Transport layer
n computing and 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 (TransmissionControl 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 Protocols.
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.
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.
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