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.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 1, 2020.

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Table of Contents

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 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
                 ...
             ...

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.

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

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:

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.

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

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:

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

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].

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:

          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

   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", Internet-Draft draft-ietf-idr-segment-routing-te-policy-07, 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", Internet-Draft draft-ietf-spring-mpls-path-segment-01, September 2019.
[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", Internet-Draft draft-ietf-spring-segment-routing-policy-03, 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.
[RFC8402] Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B., Litkowski, S. and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018.

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", Internet-Draft draft-gandhi-spring-udp-pm-02, September 2018.
[I-D.ietf-mpls-bfd-directed] Mirsky, G., Tantsura, J., Varlashkin, I. and M. Chen, "Bidirectional Forwarding Detection (BFD) Directed Return Path", Internet-Draft draft-ietf-mpls-bfd-directed-12, August 2019.

Authors' Addresses

Cheng Li Huawei Technologies Huawei Campus, No. 156 Beiqing Rd. Beijing, 100095 China EMail: chengli13@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
Ketan Talaulikar Cisco Systems EMail: ketant@cisco.com