| Internet-Draft | Validity of SR Policy Candidate Path | June 2025 |
| Chen, et al. | Expires 6 December 2025 | [Page] |
This document defines extensions to BGP to distribute the validity control parameters of a candidate path for an SR Policy.¶
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SR Policy architecture is specified in [RFC9256]. An SR Policy comprises one or more candidate paths(CP) of which at a given time one and only one may be active (i.e., installed in forwarding and usable for steering of traffic). Each CP in turn may have one or more SID-List of which one or more may be active; when multiple SID-List are active then traffic is load balanced over them.¶
[I-D.chen-spring-sr-policy-cp-validity] supplemented candidate path validity criterion in [RFC9256]. It defines two validity control parameters under candidate Path to control the validity judgment of candidate Path.¶
This document defines extensions to BGP to distribute the validity control parameters of a candidate path for an SR Policy.¶
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], a new SAFI is defined (the SR Policy SAFI with codepoint 73) as well as a new NLRI. The NLRI contains the SR Policy candidate path and, according to [I-D.ietf-idr-sr-policy-safi], the content of the SR Policy Candidate Path is encoded in the Tunnel Encapsulation Attribute defined in [RFC9012] using a new Tunnel-Type called SR Policy Type with codepoint 15. This document defines CP Validity Sub-TLV to carry the validity control parameters of a candidate path.¶
The new SR Policy encoding structure with CP Validity Sub-TLV is expressed as below:¶
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy (15)
Binding SID
SRv6 Binding SID
Preference
Priority
Policy Name
Policy Candidate Path Name
Explicit NULL Label Policy (ENLP)
CP Validity
Segment List
Weight
Segment
Segment
...
...
¶
The format of the CP Validity Sub-TLV is defined as follows:¶
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 | valid SL count| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| valid SL weight |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: CP Validity Sub-TLV
¶
where:¶
Type: to be assigned by IANA.¶
Length: the total length of the value field not including Type and Length fields. The total length MUST be 6.¶
valid SL count: 1-octet field which indicates the minimum number of valid segment Lists under the active candidate path. When the number of valid segment Lists under candidate path is greater than or equal to this field, the candidate path is considered valid. 0 indicates no requirement for SL count. 0xff indicates that the candidate path is considered valid only if all the segment Lists are valid.¶
valid SL weight: 4-octet field which indicates the minimum value of the sum of the weights of the valid segment List under the active candidate Path. When the sum of the weights of the valid segment Lists under the candidate path is greater than or equal to this field, the candidate Path is considered valid. 0 indicates no requirement for weight.0xffffffff indicates that the candidate path is considered valid only if all the segment Lists are valid.¶
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 the validity control parameters of the candidate path are configured by a controller.¶
After configuration, the SR policies carrying the validity control parameters of the candidate path 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 reception.¶
This document defines a new sub-TLV in the registry "BGP Tunnel Encapsulation Attribute sub-TLVs" to be assigned by IANA:¶
Value Description Reference
------- ------------------------- --------------
TBD CP Validity Sub-TLV This document
¶
The security considerations of BGP [RFC4271] and BGP SR policy [I-D.ietf-idr-sr-policy-safi] apply to the extensions described in this document as well. It does not introduce additional security issues compared to existing SR policy extensions. The CP Validity information is critical to determining the validity of the CP, and a wrong CP Validity information may cause unexpected forwarding actions and results.¶
Implementations need to make sure that the CP Validity information is correct to avoid unexpected forwarding actions and results. Additionally, the distribution of CP validity information from the controller to the ingress router needs to be protected. The security considereations in [I-D.ietf-idr-sr-policy-safi] apply to this distribution procedure.¶
TBD.¶