| Internet-Draft | BGP SR Policy Composite Candidate Path | February 2024 |
| Li, et al. | Expires 29 August 2024 | [Page] |
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.¶
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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-sr-policy-safi] 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.¶
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.¶
As defined in [I-D.ietf-idr-sr-policy-safi], 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
...
¶
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 candidate path.¶
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+¶
where:¶
Type: to be assigned by IANA.¶
Length: the total length of the value field not including Type and Length fields.¶
RESERVED: 2 octet of reserved bits. SHOULD be set to zero on transmission and MUST be ignored on receipt.¶
Color: 4-octet value identifying the constituent SR policy.¶
sub-TLVs currently defined:¶
An optional single Weight sub-TLV which is defined in section 2.4.4.1 in [I-D.ietf-idr-sr-policy-safi]. According to [RFC9256], the fraction of flows steered into each constituent SR Policy is equal to the relative weight of each constituent SR Policy.¶
The document does not bring new operation beyond the description of operations defined in [I-D.ietf-idr-sr-policy-safi]. The existing operations defined in [I-D.ietf-idr-sr-policy-safi] 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-sr-policy-safi], as well as the receiption.¶
Procedures and protocol extensions defined in this document do not affect the security considerations discussed in [I-D.ietf-idr-sr-policy-safi].¶
This document defines a new Sub-TLV in registries "SR Policy List Sub-TLVs" [I-D.ietf-idr-sr-policy-safi]:¶
| Value | Description | Reference |
|---|---|---|
| TBA | Constituent SR Policy Sub-TLV | This document |