Overview#Reliable protocol provides assurance of the delivery of data to the intended recipient(s), as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data.
Reliability is a synonym for assurance, which is the term used by the ITU and ATM Forum in the context of the ATM Service-Specific Coordination Function, for example for transparent assured delivery with AAL5.
Reliable protocols typically incur more overhead than unreliable protocols, and as a result, function more slowly and with less scalability. This often is not an issue for unicast protocols, but it may become a problem for reliable multicast protocols.
TCP, the main protocol used on the Internet, is a unicast Reliable protocol. UDP, often used in computer games or in other situations where speed is an issue and the loss of a little data is not as important because of the transitory nature of the data, is an unreliable protocol.
Often, a unicast Reliable protocol is also connection-oriented. For example, TCP is connection-oriented, with the virtual-circuit ID consisting of source and destination IP Address and port numbers. Some unreliable protocols are connection-oriented as well. These include ATM and frame relay. There are also reliable connectionless protocols, such as AX.25 when it passes data in I-frames. But this combination occurs rarely: reliable-connectionless is uncommon in commercial and academic networks.