Path MTU Discovery

Path MTU Discovery is a method for dynamically detecting the Maximum Transmission Unit (MTU ), and thus the maximum packet size for a given path in the network. In general, reduced with this information overhead and fragmentation of data packets are prevented.

IPv4

To determine in IPv4 networks, the maximum size that should have a data package, the location of the path must be found that allows the smallest data packets. For this purpose, an IPv4 packet is sent, in which the DF bit ( Do not Fragment ) is set and has the size of the locally set Maximum Transmission Unit. If the packet to a point in the network where only a smaller MTU can be processed, an ICMP error is type 3 code 4 ( Destination Unreachable Fragmentation Needed, DF Set) returned, which also contains its own MTU. The local computer receives this ICMP packet, and can now adapt to the returned MTU size of its messages. This is repeated until the packet size was chosen small enough so that the packet can go through the entire path without fragmentation.

IPv6

In IPv6 networks, no fragmentation of forwarded packets on routers takes place, so Path MTU Discovery is here crucial in determining whether communication by means of large packages is made. In IPv6, large packets are rejected by the routers with the ICMPv6 error type 2 ( Packet Too Big). This type ICMPv6 packet is used for IPv6 instead of ICMP Error type 3 code 4 packet in IPv4 to Path MTU Discovery.

Problems

If the ICMP type 3 code 4, or ICMPv6 type -2 packets filtered at a point of the path, for example, by a simple " ICMP deny" on a firewall, you may experience transmission problems, as in Article maximum Transmission Unit described. Another method for determining the path MTU is carried over TCP (or another protocol to packaging ). It gradually larger packets are sent, and the maximum size is determined by successfully transmitted packets.