Datagrams

A datagram is a self-contained, independent packet of data that is sent over a packet-switched network. Datagrams do not require a prior agreement or a connection between sender and receiver; each datagram is routed separately through the network and may take different paths to its destination.

Datagrams carry addressing and control information in their headers and are designed for best-effort delivery. Because each datagram is independent, routing decisions can change from hop to hop and datagrams may arrive out of order, be duplicated, or be lost. Datagram size is limited by protocol and network MTU.

In the Internet Protocol (IPv4 and IPv6), an IP datagram is the basic unit of data transfer. It contains a header with addressing, fragmentation flags, TTL, and a payload. IP datagrams may be fragmented by routers if the payload is larger than the network MTU, and they are reassembled at the destination when possible.

At the transport layer, the User Datagram Protocol (UDP) provides a datagram service. UDP datagrams carry source and destination ports and a checksum but do not guarantee delivery, ordering, or duplicate protection. A UDP datagram's payload may be up to 65,535 bytes total, including the UDP header, and in practice is often constrained by the path MTU.

Datagram networks are contrasted with connection-oriented or virtual-circuit networks, where a route is established and maintained for the duration of a session. Datagram networks favor simplicity, scalability, and robustness to routing changes but sacrifice reliability and ordering without higher-layer protocols such as TCP.

Common uses of datagrams include DNS queries, streaming, and real-time communications where low latency is valued over guaranteed delivery; many applications use UDP datagrams and implement their own error handling if needed.