Internet Core Protocols: The Definitive Guide

MTU and MRU size considerations

Although buffer sizing issues can have an impact on the size of any given segment at any given time, most of the time the deciding factor for segment sizes is based on the size of the MTU and MRU in use by the end-to-end network connection.

For example, even the weakest of systems will have a TCP receive buffer of two or more kilobytes, while the MTU/MRU for Ethernet networks is only 1500 bytes. In this case (and almost all others), the MTU/MRU of the Ethernet segment will determine the maximum segment size for that system, since it indicates the largest amount of data that can be sent in a single datagram without causing fragmentation to occur.

Typically, the MTU and MRU sizes for a particular network are the same values. For example, Ethernet networks have an MTU/MRU of 1500 bytes, and both of these values are fixed. However, many dial-up networks allow an endpoint system to define different MTU and MRU sizes. In particular, many dial-up systems set the MTU to be quite small, while also setting the MRU to be quite large. This imbalance can actually help to improve the overall performance of the client, making it snappier than a fixed, medium-sized MTU/MRU pair would allow for.

To understand why this is so, you have to understand that most dial-up systems are clients, using applications such as POP3 and TELNET to retrieve large amounts of data from remote servers. Having a small MTU size forces the client to send segments quickly, since the MTU is ...