Appendix E   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

• Requests

• Responses

• Buffered Streams

Compliance

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iPlanet Web Server 6.0 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/rfc/rfc2068.txt?number=2068

Requests

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

• Request Method, URI, and Protocol Version

• Request Headers

• Request Data

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 E-1.

Table E-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

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The server's response includes the following:

• HTTP Protocol Version, Status Code, and Reason Phrase

• Response Headers

• Response Data

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.

Some common status codes are shown in Table E-2.

Table E-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 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.
408	If the client starts a request but does not complete it within the keep-alive timeout configured in the server, then this reponse 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 (eg. certain NSAPI plug-ins) cannot handle very large amounts of data, so they will return this 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 should check the server's error log to see what happened.
503	Sent if the quality of service mechanism was enabled and bandwidth or connection limits were attained. The server will then serve requests with that code. See the "quality of service" section.

Response Headers

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

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Table E-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

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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 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 absence 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 transmit data, such as net_write (see Chapter 5 "NSAPI Function Reference."). You can specify the following Service SAF parameters that affect stream buffering, which are described in detail in Chapter 3 "Predefined SAFs and the Request Handling Process."

• UseOutputStreamSize

• flushTimer

• ChunkedRequestBufferSize

• ChunkedRequestTimeout

UseOutputStreamSize, ChunkedRequestBufferSize, and ChunkedRequestTimeout parameters also have equivalent magnus.conf directives; see "Chunked Encoding."

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Note	The UseOutputStreamSize parameter can be set to zero in the obj.conf file to disable output stream buffering. For the magnus.conf file, setting UseOutputStreamSize to zero has no effect.

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

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