IIS Request Processing

When IIS receives an HTTP request, it evaluates the URL to determine if the request is for static content (HTML), or dynamic content (ASP or ISAPI).

Request Processing Actions

Request	Action
HTML Page	IIS returns the page immediately in HTML format.
ISAPI extension	IIS loads the ISAPI DLL (if it is not already running) and the request is sent to the extension through the EXTENSION_CONTROL_BLOCK Structure data structure.
File name whose extension is mapped to a particular ISAPI extension, such as Asp.dll	IIS loads the appropriate DLL file and presents the request through the Extension_Control_Block data structure. For example, the .asp extension is mapped to Asp.dll, so that all requests for files with an .asp extension will be directed to Asp.dll. The .stm and .shtm extensions are mapped to the Ssinc.dll.
CGI application	IIS creates a new process. IIS will then provide the query string and other parameters that are included with the request through the environment and standard input (STDIN) handle for the process.