Internet Core Protocols: The Definitive Guide

Path MTU discovery

Even though TCP systems are able to determine the MTU values in use by the endpoints on a virtual circuit, they are not be able to determine the MTU sizes of the networks in between the two endpoints, which may be smaller than the MTU/MRU values in use at either of the endpoint networks. In this scenario, fragmentation would still occur, since the MTU of the intermediary system would require that the IP datagrams be fragmented.

For example, if two systems are both on Token Ring networks using four-kilobyte MTUs, but there is an Ethernet network between them with a 1500 byte MTU, then fragmentation will occur when the four-kilobyte IP datagrams are sent over the 1500 byte Ethernet network. This process will lower the overall performance of the virtual circuit and may introduce some reliability problems.

By itself, TCP does not provide any means for determining the MTU of an intermediate network, and must rely on external means to discover the problem. One solution to this problem is to use a technique called Path MTU Discovery, which incorporates the IP Don’t Fragment bit and the ICMP Destination Unreachable: Fragmentation Required error message to determine the MTU of the end-to-end IP network.

Essentially, Path MTU Discovery works by having one system create an IP packet of the largest possible size (as determined by the MTU/MRU pair for the virtual circuit), and then setting the Don’t Fragment flag on the first IP packet. If the packet is rejected by an intermediary ...