10.8. Path MTU Discovery with UDP

Let us examine the interaction between an application using UDP and the path MTU discovery mechanism (PMTUD) [RFC1191]. For a protocol such as UDP, in which the calling application is generally in control of the outgoing datagram size, it is useful if there is some way to determine an appropriate datagram size if fragmentation is to be avoided. Conventional PMTUD uses ICMP PTB messages (see Chapter 8) in determining the largest packet size along a routing path that can be used without inducing fragmentation. These messages are typically processed below the UDP layer and are not directly visible to an application, so either an API call is used for the application to learn the best current estimate of the path MTU size for each destination with which it has communicated, or the IP layer can perform PMTUD independently without the application knowing. The IP layer often caches PMTUD information on a per-destination basis and times it out if it is not refreshed.

10.8.1. Example

In the following example, we use the sock program to create a UDP datagram that produces a 1501-byte IPv4 datagram. Both our host system and the attached LAN support an MTU larger than 1500 bytes, but the outgoing link to the Internet at the router does not. The command attempts to send three UDP messages to the echo service (UDP port 7) in quick succession.