IP Message Fragmentation Process
As explained in the previous section, when an IP datagram is too large for the MTU of the underlying data link layer technology used for the next leg of its journey, it must be fragmented before it can be sent across the network. The higher-layer message to be transmitted is not sent in a single IP datagram, but rather broken down into fragments that are sent separately. In some cases, the fragments themselves may need to be fragmented further.
Fragmentation is key to implementing a network-layer internetwork that is independent of lower-layer details, but it introduces significant complexity to IP. Remember that IP is an unreliable, connectionless protocol. IP datagrams can take any of several routes on their way ...
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