Socket Communication

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This chapter explains:

• What is a socket.
• How to establish a socket communication.
• A simple example of socket server application.
• A simple example of socket client application.
• Built-in network utility functions.
• Socket module.

What Is a Socket

Socket: An concept represents one end-point of a two-way communication link between two programs running on the Internet network.

Internet Address: A unique number to identify each computer system on the Internet.

Port: A number representing an entry point of on a computer system on the Internet, where a two-way communication link can be established.

Local System: The computer system at this end of a two-way communication link.

Remote System: The computer system at the other end of a two-way communication link.

A two-way communication link on the Internet involves:

• Two computer systems, each has its own Internet address.
• A port number on each computer system that is used only by this communication link.
• An application program on each computer system that uses this communication link.
• A socket handle created in the application program that represents this communication link.
• The application program can use the socket as a logical input or output device to receive or send data from or to the application program running at the other end of the communication link.
• The application program can also query the socket for information about the communication link, like: The Internet address and port number at this end of the communication link; The Internet address and port number at the other end of the communication link.

The following diagram illustrates how an application program can use a socket to talk with other application program running on another computer system through a two-way communication link on the Internet:

Computer System I                            Computer System II

       i/o       |Address a|    Internet    |Address b|       i/o
App. A<--->Socket|Port 1   |<-------------->|   Port 2|Socket<--->App. B

Establishing a Socket Communication

In the previous section, we only discussed about how socket works with a communication link that has been established already. Now, let's see how two application programs can establish a communication link between them.

To establish a communication link, one application program must act as a server, create a server socket with a given port number, and set the server socket in the listen mode waiting for a connection request from other program.

With one program running as a server listening for a connection request at a specific port number, the other program can now create socket with a given local port number, the Internet address of the computer system where the first program is running, and the port number where the server socket is listening. At this time, a connect request will be send over to the server socket. The server socket should then accept the connect request and instantiate a socket object to complete communication link.

Perl offers several built-in functions to support socket communication:

socket() - Creates a socket handle for the specified communication domain, type and protocol. For example:

   $domain = 2; # Internet domain
   $type = 1; # Sequenced, reliable, two-way connection, byte streams
   $proto = 6; # Transmission Control Protocol (TCP)
   socket(SOCK,$domain,$type,$proto);

bind() - Binds a socket handle to a local address that represents a port on the local system. For example:

   $domain = 2; # Internet domain
   $host = pack('C4', 127,0,0,1); # localhost = 127.0.0.1
   $port = 8888; 
   $address = pack('S n a4 x8', $domain, $port, $host);
   bind(SOCK, $address);

Note that host IP address must be packed into a 4-byte number, then put it into the address structure with the port number using the following format codes:

S An unsigned short value. 16 bits
n An unsigned short in "network" (big-endian) order. 16 bits
a A string with arbitrary binary data, will be null padded.
x A null byte.

(Continued on next part...)

Part:   1   2  3  4