ISO's OSI Layered Model - Transport Layer

The OSI Model layer 4 is Transport Layer. This layer maintains flow control of data and provides for error checking and recovery of data between the source and the destination. Transport Layer messages are called segments or transport protocal data units (TPDUs). Unlike the hop-by-hop communication at network layer, transport layer is an end-to-end communication -- that is -- the two communicating hosts need not be concerned with the topology of the internet work, which lies between them. They only need to know the state of their communication.

Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer.

TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. The sender of data, first establishes a logical connection with the prospective receiver of the data, sends the data and then terminates the connection. TCP is connection oriented and uses a 3 way handshake to establish a connection before data is sent.

UDP on the other hand provides a low overhead transmission service, but with less error checking. The sender does not establish a contact with the receiver first. Whenever there is a data packet ready to be sent, it independently routes the packet to a gateway. UDP is a connectionless protocal at the transport layer.

The device at transport layer is gateway. The layer 4 examples are TCP, UDP, SPX

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