When one IPv6 node has a large amount of data to send to another node, the data is transmitted in a series of IPv6 packets. It is preferable that these packets be of the largest size that can successfully traverse the path from the source node to the destination node. This packet size is referred to as the Path MTU (PMTU), and it is equal to the minimum link MTU of all the links in a path. IPv6 provides PMTU discovery as a standard mechanism for a node to discover the PMTU of an arbitrary path.

For IPv6, intermediate routers cannot fragment packets. An implementation must either support path MTU discovery or send using IPv6 minimum link MTU. z/OS® Communications Server supports path MTU discovery.

Path MTU discovery supports multicast as well as unicast destinations. When PMTU information is learned, it is cached for a period of time and then deleted in order to learn of increases in the MTU value.