title: Computer network programming
text: # Computer network programming
  
  Computer network programming involves writing computer programs that enable processes to communicate with each other across a computer network.
  
  ## Connection-oriented and connectionless communications
  
  Very generally, most of communications can be divided into connection-oriented, and connectionless. Whether a communication is connection-oriented or connectionless, is defined by the communication protocol, and not by application programming interface (API). Examples of the connection-oriented protocols include Transmission Control Protocol (TCP) and Sequenced Packet Exchange (SPX), and examples of connectionless protocols include User Datagram Protocol (UDP), "raw IP", and Internetwork Packet Exchange (IPX).
  
  ## Clients and servers
  
  For connection-oriented communications, communication parties usually have different roles. One party is usually waiting for incoming connections; this party is usually referred to as "server". Another party is the one which initiates connection; this party is usually referred to as "client".
  
  For connectionless communications, one party ("server") is usually waiting for an incoming packet, and another party ("client") is usually understood as the one which sends an unsolicited packet to "server".
  
  ## Popular protocols and APIs
  
  Network programming traditionally covers different layers of OSI/ISO model (most of application-level programming belongs to L4 and up). The table below contains some examples of popular protocols belonging to different OSI/ISO layers, and popular APIs for them.
  
  | OSI/ISO Layer | Protocol | API |
  | - | - | - |
  | L3 (network) | IP | Raw socket |
  | L4 (transport) | TCP, UDP, SCTP | Berkeley Sockets |
  | L5 (session) | TLS | OpenSSL |
  | L7 (application) | HTTP | Various |
  