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.
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