Facebook JavaScript SDK: Read App URL

I have an app with a secure canvas URL, I'm using the JavaScript SDK. Let's say this is the url: http://ift.tt/1JQBOtn

I'd like to read in my canvas (inside of an iframe on facebook) the url of the app, so that I can read the request ids passed from notifications.

For example:

http://ift.tt/17B9Zni

How do I do that?