Network Socket - Socket States and The Client-server Model

Socket States and The Client-server Model

Computer processes that provide application services are called as servers, and create sockets on start up that are in listening state. These sockets are waiting for initiatives from client programs.

A TCP server may serve several clients concurrently, by creating a child process for each client and establishing a TCP connection between the child process and the client. Unique dedicated sockets are created for each connection. These are in established state, when a socket-to-socket virtual connection or virtual circuit (VC), also known as a TCP session, is established with the remote socket, providing a duplex byte stream.

A server may create several concurrently established TCP sockets with the same local port number and local IP address, each mapped to its own server-child process, serving its own client process. They are treated as different sockets by the operating system, since the remote socket address (the client IP address and/or port number) are different; i.e. since they have different socket pair tuples (see below).

For further details on TCP sockets, including other states of TCP sockets, see Transmission Control Protocol.

A UDP socket cannot be in an established state, since UDP is connectionless. Therefore, netstat does not show the state of a UDP socket. A UDP server does not create new child processes for every concurrently served client, but the same process handles incoming data packets from all remote clients sequentially through the same socket. This implies that UDP sockets are not identified by the remote address, but only by the local address, although each message has an associated remote address.