Cryptography Reference
In-Depth Information
Amongthe simplest forms of servers are the file servers, whereas amongthe
more advanced servers are the database servers. As mentioned earlier, use of a
file server to transfer data over a network slows the process considerably. In the
client-server model, the client sends a request to a server, which processes the
request on its own power to find the requested data rather than transferringall
the information back to the client to find its own data.
Client-Server and HTTP
: On page 219, we were introduced to HTTP
via protocol layers studied in Section 5.7. Here is how HTTP fits into the client-
server model. When Alice opens her WWW
browser
(a client software program
used for locatingand viewingdifferent types of Internet resources such as data on
a WWW site), she indirectly makes use of HTTP. Each WWW server contains
an HTTP
daemon
8.19
(pronounced
dee-muhn
), denoted by
HTTPD
, which is
a continuously runningproram (by itself under the operatingsystem), whose
sole purpose is to (wait for and) handle requests that a given computer system
receives periodically. Alice's browser is an HTTP client that makes requests to
a server by, say, openinga WWW file via the typingin of a
Uniform Resource
Locator
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(URL). By so doing, her browser formulates an HTTP request and
sends it to the IP address indicated by the URL. The HTTP daemon at the
server site receives the request and sends back a response in the form of requested
files. Unfortunately, HTTP is what is known as a
stateless protocol
, which means
that each time Alice visits a WWW site (or even when she just clicks to another
location from that site), the server sees this as her first visit. In other words, the
server forgets all that has transpired after each request unless there is a means
to somehow “stamp” Alice so that the server will remember the details of her
last visit. The followingis a mechanism for accomplishingthis task.
Cookies
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What is a cookie and how does it fit into the client-server model? In simplest
terms, a cookie is data (for future use) that is stored by a server on the client side
of a client-server model. For instance, a cookie might record Alice's preferences
when visiting, say, QQQ.com. The cookie is a means by which the server can
store its own data about Alice on Alice's own computer.
Analogy
: An analogy is a voucher Alice gets when she brings her shoes to a
cobbler, Corbett, say. If she returns for her shoes without that voucher, Corbett
will not be able to locate her shoes. To him, she could be a new customer. Alice's
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The etymology is from the Greek meaning an attendant supernatural being, on a hierarchy
between gods and humans.
8.20
A
URL
is the acronym for
Uniform Resource Locator
, which is the global address associ-
ated with given data. The first part of the URL specifies which protocol to use, and the second
part indicates the domain name. For example,
http://www.math.ucalgary.ca/˜ramollin/
in-
dicates that this is a WWW page and the HTTP protocol should be used. The second part
is the domain name where my homepage is located.
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The origin of the term “cookie” is uncertain, although its inventor, Netscape, claims it
was a name chosen at random. Some claim that it was derived from a similar Unix operating
system transaction called a “token”. On MAC computers, the cookies are kept in a list called
“magic cookie”, whereas on IBM CPUs, they are in a file called “cookies.txt”.
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