Subject: Re: [Sip] Local vs Remote ringback on 180

From: "Chris Hogg" <[email-masked]>
Date: Wed, 26 Jun 2002 10:40:06 +0100

Hi Christer,

I'm a bit late into this discussion, but I'll put my thoughts in anyway.

>>However, if you mean that it doesn't change what is sent to the listener
either
>>I think what is written in some drafts is wrong. It IS said (at least that
is my
>>understanding) that SDP/no-SDP in 180 DOES indicate if a remote ringback
tone
>>will be generated or not.

[CH] It is my understanding that previous (pre-"UNIFY" draft) versions of
the bis did indicate that you could use the presence or absence of SDP to
indicate whether or not the UAC should generate local ringback or play
media.  If my understanding is correct, this was removed because of the need
to solve many other problems (e.g. HERFP, early media etc) which all arose
out of the dependence of session state on the SIP signalling.  By creating
the offer-answer model (and the additional SIP specific offer-answer rules
within RFC3261) then independence of session state from SIP signalling state
was achieved.  I believe the current version of SIP only says that the 180
MAY be used to generate local ringback but does not say how this is achieved
since it is implementation dependent.  

I believe that the most likely way to do this currently is to initiate a new
offer-answer exchange in an UPDATE/200 OK(UPDATE) with the new offer (from
the UAS) indicating that the media stream direction is a=recvonly or
a=inactive.  Its' then up to the UAC/implementation to decide what to do now
that it knows that there is no media coming from the far end.  This could be
playing local ringback or playing an announcement or popping a window up on
a screen. I understand the reasoning behind trying to reduce the number of
messages sent to achieve this aim, however I am also concerned about
creating a special case for local ringing.

Just my 2 cents.

Best Regards,

Cheers
Chris

Chris Hogg
Nortel Networks

---

From: Christer Holmberg <[email-masked]>
Date: Thu, 27 Jun 2002 02:08:17 +0300

Hi Chris,

Chris Hogg wrote:

> >>However, if you mean that it doesn't change what is sent to the
> listener either
> >>I think what is written in some drafts is wrong. It IS said (at
> least that is my
> >>understanding) that SDP/no-SDP in 180 DOES indicate if a remote
> ringback tone
> [CH] It is my understanding that previous (pre-"UNIFY" draft) versions
> of the bis did indicate that you could use the presence or absence of
> SDP to indicate whether or not the UAC should generate local ringback
> or play media.  If my understanding is correct, this was removed
> because of the need to solve many other problems (e.g. HERFP, early
> media etc) which all arose out of the dependence of session state on
> the SIP signalling.  By creating the offer-answer model (and the
> additional SIP specific offer-answer rules within RFC3261) then
> independence of session state from SIP signalling state was achieved.
> I believe the current version of SIP only says that the 180 MAY be
> used to generate local ringback but does not say how this is achieved

Yes. My UAC MAY choose to stop forwarding any possible early media
(again, without changing the state of the session) to the caller, and
instead generate local ringback, if 180 without SDP is received. My
proposal was that a 180 without SDP should be used to indicate that the
UAS is currently not sending any early media (announcement or early
media) on the session, and that the UAC therefore MAY choose to generate
local ringback.

> I believe that the most likely way to do this currently is to initiate
> a new offer-answer exchange in an UPDATE/200 OK(UPDATE) with the new
> offer (from the UAS) indicating that the media stream direction is
> a=recvonly or a=inactive.  Its' then up to the UAC/implementation to
> decide what to do now that it knows that there is no media coming from
> the far end.  This could be playing local ringback or playing an
> announcement or popping a window up on a screen. I understand the
> reasoning behind trying to reduce the number of messages sent to
> achieve this aim, however I am also concerned about creating a special
> case for local ringing.

I don't think that is a good idea. Just because the UAS sends UPDATE,
and stop sending media, it doesn't mean the called party has been
reached. I think the 180 shall, the way it is defined today, be the
ringback "trigger".

Regards,

Christer Holmberg
Ericsson Finland

> Nortel Networks

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