Subject: [SIP] Late Loop Detection

From: "Jo Hornsby" <[email-masked]>
Date: Tue, 25 Jul 2000 16:36:28 +0100

Hi again,

I am reraising this in the hope that it can be resolved without
becoming an "OPEN ISSUE" (I really don't feel it's that significant,
as a question, but I do feel it needs answering).

I use the term "late" here to indicate that an upstream stateful
proxy did not detect a loop, and thus ended up sending a request
downstream "twice".

If a proxy detects a looped request, should it always respond with
482, even if it knows it will not be able to distinguish between
an ACK/CANCEL for the "legal leg" of the request and the "looped leg"
of the request?  This is also undesirable because it can confuse
the proxy where the request really looped from thinking that a
a transaction has completed when it has not.

The best "solution" (that I can come up with, anyway), is to
recommend that proxies create branch parameters in their Vias,
which somehow have a unique component, such that any proxy
would be able to recognise if it had generated a Via just by
looking at the branch parameter.  This way, proxies can easily
do loop detection just by looking at the branch parameter in
Vias.  Of course, this doesn't really help a proxy if it does
detect a loop "late", but it should prevent the most likely
scenario where this occurs (when Via hiding is employed).

For a more verbose and even less interesting discussion, have
at my post(s?) in the thread "[SIP] hiding via field and loop
detection" from sometime last week.

Cheers,

 - Jo.
-- 
  Jo Hornsby; IT Professional         --       mailto:[email-masked]
 Ubiquity Software Corporation        --         http://www.ubiquity.net/

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