Datagram

Datagram is a basic transfer unit used in packet-switched networks. A datagram is a self-contained packet that carries enough information to be routed from the source to the destination independently of other datagrams. In Internet protocols, the term is most often associated with IP datagrams and UDP datagrams. Datagram networks provide a connectionless service, meaning that sending a datagram does not establish a persistent path or session, and delivery is not guaranteed.

An IP datagram consists of a header and a payload. The header includes information such as version, header length, total length, identification, flags, fragment offset, Time To Live, protocol, checksum, and the source and destination addresses. The payload carries the encapsulated data, which may be an upper-layer protocol’s segment or application data. IP is a connectionless, best-effort service: routers forward each datagram independently, based solely on the destination address, without maintaining state about the sender or other datagrams.

A UDP datagram is a unit of data delivered by the User Datagram Protocol. Each UDP datagram contains a UDP header with source port, destination port, length, and checksum, followed by the payload. UDP is also connectionless and unreliable; datagrams may be lost, duplicated, or delivered out of order, and applications may implement their own mechanisms for reliability if needed.

Datagram opening contrasts with connection-oriented approaches that establish a route or channel before data transfer. Large datagrams may be fragmented into smaller units for transport, depending on the network, with reassembly occurring at the destination where supported.