Transport Layer Definition

The transport layer, the middle (i.e., fourth) layer in the OSI (open systems interconnection) seven layer model, is responsible for maintaining reliable end-to-end communications across the network. It provides full-duplex virtual circuits on which delivery is reliable, error free, sequenced, and duplicate free.

The transport layer responds to service requests from the session layer above it and issues service requests to the network layer below it to establish a conversation (i.e., a virtual connection) between two hosts. The network layer, the layer at which IP (Internet protocol) operates, is responsible for routing, which is moving packets across the network using the most appropriate paths.

The best known example of a transport layer protocol is TCP (transmission control protocol), which provides a virtually error-free point-to-point connection that allows packets to arrive at their intended destinations uncorrupted and in the correct order. Also operating at this layer are UDP (user datagram protocol), RTP (real-time transport protocol) and SCTP (stream control transmission protocol).

Created October 19, 2005.
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