Transport Layer

Below the Session Layer is the Transport Layer (layer 4). This layer in the model is responsible for the transparent transferring of data between protocols at the session layer, providing a reliable and cost-effective means of transferring data from the preceding layers. This layer determines how best to utilize the available resources below it (the network services) in order to meet the performance demands of the session layer protocols above it. The Transport Layer assigns transport addresses to each Session Layer protocol that requires its services, and then uses these addresses to establish communication between Session Layer protocols. This communication may be connection-oriented or connectionless, and may allow multiple connections to the same Session Layer protocol. This layer may additionally provide services such as data segmentation (the breaking up of large chunks of data into smaller pieces), the generation of acknowledgements (providing a message when a chunk of data is delivered), and data reordering (ensuring that data is processed in the correct order on the receiving side). Examples of protocols at this layer include X.224 (the connection-mode service protocol) and X.234 (the connectionless-mode service protocol).