The term reliability generally denotes a presence of guarantees that data sent by the sender are received by the recipient. Ideally, data that were sent would always be received and no data would be received that were not sent. This ideal is referred to as exactly once delivery semantics.

To guarantee the exactly once delivery semantics, the protocol stack has to cope with various network failures. The most frequently considered network failures are damage to a data packet, loss of a data packet, duplication of a data packet.