Tcp Ip Protocol Questions Medium
The TCP/IP Protocol handles fragmentation and reassembly of IP packets through a process known as Path MTU Discovery (PMTUD) and the IP Fragmentation and Reassembly mechanism.
When a data packet is sent over a network, it may encounter different network links with varying Maximum Transmission Units (MTUs). The MTU represents the maximum size of a packet that can be transmitted over a particular network link without fragmentation. If a packet's size exceeds the MTU of a network link, it needs to be fragmented into smaller pieces to fit within the MTU.
The TCP/IP Protocol uses PMTUD to determine the smallest MTU along the path between the source and destination. Initially, the sender sets the Don't Fragment (DF) flag in the IP header of the packet, indicating that it should not be fragmented. If a router along the path receives a packet that exceeds its MTU, it will drop the packet and send an ICMP "Fragmentation Needed" message back to the sender. This message includes the MTU of the network link where the packet was dropped.
Upon receiving the "Fragmentation Needed" message, the sender reduces the packet size to match the reported MTU and retransmits the packet. This process continues until the packet reaches its destination without being fragmented.
In cases where fragmentation is necessary, the IP Fragmentation and Reassembly mechanism is employed. The sender divides the original packet into smaller fragments, each fitting within the MTU of the network link. Each fragment is assigned a unique identification number and offset value to indicate its position within the original packet. The fragments are then individually transmitted to the destination.
Upon receiving the fragments, the destination host uses the identification number and offset values to reassemble the original packet. It ensures that all fragments are received and arranges them in the correct order based on their offset values. Once all fragments are successfully reassembled, the original packet is passed to the higher-layer protocols, such as TCP or UDP, for further processing.
In summary, the TCP/IP Protocol handles fragmentation and reassembly of IP packets through PMTUD to determine the smallest MTU along the path and the IP Fragmentation and Reassembly mechanism to divide and reassemble packets when necessary. This ensures efficient and reliable transmission of data across networks with varying MTUs.