February 2000
Intermediate to advanced
464 pages
15h 57m
English
Content preview from Internet Core Protocols: The Definitive Guide
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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 ...
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