1.3. PORTS, SOCKETS, AND SERVICES

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1.3

Ports, Sockets, and Services

The computers on the internet interact in a wide variety of ways, but their

interaction is nonetheless restricted. It would not be wise to allow any computer

on the internet to have full access to every other computer on the net because

an unscrupulous user might decide to delete all of your disk files or to otherwise

use your computer without permission.

To get around this problem, the internet is modelled on an abstract view of

the net in which each computer specifies exactly what kinds of interactions it

will allow. These types of interactions are called services and each computer

on the net can offer up to 65536 services.

These services are specified by a number from 0 to 65535 called a port.

Typically, the ports with numbers under 1024 are reserved for system services

(such as email and web page serving), but anyone is free to offer any service

they please on ports numbered greater than 1024.

A computer that offers a service to another computer is called a server

and a computer that requests a service is called a client. It is typical for

computers on the internet to be both clients and servers and the same time. The

communication between client and server is initiated by the client by specifying

the IP address of the server computer and the port number of the service to

be provided. If the specified computer is offering that service, then a special

connection called a socket is created. The socket allows the two computers to

send data back and forth between themselves.

1.3.1

Common Services on the net

Some of the more common system services are listed in Figure 1.1. Each service

has a set of rules governing how the client and server interact. These rules

are called protocols and they simply represent the conventions that the two

computers will use when communicating on that port.

You can access some of these ports from Linux using the telnet command.

For example, Figures 1.2 and 1.3 give examples of accessing the date and echo

services respectively: The date service returns the local time on the server being

queried. The echo service is used for testing whether a connection is active and

just echo back each line of text that it receives.

1.4

Web Browsers and Servers

The HTTP service is perhaps the most revolutionary service that has been de 

veloped for use on the internet. It provides a mechanism for clients to access

files on the server by giving the name of the file in the webserver folder. The

HTTP server then responds to such a request by returning several lines of in 

formation about the file (e.g. what kind of data it contains, text, image, movie,

sound, etc.) when it was last modified, how large the file is, etc. HTTP services

are generally provided on port 80.