Check_session_iframe is defined in OpenID Connect Session Management as a REQUIRED (for OpenID Connect Session Management) OpenID Connect Discovery parameter that is the URL of an OpenID Connect Provider iframe that supports cross-domain communications for session state information with the Relying Party Client, using the HTML5 postMessage API.

The page is loaded from an invisible iframe embedded in an Relying Party page so that it can run in the OpenID Connect Provider's security context. The URL accepts postMessage requests from the relevant RP iframe and uses postMessage to post back the login status of the End-User at the OpenID Connect Provider.

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