Connection-oriented protocol

A connection-oriented networking protocol is one that delivers a stream of data in the same order as it was sent, after first establishing a communication session. It may be a circuit switched connection, or a virtual circuit connection in a packet switched network. In the latter case, it identifies traffic flows by some connection identifier rather than by explicitly listing source and destination addresses. Typically, this connection identifier is a small integer (10 bits for Frame Relay, 24 for ATM, for example). This makes network switches substantially faster (as routing tables are just simple look-up tables, and are trivial to implement in hardware). The impact is so great, in fact, that even characteristically connectionless protocols, such as IP traffic, are being tagged with connection-oriented header prefixes (e.g., as with MPLS, or IPv6's built-in Flow ID field). Example of a connection-oriented protocol at the transport layer is the TCP protocol.

Note that connection-oriented protocols are not necessarily reliable protocols. ATM and Frame Relay, for example, are both examples of a connection-oriented, unreliable protocol. There are also reliable connectionless protocols as well, such as AX.25 when it passes data in I-frames. But this combination is rare, and reliable-connectionless is uncommon in commercial and academic networks.

Note that connection-oriented protocols handle real-time traffic substantially more efficiently than connectionless protocols, which is why ATM has yet to be replaced by Ethernet for carrying real-time, isochronous traffic streams, especially in heavily aggregated networks like backbones, where the motto "bandwidth is cheap" fails to deliver on its promise. Experience has also shown that over-provisioning bandwidth does not resolve all quality of service issues. Hence, (10-)gigabit Ethernet is not expected to replace ATM at this time.

Some connection-oriented protocols have been designed or altered to accommodate both connection-oriented and connectionless data.

List of Connection-oriented protocols


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