A network socket is an endpoint of a connection across computer networks. Nowadays, almost all communication between computers and distinct networks is based on the Internet Protocol, which uses sockets as a basis of communication. A socket is an abstract reference that a local program can pass to the network API to make use of a connection. Sockets are internally often represented in network programming APIs simply as integers, which identify which connection to use. In brief, we will be covering the following topics in this chapter:
- The basics of sockets
- The difference between TCP and UDP sockets
- UDP sockets
- TCP sockets
- The Telnet program
- A chat application
A socket API is an application programming interface, provided by the operating system (OS), that allows application programs to initiate, control, and use network sockets programmatically for communication. Internet socket APIs are usually based on the Berkeley sockets standard. In the Berkeley sockets standard, sockets are a form of file descriptor, adhering to the Unix philosophy that everything is a file. Thus, we can read, write, open, and close sockets in the same way as we do files. In inter-process communications, each end will have its own socket, but these may use different socket-programming APIs. However, they are abstracted by the network protocol.
A socket address is a combination of an Internet Protocol (IP) address and a port number. Internet sockets deliver and receive data packets to and from the appropriate application process or thread. On a computer with an IP address, every network-related program or utility will have its own unique socket or set of sockets. This ensures that the incoming data is redirected to the correct application.