Subject: RE: [Sipping] new draft-ietf-sipping-isup-05 (to Al Varney on ear ly media)

From: "Peterson, Jon" <[email-masked]>
Date: Thu, 15 Aug 2002 15:15:49 -0400

This summarizes from a couple of mails from you; comments below.

Jon Peterson
NeuStar, Inc.

> -----Original Message-----
> From: Al Varney [mailto:[email-masked]]
> Sent: Thursday, August 15, 2002 10:45 AM
> To: [email-masked]
> Subject: RE: [Sipping] new draft-ietf-sipping-isup-05
> 
[snip]
> 
>     My biggest concern, perhaps orthogonal to the above issue, is that 
> Draft-05 introduced "new media" text that would IGNORE in-band ringback in

> preference for local ringback (whether 183 or 180), in situations where it

> is not possible to KNOW that the in-band audio is ringback vs. some other 
> tone/announcement.
> 

Yes, there is a general bias in SIP towards local ringback, discarding
in-band information. This is something that Gonzalo's mails have spoken to.
The basic justification for this is the unnecessary expenditure of resources
(mostly bandwidth) that results from in-band alerting media.

In the PSTN, this issue is less significant, because when you send an IAM a
TCIC is reserved and that TCIC cannot be used for other purposes. But for
example in a SIP network, when an INVITE is sent the bandwidth that might be
used for a voice session over RTP could conceivably be used for other
purposes until the 200 OK is received. Sending RTP packets over the network
to carry a ring tone that could be generated locally is believed to be
undesirable, yes. Consider forking, for example - if I called five of your
phones in different locations (office, home, cell, etc) simultaneously,
getting five RTP streams worth of ringing could be very problematic for me
bandwidth-wise, and I certainly don't benefit at all from hearing these
ringtones as opposed to ones that might be generated locally. However, if I
merely get a single message that alerting is taking place (180) from each of
these setup attempts, and media transmission only begins when you pick up
one of the phones, then I could call five hundred of your phones over a
dial-up connection without a problem (albeit some, like Mr. Pierce, have
argued that this is a security bug rather than a feature).

Does that make sense? This is a pretty fundamental part of the SIP design.

[snip]
> 
>  I recall prior experience with some (business) 
> callers offering a brief "Yes" or "Hello" within the first 300-500 ms, and

> then silence.  They dislike repeating themselves.
> 

There are precondition/resource reservation systems for SIP that prevent
clipping, but do not require the transmission of backwards alerting media. I
think we have it covered, though this discussion isn't the right place for
the details on this.

[different mail]
>     Respectfully, Jon, these were NOT TRIVIAL changes.  For example:
> 
>   Old 4.5 in Draft-04 (renumbered to 5.5 in Draft-05):
>     "To allow the transmission of messages and tones before a final
>     connection has been established, SIP nodes which interwork with the
>     PSTN need to be able to establish temporary media connections during
>     this period."
> 
>   New 5.5 Early Media:
>     "In cases where interworking has not been encountered,
>     use of early media is almost always undesirable since it consumes
>     inter-machine trunk recourses to play media for which no revenue is
>     collected."
> 

I don't think those two statements are necessarily incompatible. It's true
that we need to support early media, especially when we are interworking
with legacy portions of the PSTN somewhere in the call path. It is also true
that whenever possible, we should try to not to do in-band ringback, for the
reasons I described above.

>     Equivalent changes in 7.2.5/7.2.6 now express a decided preference for

> NOT establishing early media on all calls, unlike the Draft-04 text, which

> at least leaned in the direction of providing early media.  The PSTN is an

> early-media system, and trying to make it into a media-after-answer system

> is an uphill crawl.

I agree, it is a real drag to have to support this stuff in SIP. :)

>     In other words, in my opinion, Draft-04 worked with the PSTN, and 
> Draft-05 doesn't.

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