Socket characteristics

Sockets share some common characteristics.

A socket is represented by an integer. That integer is called a socket descriptor.
A socket exists as long as the process maintains an open link to the socket.
You can name a socket and use it to communicate with other sockets in a communication domain.
Sockets perform the communication when the server accepts connections from them, or when it exchanges messages with them.
You can create sockets in pairs (only for sockets in the AF_UNIX address family).

The connection that a socket provides can be connection-oriented or connectionless. Connection-oriented communication implies that a connection is established, and a dialog between the programs follows. The program that provides the service (the server program) establishes the available socket that is enabled to accept incoming connection requests. Optionally, the server can assign a name to the service that it supplies, which allows clients to identify where to obtain and how to connect to that service. The client of the service (the client program) must request the service of the server program. The client does this by connecting to the distinct name or to the attributes associated with the distinct name that the server program has designated. It is similar to dialing a telephone number (an identifier) and making a connection with another party that is offering a service (for example, a plumber). When the receiver of the call (the server, in this example, a plumber) answers the telephone, the connection is established. The plumber verifies that you have reached the correct party, and the connection remains active as long as both parties require it.

Connectionless communication implies that no connection is established, over which a dialog or data transfer can take place. Instead, the server program designates a name that identifies where to reach it (much like a post-office box). If you send a letter to a post office box, you cannot be absolutely sure that the receiver got the letter. You might need to wait for a response to your letter. There is no active, real-time connection, in which data is exchanged.

How socket characteristics are determined

When an application creates a socket with the socket() API, it must identify the socket by specifying these parameters:

The socket address family determines the format of the address structure for the socket. This topic contains examples of each address family's address structure.
The socket type determines the form of communication for the socket.
The socket protocol determines the supported protocols that the socket uses.

These parameters or characteristics define the socket application and how it interoperates with other socket applications. Depending on the address family of a socket, you have different choices for the socket type and protocol. The following table shows the corresponding address family and its associated socket type and protocols:

Table 1. Summary of socket characteristics
Address family	Socket type	Socket protocol
AF_UNIX	SOCK_STREAM	N/A
	SOCK_DGRAM	N/A
AF_INET	SOCK_STREAM	TCP
	SOCK_DGRAM	UDP
	SOCK_RAW	IP, ICMP
AF_INET6	SOCK_STREAM	TCP
	SOCK_DGRAM	UDP
	SOCK_RAW	IP6, ICMP6
AF_UNIX_CCSID	SOCK_STREAM	N/A
	SOCK_DGRAM	N/A

In addition to these socket characteristics or parameters, constant values are defined in network routines and header files that are shipped with the QSYSINC library. For descriptions of header files, see the individual APIs. Each API lists its appropriate header file in the usage section of the API description.

Socket network routines allow socket applications to obtain information from the Domain Name System (DNS), host, protocol, service, and network files.