Communications protocol

A method of communicating datagrams between terminals (nodes) of a communications system. In systems operating over unreliable media, redundancy check data can be included in a datagram for use in verifying the integrity of the datagram when received by a terminal; and satisfactory receipt of the datagram can be confirmed by the receiving terminal issuing a response datagram to the originating terminal, for example an acknowledgement. The redundancy check data of a received original datagram (which redundancy check data includes information on the identity of the originating terminal and the content of the datagram) is utilized in the generation of new redundancy check data for the acknowledgement (or other type of response) datagram. The originating terminal receives and processes the response datagram in conjunction with its own local record of the redundancy check data of the original datagram.