BookingSync-Rack-P3p is rack middleware for inserting P3P headers into apps. This will allow your application to serve cookies to IE when in an iframe. It will also pull off the cookie on 304 responses, since IE does not like the cookie then.
Gemfile gem 'bookingsync-rack-p3p', '~> 1.0.0'
install gem install bookingsync-rack-p3p