TCP/IP Tutorial and Technical Overview - IBM Redbooks

Fragmentation
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.
104 TCP/IP Tutorial and Technical Overview