RFC 9545: Path Segment Identifier in MPLS-Based Segment Routing Networks

A Segment Routing (SR) path is identified by an SR segment list. A subset of segments from the segment list cannot be leveraged to distinguish one SR path from another as they may be partially congruent. SR path identification is a prerequisite for various use cases such as performance measurement and end-to-end 1+1 path protection. In an SR over MPLS (SR-MPLS) data plane, an egress node cannot determine on which SR path a packet traversed the network from the label stack because the segment identifiers are removed from the label stack as the packet transits the network. This document defines a Path Segment Identifier (PSID) to identify an SR path on the egress node of the path.