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Fragmentation and Reassembly
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
Fragmentation is the process of breaking a packet into smaller pieces so that they will fit into the frames of the underlying network. The receiving system reassembles the pieces into the original packets. The term MTU (maximum transmission unit) refers to the maximum amount of data that can travel in a frame. Different networks have different MTU sizes, so packets may need to be fragmented in order to fit within the frames of the network that they transit. The process is illustrated in Figure F-8.
RFC 791 (Internet Protocol, September 1981) describes fragmentation and reassembly. RFC 815 (IP Datagram Reassembly Algorithms, July 1982) also describes reassembly. RFC 1122 (Requirements for Internet Hosts-Communication Layers, November 1990) also describes the process (see sections "3.3.2 Reassembly" and "3.3.3 Fragmentation"). A must-read document to understand fragmentation is RFC 879 (The TCP Maximum Segment Size and Related Topics, November 1983). Also useful is RFC 1180 (A TCP/IP Tutorial, January 1991).
ANCHOR HERE: Figure 8 (see book)
Internetworking protocols such as IP use fragmentation because each of the networks that a packet may travel over could have a different frame size. Fragmentation occurs at routers that connect two networks with different MTUs. While it is possible to design an internal network with the same MTU size, this is not an option on the Internet, which includes thousands of independently managed interconnected networks.
Fragmentation is always undesirable because it reduces performance. In fact, fragmentation is not allowed in IPv6. Large packets are always preferable...
This topic continues in "The Encyclopedia of Networking and Telecommunications" with a discussion of the following:
Path MTU discovery is described in RFC 1191 (Path MTU Discovery, November 1990), RFC 1981 (Path MTU Discovery for IP Version 6, August 1996), and RFC 2923 (TCP Problems with Path MTU Discovery, September 2000).
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.