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2. SR P2MP Policy

An SR P2MP Policy instantiates P2MP trees between Root and Leaf nodes in an SR domain. Multiple SR P2MP Policies may have identical Root nodes and identical Leaf-node sets. Each policy has one or more candidate paths.

2.1. SR P2MP Policy Identification​

An SR P2MP Policy is uniquely identified by the tuple:

<Root, Tree-ID>
  • Root: the IP address of the Root node of the P2MP trees instantiated by the policy.
  • Tree-ID: a 32-bit unsigned integer that uniquely identifies the policy in the Root-node context.

2.2. Components of an SR P2MP Policy​

The policy contains:

  • Leaf nodes: nodes that terminate the P2MP trees.
  • Candidate paths: possible paths that define constraints and optimization objectives for PTIs.

The policy and its CPs are provisioned on a controller, on the Root node, or both, depending on the provisioning model. After provisioning, a signaling protocol instantiates them on the Root node or controller.

2.3. Candidate Paths and P2MP Tree Instances​

A CP is identified within an SR P2MP Policy by:

<Protocol-Origin, Originator, Discriminator>

The Root node selects the active CP using the tiebreaking rules from RFC 9256. A CP may have zero or more PTIs. It has no PTI if the controller cannot compute a P2MP tree that satisfies the constraints or objectives. A CP can have more than one PTI during Make-Before-Break, but one and only one PTI MUST be active for a CP. If multiple PTIs of the active CP are active at the same time, duplicate traffic may be delivered to Leaf nodes.

A PTI is identified by a 16-bit unsigned Instance-ID that is unique in the context of the SR P2MP Policy. This document updates the Replication-ID of the Replication segment control-plane identifier from RFC 9524 to:

<Root, Tree-ID, Instance-ID>

Therefore, Replication segments used to instantiate a PTI are identified in the control plane by:

<Root, Tree-ID, Instance-ID, Node-ID>

For the simple RFC 9524 use case where Replication-ID is a 32-bit number, the Root MUST be zero (0.0.0.0 for IPv4 and :: for IPv6), the Instance-ID MUST be zero, and the 32-bit Tree-ID is used to form:

<[0.0.0.0 or ::], Tree-ID, 0, Node-ID>

Because PTIs can have different topologies, each PTI has its own Replication segment and a unique Replication-SID in the data plane at a given SR-domain node. Explicit static tree topologies may be provisioned through NETCONF/YANG or CLI, but their provisioning and Root-node signaling procedures are outside this document.

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