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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)

IP Datagram Options and Option Format 1 IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview

IP's main responsibility is to deliver data between internetworked devices. As we saw in the preceding section, this requires that data received from higher layers be encapsulated into IP datagrams for transmission. These datagrams are then passed down to the data link layer where they are sent over physical network links.

In order for this to work properly, each datagram must be small enough to fit within the frame format of the underlying technology. If the message is bigger than the maximum frame size of the underlying network, it may be necessary to break up an IP message into several datagrams, a process called fragmentation. The datagrams are then sent individually and reassembled into the original message.

The Internet Protocol is designed to manage datagram size, and to allow fragmentation and reassembly in a seamless manner. In this section I explore issues related to managing the size of IP datagrams. I start with an overview of datagram size issues and the important concept of a network's maximum transmission unit (MTU), discussing why fragmentation is necessary. I then describe the process by which IP messages to be transmitted are fragmented by the source device and possibly routers along the path to the destination, and then outline how they are reassembled by the recipient.

IP Datagram Options and Option Format 1 IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview