A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The delivery, arrival time, and order of arrival of datagrams need not be guaranteed by the network. History In the early 1970s, the term datagram was created by combining the words data and telegram by the CCITT rapporteur on packet switching, Halvor Bothner-By. While the word was new, the concept had already a long history. In 1962, Paul Baran described, in a RAND Corporation report, a hypothetical military network having to resist a nuclear attack. Small standardized "message blocks", bearing source and destination addresses, were stored and forwarded in computer nodes of a highly redundant meshed computer network. "The network user who has called up a "virtual connection" to an end station and has transmitted messages ... might also view the system as a black box providing an apparent circuit connection". In 1967, Donald Davies published a seminal article in which he introduced the now largely used words packet and packet switching. His core network is similar to that of Paul Baran although it has been independently designed. To deal with datagram permutations (due to dynamically updated routing preferences) and to datagram losses (unavoidable when fast sources send to a slow destinations), he assumes that "all users of the network will provide themselves with some kind of error control" (what will be called later on a pure datagram service). His target is, for the first time in packet switching, a "common-carrier communication network". To support remote access to computer services by user terminals, which at that time transmitted in general character by character, he included at the network periphery interface computers that convert character flows into packet flows and conversely. In 1970, Lawrence Roberts and Barry D. Wessler published an article about ARPANET, the first multi-node packet-switching network. An accompanying paper described its switching nodes (the IMPs) and its packet formats. The network core performed datagram switching as in Baran's and Davies' model, but provision was added within the network, at its periphery, to deal with datagram losses and permutations. A reliable message transfer service was thus offered to user computers, thus greatly simplifying their own work, and keeping it less dependent on further research. In 1973, Louis Pouzin presented his design for Cyclades, the first real size network