IP Fragmentation

In this tutorial, we are going to see What is the IP Fragmentation and how it works? As we have seen previously, the maximum size of a datagram is 65536 bytes. However, this value is never reached because networks do not have enough capacity to send such large packets. Also, networks on the Internet use different technologies, so the maximum size of a datagram varies depending on the type of network.
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The maximum size of a frame is called the MTU (Maximum Transfer Unit), which will cause the datagram to be fragmented if it is larger than the MTU of the network.

Network type	MTU (in bytes)
Arpanet	1000
Ethernet	1500
FDDI	4470

 

IP Fragmentation:

The fragmentation of a datagram is done at the level of the routers, i.e. during the transition from a network whose MTU is important to a network whose MTU is lower. If the datagram is too big to pass on the network, the router will fragment it, which means cutting it into fragments of sizes smaller than the MTU of the network and in such a way that the size of the fragment is a multiple of 8 bytes.
 

 
The router will then send these fragments independently and encapsulate them (add a header to each fragment) in such a way as to take into account the new fragment size. In addition, the router adds information so that the destination machine can reassemble the fragments in the correct order. However, there is no guarantee that the fragments will arrive in the correct order since they are routed independently of each other.
 
 
To deal with fragmentation, each datagram has several fields that allow it to be reassembled:

• fragment offset field (13 bits): field allowing to know the position of the beginning of the fragment in the initial datagram. The unit of measurement for this field is 8 bytes (the first fragment has 0 as a value).
• identification field (16 bits): number assigned to each fragment in order to allow their reassembly.
• total length field (16 bits): it is recalculated for each fragment.
• flag field (3 bits): it is composed of three bits:
  • The first is not used.
  • The second (called DF: Don’t Fragment) indicates if the datagram can be fragmented or not. If a datagram has this bit set to one and the router cannot route it without fragmenting it, then the datagram is rejected with an error message
  • The last one (called MF: More Fragments) indicates if the datagram is a data fragment (1). If the indicator is zero, it indicates that the fragment is the last one (so the router should have all the previous fragments) or the datagram has not been fragmented

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