Chapter 27
WWW and HTTP
27.1
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27-1 ARCHITECTURE
The WWW today is a distributed client/server service,
in which a client using a browser can access a service
using a server. However, the service provided is
distributed over many locations called sites.
Topics discussed in this section:
Client (Browser)
Server
Uniform Resource Locator
Cookies
27.2
Figure 27.1 Architecture of WWW
27.3
Figure 27.2 Browser
27.4
Figure 27.3 URL
http://
https://
ftp://
27.5
80 by default
Cookies


27.6
The WWW was originally designed as a stateless entity.
Cookies are needed for extending functionalities of the
Web, such as:
 To remember past client in order to show a
customized webpage.
Cookies: Creation and storage



27.7
When a server receives a request from a client, it stores
information about the client in a file or a string.
The server includes the cookie in the response that it
sends to the client.
When the client receives the response, the browser
stores the cookie in the cookie directory.
27-2 WEB DOCUMENTS
The documents in the WWW can be grouped into three
broad categories: static, dynamic, and active. The
category is based on the time at which the contents of
the document are determined.
Topics discussed in this section:
Static Documents
Dynamic Documents
Active Documents
27.8
Figure 27.4 Static document

27.9
Uses the Hypertext Markup Language
(HTML)
Figure 27.5 Boldface tags
27.10
Figure 27.7 Beginning and ending tags
Example:


27.11
<a href="Chapter3-part3.ppt">Chapter3-part3</a>
<img src="../images/smallUCF.gif" width="200"
border="0" height="76">
Figure 27.8 Dynamic document using CGI
27.12
Figure 27.9 Dynamic document using server-site script
27.13
Note
Dynamic documents are sometimes
referred to as server-site dynamic
documents.
27.14
Figure 27.10 Active document using Java applet
27.15
Figure 27.11 Active document using client-site script
27.16
Note
Active documents are sometimes
referred to as client-site dynamic
documents.
27.17
27-3 HTTP
The Hypertext Transfer Protocol (HTTP) is a protocol
used mainly to access data on the World Wide Web.
Topics discussed in this section:
HTTP Transaction
Persistent Versus Nonpersistent Connection
27.18
Figure 27.12 HTTP transaction
HTTP uses the services of TCP on wellknown port 80.
27.19
Figure 27.13 Request and response messages (all in Plain Text)
27.20
Table 27.1 Methods
27.21
Table 27.2 Status codes
27.22
Table 27.2 Status codes (continued)
27.23
Figure 27.15 Header format
27.24
Table 27.3 General headers
27.25
Table 27.4 Request headers
27.26
Table 27.5 Response headers
27.27
Table 27.6 Entity headers
27.28
Figure 27.16 Example 27.1
27.29
Figure 27.17 Example 27.2 (client sends data to server)
27.30
Example 27.3 (continued)
27.31
Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
telnet www.cs.ucf.edu 80 Opens TCP connection to port 80
(default HTTP server port) at cs.ucf.edu.
Anything typed in sent
to port 80 at www.cs.ucf.edu
2. Type in a GET HTTP request:
GET /~czou/CNT3004/example.html HTTP/1.1
Host: www.cs.ucf.edu
By typing this in (hit carriage
return twice), you send
this minimal (but complete)
GET request to HTTP server
3. Look at response message sent by HTTP
server!
27.32
Web Browser’s Operation

First, get the basis static HTML file


/~czou/CNT3004/example.html
Second, interpret HTML to find all
contained “objects”

Images, java applets, flash,….



27.33
<img src="../images/smallUCF.gif“ >
<img
src=“http://upload.wikimedia.org/wikipedia/commons/6/63/
Wikipedia-logo.png” >
Third, get those objects via HTTP
Let’s look at HTTP in action

Telnet example


“GET” must be Capital letters!
Must have “host” header!

For web proxy reason


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27.34
A proxy can know where to forward the GET request
What if type in “HTTP/1.0” ?
Wireshark example
Note
HTTP version 1.1 specifies a persistent
connection by default.
27.35
Persistent vs Nonpersistent
Connection



27.36
In a nonpersistent connection, one
TCP connection is made for each
request/response.
In a persistent connection, the server
leaves the connection open for more
requests after sending a response.
The server can close the connection at the
request of a client or if a time-out has
been reached.
Wireshark example