This came up at sipit 16.
draft-ietf-sip-session-timer-15.txt says in section 7.1:
"The UAC MAY include the refresher parameter with value 'uac' if it wishes to perform the refreshes. However, it is RECOMMENDED that the parameter be omitted, so that it can be selected by the negotiation mechanisms described below."
I came across an implantation that sets the refresher parameter to 'uas' when initiating the INVITE. The claim was that the draft does not explicitly forbid that.
The problem with that is: what if the UAS does not support session timer. The spec says in section 8.2 (proxy processing responses)
"If, however, the proxy remembers that the UAC did support session timer, additional processing is needed.
Because there is no Session-Expires or Require header field in the response, the proxy knows it is the first session-timer-aware proxy to receive the response. This proxy MUST insert a Session-Expires header field into the response with the value it remembered from the forwarded request."
Some proxies may interpret that to mean the whole value, including the refresher parameter that contained 'uas' which will cause all sorts of funny things to happen.
Now, the draft does go on to say
" It (the proxy) MUST set the value of the 'refresher' parameter to 'uac'."
so the problem my fix itself, but i think we need to clarify this: either say that the UAC MUST NOT set the refresh parameter to 'uas' (which is what I prefer) or add some text clarifying the scenario and behaviour for proxies.
Regards, Hisham
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