UDP and datagram sockets

Datagram sockets are typically used to send and receive UDP datagrams. They are connectionless, and need only minimal state in the operating system. The communication is not reliable and there is no congestion control or flow control. UDP is typically used for different kinds of signaling or service discovery protocols, where individual messages are small, or for DNS name resolution. UDP is also used for data where delay variation is undesirable, such as realtime audio/video calls or multiplayer games. Recently UDP has been used for HTTP transfers with QUIC protocol, where the reliability and congestion control is built on a separate layer on top of UDP.

simple-udp is a simple example demonstrating sending and receiving data between a UDP client and a UDP server. In Rust, UDP socket can be created with the bind call, that at the same time binds it to a local address an port. In Posix C API there would be a separate socket call for creating the socket, and a separate bind call for binding it.

UDP socket does not need to be connected, but once created, the application can start sending or receiving data right away. Therefore, there are separate calls for sending data from an unconnected socket (send_to) and for receiving data using an unconnected socket (recv_from). Because the socket is not connected, the same socket can be used for sending to multiple different destinations or receiving from multiple different sources.