the front channel logout spec defines: Session Continuous period of time during which an End-User accesses a Relying Party relying on the Authentication of the End-User performed by the OpenID Provider. But the spec also says: [The sid's] contents are opaque to the RP. And this value seems to bear no clear relationship to what the user agent and the RP decide the session should be. The sid is just a value (claim) from the OP that it regurgitates when it sends a logout.
Issue #1024 resolved