the forwarding path for packets at the ingress node.  An SR
   Policy is associated with one or more candidate paths.  A candidate
   path is either dynamic, explicit or composite.  This document defines
   extensions to BGP to distribute SR policies carrying composite
   candidate path information.  So that composite candidate paths can be
   installed when the SR policy is applied.

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 August 6, 2021.

Copyright Notice

   Copyright (c) 2021 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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   to this document.  Code Components extracted from this document must



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Constituent SR Policy Attributes in SR Policy . . . . . . . .   3
     3.1.  Constituent SR Policy Sub-TLV . . . . . . . . . . . . . .   4
   4.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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].  In order to distribute SR
   policies to the headend, [I-D.ietf-idr-segment-routing-te-policy]
   specifies a mechanism by using BGP.

   An SR Policy is associated with one or more candidate paths.  A
   composite candidate path acts as a container for grouping of SR
   Policies.  As described in section 2.2 in
   [I-D.ietf-spring-segment-routing-policy], the composite candidate
   path construct enables combination of SR Policies, each with explicit
   candidate paths and/or dynamic candidate paths with potentially
   different optimization objectives and constraints, for a load-
   balanced steering of packet flows over its constituent SR Policies.

   This document defines extensions to Border Gateway Protocol (BGP) to
   distribute SR policies carrying composite candidate path information.
   So that composite candidate paths can be installed when the SR policy
   is applied.

2.  Terminology

   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



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   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Constituent SR Policy Attributes 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
                   SRv6 Binding SID
                   Preference
                   Priority
                   Policy Name
                   Policy Candidate Path Name
                   Explicit NULL Label Policy (ENLP)
                   Segment List
                       Weight
                       Segment
                       Segment
                       ...
                   ...

   As described in section 2.2 in
   [I-D.ietf-spring-segment-routing-policy], the endpoints of the
   constituent SR Policies and the parent SR Policy MUST be identical,
   and the colors of each of the constituent SR Policies and the parent
   SR Policy MUST be different.  Therefore a constituent SR Policy is
   referenced only by color in the composite candidate path since its
   headend and endpoint are identical to the parent SR policy.


















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   SR policy with composite candidate 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
                   SRv6 Binding SID
                   Preference
                   Priority
                   Policy Name
                   Policy Candidate Path Name
                   Explicit NULL Label Policy (ENLP)
                   Segment List
                       Weight
                       Segment
                       Segment
                       ...
                   Constituent SR Policy
                       Weight
                   ...

3.1.  Constituent SR Policy Sub-TLV

   The Constituent SR Policy sub-TLV encodes a single composite path
   towards the endpoint.  The Constituent SR Policy sub-TLV is an
   optional sub-TLV of BGP Tunnel Encapsulation Attribute, and MAY
   appear multiple times in the SR Policy encoding.  The ordering of
   Constituent SR Policy sub-TLVs does not matter.  The Constituent SR
   Policy sub-TLV MAY contain a Weight sub-TLV.

   Since a candidate path is either dynamic, explicit or composite, the
   Constituent SR Policy sub-TLV and the Segment List sub-TLV SHOULD NOT
   appear in the same BGP Tunnel Encapsulation Attribute.

   The Constituent SR Policy 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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Color                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            sub-TLVs                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   where:

   o  Type: to be assigned by IANA.

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

   o  RESERVED: 2 octet of reserved bits.  SHOULD be set to zero on
      transmission and MUST be ignored on receipt.

   o  Color: 4-octet value identifying the constituent SR policy.

   o  sub-TLVs currently defined:

      *  An optional single Weight sub-TLV which is defined in section
         2.4.4.1 in [I-D.ietf-idr-segment-routing-te-policy].  According
         to [I-D.ietf-spring-segment-routing-policy], the fraction of
         flows steered into each constituent SR Policy is equal to the
         relative weight of each constituent SR Policy.

4.  Operations

   The document does not bring new operation beyond 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 SR policies carrying composite
   candidate path information are configured by a controller.

   After configuration, the SR policies carrying path composite
   candidate path information 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.

5.  Security Considerations

   Procedures and protocol extensions defined in this document do not
   affect the security considerations discussed in
   [I-D.ietf-idr-segment-routing-te-policy].

6.  IANA Considerations

   This document defines a new Sub-TLV in registries "SR Policy List
   Sub-TLVs" [I-D.ietf-idr-segment-routing-te-policy]:





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   +-------+-------------------------------+---------------+
   | Value | Description                   | Reference     |
   +-------+-------------------------------+---------------+
   | TBA   | Constituent SR Policy Sub-TLV | This document |
   +-------+-------------------------------+---------------+

7.  Contributors

   In addition to the authors listed on the front page, the following
   co-authors have also contributed to this document:

   Yuanxiang Qiu
   Liping Yang
   Yang Wang

8.  References

8.1.  Normative References

   [I-D.ietf-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
              Rosen, E., Jain, D., and S. Lin, "Advertising Segment
              Routing Policies in BGP", draft-ietf-idr-segment-routing-
              te-policy-11 (work in progress), November 2020.

   [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>.

8.2.  Informative References

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", draft-
              ietf-spring-segment-routing-policy-09 (work in progress),
              November 2020.





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Authors' Addresses

   Hao Li
   H3C

   Email: lihao@h3c.com


   Mengxiao Chen
   H3C

   Email: chen.mengxiao@h3c.com


   Changwang Lin
   H3C

   Email: linchangwang.04414@h3c.com

































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