Internet Core Protocols: The Definitive Guide

Fragmentation and Reassembly

Every network has certain characteristics that are specific to the medium in use on that network. One of the most important characteristics is the maximum amount of data that a network can carry in a single frame (called the Maximum Transmission Unit, or “MTU”). For example, Ethernet can pass only 1500 bytes in a single frame, while the typical MTU for 16-Mb/s Token Ring is 17,914 bytes per frame.

RFC 791 specifies that the maximum allowed MTU size is 65,535 bytes, and that the minimum allowed MTU size is 68 bytes. No network should advertise or attempt to use a value that is greater or lesser than either of those values. Several RFCs define the specific default MTU values that are to be used with different networking topologies. Table 2.5 lists the common MTU sizes for the most-common media types, and also lists the RFCs (or other sources) that define the default MTU sizes for those topologies.

Table 2.5. Common MTU Sizes and the Related RFCs

Topology	MTU (in bytes)	Defined By
Hyperchannel	65,535	RFC 1374
16 Mb/s Token Ring	17,914	IBM
802.4 Token Bus	8,166	RFC 1042
4 Mb/s Token Ring	4,464	RFC 1042
FDDI	4,352	RFC 1390
DIX Ethernet	1,500	RFC 894
Point-to-Point Protocol (PPP)	1,500	RFC 1548
802.3 Ethernet	1,492	RFC 1042
Serial-Line IP (SLIP)	1,006	RFC 1055
X.25 & ISDN	576	RFC 1356
ARCnet	508	RFC 1051

Since an IP datagram can be forwarded across any route available, every IP packet that gets generated by a forwarding device has to fit the packet within the available MTU space of the underlying ...