Connection-Oriented and Connectionless Protocols
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Connection-Oriented and Connectionless Protocols in TCP/IP
Looking again at TCP/IP, it has two main protocols that operate at the transport layer of the OSI Reference Model. One is the Transmission Control Protocol (TCP), which is connection-oriented; the other, the User Datagram Protocol (UDP), is connectionless. TCP is used for applications that require the establishment of connections (as well as TCPs other service features), such as FTP; it works using a set of rules, as described earlier, by which a logical connection is negotiated prior to sending data. UDP is used by other applications that don't need connections or other features, but do need the faster performance that UDP can offer by not needing to make such connections before sending data.
Some people consider this to be like a simulation of circuit-switching at higher network layers; this is perhaps a bit of a dubious analogy. Even though a TCP connection can be used to send data back and forth between devices, all that data is indeed still being sent as packets; there is no real circuit between the devices. This means that TCP must deal with all the potential pitfalls of packet-switched communication, such as the potential for data loss or receipt of data pieces in the incorrect order. Certainly, the existence of connection-oriented protocols like TCP doesn't obviate the need for circuit switching technologies, though you will get some arguments about that one too. J
The principle of layering also means that there are other ways that connection-oriented and connectionless protocols can be combined at different levels of an internetwork. Just as a connection-oriented protocol can be implemented over an inherently connectionless protocol, the reverse is also true: a connectionless protocol can be implemented over a connection-oriented protocol at a lower level. In a preceding example, I talked about Telnet (which requires a connection) running over IP (which is connectionless). In turn, IP can run over a connection-oriented protocol like ATM.
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