>3 apr 2011 kl. 13.23 skrev Iñaki Baz Castillo:
>
>> 2011/3/31 Olle E. Johansson <o...@edvina.net>:
>>> If you are sending only ringback, I would recommend sending 180 with SDP
>>> instead of 183. If you're sending 183, I can't move my state machine to
>>> ringing state, which would help a lot of 3rd party apps. If you send
>>> ringback in 183 - they won't notice the ringing and would see the call
>>> going from calling state directly to answer, which is confusing. Sending
>>> ringback with 183 and not sending 180 is a problem for many apps.
>>>
>>> Use 183 only if you have an operator message to play. Otherwise, add SDP to
>>> the 180 Ringing.
>>
>> Hi Olle. So you propose that ringback (just pure ringback with no
>> voice announcement) should be always sent in a 180 response (with or
>> without SDP, as usually SIP phones generate 180 with no SDP, of
>> course).
>Yes. I am not saying that you should always send ringback with 180, but
>if you do want to send ringback in audio, use 180.
I should tell you that this is opposite how almost all commercial
products work; 180 is normally used without SDP for telling the caller
to generate internal ringback, and 183 with SDP is used for providing
remote ringback (usually from a SIP->PSTN gateway). Most UA devices
don't send ringback themselves and just send 180 to the server.
>> And just use 183 (always with SDP for sure) when the media contains an
>> announcement ("the number you are calling is not available" and so).
>exactly.
>
>> Am I right?
>Yes.
>
>183 doesn't say much about the state change, but 180 is actually a
>state change indicating that something is alerting the target about an
>incoming call. This is very important in gateway situations, like a
>b2bua like Asterisk.
A quote from this list from 2008 (use the archives!!!):
RFC3960 gateway model gives guidelines about the, let's say, "correct"
behavior. The main rule says more or less "media has always the precedence".
About the scenario you mentioned in the text, i.e. 183/SDP and then 180 (no
SDP I assume) you should play a local ringback ONLY in the case you don't
receive any media (despite the media channel has been established).
In the reverse case, i.e. 180/noSDP and the 183/SDP the rule is the same,
meaning you should play the media. Should the media missing, you would
continue to play the local ringback due to the 180.
This is what RFC3960 says. In the real world (especially when you
interoperate to the PSTN) it doesn't work in some case. My recommendation is
to always use the latest received message as the most significant. Then, in
the first case once receiving the 180 I would suggest to play a local
ringback stopping playing the media, in the second one to stop the local
ringback and play the media (if any). Using this approach, I didn't find yet
any failing case when interoperating with PSTN (this is the most complex
scenario usually) in the operator's network I'm working with.
--
Randell Jesup, Worldgate (developers of the Ojo videophone)
rje...@wgate.com
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