The term unicast describes communication where data is sent from a single sender to a single recipient. The term unreliable denotes an absence of guarantees that data sent by the sender are received by the recipient. To avoid the extreme of no communication being a special case of unreliable communication, best effort guarantees are assumed implicitly.

Unreliable unicast communication is provided by the network hardware. Typically, however, the network hardware can only communicate directly with other nodes connected to the same network segment. Additionally, the size of the data packets can be limited. To circumvent these limitations, addressing, routing, fragmentation and reassembly mechanisms are added by the unreliable unicast layer.