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OSI Model Transport Layer (Layer 4)

The transport layer implements reliable internetwork data transport services that are transparent to upper layers. Transport-layer functions typically include flow control, multiplexing, virtual circuit management, and error checking and recovery.

Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process. Multiplexing enables data from several applications to be transmitted onto a single physical link. Virtual circuits are established, maintained, and terminated by the transport layer. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves taking an action, such as requesting that data be retransmitted, to resolve any errors that occur.

Some transport-layer implementations include Transmission Control Protocol, Name Binding Protocol, and OSI transport protocols. Transmission Control Protocol (TCP) is the protocol in the TCP/IP suite that provides reliable transmission of data. Name Binding Protocol (NBP) is the protocol that associates AppleTalk names with addresses. OSI transport protocols are a series of transport protocols in the OSI protocol suite.