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Transport Protocols and Services
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
Transport protocols occupy layer 4 of the OSI protocol model. The protocols at this level provide connection-oriented sessions and reliable data delivery services. The transport layer sits on top of layer 3 networking services. In the Internet Protocol suite, TCP provides transport services, while IP provides network services. In Novell's SPX/IPX protocol suite, SPX (Sequenced Packet Exchange) provides transport services, while IPX (Internetwork Packet Exchange) provides network services. NetBIOS is also a transport layer protocol.
Network layer protocols like IP provide best effort services-that is, they deliver packets but don't guarantee that the packets will actually be delivered. Think about the postal service. It delivers letters, but you don't know that it was received unless you arrange to have a delivery confirmation returned to you. Some applications, such as real-time voice and video, do not need TCP's services. In a real-time stream, it does not make sense to recover a lost packet. Speed of delivery is more important, so UDP (a limited services transport protocol) is used.
Transport layer protocols provide delivery guarantees that are essential for file transfers and mission-critical applications. TCP uses IP, but adds the reliability services at the cost of more overhead and slightly reduced performance. These services operate over a "virtual connection" that is established between sender and receiver. When a session begins, the sender uses a handshaking technique to establish a connection with the receiver. During the session, sender and receiver engage in a dialog that manages the flow of data to prevent from overflowing the receiver and confirms the receipt of TCP segments. A communication session goes like this:
The services provided by transport protocols are listed below with links to appropriate topics. You should also refer to "TCP (Transmission Control Protocol)" for detailed information on the Internet's transport protocol. A related section is "UDP (User Datagram Protocol)."
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.