> Suppose a session-timer of 30 minutes duration 
> has been negotiated between two UAs.
> A session-expires value of 30 minutes was 
> negotiated and UA1 is the refresher.
> UA1 starts a timer of value 15 minutes and 
> UA2 starts a timer of value 30
> minutes  minus 10 seconds, as per the 
> recommendations in the spec.
> 
> Now 10 minutes after the first INVITE 
> negotiation, UA1 sends a re-INVITE
> (not a timer refresh) to UA2 in order 
> (to negotiate a codec change, put
> party on-hold etc.) and added the 
> Session-Expires header with the same value
> that was previously negotiated (30 minutes).
> 
> For some reason, the re-INVITE was rejected 
> with some 4xx/5xx response (something other 
> than 408 and 481). What should the UAC do now?

Assuming the re-invite was not important,
the call stays up.  If the failure response
should trigger a subsequent request 
(401, 407, ...), the UAC should act accordingly.
 
> What happens to the previously negotiated value
> of Session-Timer?

It is still timing.  Because the re-invite
failed, it does not alter the previously
negotiated timing.

> As per my understanding, if the response to 
> a re-INVITE with a Session-Expires header is 
> not a 2xx with a session_expires header, the
> session timer is cancelled. Is this correct?

No.  A non-2xx final response to a re-invite
does not alter the current session timing.
A 2xx without session-expires means the
session timer is cancelled.

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