Frames: how to get the url of the top frame?

I am providing content for an other site. My content is getting shown in side a frame. My content contains links to other content I provide. It also has forms that the user can use to submit information. The form's submit action depends on some information that the main site send to me in the url, i.e., http://bigsite.com/index.html?submit=actionName The problem is that cannot find a way to access the URL form inside the frame. All the $_SERVER vars are relative to my frame. Is there a way for me to access the top (parent?) frame's URL? Thanks, Jean-Christian Imbeault