Sun Java System Web Server 7.0 NSAPI Developer's Guide

Appendix A Hypertext Transfer Protocol

The Hypertext Transfer Protocol (HTTP) is a protocol (a set of rules that describes how information is exchanged) that allows a client (such as a web browser) and a web server to communicate with each other.

HTTP is based on a request-response model. The browser opens a connection to the server and sends a request to the server. The server processes the request and generates a response, which it sends to the browser. The server then closes the connection.

This chapter provides a short introduction to a few HTTP basics. For more information on HTTP, see the IETF home page at:

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

This chapter has the following sections:

Compliance

Sun Java System Web Server supports HTTP/1.1. 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 2616) at: http://www.ietf.org/rfc/rfc2616.txt

Requests

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

Request Method, URI, and Protocol Version

A browser can request information using a number of methods. The commonly used methods are:

GET -- Requests the specified resource, such as a document or image
HEAD -- Requests only the header information for the document
POST -- Requests that the server accept some data from the browser, such as form input for a CGI program
PUT -- Replaces the contents of a server’s document with data from the browser

Request Headers

The browser can send headers to the server. Most of these request headers are optional.

The following table lists some of the commonly used request headers.

Table A–1 Common Request Headers


Request Header	Description
`Accept`	File types the browser can accept.
`Authorization`	Used if the browser wants to authenticate itself with a server; information such as the user name and password are included.
`User-Agent`	Name and version of the browser software.
`Referer`	URL of the document.
`Host`	Internet host and port number of the resource being requested.

Request Data

If the browser has made a POST or PUT request, it sends data after the blank line following the request headers. If the browser sends a GET or HEAD request, there is no data to send.

Responses

The server’s response includes the following:

HTTP Protocol Version, Status Code, and Reason Phrase

The server sends back a status code, which is a three-digit numeric code. The five categories of status codes are:

100-199 a provisional response.
200-299 a successful transaction.
300-399 the requested resource should be retrieved from a different location.
400-499 an error was caused by the browser.
500-599 a serious error occurred in the server.

The following table lists some common status codes.

Table A–2 Common HTTP Status Codes


Status Code	Meaning
`200`	OK; request has succeeded for the method used (`GET`, `POST`, `HEAD`).
`201`	The request has resulted in the creation of a new resource reference by the returned URI.
`206`	The server has sent a response to byte range requests.
`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.
`400`	Sent if the request is not a valid HTTP/1.0 or HTTP/1.1 request. For example HTTP/1.1 requires a host to be specified either in the `Host` header or as part of the URI on the request line.
`401`	Unauthorized. The user requested a document but did not provide a valid user name or password.
`403`	Forbidden. Access to this URL is forbidden.
`404`	Not found. The document requested is not on the server. This code can also be sent if the server is configured to protect the document for unauthorized personnel.
`408`	If the client starts a request but does not complete it within the keep-alive timeout configured in the server, then this response will be sent and the connection closed. The request can be repeated with another open connection.
`411`	The client submitted a `POST` request with chunked encoding, which is of variable length. However, the resource or application on the server requires a fixed length - a `Content-Length` header to be present. This code tells the client to resubmit its request with content-length.
`413`	Some applications (e.g., certain NSAPI plug-ins) cannot handle very large amounts of data, so returns this error code.
`414`	The URI is longer than the maximum the web server is willing to serve.
`416`	Data was requested outside the range of a file.
`500`	Server error. A server-related error occurred. The server administrator must check the error log in the server.
`503`	Sent if the quality of service mechanism was enabled and bandwidth or connection limits were attained. The server then serves requests with that code.

Response Headers

The response headers contain information about the server and the response data.

The following table lists some common response headers.

Table A–3 Common Response Headers


Response Header	Description
`Server`	Name and version of the web server.
`Date`	Current date (in Greenwich Mean Time).
`Last-Modified`	Date when the document was last modified.
`Expires`	Date when the document expires.
`content-length`	Length of the data that follows (in bytes).
`content-type`	MIME type of the data that follows.
`WWW-Authenticate`	Used during authentication and includes information that tells the browser software what is necessary for authentication (such as user name and password).

Response Data

The server sends a blank line after the last header. It then sends the response data such as an image or an HTML page.