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HTTP Operational Model and Client/Server Communication
(Page 1 of 3)
The Hypertext Transfer Protocol is
the application-layer protocol that implements the World Wide Web. While
the Web itself has many different facets, HTTP is only concerned with
one basic function: the transfer of hypertext documents and other files
from Web servers to Web clients. In terms of actual communication, clients
are chiefly concerned with making requests to servers, which respond
to those requests.
Thus, even though HTTP includes a
lot of functionality to meet the needs of clients and servers, when
you boil it down, what see is a very simple, client/server, request/response
protocol. In this respect, HTTP more closely resembles a rudimentary
protocol like BOOTP
than it does other application-layer protocols like FTP
which all involve multiple communication steps and command/reply sequences.
Basic HTTP Client/Server Communication
In its simplest form, the operation
of HTTP involves only an HTTP client, usually a Web browser on
a client machine, and an HTTP server, more commonly known as a Web
server. After a TCP connection is created, the two steps in communication
are as follows:
- Client Request: The HTTP client
sends a request message formatted according to the rules of the HTTP
standardan HTTP Request. This message specifies the resource
that the client wishes to retrieve, or includes information to be provided
to the server.
- Server Response: The server
reads and interprets the request. It takes action relevant to the request
and creates an HTTP Response message, which it sends back to
the client. The response message indicates whether the request was successful,
and may also contain the content of the resource that the client requested,
Figure 315: HTTP Client/Server Communication
In its simplest form, HTTP communication consists of an HTTP Request message sent by a client to a server, which replies with an HTTP Response.
In HTTP/1.0, each TCP connection
involves only one such exchange, as shown in Figure 315;
in HTTP/1.1, multiple exchanges are possible, as we'll see in the
next topic. Note also that the server
may in some cases respond with one or preliminary responses prior to
sending the full response. This may occur if the server sends a preliminary
response using the 100 Continue status code prior to the
real reply. See
the topic on HTTP status codes for more information.
Key Concept: HTTP is a client/server-oriented, request/reply protocol. Basic communication consists of an HTTP Request message sent by an HTTP client to an HTTP server, which returns an HTTP Response message back to the client.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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