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

From: "Mark Watson" <[email-masked]>
Date: Mon, 1 Jul 2002 14:35:04 +0100

All,

I'm confused by this discussion, because there seems to be a desire to
extrapolate some semantics about the *media* from a message which does not
contain SDP. Apologies if I have missed the issue at hand, or if the
following is obvious, but...

The unify model means that media and session state are completely
independent, so, to talk about '180 with SDP' or '180 without SDP' makes no
sense.

You can talk about the effect 180 has on the session state (indicates that
called party is being alerted), but this has nothing to do with the media
state (if there was media before, there still is, if there was no media
before, there still isn't).

You can talk about the effect that SDP offer/answer exchanges have on the
media, but this operates independently of the SIP message those SDP bodies
arrive in.

So,

1) the UAC knows whether there is media or not (based on SDP offer/answer
exchanges so far)
2) if 180 has been received, the UAC knows that the called party is alerting

If the UAC knows that the called party is alerting AND knows that there is
media, it is a local matter what to do. Personally, I cannot see an argument
for doing anything other than playing the media (it might contain 'ringing
with call waiting announcement').

A PROBLEM arises if there is a UAS which negotiates media but then sends
silence (no packets) - there ARE some reasons why it might want to do this
to do with reducing clipping on answer. The result is that silence would be
all the caller would hear even though the UAC may know that the called party
is being alerted. I think we just have to say that it is foolish for the UAS
to negotiate media and then send silence - it should put the media on hold
if it has nothing to send.

Regards,

Mark

> -----Original Message-----
> From: Christer Holmberg [mailto:[email-masked]]
> Sent: 01 July 2002 12:26
> To: Alexeitsev, D
> Cc: [email-masked]; Hogg, Chris [MOP:MA01:EXCH]
> Subject: Re: [Sip] Local vs Remote ringback on 180
> 
> 
> 
> Hi,
> 
> "Alexeitsev, D" wrote:
> 
> > Hello All,
> >
> > Some comments/questions inline
> >
> > Greetings,
> > Denis Alexeitsev
> >
> > -----Ursprüngliche Nachricht-----
> > Von: Christer Holmberg [mailto:[email-masked]]
> > Gesendet am: Freitag, 28. Juni 2002 18:53
> > An: Chris Hogg
> > Cc: Flemming Andreasen; Jonathan Rosenberg; Mark Watson; 
> [email-masked];
> > Bob Penfield
> > Betreff: Re: [Sip] Local vs Remote ringback on 180
> >
> > [CHH] The UAS may still send early media, but since it 
> sends 180 without
> > SDP it allows the UAC to generate local ringback tone, so 
> the UAC now
> > can assume that it can "cut" the early media from the 
> caller during that
> > time. If the UAS does NOT want the UAC to generate local 
> ringback, the
> > 180 would also contain SDP (the same SDP which was used in 
> the 183 to
> > establish the early dialog, ie the SDP is not a new offer/answer -
> > simply a ringback "indicator").
> >
> > [DA]I have a question about the need to receive the SDP in 
> any responses in order to play remote ringback. As the 
> offer/answer draft says:
> > "Once the offerer has sent the offer, it MUST be prepared 
> to receive media"
> >
> > The SDP coming from the other side is used to send the 
> media, that is unimportant in the ringback case. Am I missing 
> something here?
> 
> [CHH] For the specific ringback tone case one can argue that 
> an SDP is not needed for the media itself (unless you need it 
> for RTCP purpose,
> or something).
> 
> However, there is no specific "remote ringback session" in 
> SIP. You establish a generic early dialiog, by sending an 
> offer and receiving an
> answer, which can then be used for any kind of early media 
> (ringback tone, announcement,...), in any direction 
> (depending on the "direction"
> indicators - sendrecv, sendonly, inactive etc etc).
> 
> Regards,
> 
> Christer Holmberg
> Ericsson Finland
> 
> 
> 
> _______________________________________________
> Sip mailing list  https://www1.ietf.org/mailman/listinfo/sip
> This list is for NEW development of the core SIP Protocol
> Use [email-masked] for questions on current sip
> Use [email-masked] for new developments on the application of sip
>

---

From: Christer Holmberg <[email-masked]>
Date: Mon, 01 Jul 2002 17:45:09 +0300

Hi Mark,

> extrapolate some semantics about the *media* from a message which does
> not contain SDP. Apologies if I have missed the issue at hand, or if
> the following is obvious, but...
>
> independent, so, to talk about '180 with SDP' or '180 without SDP'
> makes no sense.

