http.sys is Microsoft Windows HTTP protocol stack file
The HTTP listener is implemented as a kernel-mode device driver called the HTTP protocol stack (HTTP.sys). IIS uses HTTP.sys, which is part of the networking subsystem of the Windows operating system, as a core component.
When IIS runs in worker process isolation mode, HTTP.sys listens for requests and queues those requests in the appropriate queue. Each request queue corresponds to one application pool. An application pool corresponds to one request queue within HTTP.sys and one or more worker processes.
If a defective application causes the user-mode worker process to terminate unexpectedly, HTTP.sys continues to accept and queue requests, provided that the WWW service is still running, queues are still available, and space remains in the queues.
