Subject: RE: [Sipping] draft-burger-sipping-em-rqt-00.txt

From: "Eric Burger" <[email-masked]>
Date: Fri, 2 Nov 2001 08:32:34 -0500

----- Naoto MAKINAE wrote -----
> Dear Neil and Eric,
>
> I have some comments on the proposed scenarios.
>
> - In 6.1, I do not agree on the use of 486 Busy Here response.
>   486 should be issued by the called endsystem. Why not
>   simply establish a both-way RTP media stream between
>   the caller and the media server after 200 OK and ACK, and
>   then release the call after the annnoucement, if you do not
>   insist on the caller's experience of incomplete call?

In the scenario described in 6.1, the media server *is* the end system.
Thus it is entirely appropriate for the media server to respond with 486.

Think of what 200 OK means to the UAC.  The UAC asks to start a session with
an end point.  The intervening network (SIP Proxy) determines that the UAC
will not get to the end point.  If the network responds to the UAC with a
200 OK, the network has just lied to the UAC.  Early media signaling
accurately reflects the state of the call.

Returning to your point: if the end-to-end network does not require accurate
call progress ("if you do not insist on the caller's experience of
incomplete call"), then 200 OK would be fine.  However, I would then offer
that we could dump 181, 182, and 183, as well.  Everything would just be 200
OK.

I am sorry that I was not clear on the point of retaining SIP semantics
throughout the network.  Would this language clarify the point?

"One requirement of any well behaved network element is to preserve the
meaning and semantics of call state throughout the network."

>
>   If you motivation of the scenario described in 6.1 is NOT to
>   charge for an incomplete call, could you provide assumed
>   charging models? If a PSTN caller could make a call to a free phone
>   number, which is actually a SIP phone,  and of course without charge
>   with some charging arrangement, then the caller in 6.1 could also
>   hear an announcemnet after 200 OK and ACK without charge
>   with the same arragement.

That is quite true.  However, I would offer that even in the freephone case,
responding with a 200 OK when *not* connecting with the target service is
incorrect.  As we all have said many times, we should not build the protocol
around broken billing models.  In this case, the "broken" model is that in
freephone, we happen to never charge.  However, that does not free us from
providing accurate signaling to the UAC!

>
> - In 6.2.1, an analogy of answering machine in PSTN suggests that
>   the called UAS should respond with 200 OK, receive ACK,
>   establish a both-way RTP media stream, play an annoucement,
>   and optionally record the caller's message.

That would be correct behavior for a call answering service.  However, the
example was for a vacant or Do Not Disturb message.  In the former case, the
network connects the UAC with an agent of the target end point.  Thus 200 OK
is the appropriate response.  In the latter case, the network is *not* able
to start the session, and thus 200 OK is *not* the appropriate response.

>
> - In 6.2.2, why not redirect the call to the media server?

That would be one outcome.  For example, if the end point redirects the call
to an answering service, then the call would be completed normally.  The
point of 6.2.2, where the call is redirected to a media server, is that it
is for the non-completion case.

>
> I am not objecting to the idea of ealry media. I just do not see
> enough justification for putting more complexity into SIP.

I would offer that by not overloading the meaning of a connected call, we
are making SIP simpler.  How does Early Media make SIP more complex?

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