Internet-Draft BGP SR Policy Extensions for template May 2024
Zhang, et al. Expires 15 November 2024 [Page]
Network Working Group
Intended Status:
Standards Track
K. Zhang
Z. Hu
J. Dong
Q. Gao

BGP SR Policy Extensions for template


Segment Routing(SR) Policies can be advertised using BGP. An SR Policy may has lots of attributes, and as the application and features evolve, the SR Policy may need have more and more attribute attributes. To avoid modifying BGP when attributes are added to an SR Policy, we can define a template. The identifier and content of the template are defined by the receiver of the SR Policy. The advertiser of an SR policy only needs to know the ID of the template. When advertising SR policy, the advertiser carries the template ID in the tunnel encapsulation information of the SR policy. After receiving the SR Policy information, the receiver obtains the corresponding template and content according to the template ID, thereby obtaining abundant constraint configuration 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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

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

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This Internet-Draft will expire on 15 November 2024.

1. Introduction

[I-D.ietf-idr-segment-routing-te-policy]defines some attributes encoding of the SR Policy path. However, in actual applications, there are many other attributes of SR Policy path. These attributes are valid only on the device where the SR Policy path is installed. Such attributes may include backup protection, Bidirectional Forwarding Detection information, traffic statistics collection, or in-situ Flow Information Telemetry detection information, etc. If these attributes are directly delivered through BGP, the BGP SR Policy protocol may change frequently. This document defines a general method to carry the path attributes of SR Policies.

2. Terminology

SR Policy: An ordered list of segments.

Candidate Path: the unit for signaling of an SR Policy to a headend via protocol extensions like Path Computation Element (PCE) Communication Protocol (PCEP) [RFC8664] [I-D.ietf-pce-segment-routing-policy-cp] or BGP SR Policy [I-D.ietf-idr-segment-routing-te-policy].

SRPM: SR Policy Module.

Template: A collection of attributes sets.

Template ID: The identifier of a template.

3. Template ID defination

To support the attributes extension of SR Policies, this document defines a constraint template identifier. The constraint template ID is valid only for the recipient. The SR policy publisher only needs to carry the template ID when publishing the SR policy. The receiver of the SR Policy may create a template corresponding to the template identifier in advance before receiving the SR Policy, or may define a corresponding template after receiving the template definition of the SR Policy. The template can contain any attributes on the SR Policy path, including but not limited to backup protection, Bidirectional Forwarding Detection information, traffic statistics collection, or in-situ Flow Information Telemetry detection information, etc. After receiving the SR Policy information, the receiver matches the template information based on the template ID and adds attributes to the SR Policy based on the attributes defined in the template.

Template ID is an local identifier, just to use on the headend of the SR Policy. And it is a local configured identifier, need to be unique only on the headend device. We need no further process to coordinate the template ID between multiple routers.

A template represents a configuration plan intent. In a network managed by a controller, the template can be configured by controller and template ID is specified during the configuration. Therefore, the template ID is planned on the entire network. All policies can use the same template as long as they require the same configuration attributes, regardless of whether they are on the same device.

For example, a policy has two candidate paths with preference 200 and 100. The path with preference 200 has a higher preference and is used as the primary path. The BFD detection time is 10 ms x 3. The candidate path with preference 100 is used as the backup path, uses BFD detection, and the BFD detection time is 50 ms x 3. Then we can define two templates:

    Template 1:
        Hotstandby backup Enable
        BFD Seamless Enable
        BFD min-tx-interval 10
        BFD detect-multiplier 3

    Template 2:
        BFD Seamless Enable
        BFD min-tx-interval 50
        BFD detect-multiplier 3

    CP1<E1, C1>:
        <preference = 200,
         SL(Weight=W1): <S1, S2, S3>,
         SL(Weight=W2): <S4, S5, S6>,
         TemplateID = 1>

    CP2<E1, C1>:
        <preference = 100,
        SL(Weight=W1): <S7, S8, S9>,
        SL(Weight=W2): <S10, S11, S12>,
        TemplateID = 2>

We can see that the template defines some common attributes and can be used by different SR Policies.

