Transport Layer

The fourth layer of the OSI Reference Model is the transport layer. The transport layer is responsible for the actual mechanics of a connection where it can provide both reliable and unreliable delivery of data. For reliable connections the transport layer is responsible for error detection and correction: When an error is detected the transport layer will resend the data thus providing the correction. For unreliable connections the transport layer provides only error detection error correction is left up to one of the higher layers (typically the application layer). In this sense unreliable connections attempted to provide a best effort delivery if the data makes it there that great and if it dose not oh well! Example of a reliable transport protocol are TCP/IP Transmission Control Protocol (TCP) and IPX SPX (Sequenced Packet Exchange) protocol. TCP/IP User Datagram Protocol (UDP) is an example of a protocol that uses unreliable connections. Actually IPX and IP themselves are example of protocols that provide unreliable connections even though they operate at the network and not transport layer. In IPX case if a reliable connection is needed SPX is used. For IP if a reliable connection is mechanics is discussed in more depth in the section Transport Layer later in this chapter.