IP Fragmentation

The physical network layer normally imposes an upper limit on the size of the frame that can be transmitted. Whenever the IP layer receives an IP datagram to
send, it determines which local interface the datagram is being sent on (routing), and queries that interface to obtain its MTU. IP compares the MTU with the datagram size and performs fragmentation, if necessary. Fragmentation can take place either at the original sending host or at an intermediate router. When an IP datagram is fragmented, it is not reassembled until it reaches its final destination.
The IP layer at the destination performs the reassembly. The goal is to make fragmentation and reassembly transparent to the transport layer (TCP and UDP), which it is, except for possible performance degradation. It is also possible for the fragment of a datagram to again be fragmented (possibly more than once). The information maintained in the IP header for fragmentation and reassembly provides enough information to do this. Recalling the IP header, the following fields are used in fragmentation. The identification field contains a unique value for each IP datagram that the sender transmits. This number is copied into each fragment of a particular datagram.
The flags field uses one bit as the “more fragments” bit. This bit is turned on for each fragment comprising a datagram except the final fragment. The fragment offset field contains the offset of this fragment from the beginning of the original datagram. Also, when a datagram is fragmented the total length field of each fragment is changed to be the size of that fragment. Finally, one of the bits in the flags field is called the “don’t fragment” bit. If this is turned on, IP will not fragment the datagram. Instead the datagram is thrown away and an ICMP error is sent to the originator.
When an IP datagram is fragmented, each fragment becomes its own packet, with its own IP header, and is routed independently of any other packets. This makes it possible for the fragments of a datagram to arrive at the final destination out of order, but there is enough information in the IP header to allow the receiver to reassemble the fragments correctly. Although IP fragmentation looks transparent, there is one feature that makes it less than desirable: if one fragment is lost the entire datagram must be retransmitted. To understand why this happens, realize that IP itself has no timeout and retransmission-that is the responsibility of the higher layers.
 When a fragment is lost that came from a TCP segment, TCP will time out and retransmit the entire TCP segment, which corresponds to an IP datagram. There is no way to resend only one fragment of a datagram. Indeed, if the fragmentation was done by an intermediate router, and not the originating system, there is no way for the originating system to know how the datagram was fragmented, For this
reason alone, fragmentation is often avoided.
The source and destination ports. The protocol could be printed, since it’s in the IP header that’s copied into the fragments. The port numbers, however, are in the UDP header, which only occurs in the first fragment.          

 Example of IP fragmentation.

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