BGP Extensions of SR Policy for Composite Candidate Path

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.¶

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 [RFC9256]. 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 [RFC9256], 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.¶

[I-D.draft-jiang-spring-parent-sr-policy-use-cases] describes some use cases for SR policy group composite candidate path.¶

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 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 [RFC9256], 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.¶

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
                ...

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                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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]:¶

7. References