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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)                      9  IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly

IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview 1 2 3 4 IP Message Reassembly Process

IP Header Flags Related to Fragmentation

In addition to the fields above, there are a couple of flags in the IP header related to fragmentation.

If a datagram containing options must be fragmented, some of the options may be copied to each of the fragments. This is controlled by the Copied flag in each option field.

This flag can be set to 1 by a transmitting device to specify that a datagram not be fragmented in transit. This may be used in certain circumstances where the entire message must be delivered intact as pieces may not make sense. It may also be used if the destination device has a limited IP implementation and can't reassemble fragments, and is also used for testing the maximum transmission unit (MTU) of a link. Normally, however, devices don't care about fragmentation and this field is left at zero.

What happens if a router encounters a datagram too large to pass over the next physical network but with the Don't Fragment bit set to 1? It can't fragment the datagram and it can't pass it along either, so it is “stuck”. It will generally drop the datagram, and then send back a special ICMP Destination Unreachable error message: “Fragmentation Needed and Don't Fragment Bit Set”. This is used in MTU Path Discovery as described in the preceding section.

IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview 1 2 3 4 IP Message Reassembly Process