Reliability (computer networking)

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In computer networking, a reliable protocol is one that ensures reliability properties with respect to the delivery of data to the intended recipient(s), as opposed to an unreliable protocol, which does not guarantee that data will be delivered intact, or that it will be delivered at all.

A reliable multicast protocol may ensure reliability on a per-recipient basis, as well as provide strong reliability properties that relate the delivery of data to different recipients, such as e.g. total order, atomicity, or virtual synchrony.

Reliable protocols typically incur more overhead than unreliable protocols, and as a result, are slower and less scalable. This often isn't an issue for unicast protocols, but it may be a problem for multicast protocols.

TCP, the main protocol used in the Internet today, is a reliable unicast protocol. UDP, often used in computer games or other situations where speed is an issue and the loss of a little data is not, is an unreliable protocol.

Examples of reliable multicast protocols include Reliable Multicast Transport Protocol (RMTP), Scalable Reliable Multicast (SRM), QuickSilver Scalable Multicast (QSM) and SMART Multicast (SMART) (Secure Multicast for Advanced Repeating of Television).

Often, a reliable unicast protocol is also connection-oriented. For example, the TCP/IP protocol is connection-oriented, with the virtual circuit ID consisting of source and destination IP addresses and port numbers. Some unreliable protocols are connection-oriented as well. These include ATM and Frame Relay, on which a substantial part of all Internet traffic is passed.