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TCP/IP Guide by Charles M. Kozierok

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IP Message Reassembly

When a datagram is fragmented, it becomes multiple fragment datagrams. The destination of the overall message must collect these fragments and reassemble them into the original message.

While reassembly is the complement to fragmentation, the two processes are not symmetric. A primary differentiation between the two is that intermediate routers can fragment a single datagram or further fragment a datagram that is already a fragment, but intermediate devices do not perform reassembly; reassembly happens only at the message's ultimate destination. Thus, if a datagram at an intermediate router on one side of a physical network with an MTU of 1,300 bytes causes fragmentation of a 3,300-byte datagram, the router on the other end ...

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