public static Route MapRoute(this RouteCollection routes, string name, 
          string url, object defaults, object constraints, string[] namespaces)
{        
    if (routes == null)        
    {        
        throw new ArgumentNullException("routes");        
    }        
    if (url == null)        
    {        
        throw new ArgumentNullException("url");        
    }        

    Route route = new Route(url, new MvcRouteHandler())        
    {        
        Defaults = CreateRouteValueDictionary(defaults),        
        Constraints = CreateRouteValueDictionary(constraints),        
        DataTokens = new RouteValueDictionary()        
    };        

    if ((namespaces != null) && (namespaces.Length > 0))        
    {        
        route.DataTokens["Namespaces"] = namespaces;        
    }        

    routes.Add(name, route);        

    return route;        
}        

...

 public class MvcRouteHandler : IRouteHandler
  {
    protected virtual IHttpHandler GetHttpHandler(RequestContext requestContext)
    {
      requestContext.HttpContext.SetSessionStateBehavior(GetSessionStateBehavior(requestContext));
      return new MvcHandler(requestContext);
    }
  }