Subject: RE: Deprecate loop detection, was: Re: [Sip] bis draft: Pre-existing Route headers

From: "Chiou, Mark" <[email-masked]>
Date: Tue, 8 Jan 2002 12:48:21 -0600

Hi,

The reason to use Record-route and Route is to ensure the Proxy Server
to receive all the SIP request/response messages. You know without
Record-route and Route, the ACK request can go different route such that
a Proxy Server won't be able to capture the answer time. The same reason
for capturing disconnect time when UAC and UAS release (i.e. BYE
request) the call.

The Max-forwards is decreased by one is fine as long as it can be used
when interworking with PSTN call forwarding features.

We are talking the same thing. proxy does need a loop detection
mechanism.

Mark Chiou
Santera Systems

-----Original Message-----
From: Christer Holmberg [mailto:[email-masked]]
Sent: Tuesday, January 08, 2002 12:29 PM
To: Chiou, Mark
Cc: Jonathan Rosenberg; Drage, Keith (Keith); [email-masked]
Subject: Re: Deprecate loop detection, was: Re: [Sip] bis draft:
Pre-existing Route headers

Hi,

>**************
>No, we do need the route header for Proxy Servers. The record-route and
>route are used by the Proxy Server such that the Proxy server knows
when
>the call is answered and disconnected in order to do billing
>************** Mark Chiou

Maybe I don't get the point, but why do you need the Route and
Record-Route headers to know when a call is answered and disconnected?

>**************
>Do I miss something, I thought the MAX-Forwards is used when a call is
>forwarded. In PSTN world, the default number allowing a call to be
>forwarded is 5. This means a call can only be forwarded (regarding any
>combination of forwarding flavor: CFU, CFB, CFNA, etc) up to 5 times.
>The Proxy Server shall check the max-Forward count before forwarding
the
>call and increment max-Forward by one in the message toward the next
>Server.

In SIP the MAX-Forwards value is DECREASED by one by proxies. When it
reaches zero, an error response is then sent back to the sender.

>The loop detection is used when a call is ended up forwarded to itself.
>For example, A calls B and B CFU to C and C CFNA to A.
>************** Mark Chiou

Yes, but what we are discussing here is if the loop detection mechanism
used by proxies is needed or not. 

If a client (in your case A) initiates the call, and it is routed back
to that client, that mechanism is not needed to detect the loop.

Regards,

Christer Holmberg
Ericsson Finland

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---

From: "Chiou, Mark" <[email-masked]>
Date: Tue, 8 Jan 2002 15:39:08 -0600

I see your point, but I thought the internet is no boundary such that
the hops should be less than the SS7 networks.

For example, a call for Dallas US to Beijing China, The call might only
needs one national gateway in Dallas, one international gateway in
Seattle US, one in Japan to tandem to China for PSTN network. This is
the best scenario of course, it requires 5 to 6 switches.
For IP world, If the IP address can be resulted to China directly, then
only 2 to 3 Proxies are needed.
Am I missing something? 

Plus, do we consider the performance for two many hops?

Mark Chiou

-----Original Message-----
From: Cullen Jennings [mailto:[email-masked]]
Sent: Tuesday, January 08, 2002 3:21 PM
To: Chiou, Mark
Cc: [email-masked]
Subject: RE: Deprecate loop detection, was: Re: [Sip] bis draft:

>  Default value to 20 for Max-forwards is way too much.
>
>  Mark Chiou
>  Santera

I don't think that 20 is too much. It is amazing how many proxies one
goes
through in some deployed systems today. When you start considering ASP's
and
service route ideas it is going to get even larger than it is today.

Cullen

---

From: "Zmolek, Andrew (Andrew)" <[email-masked]>
Date: Thu, 17 Jan 2002 20:00:40 -0700

Jonathan said:

> 2. How does it know that the request received a second time is the
same? 
> Your PBX probably looks at DNIS or something. This is equivalent to 

Actually, the PBX often has no way of knowing that a loop is happening
when it gets locked into a situation where two switches are forwarding
to each other. Some older (but well-used) trunk types don't present
DNIS, and even on ISDN it can be unreliable. The point is that there are
forwarding loop cases that are almost impossible to avoid (particularly
when gatewaying is involved), and this 15-second hack prevents a pretty
easy PBX DoS attack. Would a gatewayed SIP environment be any smarter
about this when there are two gateways forwarding to each other? 

> looking at the request-URI in SIP, and checking if there is another 
> transaction in progress for that same r-uri. This would require an 
> additional mapping of all transactions using purely the r-uri as the 
>key. The alternative is to include the original r-uri in a hash in the 
via, and then, well, we are right back to the originally specified 
algorithm.

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