4. SR Policy and Tunnel Encapsulation Attribute Update

As the template ID is defined, the tunnel attribute encapsulation of the BGP SR Policy needs to be updated.

The SR Policy Encoding structure is as follows:

SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>

   Tunnel Encaps Attribute (23)
      Tunnel Type: SR Policy
        Binding SID
        Policy Name
        Policy Candidate Path Name
        Explicit NULL Label Policy (ENLP)
        Template ID
        Segment List

Where Tempate ID indicates the template ID for the SR Policy candidate path.

4.1. Template ID sub-TLV

A new sub-TLV called Template ID sub-TLV is defined. Template ID sub-TLV specifies the template ID of an SR policy candidate path. Each sub-TLV is encoded as shown in Figure 1.

0               1               2               3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
|      Type     |    Length     |     Flags   |N|   RESERVED   |
|                   Template ID(4 octets)                      |
//                  Template Name (optional)                  //
Figure 1: Figure 1: Template ID Sub-TLV

Type: Template ID, 1 octet, TBD.

Length: 6.

Flags: 1 octet of flags.


        N-Flag: This flag, when set, indicates the presence of the Template
        Name encoding.

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

Template ID: a 4-octet value.

Template Name: Template MAY also be assigned with a template name, such template name MUST NOT be considered as identifiers for a template. The size of the tempalte name for the template is limited to 255 bytes.

5. SR Policy Operations

5.1. Advertisement of SR Policies

When BGP advertises an SR Policy, different candidate paths of the same SR Policy may have different template IDs or the same template ID, depending on the attributes required by the candidate paths of the SR Policy.

Reflectors just need to advertise the route of SR, no need to process it.

5.2. Reception of an SR Policy

SR Policy is only to be processed on the SR Policy headend, reflectors just need to reflect the route of SR Policy, no need to process it. To make this possible, an attribute needs to be attached to the advertisement that enables a BGP speaker to determine whether it is intended to be a headend for the advertised policy. This is done by attaching one or more Route Target Extended Communities to the advertisement [RFC4360]. This process is defined in [I-D.ietf-idr-segment-routing-te-policy]. This draft does not add any extra process in this process.

Once BGP on the receiving node has determined that the SR Policy NLRI is usable, it passes the SR Policy candidate path to the SRPM. The SRPM then determine how to use the template ID in SR Policy. The SRPM find the local configured template by template ID, and get all the attributes that is configured in the template, and then process the candidate path with these attributes. For example, if the template configure seamless bfd, then the SRPM can create sbfd sessions for each Segment List in the candidate path. If there is no template find, the SRPM should ignore the template ID and use the candidate path as there is no template ID.

6. Acknowledgements


7. IANA Considerations

This document requests that IANA allocates a new sub-TLV type as defined in Section 4.1 from the "Sub-TLVs for SR Policy" registry as specified.

Value                  Description                  Reference
---------------------- ---------------------------- --------------
TBD                    SR Policy Template ID        This document
Figure 2: Figure 2: Template ID sub-TLV

8. Security Considerations

These extensions to BGP SR Policy do not add any new security issues to the existing protocol.

9. References

Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and D. Jain, "Advertising Segment Routing Policies in BGP", Work in Progress, Internet-Draft, draft-ietf-idr-segment-routing-te-policy-26, , <>.
Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H. Bidgoli, "Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing (SR) Policy Candidate Paths", Work in Progress, Internet-Draft, draft-ietf-pce-segment-routing-policy-cp-15, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended Communities Attribute", RFC 4360, DOI 10.17487/RFC4360, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., and J. Hardwick, "Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing", RFC 8664, DOI 10.17487/RFC8664, , <>.

Authors' Addresses

Ka Zhang
Huawei Bld., No.156 Beiqing Rd.
Zhibo Hu
Huawei Bld., No.156 Beiqing Rd.
Jie Dong
Huawei Bld., No.156 Beiqing Rd.
Qiangzhou Gao
Huawei Bld., No.156 Beiqing Rd.