BGP SR Policy Extensions for Performance-Aware Path Selection

To enable the headend node to do performance-aware path selection, this document proposes an extension to the BGP SR Policy protocol by defining a new optional Metric Sub-TLV within the BGP Tunnel Encapsulation Attribute. The introduced Metric Sub-TLV encodes performance parameters (such as latency, bandwidth, reliability, etc.) for SR Policy paths. This specification also updates the BGP route selection procedures in RFC4271, modifying the Breaking Ties (Phase 2) logic to prioritize the metrics for SR Policy paths. Key contributions include: * Introduce Metric Sub-TLV in BGP SR Policy * Update the tie-breaking procedure for BGP route selection