Destination Originates Internet Control Message Protocol (ICMP) Packet Too Big (PTB) Messages

This document defines procedures that enhance Path MTU Discovery (PMTUD), so that it no longer relies on the network's ability to deliver an ICMP Packet Too Big (PTB) message from a downstream router to an IPv6 source node. According to these procedures, selected packets carry a new IPv6 Destination option. When a downstream router cannot forward one of these packets because of MTU issues, it truncates the packet, marks it to indicate that it has been truncated, and forwards it towards the destination node. When the destination node receives a packet that has been truncated, it sends an ICMP PTB message to the source node. The source node uses MTU information contained by the ICMP PTB message to update its PMTU estimate. The destination node does not deliver the truncated packet to upper layer protocols unless specifically requested to do so by the upper layer protocol. The procedures described herein rely on the network's ability to deliver an ICMP PTB message from the packet's destination to the packet's source.