It is important to note that the server sends requested les to clients without storing any state in-
formation about the client. If a particular client asks for the same object twice in a period of a few
seconds, the server does not respond by saying that it just served the object to the client; instead,
the server resends the object, as it has completely forgotten what it did earlier. Because an HTTP
server maintains no information about the clients, HTTP is said to be a stateless protocol. The
latter feature simpli es server design and has permitted engineers to develop high-performance
Web servers that can handle thousands of simultaneous TCP connections. However, it is often
desirable for a Web site to identify users, either because the server wishes to restrict user access or
because it wants to serve content as a function of the user identity. For these purposes, HTTP uses
cookies. Cookies allow sites to keep track of users. Most major commercial Web sites use cookies
today. As shown in Figure 1, cookie technology has four components: (1) a cookie header line in
the HTTP response message; (2) a cookie header line in the HTTP request message; (3) a cookie
le kept on the user s end system and managed by the user s browser; and (4) a back-end database
at the Web site. Using Figure 1, let s walk through an example of how cookies work. Suppose
Susan, who always accesses the Web using Internet Explorer from her home PC, contacts Ama-
zon.com for the rst time. Let us suppose that in the past she has already visited the eBay site.
When the request comes into the Amazon Web server, the server creates a unique identi cation
number and creates an entry in its back-end database that is indexed by the identi cation number.
The Amazon Web server then responds to Susan s browser, including in the HTTP response a
Set-cookie: header, which contains the identi cation number.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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