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

Responsible for packet handling.
Ensures error-free delivery.
Repackages messages, divides messages into smaller packets, and handles error handling.

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.

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 Transmission Control Protocol, while UDP stands for User Datagram Protocol.

Transport layer concepts:

Connection-oriented - an acknowledgement (ACK) verifies that the host has received each segment of the message, reliable delivery service. If the ACK is not received after a given time period, then the data is resent. If segments are not delivered to the destination device correctly, then the Transport layer can initiate retransmission or inform the upper layers. Uses segmentation, flow control, and error checking to insure packet delivery the purpose of name resolution, either to an IP/IPX address or a network protocol name resolution helps upper layer services communicate segment destinations with lower layer services.

More information on the Transport Layer is contained in the pages on TCP/IP under the functions of TCP.

For more information on Networking Basics, the OSI Model and Server Administration:
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