Transport Layer

telecommunications, the transport layer is the second highest layer in the four and five layer TCP/IP reference models, where it responds to service requests from the application layer and issues service requests to the Internet layer. It is also the name of layer four of the seven layer OSI model, where it responds to service requests from the session layer and issues service requests to the network layer. The definitions of the transport layer are slightly different in these two models. This article primarily refers to the TCP/IP model. See also the OSI model definition of the transport layer.

A transport protocol is a protocol on the transport layer. The two most widely used transport protocols on the Internet are the connection oriented TCP (Transmission Control Protocol), and UDP (User Datagram Protocol). TCP is the more complicated and most common. Other options are the Datagram Congestion Control Protocol (DCCP) and Stream Control Transmission Protocol (SCTP).

The transport layer is typically handled by processes in the host computer operational system, and not by routers and switches. The transport layer usually turns the unreliable and very basic service provided by the Network layer into a more powerful one.

In the TCP/IP model, the transport layer is responsible for delivering data to the appropriate application process on the host computers. This involves statistical multiplexing of data from different application processes, i.e. forming data packets, and adding source and destination port numbers in the header of each transport layer data packet. Together with the source and destination IP address, the port numbers constitutes a network socket, i.e. an identification address of the process-to-process communication. In the OSI model, this function is supported by the session layer.

Some transport layer protocols, for example TCP but not UDP, support virtual circuits, i.e. provide connection oriented communication over an underlying packet oriented datagram network. A byte-stream is delivered while hiding the packet mode communication for the application processes. This involves connection establishment, dividing of the data stream into packets called segments, segment numbering and reordering of out-of order data.

Finally, some transport layer protocols, for example TCP but not UDP, provides end-to-end reliable communication, i.e. error recovery by means of error detecting code and automatic repeat request (ARQ) protocol. The ARQ protocol also provides flow control, which may be combined with congestion avoidance.

UDP is a very simple service, and does not provide virtual circuits, nor reliable communication, leaving these to the application. The UDP packets are called datagrams rather than segments.

TCP is used for HTTP web browsing and email transfer. UDP may be used for multicasting and broadcasting, since retransmissions are not possible to a large amount of hosts. UDP typically gives higher throughput and shorter latency, and is therefore often used for realtime multimedia communication where packet loss occasionally can be accepted, for example IP-TV and IP-telephony, and for online computer games.

In many non-IP-based networks, for example X.25, Frame Relay and ATM, the connection oriented communication is implemented at network layer or data link layer rather than the transport layer. In X.25, in telephone network modems and in wireless communication systems, reliable node-to-node communication is implemented at lower protocol layers.

In the OSI/X.25 protocol suite, there are five classes of the OSI transport protocol, ranging from class 0 (which is also known as TP0 and provides the least error recovery) to class 4 (which is also known as TP4 and is designed for less reliable networks, similar to the Internet).