Sockets

Sockets are endpoints for inter-process communication. They provide a standardized interface for exchanging data across a network or between processes on the same host. A socket represents a bidirectional channel rather than the data itself.

Network sockets can be stream-oriented or datagram-oriented. Stream sockets use TCP to provide reliable, connection-oriented delivery; datagram sockets use UDP to deliver discrete messages without guarantees. Some environments also support other protocols such as SCTP.

Sockets are addressed by a combination of address and port. IP sockets use an IP address and a port number, while Unix-domain (local) sockets use a filesystem path. IPv4 and IPv6 are supported, with ports ranging to 65535.

Typical usage involves creating a socket, binding it to an address, and either listening for connections (server) or connecting to a remote endpoint (client). For servers, accept returns a new socket for the connection. Data are sent and received through the socket, and the endpoint is closed when done.

APIs differ by platform. The POSIX/BSD sockets API is common on Unix-like systems; Windows provides Winsock. Common operations include socket, bind, listen, accept, connect, send, recv, and close (or closesocket). Non-blocking and asynchronous I/O enable scalable designs.

Security and performance considerations include encryption (for example TLS over sockets), authentication, firewall rules, and tuning of buffering and congestion control. Sockets are foundational to most network services, including web servers, email, file transfer, and remote procedure calls.