What is fragmentation ? Explain its significations.

When an IP datagram travels from one host to another, it can pass through different physical networks. Each physical network has a maximum frame size. This is called the maximum transmission unit (MTU). It limits the length of a datagram that can be placed in one physical frame. IP implements a process to fragment datagrams exceeding the MTU. The process creates a set of datagrams within the maximum size. The receiving host reassembles the original datagram. IP requires that each link support a minimum MTU of 68 octets. This is the sum of the maximum IP header length (60 octets) and the minimum possible length of data in a non-final fragment (8 octets). If any network provides a lower value than this, fragmentation and reassembly must be implemented in the network interface layer. This must be transparent to IP. IP implementations are not required to handle unfragmented datagrams larger than 576 bytes. In practice, most implementations will accommodate larger values. An unfragmented datagram has an all-zero fragmentation information field. That is, the more fragments flag bit is zero and the fragment offset is zero. The following steps fragment the datagram:

1. The DF flag bit is checked to see if fragmentation is allowed. If the bit is set, the datagram will be discarded and an ICMP error returned to the originator. 2. Based on the MTU value, the data field is split into two or more parts. All newly created data portions must have a length that is a multiple of 8 octets, with the exception of the last data portion. 3. Each data portion is placed in an IP datagram. The headers of these datagrams are minor modifications of the original: The more fragments flag bit is set in all fragments except the last. The fragment offset field in each is set to the location this data portion occupied in the original datagram, relative to the beginning of the original unfragmented datagram. The offset is measured in 8-octet units. If options were included in the original datagram, the high order bit of the option type byte determines if this information is copied to all fragment datagrams or only the first datagram. For example, source route options are copied in all fragments. The header length field of the new datagram is set. – The total length field of the new datagram is set. – The header checksum field is re-calculated. 4. Each of these fragmented datagrams is now forwarded as a normal IP datagram. IP handles each fragment independently. The fragments can traverse different routers to the intended destination. They can be subject to further fragmentation if they pass through networks specifying a smaller MTU. At the destination host, the data is reassembled into the original datagram. The identification field set by the sending host is used together with the source and destination IP addresses in the datagram. Fragmentation does not alter this field. In order to reassemble the fragments, the receiving host allocates a storage buffer when the first fragment arrives. The host also starts a timer. When subsequent fragments of the datagram arrive, the data is copied into the buffer storage at the location indicated by the fragment offset field. When all fragments have arrived, the complete original unfragmented datagram is restored. Processing continues as for unfragmented datagrams. If the timer is exceeded and fragments remain outstanding, the datagram is discarded. The initial value of this timer is called the IP datagram time to live (TTL) value. It is implementation-dependent. Some implementations allow it to be configured. The netstat command can be used on some IP hosts to list the details of fragmentation.