Path MTU Discovery

IPv6 routers along a path no longer perform fragmentation as in IPv4. Fragmentation is performed at the source IPv6 node when necessary.

The main purpose of PMTUD discovery process is determining the most optimum (maximum) MTU for a path to eliminate the need of fragmentation. PMTUD allows IPv4 and IPv6 nodes to dynamically discover and adjust to differences in the MTU of the links along a path accordingly.

An ICMPv6 Type 2 – Packet Too Big error message will be sent by a router when it cannot forward a packet that is larger that the MTU of the outgoing link to the destination.

When an IPv6 node attempts to send a packet at the size specified by the upper layers and receives an ICMP Packet Too Big error message (which would contains a recommended MTU), it tells the upper layer to discard the packet and use the new MTU. Each device needs to track the MTU size for each session. The tracking of the MTU size can be built by creating a cache based on destination address, flow label, or source address (if source routing is being performed).

A host ages cached MTU values and performs PMTUD every 10 minutes to see if the MTU has increased along the path, as there can be more appropriate MTU when routing paths change.

Path MTU Discovery