UDP is a simple transport-layer protocol. It is described in RFC 768 [Postel 1980]. The application writes a message to a UDP socket, which is then encapsulated in a UDP datagram, which is then further encapsulated as an IP datagram, which is then sent to its destination. There is no guarantee that a UDP datagram will ever reach its final destination, that order will be preserved across the network, or that datagrams arrive only once.
The problem that we encounter with network programming using UDP is its lack of reliability. If a datagram reaches its final destination but the checksum detects an error, or if the datagram is dropped in the network, it is not delivered to the UDP socket and is not automatically retransmitted. If we want to be certain that a datagram reaches its destination, we can build lots of futures into our application: acknowledgments from the other end, timeouts, retransmissions, and the like.
Each UDP datagram has a length. The length of a datagram is passed to the receiving application along with the data. We have already mentioned that TCP is a byte-stream protocol, without any record boundaries at all, which differs from UDP.
We also say that UDP provides a connectionless service, as there need not be any long-term relationship between a UDP client and server. For example, a UDP client can create a socket and send a datagram to a given server and the immediately send another datagram on the same socket to a different server. Similarly, a UDP server can receive several datagrams on a single UDP socket, each from a different client.
以上摘自 «Unix Network Programming Vol 1» 2.3节。
