Subject: RE: [SIP] Early media replacement.

From: "Christian Huitema" <[email-masked]>
Date: Tue, 9 Jan 2001 09:28:11 -0800

So far, we have heard three arguments for 183. I would like to take
these
arguments apart, because lumping everything as 'early media' does not
make a lot of sense.

The first argument is that in many cases it is nice to hear the actual
call progress tones and messages, instead of letting the calling UA
synthesize them. This does not by itself mandates the transfer of
SDP information back to the caller; in fact, if it did mandate such
a transfer, we would have a race condition between the beginning
of the progress message and the arrival of the SDP. On the other hand,
if we assume that the 180 code implicitly request the UA to start to
synthesize ringing, then having a different progress code can help.

The second argument is that mixing multiple progress tones is confusing.
But we definitely want to be able to "ring the five telephones in the
house and pick the first one that gets lifted." I think that the
consensus
here is that having a 183 response does not help one bit. If we have
multiple simultaneous ringing, the user is probably better off with
synthesized ringing. So, may be what we need is a rule that says "don't
send multimedia if you received a forked INVITE."

The third argument is that it is in some cases nice to start a two-ways
session without actually starting to bill the call. That one is, IMHO,
bogus. If one wants to provide billing indications, one should do so
by carrying explicit "start of billing" marks. One possibility could
be to answer the call with a 200 OK, maybe with a special header that
says "the call is set temporarily, don't bill yet," and rely on an INFO
message or maybe a subsequent re-invite to actually start billing. In
any case, all procedures that allow for a two-way session without 
billing provide an opening for fraud.

Note that if one wants to actually establish a two-ways session, then
one must in practice make sure that there is only one leg in the call.
This means that this the call was forked, then all branches that were
not selected must be cancelled. This is the semantic of the 200 OK.
This is not the semantic of the 180 message.

In short, there may be a role for a special progress message that 
informs the UA of the call state, but asks the UA to not synthesize 
ringing. There is no obvious need to carry an SDP payload in these
messages; the handling of fork/cancel is much better done if only
the 200 OK can carry an SDP.

-- Christian Huitema

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From: Rick Dean <[email-masked]>
Date: Tue, 9 Jan 2001 14:02:03 -0600

> send multimedia if you received a forked INVITE."

Five SIP phones?  then there is no problem, unless they
independently go through PSTN gateway B channels,
a terrible design from the start.
SIP phones don't need to send early media because
they know for sure the call state.

The early media problem is PSTN gateways
which do not know the call state.  Please treat the 
disease not the symptom.

--
Rick

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From: Michael Thomas <[email-masked]>
Date: Thu, 11 Jan 2001 09:33:00 -0800 (PST)

> billing provide an opening for fraud.

   Actually, I have a lot of doubts that this belongs in
   the call signaling plane at all for this very reason.
   If the UA's are trustworthy, they can just send the 
   accounting messages themselves. If they are not and there
   are no policers at the edge of the trust
   boundary to police the session, then nothing can be
   done. The policing is generally of the QoS flow variety
   at edge routers for RTP, but more generally one shouldn't
   expect that the session and signaling plane have any
   necessary connection with one another. As such, I think
   that any reliable accounting messages must to be placed
   at a trustworthy device where the actual
   session flows through so that it can reliably
   determine the beginning and end of the session.

		 Mike