Subject: [SIP] loop detection ??

From: Bertil Engelholm <[email-masked]>
Date: Mon, 23 Oct 2000 16:42:19 +0200

Hi,

I have a question regarding loop detection.

In the description of VIA-headers it says that the loop detection
value MAY be a hash of TO,FROM,CALL-ID,CSEQ and REQUEST-URI.

So I wonder if it's not enough to only use the REQUEST-URI. Can
there exist cases where someone changes any of the other fields
(and thereby change CALL-LEG) and continues with the VIA-list 
that existed before the change ? Is this allowed ?

If someone does this I guess they have to be prepared to change
the values back when the response is sent back. Otherwise the
caller will not be able to recognise the CALL-LEG ?

-- 
Bertil Engelholm
AXE Research and Development        voice : +46 8 727 3499
SIP Security                        Fax   : +46 8 647 8276
S-126 25 Stockholm Sweden           E-mail:
[email-masked]

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From: "Jo Hornsby" <[email-masked]>
Date: Tue, 24 Oct 2000 09:08:32 +0100

> So I wonder if it's not enough to only use the REQUEST-URI.

Yes.  Technically, the branch parameter must be a function of
the Request-URI, but this is the only component of a message
that must be used.

> that existed before the change ? Is this allowed ?

This is sort of allowed, although the entity that did this
would not be a proxy (more like a B2BUA).  In such cases,
all bets would be off (it might have also changed the Vias,
thus it becomes impossible to detect loops in such cases).

> caller will not be able to recognise the CALL-LEG ?

Very much so.

 - Jo.