Routing And Switching Questions Long
IP packet fragmentation is a process that occurs when a packet is too large to be transmitted over a network without being divided into smaller fragments. This fragmentation process is necessary because different networks have different maximum transmission unit (MTU) sizes, which define the maximum size of a packet that can be transmitted without being fragmented.
When a packet is larger than the MTU of a network it needs to traverse, it is fragmented into smaller pieces at the source host before being transmitted. Each fragment contains a portion of the original packet's data, along with a fragment header that provides information about the fragment's position within the original packet.
The process of IP packet fragmentation involves the following steps:
1. Source host determination: The source host determines the MTU of the network it needs to send the packet to. It compares the size of the packet with the MTU and identifies if fragmentation is required.
2. Fragmentation: If the packet size exceeds the MTU, the source host divides the packet into smaller fragments. Each fragment contains a fragment header that includes information such as the identification number, offset, and a flag indicating whether more fragments are expected.
3. Transmission: The source host transmits the fragments individually to the destination host. These fragments may take different paths through the network and may arrive at the destination host out of order.
4. Reassembly: Upon receiving the fragments, the destination host uses the identification number and offset information in the fragment headers to reassemble the original packet. It stores each fragment in a temporary buffer until all fragments are received.
5. Fragment handling by routers: Routers play a crucial role in handling IP packet fragmentation. When a router receives a fragment, it examines the fragment header to determine the destination address and the identification number. It then checks its routing table to determine the next hop for the packet.
6. Fragment forwarding: Routers forward each fragment based on the destination address and the next hop determined from the routing table. The fragments may take different paths through the network, and each router along the path performs the same process of examining the fragment header, determining the next hop, and forwarding the fragment.
7. Fragment reassembly: The final router in the path, which is typically the destination host's router, receives the fragments and reassembles them into the original packet. It uses the identification number and offset information in the fragment headers to correctly order the fragments and reconstruct the original packet.
It is important to note that IP packet fragmentation can introduce additional overhead and can impact network performance. Therefore, it is generally recommended to avoid fragmentation whenever possible by adjusting the packet size or using techniques such as Path MTU Discovery (PMTUD) to determine the maximum MTU along the path.