2.9 Path MTU

2.9 Path MTU

When two hosts on the same network are communicating with each other, it is the MTU of the network that is important. But when two hosts are communicating across multiple networks, each link can have a different MTU. The important numbers are not the MTUs of the two networks to which the two hosts connect, but rather the smallest MTU of any data link that packets traverse between the two hosts. This is called the path MTU.

The path MTU between any two hosts need not be constant. It depends on the route being used at any time. Also, routing need not be symmetric (the route from A to B may not be the reverse of the route from B to A), hence the path MTU need not be the same in the two directions.

RFC 1191 [Mogul and Deering 1990] specifies the "path MTU discovery mechanism," a way to determine the path MTU at any time. We'll see how this mechanism operates after we've described ICMP and IP fragmentation. In Section 11.6 we'll examine the ICMP unreachable error that is used with this discovery mechanism and in Section 11.7 we'll show a version of the traceroute program that uses this mechanism to determine the path MTU to a destination. Sections 11.8 and 24.2 show how UDP and TCP operate when the implementation supports path MTU discovery.



TCP.IP Illustrated, Volume 1. The Protocols
TCP/IP Illustrated, Vol. 1: The Protocols (Addison-Wesley Professional Computing Series)
ISBN: 0201633469
EAN: 2147483647
Year: 1993
Pages: 378

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