Re: [Sip] Early media as an endpoint application

["Christer Holmberg" <christer.holmberg at ericsson.com>]

Hi, 

>>I'm sorry but it seems pretty far-fetched to try and blame SIP
shortcomings on "Bell-heads".  Since when have they (we) >>been that
influential?
>
>Since we were busy defining the SIP-PSTN interworking model back in
about 1998, and came up with the "early media" 
>approach. Mots of us working on SIP back then were bell-headed, even if
we didn't think so at the time. It's a matter of 
>accepting the preconceived notions that the telephone system was
reasonable. I'm quite guilty of it myself.  
>Remember, Henry and I were sponsored by MCI back then, and our mission
was to make something that interopped with the 
>PSTN and that the business people could understand how to make a
business out of.

I think we should all be thankful for your mission back then. Otherwise
we would all now be working on H.323 version 56.

...and they don't serve cookies in SG16.

Whether the protocol solution for early media could have been done in
another way is of course one question, but I agree with Tim that
use-cases where early media is needed is nothing PSTN specific. I think
there are useful IP services (no matter whether they are implemented in
the network or in Henry's pure/clean IP-IP world) which use early media.

>A lot of the insanity here stems back to obtuseness on my part at the
interim meeting we held with the DCS cable team 
>back in 1999, where we came up with the idea of QOS preconditions
negotiated with reliable- provisionals rather than just 
>using a two-stage invite. We should have had a first INVITE for QoS,
and a re-invite for media, and probably would have 
>if we hadn't had people thinking that the first 200 OK meant "start
billing", 

Instead of the first 200 OK you could have used 202, and 200 OK whenever
the billing was supposed to start.

...if there would have been a 202 at that time, that is :)

>or if we had realized that gateway/precondition forking really requires
a B2BUA, not a proxy. But JDR and I really 
>believed in proxies back then . . .

If we could turn back time...

Regards,

Christer




> My point to Henry was just that this "problem" is not specific to PSTN

> gateways; so it wouldn't be appropriate to confine its solution to 
> such a device.  The need for media interaction prior to call 
> completion (which as Martin notes can be 2-way) arises from the 
> intermediated service model; which is as applicable to VoIP as to 
> circuit switched networks.
>

So we need to be able to differentiate the "intermediated" session from
the "non-intermediated" session. Insisting that the "session"  
hasn't started (despite the fact that media is flowing) until some
arbitrary downstream event has occurred is just silly.

We've massed layers of complexity (early media, early session,
preconditions)  onto the rather elegant base protocol just to compensate
for this misunderstanding. And frankly, it hasn't helped things all that
much -- implementations are now insanely difficult, I'm confused by it,
and AFAIK, nobody's stuff actually works 100% to the specs. Why should
we be happy with this? As (I think it was Douglas Adams that said) it is
easy to become blind to the essential uselessness of these things due to
the sense of wonder and fulfillment one gets from getting them to do
anything at all.

--
Dean

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