Rails 3 request dispatch cycle

I was looking at the rails 3 architecture in order to understand the process of dispatching a request . The whole process is quite simple. Application is a rack application which finally delegates its call message to the call method of ActionDispatch::Routing::RouteSet which dispatches appropriate action of necessary controller. It takes controller and action names from the hash stored in rack env by the key "action_dispatch.request.path_parameters".

So the question is: Who sets this hash? Who parses request uri and determines action and controller names?

I was not able to find this code. I see that during route configuration ActionDispatch::Routing::Mapper object is used to construct information about defined routes and stores it in ActionDispatch::Routing::RouteSet. But how this information is used during the request to find appropriate action and controller. Is rack also somehow involved here?

Look like it happens via rack-mount these days