Subject: [lisp] Confirming consensus behind echo noncing

From: Sam Hartman <[email-masked]>
Date: Fri, 4 Sep 2009 09:03:18 -0400 (EDT)

Echo nonces were added to either -02 or -03.  They had been briefly
discussed on the list, but at the time the plan was to present them at
IETF 75 and have a consensus call after that.  So, this is one of the
rare times when we agreed to add some text to the document before we
had a consensus behind it.

So, echo nonces have been discussed somewhat on the list, in the draft
for two versions, discussed extensively at IETF 75.  So far we've seen
no objections.  Normally, silence is not the same as consensus, but I
don't really think we've had silence on the topic.  Enough people have
been considering and discussing the implications of echo nonces
without raising any objections that I think it is safe to say that
several WG participants have evaluated and understood the proposal.

However, I wanted to formally confirm that there are no objections.
If we do not see strong objections by next Friday then I'll conclude
that we have support to include echo nonces at this time.

All the locator reachability algorithms are up for experimentation.  I
think it would be fair to re-open the question of whether any given
reachability algorithm is appropriate based on practical experience.

---

From: Sam Hartman <[email-masked]>
Date: Fri, 04 Sep 2009 15:24:51 -0400

Speaking very much as an individual.  I'm not objecting to echo
nonces.  However, I personally think we'll find they are not very
useful.

In particular,  they are not useful in any of the following situations

* square routing
* triangle routing (a sends to b, c sends to a)
* cannot detect a full path failure: in order to conclude you cannot reach someone you need to get packets from them

I think that triangle and square routing will be very common unless we
take active steps to avoid them.  It seems likely that in any
situation where you have multiple rlocs of the same priority you'll
likely run into that case if you have a small number of flows.

Long term, especially when we take security considerations into
account, I think we'll end up with required control plane probing of
locators with possible optimizations through the data plane.  In that
environment, I think echo nonces will serve no purpose.  However this
is just my opinion.

I can't reason about or think about the performance implications until
I understand the deployment model of LISP.  In particular, the
performance concerns that matter for probing on CPEs seem very
different than say XTRs at an Amazon data center.

Regardless of the above, I think getting data on echo nonces can do no harm.

---

From: Dino Farinacci <[email-masked]>
Date: Fri, 4 Sep 2009 15:36:18 -0700

> nonces.  However, I personally think we'll find they are not very

Echo-noncing is useful because it can avoid explicit control-plane  
probing. And we envision people will use active-active multi-homing so  
traffic will be flowing in both directions between a locator pair.

> * triangle routing (a sends to b, c sends to a)

The top 2 cases are one in the same.

> * cannot detect a full path failure: in order to conclude you cannot  
> reach someone you need to get packets from them

Echo-noncing is a unilateral protocol. So the other side will detect  
its forward path is down. You don't have to detect your return path is  
down.

> likely run into that case if you have a small number of flows.

You run into the case if one side is doing active-active and the other  
side is doing active-backup. But that means that echo-noncing can be  
used on each active side of each site.

Dino

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