Electronic Mail and Internet Security - Codes: The Guide to Secrecy from Ancient to Modern Times - page 321

8.5 Client-Server Model and Cookies

Any general statement is like a cheque drawn on a bank. Its value depends
on what there is to meet it.

Ezra Pound (1885-1972), American poet
— from The ABC of Reading (1934)

We have informally discussed client-server model applications throughout
the text such as Kerberos in Section 5.2 and SSL in Section 5.7. Now we look
at the general nature of such models, largely from the perspective of “cookies”,
which we will define and study in detail below. The so-called client-server model
is one of the central features of Internetworking. It is time to settle on a general
definition of these terms.

Client-Server Model
A client , when considered as part of software, is a computer program (al-
though we may use Alice and/or Bob in these roles), which relies on a server
to perform some operations. (In the client-server model, the term “program”
may be replaced by the term “computer” on which the program runs, sometimes
called a “host computer”, but this computer is typically employed for more tasks
than just the client-server architecture). Think of a client as a “requester of ser-
vices”. A server in this context, is a computer program that provides access
(for the client) to WWW formats and protocols (or to where HTML documents
are stored). Think of a server as a “provider of services”. The client-server
model is a relationship between two programs in which one program, the client,
makes a request of the other program, the server, which fulfills the request.

Client-Server Origin and Role : The client-server model was introduced
in the 1980s as message-based modular software, intended for use over a network.
The motivation was to improve functionality, versatility, interoperability, and
scalability over a single mainframe computer with time sharing. It is possible to
configure the client-server architecture so that it operates on a single computer;
in other words, the same machine serves the role of both client and server.
However, the intention for, and full value of, the client-server model is realized
over a network with physically separated client and server machines. This is
because the client-server model was introduced largely to address the limitations
of file-sharingarchitecture where the server downloaded files from the shared
location to the desktop environment. This type of architecture was strained by
a large number of online users, and large volumes of data. The client-server
architecture, in contrast, was a means by which the file server was replaced by
the database server. By employinga database management system (DBMS),
user enquiries could be answered directly, thereby reducingnetwork traJc via
an enquiry and response rather than total file transfer. The term Intranet means
the employment of Internet technology for a given organization to implement
client-server applications. To do this, a corporation for example, would merely
have to change its code on an HTTP server, as opposed to updating, the client
code on numerous desktop computers in its organization.