IP Fragmentations

In data link layer or MAC layer, there is a quantity called maximum transmission unit (MTU), which is the maximum length of a data link frame. For example, for Ethernet, it's 1500 bytes. MTU can be shorter for other link layer interfaces. Because an IP datagram can be as long as 65535 bytes, some IP datagram may be too large to fit into an MTU.

In this situation, IP datagram has to be fragmented. Here are some of the important issues in fragmentations:

• Once an IP is fragmented, it will not be reassembled until it reaches the destination;
• A fragmented IP datagram can be further fragmented if necessary.
• The fragmentation process is transparant to upper layer protocols. That is, the IP datagram will be reassembled at the destination before being delivered to the upper layer, such as TCP or UDP;
• Fragmented IP datagrams will be independently routed from the place of fragmentation. In other words, each fragment is treated as a new IP datagram.
• If an IP fragment is lost or corrupted, whether at the destination or at the intermediate routers, the entire original IP datagram is thrown away on the scene in order not to complicate things further down the link. Again, IP layer doen't retransmit, but only to notify upper layers of the loss through the use of ICMP message.
• Each new IP fragment contains the same IP header as the original, especially the source and destination address, and IP fields. However, the field is changed depending on whether the fragment is the last fragment. Also, fields contains the offset of this fragment from the beginning of the original datagram. This is used for the reassembly at the destination.
• The field in each new fragment has to be changed to the actual length of the fragment.