Internet Core Protocols: The Definitive Guide

Notes on Fragmentation

As discussed in Section 2.1.5 earlier in this chapter, each of the different network topologies have different Maximum Transfer Unit (MTU) sizes, which represent the maximum amount of data that can be passed in a single frame. On Ethernet networks, the MTU is typically 1500 bytes, while 16 Mb/s Token Ring has a default MTU size of 17,914 bytes. Some networks have smaller MTUs, with the minimum allowed value being just 68 bytes.

Whenever an IP datagram needs to be sent across a network to another device, the datagram must be small enough to fit within the MTU size constraints of the local network. For example, if the local network is Ethernet, then the IP datagram must be 1500 bytes or less in order for the datagram to get sent across that network. If the datagram is larger than 1500 bytes, then it must be split into multiple fragments that are each small enough to be sent across the local network.

Most of the time, datagrams do not require fragmentation. On local networks, every device uses the same MTU size, so local packets are never fragmented. And most of the networks in use on the Internet (either as destination networks or intermediate ISP networks) are capable of handling packets that are 1500 bytes in length, which is the largest size that most dial-up clients will generate. The only times that fragmentation typically occurs is on mixed local networks that have Ethernet and Token Ring (or other large-frame networks), or when a host on an Ethernet ...