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Versions: (draft-li-idr-sr-policy-path-segment) 00 01

Interdomain Routing Working Group                                  C. Li
Internet-Draft                                                     Z. Li
Intended status: Standards Track                     Huawei Technologies
Expires: February 12, 2021                                       H. Chen
                                                           China Telecom
                                                                W. Cheng
                                                            China Mobile
                                                          K.  Talaulikar
                                                           Cisco Systems
                                                         August 11, 2020


      SR Policy Extensions for Path Segment and Bidirectional Path
                draft-ietf-idr-sr-policy-path-segment-01

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 unidirectional 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

   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
   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



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   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 February 12, 2021.

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
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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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   3.  Path Segment in SR Policy . . . . . . . . . . . . . . . . . .   3
     3.1.  SR Path Segment Sub-TLV . . . . . . . . . . . . . . . . .   5
   4.  SR Policy for Bidirectional Path  . . . . . . . . . . . . . .   6
     4.1.  Reverse Path Segment List Sub-TLV . . . . . . . . . . . .   7
   5.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Existing Registry: BGP Tunnel Encapsulation Attribute
           sub-TLVs  . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   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 carrying 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].

2.1.  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.

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:














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      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
                    ...
                ...

   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 identifying an SR path(specified by SID list) from
   the headend and the tailend.  Also, it can be used for identifying an
   SR candidate path in some use cases if needed.  This document defines
   a new Path Segment sub-TLV within Segment List sub-TLV, the details
   will be described at section 3.1.  The new SR Policy encoding
   structure with Path Segmentg sub-TLV is expressed as below:

























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      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
                    Path Segment
                    Segment
                    Segment
                    ...
                Segment List
                    Weight
                    Path Segment
                    Segment
                    Segment
                    ...
                ...


   The Path Segment is used to identified an SR path, and it can be used
   in OAM or IOAM use cases.  When all the SID Lists within a candidate
   path share the same Path Segment ID, the Path Segment can be used to
   collect the aggregated information of the candidate path.  Multiple
   Path Segment MAY be included in a Segment List for different use
   cases, all of them SHOULD be inserted into the SID List.

3.1.  SR Path Segment Sub-TLV

   This section defines an SR Path Segment sub-TLV.

   An SR Path Segment sub-TLV is included in the segment list sub-TLV to
   identify an SID list.  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     |    Flags      |      ST       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              Path Segment ID (Variable depends on ST)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        Figure 1. Path Segment sub-TLV





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   Where:

   Type: to be assigned by IANA.

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

   Flags: 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.  The rest bits of Flag are 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 ID: The Path Segment ID 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], or other IDs
   that identifies a path.  When ST is 0, the Path Segment ID is a SR-
   MPLS Path Segment, and format is shown below.

      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     |    Flags      |      ST=0     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Path Segment Label            | TC  |S|       TTL     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Figure 2. SR-MPLS Path Segment sub-TLV


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 a Reverse
   Segment List sub-TLV to describe the reverse path associated with the



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   forward path specified by the Segment List.  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)
               Segment List
                   Weight
                   Path Segment
                   Segment
                   Segment
                   ...
                   Reverse Segment List
                       Path Segment
                       Segment
                       Segment
                       ...


4.1.  Reverse Path Segment List Sub-TLV

   A Reverse Path Segment List sub-TLV is defined to specify an SR
   reverse path associated with the path specified by the Segment List,
   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 3. SR Reverse Path Segment List Sub-TLV


   where:

   Type: TBA.

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





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   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  One or more mandatory SR Path Segment sub-TLVs that contains the
      Path Segments of the reverse SR path.

   o  One 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 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 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
   reception.

   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:








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

   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
          -------------------------------------------------------------
          TBA         Path Segment sub-TLV                  This document
          TBA         Reverse Segment List sub-TLV          This document


7.  Security Considerations

   TBA

8.  Contributors

      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








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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., Talaulikar, K., Mattes, P.,
              Rosen, E., Jain, D., and S. Lin, "Advertising Segment
              Routing Policies in BGP", draft-ietf-idr-segment-routing-
              te-policy-09 (work in progress), May 2020.

   [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-02 (work in progress),
              February 2020.

   [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-08 (work in progress),
              July 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>.

10.2.  Informative References









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   [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 for MPLS Label Switched Paths (LSPs)", draft-ietf-
              mpls-bfd-directed-15 (work in progress), August 2020.

Authors' Addresses

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

   Email: c.l@huawei.com


   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



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   Ketan Talaulikar
   Cisco Systems

   Email: ketant@cisco.com















































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