Re: [Sip] Question on loop detection

["Dale Worley" <dworley at nortel.com>]

On Thu, 2009-02-05 at 14:03 +0000, Elwell, John wrote:
> It is true this is a merge rather than a loop situation, but the
> conditions I describe are conditions for sending a 482 "loop detected".

RFC 3261 is correct but not at all clear in this regard.  There are
*two* error conditions, and RFC 3261 calls both "482 (Loop Detected)".
But in reality, there is "482 (Loop Detected)" and "482 (Merged
Request)".

482 Merged Request is generated by UASs, as described in section
8.2.2.2, and it is based on noticing that two incoming requests
originated as the same UAC request.  But the gotcha is that 8.2.2.2
describes UAS behavior, not proxy behavior.

482 Loop Detected is generated by proxies, as described in section 16.3
item 4, and is based on detecting that this request has an "upstream
ancestor" request that passed through this same proxy.

In regard to a B2BUA acting as an SBC, or a "quasi-proxy" as I like to
call it, it needs to apply the loop detection logic, not the merged
request logic.

Dale