Chapter 17. HTTP

Team-Fly    

 
Webmaster in a Nutshell, 3rd Edition
By Robert Eckstein, Stephen Spainhour
Table of Contents
Part VII:  HTTP


The Hypertext Transfer Protocol (HTTP) is the language web clients and servers use to communicate with each other. It is essentially the backbone of the World Wide Web. While HTTP is largely the realm of server and client programming, a firm understanding of HTTP is also important for CGI programming. In addition, sometimes HTTP filters back to the usersfor example, when server error codes are reported in a browser window.

This chapter covers all the basics of HTTP. For absolutely complete coverage of HTTP and all its surrounding technologies, see HTTP: The Definitive Guide by David Gourley and Brian Totty, with Marjorie Sayer, Sailu Reddy, and Anshu Aggarwal (O'Reilly).

All HTTP transactions follow the same general format. Each client request and server response has three parts : the request or response line, a header section, and the entity body. The client initiates a transaction as follows :

  1. The client contacts the server at a designated port number (by default, 80). It sends a document request by specifying an HTTP command called a method , followed by a document address, and an HTTP version number. For example:

     GET /index.html HTTP/1.1 

    This makes use of the GET method to request the document index.html using Version 1.1 of HTTP. HTTP methods are discussed in more detail later in this chapter.

  2. Next , the client sends optional header information to inform the server of its configuration and the document formats it will accept. All header information is given line by line, each with a header name and value. For example, this header information sent by the client indicates its name and version number and specifies several document preferences:

     User-Agent: Mozilla/4.05(WinNT; I) Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */* 

    The client sends a blank line to end the header.

  3. After sending the request and headers, the client may send additional data. This data is mostly used by CGI programs that use the POST method. It may also be used by clients like Netscape Navigator Professional Edition to publish an edited page back onto the web server.

The server responds in the following way to the client's request:

  1. The server replies with a status line containing three fields: HTTP version, status code, and description. The HTTP version indicates the version of HTTP the server is using to respond.The status code is a three-digit number that indicates the server's result of the client's request. The description following the status code is simply human-readable text that describes the status code. For example:

     HTTP/1.1 200 OK 

    This status line indicates that the server uses Version 1.1 of HTTP in its response. A status code of 200 means that the client's request was successful, and the requested data will be supplied after the headers.

  2. After the status line, the server sends header information to the client about itself and the requested document. For example:

     Date: Fri, 20 Sep 1998 08:17:58 GMT Server: NCSA/1.5.2 Last-modified: Mon, 17 Jun 1998 21:53:08 GMT Content-type: text/html Content-length: 2482 

    A blank line ends the header.

  3. If the client's request is successful, the requested data is sent. This data may be a copy of a file or the response from a CGI program. If the client's request could not be fulfilled, the additional data may be a human-readable explanation of why the server could not fulfill the request.

In HTTP 1.0, after the server has finished sending the requested data, it disconnects from the client, and the transaction is over unless a Connection: Keep Alive header is sent. Beginning with HTTP 1.1, however, the default is for the server to maintain the connection and allow the client to make additional requests . Since many documents embed other documents (inline images, frames , applets, etc.), this saves the overhead of the client having to repeatedly connect to the same server just to draw a single page. Under HTTP 1.1, therefore, the transaction might cycle back to the beginning, until either the client or server explicitly closes the connection.

Being a stateless protocol, HTTP does not maintain any information from one transaction to the next, so the next transaction needs to start all over again. The advantage is that an HTTP server can serve a lot more clients in a given period of time, since there's no additional overhead for tracking sessions from one connection to the next. The disadvantage is that more elaborate CGI programs need to use hidden input fields (as described in Chapter 6), or external tools such as cookies (described later in this chapter) to maintain information from one transaction to the next.


Team-Fly    
Top


Webmaster in a Nutshell
Webmaster in a Nutshell, Third Edition
ISBN: 0596003579
EAN: 2147483647
Year: 2002
Pages: 412

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net