NETWORK ~ IPv4 fragmentation
In this article we will learn about the IP fragmentation and reassembly process used by IPv4. We will look at the purpose of IP fragmentation, the relevant fields contained in the IP header and the role of the maximum transmission unit (MTU) in determining when IP fragmentation will be used.
As specified in RFC 791, the IP fragmentation and reassembly process occurs at the IP layer and is transparent to higher layer protocols. When a data block is prepared for transmission, the transmission device examines the MTU of the network over which the data is to be sent or transferred. If the size of the data block is less than the MTU of that network, the data is transmitted according to the rules of that particular network. But what happens when the amount of data is larger than the MTU of the network? This is where one of the functions of the IP layer, commonly known as fragmentation and reassembly, will come into play.
Maximum Transmission Unit (MTU):
There are different network transmission architectures, each with a physical limit on the number of bytes of data that can be contained in a given frame. This physical limit is described in many specifications and is referred to as the maximum transmission unit or MTU of the network. An example of such an MTU would be IEEE 802.3 Ethernet; According to the specification, the maximum number of data bytes that can be contained in a frame is 1500. The following table lists the MTUs for several common network types (from RFC 1191):
Network architecture MTU in bytes
Ethernet 802.3 >>>>>>>>> 1500 Byte
4MB Token Ring >>>>>>>>> 4464 Byte
16MB Token Ring >>>>>>>>> 17914 Byte
FDDI >>>>>>>>> 4352 Byte
X.25 >>>>>>>>> 576 Byte
The MTU becomes important in two cases:
- When the size of the data block being transmitted is larger than the MTU.
- When the data must traverse multiple network architectures, each with a different MTU. Such an example would be the case if the data came from a network (4MB Token Ring) (MTU = 4464 Byte) connected to another network (4MB Token Ring) (MTU = 4464 Byte) via a network (Ethernet) (MTU = 1500 Byte).
Whatever the situation, the rules followed by IP fragmentation remain the same.
IP Fragmentation and Reassembly:
Let's go back to the previously mentioned example of a block of data coming from a network (4MB Token Ring) (MTU = 4464 Byte) connected to another network (4MB Token Ring) (MTU = 4464 Byte) via a network (Ethernet) (MTU = 1500 Byte).
At the time of transmission, the data block met the MTU restriction for the ring network of 4MB. Yet, the router connecting the ring network to the Ethernet network is forced to transfer this large block over a network with a smaller MTU. So how does Router 1 handle this MTU situation? The answer is simple, it will simply follow the rules of IP fragmentation.
Once the data has reached Router 2, it then performs fragment reassembly exactly as described above and forwards the reassembled block of data to the network with the new MTU.
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