> Here callee should assume that peer supports 
> session timer or not because supported: timer 
> is not present in request.

Concerning "timer", the proxy/UAS receiving the request cannot assume that the 
UAC still supports "timer" from a negotiation perspective.  If the UAC still 
supports it, it must explicitly be indicated within the UPDATE/re-INVITE.  This 
is because the RFC uses the received request instead of dialog to determine if 
the UAC supports "timer".


> Here callee should assume that peer does not 
> support session timer supported: 100rel

Concerning "timer", the proxy/UAS receiving the request explicitly knows that 
the UAC does not support "timer".


> a. Please let me know what should be correct 
> behavior for above both cases.

Assuming that you want to allow the call to continue, process the 
UPDATE/re-INVITE as though the UAC does not support timer.


> b. Application scenario where supported value 
> is changed in between the session.

This is generically valid.  However some interpret rfc4028 section 7.4 as 
though it may not be valid to change Supported, Require, and Proxy-Require 
within some situations.


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