Part 1:Socket Programming in PERL
In this article, Rahul shows us how to create a client-server socket program in Perl and then demonstrates it by pinging the server.
What
is a socket? Just another bit of computer jargon? Devling a little into
networking history, it is a Berkeley UNIX mechanism of creating a
virtual duplex connection between processes. This was later ported on
to every known OS enabling communication between systems across
geographical location running on different OS software. If not for the
socket, most of the network communication between systems would never
ever have happened.
Taking a closer look; a typical computer
system on a network receives and sends information as desired by the
various applications running on it. This information is routed to the
system, since a unique IP address is designated to it. On the system,
this information is given to the relevant applications which listen on
different ports. For example a net browser listens on port 80 for
information. Also we can write applications which listen and send
information on a specific port number.
For now, let's sum up that a socket is an IP address and a port, enabling connection.
Part 2:Types of Sockets
There are just two types of sockets: connection
oriented and connection-less. There are other types but this
classification is fair enough to get started, a socket has a domain (UNIX or internet), a connection type (connection oriented or connection less) and a protocol (TCP or UDP). Client Server Architecture
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A connection oriented or a stream socket is a reliable two way communication.
If you send three packets, say 1, 2 and 3, they are received in the
same order they were sent. They achieve this high level of transmission
quality by using TCP for error free reliable communication. The
ubiquitous telnet application uses stream sockets.
The
connection-less sockets or stream-less sockets use IP for routing but
use the UDP. They are connectionless, since the connection need not be
open as in stream sockets, the packet formed is given a destination IP
address and than transmitted. This method is mostly used for
packet-to-packet transfer as in ftp applications.
How a Packet is Formed
We will use an example of UDP packet for some light stuff on networking
theory. It's Data Encapsulation. A packet is formed by encapsulating
the data in a header at each level as it passes through the layers
(protocol stack). At the receiving end the headers are stripped off as
the packet travels up the layers to get the data.
Basically, at each layer, the protocol adds a header to the payload to perform the required functionality.
It's a client-server world, today. Just about everything on the network deals with client processes talking to server
processes and vice versa. Take the ubiquitous telnet, for instance.
When you connect to a remote host on port 23 with telnet (the client),
a program on that host (called telnetd, the server) springs to life. It
handles the incoming telnet connection, sets up a login prompt, etc.
Note that the client-server pair can speak in streaming or stream-less,
or anything else (as long as they are speaking the same thing).
Some good examples of client-server pairs are telnet/telnetd, ftp/ftpd.
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