IP datagram

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.

The term datagram appeared first within the project CYCLADES, a packet-switched network created in the early 1970s, and was coined by Louis Pouzin by combining the words data and telegram. CYCLADES was the first network to make the hosts responsible for the reliable delivery of data, rather than the network itself, using unreliable datagrams and associated end-to-end protocol mechanisms.

The inspiration for datagrams had two sources. One was Donald Davies' studies. He had done some simulation of datagram networks, although he had not built any, and it looked technically viable. The second inspiration was I like things simple. I didn't see any real technical motivation to overlay two levels of end-to-end protocols. I thought one was enough.

These concepts were later adopted for the creation of the (IP) and other network protocols.

RFC 1594 defines the term Datagram as follows:

“A self-contained, independent entity of data carrying sufficient information to be routed from the source to the destination computer without reliance on earlier exchanges between this source and destination computer and the transporting network.”

A datagram needs to be self-contained without reliance on earlier exchanges because there is no connection of fixed duration between the two communicating points as there is, for example, in most voice telephone conversations.

Datagram service is often compared to a mail delivery service, the user only provides the destination address, but receives no guarantee of delivery, and no confirmation upon successful delivery. Datagram service is therefore considered unreliable. Datagram service routes datagrams without first creating a predetermined path. Datagram service is therefore considered connectionless. There is also no consideration given to the order in which it and other datagrams are sent or received. In fact, many datagrams in the same group can travel along different paths before reaching the same destination.

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