Socket

Introduction

The endpoint in an interprocess communication is called a socket, or a network socket for disambiguation. Since most communication between computers is based on the Internet Protocol, an almost equivalent term is Internet socket. The data transmission between two sockets is organised by communication protocols.usually implemented in the operating system of the participating computers. Application programs write to and read from these sockets. Therefore, network programming is essentially socket programming.

Programming with TCP/IP sockets

There are a few steps involved in using sockets:

1. Create the socket
2. Identify the socket
3. On the server, wait for an incoming connection
4. On the client, connect to the server’s socket
5. Send and receive messages
6. Close the socket

Step 1. Create a socket

A socket, s, is created with the socket system call:

All the parameters as well as the return value are integers:

domain, or address family —

communication domain in which the socket should be created. Some of address families are AF_INET (IP), AF_INET6 (IPv6), AF_UNIX (local channel, similar to pipes), AF_ISO (ISO protocols), and AF_NS (Xerox Network Systems protocols).

type —

type of service. This is selected according to the properties required by the application: SOCK_STREAM (virtual circuit service), SOCK_DGRAM (datagram service), SOCK_RAW (direct IP service). Check with your address family to see whether a particular service is available.

protocol —

indicate a specific protocol to use in supporting the sockets operation. This is useful in cases where some families may have more than one protocol to support a given type of service. The return value is a file descriptor (a small integer). The analogy of creating a socket is that of requesting a telephone line from the phone company.

For TCP/IP sockets, we want to specify the IP address family (AF_INET) and virtual circuit service (SOCK_STREAM). Since there’s only one form of virtual circuit service, there are no variations of the protocol, so the last argument, protocol, is zero.

Step 2. Indentify (name) a socket

When we talk about naming a socket, we are talking about assigning a transport address to the socket (a port number in IP networking). In sockets, this operation is called binding an address and the bind system call is used for this. The analogy is that of assigning a phone number to the line that you requested from the phone company in step 1 or that of assigning an address to a mailbox.

The transport address is defined in a socket address structure. Because sockets were designed to work with various different types of communication interfaces, the interface is very general. Instead of accepting, say, a port number as a parameter, it takes a sockaddr structure whose actual format is determined on the address family (type of network) you’re using. For example, if you’re using UNIX domain sockets, bind actually creates a file in the file system.

The system call for bind is:

#include <sys/socket.h>

int bind(int socket, const struct sockaddr *address, socklen_t address_len);

Step 3a. Connect to a server from a client

If we’re a client process, we need to establish a connection to the server. Now that we have a socket that knows where it’s coming from, we need to tell it where it’s going to. The connect system call accomplishes this.