I think that the order of the description about iframe should reverse.
The implementer may be easy to understand a description along the real data flow
(OP -> RP -> RP iframe -> OP iframe -> RP iframe...)
4. Session Status Change Notification
- Two iframe is necessary for this implementation
- RP iframe knows id of OP iframe
- Polling by repeating postMessage
4.1 RP iframe
- This iframe MUST know the ID of the OP iframe so that it can postMessage to the OP iframe.
- The postMessage includes client_id and session_state
- The received data would either be changed or unchanged.
4.2 OP iframe
- The OP iframe has access to Browser state at the OP...
- The OP iframe MUST recalculate it from the previously obtained Client ID, the source origin URL (from the postMessage), and the current OP Browser state.
- The response is ...