MTU propagation over EVPN Overlays

Path MTU Discovery between end-host-devices/Virtual-Machines/servers/ workloads connected over an EVPN-Overlay Network in Datacenter/Campus/enterprise deployment, is a problem, yet to be resolved in the standards forums. It needs a converged solution to ensure optimal usage of network and computational resources of the networking elements, including underlay routers/switches, constituting the overlay network. This documents takes leads from the guidelines presented in [RFC4459]. The overlay connectivity can pan across various sites (geographically seperated or collocated) for realizing a Datacenter Interconnect or intersite VPNs between campus sites (buildings, branch offices etc). This literature intends to solve problem of icmp error propagation from an underlay routing/switching device to an end-host (hooked to EVPN overlay), thus facilitating "accurate MTU" learnings. This document also leverages the icmp multipart message extension, mentioned in [RFC4884] to carry the original packet in the icmp PDU.