Support for Path MTU (PMTU) in the Path Computation Element (PCE) Communication Protocol (PCEP)

The Path Computation Element (PCE) provides path computation functions in support of traffic engineering in Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) networks. The Source Packet Routing in Networking (SPRING) architecture describes how Segment Routing (SR) can be used to steer packets through an IPv6 or MPLS network using the source routing paradigm. A Segment Routed Path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), explicit configuration, or a Path Computation Element (PCE). Since the SR does not require signaling, the path maximum transmission unit (MTU) information for the SR path is unavailable at the headend. This document specifies the extension to PCE Communication Protocol (PCEP) to carry path MTU as a new metric type in the PCEP messages for SR, but not limited to it. This document also updates RFC 5440 to allow metric bounds to be minimum as needed in the case of path MTU.