In telecommunications, connection-oriented describes a means of transmitting data in which the devices at the end points use a preliminary protocol to establish an end-to-end connection before any data is sent, and in which data is sent over the same path during the communication. Connection-oriented protocol service are often but not always "reliable" network service, that provides guarantees that data will arrive in the proper sequence.

Circuit mode communication, for example the public telephone network, ISDN and SONET/SDH, are examples of connection-oriented communication.

Packet mode communication may also be connection-oriented, and is called virtual circuit mode communication. A connection-oriented packet switched protocol does not have to provide each packet with routing information (complete source and destination address), but only with a channel/data stream number, often denoted virtual circuit identifier (VCI). Routing information may be provided to the network nodes during the connection establishment phase, where the VCI is defined in tables in each node.

The alternative to connection-oriented transmission is connectionless packet-mode communication, also known as datagram communication, in which data is sent from one end point to another without prior arrangement, and no guarantees are provided. In datagram switching, each data packet must contain complete address information, since packets are routed individually. The packets may be delivered along different paths and without any guarantees, according to a best-effort policy.

Connectionless protocols are usually described as stateless because the end points have no protocol-defined way to remember where they are in a "conversation" of message exchanges. Because they can keep track of a conversation, connection-oriented protocols are sometimes described as stateful.

Examples of connection-oriented packet mode communication, i.e. virtual circuit mode communication:
* The Transmission Control Protocol (TCP) is a connection-oriented reliable protocol that is based on a datagram protocol (the IP protocol).
* X.25 was a connection-oriented reliable network protocol.
* Frame relay is a connection-oriented unreliable data link layer protocol.
* Asynchronous Transfer Mode
* Multiprotocol Label Switching

ee also

* Packet switching
* Statistical multiplexing
* Connectionless protocol
* Connectionless mode transmission
* Packet
* Virtual circuit

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