X-Domain and P3P Headers

I have a website A.com and a domain at B.com with a widget inside an iframe getting data from A.com. I w开发者_如何学编程ant to allow x-domain cookies to be passed from a.com to inside the iframe using ASP.NET

My understanding is that - I can do this in IE using P3P Headers - such that the A.com cookie is passed to the iframe and session | cookie data is preserved. Is this correct or am I totally wrong ?