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iPlanet Web Server, Enterprise Edition NSAPI Programmer's Guide



Appendix G   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 appendix 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 appendix has the following sections:



Compliance

Netscape Enterprise Server 3.x and iPlanet Web Server 4.x support 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



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 include the following:

  • 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 are optional. Some commonly used request headers are shown in Table G-1.


Table G-1    Common request headers

Request header

Description

Accept  

The file types the browser can accept.  

Authorization  

Used if the browser wants to authenticate itself with a server; information such as the username and password are included.  

User-agent  

The name and version of the browser 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 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.


    Table G-2    Common HTTP status codes

    Status code

    Meaning

    200  

    OK; successful transaction.  

    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 Headers

The response headers contain information about the server and the response data. Common response headers are shown in Table G-3.

.

Table G-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 browser software what is necessary for authentication (such as username 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.



Buffered Streams



New in iPlanet Web Server 4.1.

Buffered streams improve the efficiency of network I/O (for example the exchange of HTTP requests and responses) especially for dynamic content generation. Buffered streams are implemented in iPlanet Web Server 4.1 as transparent NSPR I/O layers, which means even existing NSAPI modules can use them without any change.

The buffered streams layer adds following features to the iPlanet Web Server:

  • Enhanced keep-alive support: When the response is smaller than the buffer size, the buffering layer generates the content-length header so that client can detect the end of the response and re-use the connection for subsequent requests.

  • Response length determination: If the buffering layer cannot determine the length of the response, it uses HTTP 1.1 chunked encoding instead of the content-length header to convey the delineation information. If the client only understands HTTP 1.0, the server must close the connection to indicate the end of the response.

  • Deferred header writing: Response headers are written out as late as possible to give the servlets a chance to generate their own headers (for example, the session management header set-cookie).

  • Ability to understand request entity bodies with chunked encoding: Though popular clients do not use chunked encoding for sending POST request data, this feature is mandatory for HTTP 1.1 compliance.

The improved connection handling and response length header generation provided by buffered streams also addresses the HTTP 1.1 protocol compliance issues where absense of the response length headers is regarded as a category 1 failure. In previous Enterprise Server versions it was the responsibility of the dynamic content generation programs to send the length headers. If a CGI script did not generate the content-length header, the server had to close the connection to indicate the end of the response, breaking the keep-alive mechanism. However, it is often very inconvenient to keep track of response length in CGI scripts or servlets, and as an application platform provider, the web server is expected to handle such low-level protocol issues.

Output buffering has been built in to the functions that get data, which are net_read and netbuf_grab (see Chapter 5 "NSAPI Function Reference."). To specify parameters that affect stream buffering, you can set the following variables, which are described in detail in Appendix B "Variables in magnus.conf."

To override the default parameters when invoking a SAF that uses one of the functions net_read or netbuf_grab, you would specify the value of the parameter in obj.conf, for example:

Service fn="my-service-saf" type=perf UseOutputStreamSize=8192


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Last Updated November 20, 2000