I agree, but those are the terms used by the drafts I've refered to. The
ISUP/SIP mapping draft says that "180 with SDP" means that a remote
ringback will be generated, and bis-09 says that "180 without SDP" MAY
cause the UAC to generate a local ringback.

Of course, those statements may sometimes be valid, if we receive either
a 180 without SDP, OR we receive 180 with SDP - and no early dialog
still exists. My orignial question was what we do if we receive 180
without SDP, but we DO have an early dialog established. It has been
argued that the UAC should not generate local ringback in that case -
instead assume the UAS does it since there is an early dialog
established (if the UAS wants the UAC to generate local ringback it
would have to put the media on hold using UPDATE). IF that's the case, I
think it should me more clarified. Clarification is the only thing I've
asked for - no changes in the protocol.

> You can talk about the effect 180 has on the session state (indicates
> that called party is being alerted), but this has nothing to do with
> the media state (if there was media before, there still is, if there
> was no media before, there still isn't).

I've said all the time that the session state is not affected, only that
the 180 affects what the application does with the media.

So, what you just wrote means, that in the case we have an established
early dialog it really doesn't matter, from a ringback point of view, if
there is a SDP in the 180 or not. Is that the "official" clarification
I've asked for? :)

> You can talk about the effect that SDP offer/answer exchanges have on
> the media, but this operates independently of the SIP message those
> SDP bodies arrive in.

True. That is why I also asked if the SDP is a good "ringback indicator"
in the first place...

> So,
>
> 1) the UAC knows whether there is media or not (based on SDP
> offer/answer exchanges so far)
> If the UAC knows that the called party is alerting AND knows that
> there is media, it is a local matter what to do. Personally, I cannot
> see an argument for doing anything other than playing the media (it
> might contain 'ringing with call waiting announcement').

I think it was earlier said that the decission can't be based on if
there is media or not. There may be RTP packets, but that doesn't mean
they contain any "real" media.

> A PROBLEM arises if there is a UAS which negotiates media but then
> sends silence (no packets) - there ARE some reasons why it might want
> to do this to do with reducing clipping on answer. The result is that
> silence would be all the caller would hear even though the UAC may
> know that the called party is being alerted. I think we just have to
> say that it is foolish for the UAS to negotiate media and then send
> silence - it should put the media on hold if it has nothing to send.

Like I said earlier, that would probably not happend when the same
server establishes the early dialog and sends the 180.  I earlier gave
an example where a B2BUA sends the announcement ("you have now reached
the X company"), the contacts a SIP phone which sends 180 (the SIP phone
does NOT generate early media). Now, the B2BUA gets the 180, stop
playing the announcement, and forwards the 180 to the UAC. Now, since
there are no more announcements, and the SIP phone doesn't generate
remote ringback, and the UAC doesn't generate local ringback, there is
no ringback at all. In this case the B2BUA would have to send UPDATE
(media on hold), then forward the 180 (the UAC will now generate local
ringback), and when the SIP phone answers it would have to send another
UPDATE (media un-hold) and then forward the 200.

An issue related to this I brought up earlier, is if the UAS will have
to buffer the 200 until both UPDATE transactions have been completed.
Any comments on that?

Also, in a non-PRACK environment, is a 18x "allowed" to establish an
early dialog in the first place? Bis-09 says that the answer must be
sent in a reliable response, which is 200 OK. Yes, bis-09 DOES say that
the answer MAY also be sent earlier, unreliably in a 18x, but when is
the early dialog actually "officially" established? Again, the UAC could
make a ringback decission based on if there is a SDP in the 180 or not,
but a unreliably sent 180 may not even arrive...

Regards,

Christer Holmberg
Ericsson Finland