Subject: Re: [SIP] Authentication and billing through proxy/re-direct server

From: Henning Schulzrinne <[email-masked]>
Date: Fri, 06 Jul 2001 22:44:01 -0400

Not to argue with the basic points made, but maybe some operational
reality would help.

I think it's worth remembering that different applications have
different requirements. For example, in a PBX rebilling application,
something that's within, say, 5% accurate is going to be good enough.
For these applications, a TCP connection between proxy server and
gateway, with restricted source address trust on the gateway, is in the
"good enough" category.

Also, speaking from experience, almost all small gateways don't have the
ability to do authentication or call/packet-level accounting (and would
be significantly more expensive or complicated if they did). Thus, you
probably have a choice: spend $10k on a carrier-strength gateway or on
occasion measure a phone call wrong by a few seconds. Given the
cents/minute we're talking about, this seems like a fair trade-off.
Ideally, even simple gateways would support at least some shared-secret
authentication or IPsec.

Many hotels measure calls starting N seconds after ringing, regardless
of whether somebody picked up or not. Certainly not precise and allows
"cheating", but in the scheme of things, not one of the major problems
in the world.

If the gateway is close to the accounting proxy server, packet loss is
very unlikely.

Accounting is like voting or audio equipment: beyond a certain limit of
accuracy, costs grow exponentially and it's much better to accept that
life isn't completely fair. In real life, we don't weigh mail for
postage with the same precision as gold. Given that most calls are still
metered on a minute-by-minute basis, random errors in the sub-minute
range are not likely to be significant.

It's probably much more important to have rough alarms in place, to, for
example, catch if a call inadvertently never gets hung up or some fraud
is in progress.

Michael Thomas wrote:
> 
> As Jonathan points out, this is trivially defrauded.
> 
> The rule is simple: if you don't see the begin/end
> of the actual media stream (either directly or
> through some router that controls, say, QoS
> resources), or whatever other kind of session you

In almost all cases, what matters is not the media stream, but the PSTN
signaling, since that determines the actual cost of call termination and
out-of-pocket expenses to other providers. It doesn't matter whether the
caller was talking or silent the whole time.

> might be basing session duration based billing on,
> then you are just whistling in the dark. Moving
> the call statefulness to a PDP only obfuscates
> which box got defrauded.
> 
>           Mike
>

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---

From: Michael Thomas <[email-masked]>
Date: Wed, 11 Jul 2001 10:24:41 -0700 (PDT)

> caller was talking or silent the whole time.

While I don't disagree with your time-and-place
kind of argument re PBX's, etc, there's a fairly
trivial scenario which doesn't involve the PSTN
that could lead to QoS fraud with untrustworthy
cooperating UA's. The DQoS/media authorization
draft talks about this ad-nauseum, but the basic
point is that if you don't have some sort of choke
on the media, then you're pretty wide open to
people sending a BYE to the signaling
intermediaries while happily honoring an
X-Not-Really end to end.

		    Mike

---

From: Michael Thomas <[email-masked]>
Date: Thu, 12 Jul 2001 07:08:06 -0700 (PDT)

Henning G. Schulzrinne writes:
 > Assuming that the proxy server and gateway are in the same trust domain,
 > this is fairly hard, since I would log the BYE request only if there's
 > been a 200 from the right party. Yes, you need to deal with max-hops,
 > but that would not trigger a 200.

   That's not what I'm assuming. I'm assuming that we're
   two bad guys colluding to get free enhanced quality of
   service from my service provider, or you-name-the-service
   which relies on accurately knowing the begin and end of
   calls (like, oh say, calling cards). Unless you have 
   some access to the media, we can always tell the
   service provider that the call is over and merrily
   take advantage of our fR33q0S.

		  Mike