Reliability

In computer networking, a reliable protocol is one that ensures that reliability properties with respect to the delivery of data to the intended recipient(s), as opposed to an unreliable procedure, which does not guarantee that data will be delivered intact, or that it will be delivered at all. A reliable multicast protocol may ensure consistency 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 normally 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 difficulty 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 situation where speed is an issue and the loss of a little fact is not, is an unreliable unicast protocol.