When you send a packet that is bigger than the Maximum Transmission Unit (MTU) of a network, the packet must be fragmented before transmission. Fragmentation is the process of breaking the packet into smaller pieces to fit within the MTU size of the network.

Here's what happens when a packet exceeds the MTU:

1. The sender or an intermediate router identifies that the packet size exceeds the MTU of the network.
2. The packet is broken down into smaller fragments, each of which is within the MTU limit.
3. Fragmentation is typically performed at the IP layer, and each fragment has its own IP header with relevant information, such as the source and destination IP addresses, as well as a fragmentation offset and flags.
4. The fragments are transmitted separately over the network.
5. When the fragments reach the destination, they are reassembled into the original packet by the receiving device. This process is called reassembly.
6. If any of the fragments are lost or corrupted during transmission, the entire packet may need to be retransmitted, as IP fragmentation does not provide error correction or retransmission capabilities.

It's important to note that fragmentation can lead to performance issues and increased overhead. To avoid or minimize fragmentation, it is recommended to use Path MTU Discovery (PMTUD) to determine the smallest MTU along the path between the sender and receiver and adjust the packet size accordingly. Additionally, some protocols like TCP have built-in mechanisms to adjust the segment size dynamically to avoid fragmentation.
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