Subject: RE: [Ieprep] Internet faults, VoIP demo, etc and others

From: "King, Kimberly S." <[email-masked]>
Date: Mon, 2 Dec 2002 16:38:14 -0500

On Monday, Dec 2, 2002, at 10:30 America/Montreal, [email-masked] 
wrote: 
> [MAP] So take this simple case and apply an emergency. (I'd like to 
> see the performance last week right after the bombing in Kenya!) 
> 
> If 10 or 20 or whatever number of those "several thousands" of end 
> users tried to make a VoIP call via that single 384K link, what would 
> have happened? It is my understanding that, at some point, the 
> overload would have caused either: 
> 
> 1. all calls to be of unacceptable quality 
> 2. new call attempts would have been rejected 

[MAP] I'll repeat the question which I don't believe I've gotten any
concrete answer to: In the case above of the 10 callers from Nairobi, if
one of them was authorized to make emergency calls and the other 9
weren't, what specific standardized mechanism (not products) could be
used to provide the required service across the congested 384 kbit/s
satellite link? (The required service is that their call gets set up and
they can talk.) If you don't believe that 10 users would cause
congestion, then make it 20 or however many of the several thousand it
would take. 

Mike,

The answers to your question depends upon the assumptions you make (i.e.,
problem context).  Do you assume your 10 or however many users are just
random folks who happen to be using the same access link owned by an
arbitrary ISP?  Then your answer is very likely to be "the call quality for
all users is going to be poor".  

Let's us examine why...well, we could try to identify emergency users with
SIP signaling and try to use "call admission control" in the telco sense.
Fine except your 10 users might use 10 different SIP service providers
completely unconnected to the ISP who happens to be providing the access
link.  Those SIP providers certainly have no way of knowing what other SIP
service providers are doing nor do they track bandwidth utilization across
some random link (they have no way to do this) hence they can't provide a
"call admission control".  So, you can then assume all 10 users are allowed
to setup calls.  Your access link provider is quite likely to be just moving
packets.

Now call quality has to do with lots of things including jitter and loss.
Trying to stuff more voice packets (UDP) into a slow link is going to result
in drops on both ends of the tiny pipe.  There is no way to identify
emergency traffic packets.  This is because there are 10,000 or so ISPs and
it is almost always true that all packets are created equal at the transport
layer.  Trying to change this for an arbitrary ISP over an arbitrary link is
just about impossible.  Remember identification mechanisms like CoS are the
exception, not the norm and the assumption that this is just some arbitrary
connection means no CoS is available.

So, with these set of assumptions, the answer is the call quality will be
poor for all until some people hang up.

But, under a different set of assumptions, the answer is different.  Assume
all 10 users have a local soft PBX that provides a "call admission control"
for all off net calls.  It also tracks who's got priority and can even
preempt lower priority calls.  Then 10 users try to setup but only the
emergency worker and the max number the access link can support are allowed
to complete.  This could actually support Assured Service.  

See how important is it to define the context?

Kimberly

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---

From: Bill Woodcock <[email-masked]>
Date: Thu, 5 Dec 2002 05:26:21 -0800 (PST)

On Monday, Dec 2, 2002, [email-masked] wrote:
    > So take this simple case and apply an emergency. (I'd like to

There was no audible or measurable difference.  I don't think that you're
getting that "emergency" means something very different to you than to the
engineering community.  An "emergency" to us would tend to mean something
that has a deleterious impact on the network.  Not something that causes
TV reporters to interrupt advertisements, necessarily.  Those tend to not
generally be associated with any serious impact on our operations.

    > > have happened?

Correct, that's a reasonable assumption.

    > I'll repeat the question which I don't believe I've gotten any
    > weren't...

Authorized by whom?

    > what specific standardized mechanism (not products) could be
    > they can talk.)

If they were authorized by _me_, that is, if they were paying _me_ to
deliver their call in preference to the other nine callers, I'd
policy-route their SIP traffic into a separate PVC at the CPE router.
That's what I did back in the distant past, when congestion still occurred
occasionally, and it seemed to work quite satisfactorily at the time.

Of course, that assumes xsomeone competent controlling a reasonable CPE
router, neither of which are useful assumptions.  It also assumes that the
person performing the authorization is someone with influence over network
operations, which is also not a useful assumption.

      On Mon, 2 Dec 2002, King, Kimberly  S. wrote:
    > But, under a different set of assumptions, the answer is different.

Dr. King's examples of two different contexts under which there might or
might not be useful ways of controlling access are very good ones.  Again,
it's important to note, as she points out, that this function is quite
feasible at the application layer, in the SIP end-nodes.  The proxies
really don't have anything to do with it.  It's the phones themselves, the
end-nodes that control the amount of data in and out of each one.  And
they're the only devices that have any visibility whatsoever into the
degree of congestion that they may be experiencing, end-to-end.

                                -Bill