A datagram is a self-contained data unit with user and control data. The user data it contains can be transported with the control data, the source address and the destination address, by means of a datagram service from a data source to a data sink via a network.
In addition to the source address and the destination address, datagrams can contain further data fields for datagram length and error detection. Datagrams are characterized by their low protocol overhead and essentially contain the receiver and sender addresses as well as the user data. The transmission is connectionless.
The OSI terminology distinguishes datagrams with regard to the OSI layer. Thus, datagrams of the link layer are called data frames and datagrams of the network layer are called data packets. A datagram could also be interpreted as a data telegram. In field buses, the term telegram is therefore used for the datagram.