[OpenSIPS-Users] Accounting: How to avoid a fraudulent BYE with lower CSeq?

[Jiri Kuthan]

Adrian Georgescu wrote:
> I beg to differ, but this is just my humble opinion based on my 
> experience with my particular customers.
> 
> The most economic and future-proof way to perform accounting for SIP 
> sessions is the SIP Proxy server alone.

This may be probably ok, as long as you don't intend to use such accounting
data for billing. (which may be still useful)

The trouble is that proxy-produced accounting data is remarkable 
incomplete and
inaccurate. It does not include QoS info, PSTN info, and they are 
sensitive to
the attacks mentioned  before that make a BYE work for a GW but not for 
a proxy
and vice versa, or other ways how BYE can be broken due to an error or 
fraud.


> 
> My personal experience is that gateways come and go in a provider 
> configuration and they are in many cases under the control of a 
> third-party that provides the PSTN termination service. When you do LCR 
> across many different gateways, which are not even yours the 
> only aggregation point for traffic is the SIP proxy 
> that authenticates and authorizes the requests. Over time, the gateways 
> change hands, get upgraded or removed much more often then the proxy 
> itself, which maintains its central role over time. 

There is certainly some invariable in a system but to my best knowledge
that's the DB backend (for example RADIUS) which gets almost never touched,
not a proxy server. The DB is the piece that is invariable, untouchable,
central in every respect, and therefore used for aggregation of usage data,
as directly as possible. I see little value on putting a SIP proxy on the
way from the service box knowing ALL call data and the final destination
of the usage data (some database).

(I agree proxy is the best place for authorization and authentication 
but that's
a different story than accounting.)

> Secondly, once you
> do more the voice like video and other services that require billing and 
> are not PSTN related, the SIP Proxy is the only network element that has 
> access to the signalling and is able to generate accounting tickets.

That seems appealing indeed, it is just that I have encountered very few 
(still some
though) who would be seriously billing for on-net calls on a per-minute 
basis.
(they haven't found a way to do sell credibly a single usrloc lookup
on a per-minute basis or didn't consider the on-net share of traffic 
significant or
thought the CDR producing expense was just not worth it) It makes sense 
as you say
to produce CDRs in a proxy if termination is provided by a third party, 
but to my
best knowledge these are based on their inaccuracy used for 
reconciliation rather
than as source of authoritative data.


> 
> Solving the accounting related problems at the SIP Proxy level is a 
> worthwhile investment 


Yes, but only if you don't care about accuracy and completeness of the 
usage data,
i.e., you don't do billing. Otherwise the customer-care cost is 
unpayable in addition
to the expense of doing it at all. The per-minute margins are so poor 
and accurate
CDR processing is such an expense, that it alone explains the increasing 
flat-rate
offerings. We have been doing it only in the reconciliation case you 
mentioned,
merely as non-authoritative data.

If you however do have a scenario, in which accuracy and completeness 
matters for
billing sake, investment in proxy-based CDR production seems to me only 
likely to
produce liability.


-jiri

> while other options are just temporary fixes that
> work in a particular case for a limited amount of time and that is a 
> waste of money.
> 
> Adrian
> 
> On Jan 7, 2009, at 2:25 AM, Jiri Kuthan wrote:
> 
>> authentication does not provide actually value here. dialog would not
>> either, since
>> the same trick can be achieved for example by low max-forwards. IMO the
>> proper
>> choice is accounting from the gateway, which provides the actual service.
>> A proxy can only provide an approximation which is inherentely to some
>> extent
>> more error-prone than the box doing the actual job.
>>
>> -jiri
>>
>> Bogdan-Andrei Iancu wrote:
>>> Hi Iñaki,
>>>
>>> Have you consider requesting auth for the BYE ? from SIP point of view
>>> is perfectly valid....
>>>
>>> Regards,
>>> Bogdan
>>>
>>> Iñaki Baz Castillo wrote:
>>>> Hi, I'm thinking in the following flow in which the caller/attacker
>>>> would get an unlimited call (but a limited CDR duration):
>>>>
>>>> --------------------------------------------------------------------------
>>>> attacker                     OpenSIPS (Acc)                    gateway
>>>>
>>>> INVITE (CSeq 12)  ------>
>>>> <-------- 407 Proxy Auth
>>>>
>>>> INVITE (CSeq 13)  ------>
>>>>                                              INVITE (CSeq 13)  ------>
>>>>                                              <------------------- 200 Ok
>>>> <------------------- 200 Ok
>>>>                          << Acc START >>
>>>> ACK (CSeq 13) ----------->
>>>>                                              ACK (CSeq 13) ----------->
>>>>
>>>> <******************* RTP ************************>
>>>>
>>>> # Fraudulent BYE !!!
>>>> BYE (CSeq 10) ----------->
>>>>                          << Acc STOP >>
>>>>                                              BYE (CSeq 10) ----------->
>>>>                                              <-- 500 Req Out of Order
>>>> <-- 500 Req Out of Order
>>>> --------------------------------------------------------------------------
>>>>
>>>> The call hasn't finished, but OpenSIPS has ended the accounting for
>>>> this call since it received a BYE. And this BYE will generate a
>>>> correct ACC Stop action (since it matches From_tag, To_tag and
>>>> Call-ID).
>>>>
>>>> I think this is *VERY* dangerous and I hope I'm wrong.
>>>>
>>>> Would help the dialog module here? does the dialog module check the
>>>> CSeq of the BYE in some way and could it prevent OpenSIPS from
>>>> generating the ACC STOP action? (I don't think so).
>>>>
>>>> Any idea?
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>>
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