When sending a data packet, the length of the packet can exceed MTU (Maximum Transmission Unit) of the network interface. When that is the case, the packet is split into fragments no larger than MTU, identified by their offset within the data packet. The fragments are sent separately and reassembled at the recipient.

Different network hardware can use different MTU. Fragmentation can therefore occur at each router that is connected to network segments with different MTU values. The sender of a data packet can discover the minimum of the MTU values on the path the packet travels, denoted as PMTU (Path Maximum Transmission Unit), and split the packet into fragments no larger than PMTU to minimize fragmentation.

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