Layer 4: The Transport Layer

The Transport Layer provides a service similar to the Data Link Layer, in that it is responsible for the end-to-end integrity of transmissions. Unlike the Data Link Layer, however, the Transport Layer is capable of providing this function beyond the local LAN segment. It can detect packets that are discarded by routers and automatically generate a retransmit request. In other words, the Transport Layer provides true end-to-end reliability.

Another significant function of the Transport Layer is the resequencing of packets that may have arrived out of order. This can happen for a variety of reasons. For example, the packets may have taken different paths through the network, or some may have been damaged in transit. In any case, the Transport Layer is capable of identifying the original sequence of packets, and must put them back into that sequence before passing their contents up to the Session Layer.