Appendix B   HyperText Transfer Protocol


This appendix provides a short introduction to a few HyperText Transfer Protocol (HTTP) basics. For more information on HTTP, see the Internet Engineering Task Force (IETF) home page at:

http://www.ietf.org/home.html

This appendix contains the following sections:

• About HyperText Transfer Protocol (HTTP)

• Requests

• Responses

About HyperText Transfer Protocol (HTTP)

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The HyperText Transfer Protocol (HTTP) is a protocol (a set of rules that describe how information is exchanged on a network) that allows a web browser and a web server to "talk" to each other using the ISO Latin1 alphabet, which is ASCII with extensions for European languages.

HTTP is based on a request/response model. The client connects to the server and sends a request to the server. The request contains the following: request method, URI, and protocol version. The client then sends some header information. The server's response includes the return of the protocol version, status code, followed by a header that contains server information, and then the requested data. The connection is then closed.

The iPlanet Web Server 4.x supports HTTP 1.1. Previous versions of the server supported HTTP 1.0. The server is conditionally compliant with the HTTP 1.1 proposed standard, as approved by the Internet Engineering Steering Group (IESG) and the Internet Engineering Task Force (IETF) HTTP working group. For more information on the criteria for being conditionally compliant, see the Hypertext Transfer Protocol—HTTP/1.1 specification (RFC 2068) at:

http://www.ietf.org/html.charters/http-charter.html

Requests

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A request from a client to a server includes the following information:

• Request method

• Request header

• Request data

Request Method

A client can request information using a number of methods. The commonly used methods include the following:

• GET—Requests the specified document

• HEAD—Requests only the header information for the document

• POST—Requests that the server accept some data from the client, such as form input for a CGI program

• PUT—Replaces the contents of a server's document with data from the client

Request Header

The client can send header fields to the server. Most are optional. Some commonly used request headers are shown in Table B-1.

Table B-1    Common request headers 

Request header	Description
Accept	The file types the client can accept.
Authorization	Used if the client wants to authenticate itself with a server; information such as the username and password are included.
User-agent	The name and version of the client software.
Referer	The URL of the document where the user clicked on the link.
Host	The Internet host and port number of the resource being requested.

Request Data

If the client has made a POST or PUT request, it can send data after the request header and a blank line. If the client sends a GET or HEAD request, there is no data to send; the client waits for the server's response.

Responses

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The server's response includes the following:

• Status code

• Response header

• Response data

Status Code

When a client makes a request, one item the server sends back is a status code, which is a three-digit numeric code. There are four categories of status codes:

• Status codes in the 100-199 range indicate a provisional response.

• Status codes in the 200-299 range indicate a successful transaction.

• Status codes in the 300-399 range are returned when the URL can't be retrieved because the requested document has moved.

• Status codes in the 400-499 range indicate the client has an error.

• Status codes of 500 and higher indicate that the server can't perform the request, or an error has occurred.

Table B-2 contains some common status codes.

Table B-2    Common HTTP status codes

Status code	Meaning
200	OK; successful transmission. This is not an error.
302	Found. Redirection to a new URL. The original URL has moved. This is not an error; most browsers will get the new page.
304	Use a local copy. If a browser already has a page in its cache, and the page is requested again, some browsers (such as Netscape Navigator) relay to the web server the "last-modified" timestamp on the browser's cached copy. If the copy on the server is not newer than the browser's copy, the server returns a 304 code instead of returning the page, reducing unnecessary network traffic. This is not an error.
401	Unauthorized. The user requested a document but didn't provide a valid username or password.
403	Forbidden. Access to this URL is forbidden.
404	Not found. The document requested isn't on the server. This code can also be sent if the server has been told to protect the document by telling unauthorized people that it doesn't exist.
500	Server error. A server-related error occurred. The server administrator should check the server's error log to see what happened.

Response Header

The response header contains information about the server and information about the document that will follow. Common response headers are shown in Table B-3.

Table B-3    Common response headers

Response header	Description
Server	The name and version of the web server.
Date	The current date (in Greenwich Mean Time).
Last-modified	The date when the document was last modified.
Expires	The date when the document expires.
Content-length	The length of the data that follows (in bytes).
Content-type	The MIME type of the following data.
WWW-authenticate	Used during authentication and includes information that tells the client software what is necessary for authentication (such as username and password).

Response Data

The server sends a blank line after the last header field. The server then sends the document data.