Subject: [Sipping] Re: [Sip] Loop detection; Not a bad thing to have

From: Adam Roach <[email-masked]>
Date: Fri, 24 Mar 2006 10:07:29 -0600

On December 30, 2002 at 10:50 AM, Jonathan Rosenberg wrote:

> The problems we had with loop detection were that it became
> increasingly unclear about what a loop was, as opposed to a spiral.
> Initially (in rfc2543), a loop was any request that arrived at the
> same proxy (there were no spirals). It became clear that this was
> broken, so we had to consider changes in the request URI. Then, we ran
> into cases were other fields would change, such as Route headers, and
> those had to be taken into account as well. Real implementations were
> frequently getting this wrong, hampering interop and proper
> functioning of applications. As we began to realize that more and more
> headers might need to be taken into consideration, it became clear
> that properly doing loop detection was simultaneously ill-defined and
> complex to implement (potentially requiring hashes of the entire
> message).
>
> That is coupled with the fact that such loop conditions will probably
> be rare in deployed networks. Unlike IP routing, SIP routing is
> considerably more static, and I consider the likelihood of real loops
> a rare one outside of a lab. Thus, its appropriate to handle them in a
> way which perhaps consumes more resources, at the cost of much lower
> complexity, and more importantly, proper operation in the normal case.
> Thats the max-forwards mecahnism. It is much easier to implement,
> always correct in the normal case (i.e., it won't mis-detect loops),
> and much more interoperable.
>
> In other words - this is good engineering practice. We don't worry
> about optimally solving a corner case, we optimize for the baseline case.

Once again, I find myself agreeing with Jonathan about loop detection.
We should ensure that the amount of work done at a proxy for loop
detection is minimized.

/a

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---

From: Adam Roach <[email-masked]>
Date: Fri, 24 Mar 2006 10:38:41 -0600

Vijay K. Gurbani wrote:

> Adam Roach wrote:
> Adam: So just to be sure -- you are advocating that we do need
> loop detection, just that the mechanism to do so should be
> minimally invasive to the processing of a proxy.  Yes?

Right.

> Jonathan's emails that you quote seem to be leaning in the
> direction of not having loop detection.

Correct. I'm agreeing with his statement of the problem. We now know
that the proposed solution doesn't work -- but that doesn't mean that
the objections Jonathan raises have magically solved themselves in the
interim.

> PS: You did dig deep into the past emails (2002, wow!).

2002 is the last time we had a conversation around the complexity of the
loop detection solution currently in RFC 3261. It is important to retain
our institutional knowledge -- otherwise we have to re-argue the same
points each time an issue comes up.

/a

---

From: "Vijay K. Gurbani" <[email-masked]>
Date: Fri, 24 Mar 2006 10:25:36 -0600

- vijay
-- 
Vijay K. Gurbani  vkg@{lucent.com,research.bell-labs.com,acm.org}
Bell Laboratories, Lucent Technologies, Inc.
2701 Lucent Lane, Rm. 9F-546, Lisle, Illinois 60532 (USA)
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