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Transport layer

In computing, the transport layer is level four of the seven level OSI model. It responds to service requests from the session layer and issues service requests to the network layer.

The transport layer provides transparent transfer of data between hosts. It is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. In the IP protocol Stack this function is achieved by the connection oriented Transmission Control Protocol (TCP) or the datagram type User Datagram Protocol (UDP). The purpose of the Transport layer is to provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer.

The transport layer turns the unreliable and very basic service provided by the Network layer into one worthy of the term 'Communication'. There is a long list of services that can be optionally provided at this level. None of them are compulsory, because not all applications want all the services available. Some can be wasted overhead, or even counterproductive in some cases.

On the internet there are a variety of Transport services, but the two most common are TCP and UDP. TCP is the more complicated, providing a connection and byte oriented stream which is almost error free, with flow control, multiple ports, and same order delivery. UDP is a very simple 'datagram' service, which provides limited error reduction and multiple ports. TCP stands for Transport Control Protocol, while UDP stands for User Datagram Protocol.

Some things, such as connection orientation can be implemented at either Transport or Network layer. The idea is that the Network layer implements whatever set of options is easiest: for some underlying networks it is easiest to implement connectionless communication, while for others it is easiest to implement connection oriented communication. The Transport layer uses this simplest set of options to implement whatever combinations of options are actually desired.