HTTP Protocol Stack and Worker Processes

HTTP Protocol Stack (HTTP.sys)

1. Kernel-mode caching. Requests for cached responses are served without switching to user mode
2. Kernel-mode request queuing. Requests cause less overhead in context switching, because the kernel forwards requests directly to the correct worker process. If no worker process is available to accept a request, the kernel-mode request queue holds the request until a worker process picks it up
3. When a worker process fails, service is not interrupted; the failure is undetectable by the user because the kernel queues the requests while the WWW service starts a new worker process for that application pool
4. Requests are processed faster because they are routed directly from the kernel to the appropriate user-mode worker process instead of being routed between two user-mode processes

Reference

Worker Processes (IIS 6.0)

A worker process is user-mode code whose role is to process requests, such as processing requests to return a static page, invoking an ISAPI extension or filter, or running a Common Gateway Interface (CGI) handler. In both application isolation modes, the worker process is controlled by the WWW service. However, in worker process isolation mode, a worker process runs as an executable file named W3wp.exe, and in IIS 5.0 isolation mode, a worker process is hosted by Inetinfo.exe.

Worker processes use HTTP.sys to receive requests and to send responses by using HTTP. Worker processes also run application code, such as ASP.NET applications and XML Web services. You can configure IIS to run multiple worker processes that serve different application pools concurrently. This design separates applications by process boundaries and helps achieve maximum Web server reliability.

Reference