The datagram service is a connectionless service of a packet-switched network with low protocol overhead. Datagrams are transmitted from the sender to the receiver without an acknowledgement mechanism.
For the transmission of a datagram, only the complete receiver address and the sender address must be specified. Since the datagram is treated as a separate entity in the network, no protocol overhead is required to establish the connection. In meshed datagram networks, there are multiple paths from the sender to the receiver. Routing is determined exclusively by the switching nodes, the Interface Message Processors( IMP), depending on the traffic situation. Therefore, data packets may arrive at the destination in an irregular sequence or not at all. The sequence number makes it possible for the receiver to reassemble the data stream correctly. The recipient can request lost data packets from the sender.
If more than one data packet with the same sequence number arrives in this way, all but one of the others are discarded. Typical transport protocols for datagram services are the User Datagram Protocol( UDP), Datagram Delivery Protocol( DDP) and the Datagram Transport Protocol( DTP).