On 24 Mar 2015, at 14:32, Juliusz Chroboczek wrote:
Dear all,
At todays meeting, the claim was made (at least twice) that adding
dynamically computed metrics to IS-IS is "just a feature". I strongly
disagree with this assessment -- it's an open research problem, and
a difficult one at that.
I think Margaret tried to convey that it would not be as simple as
copying the work from babel, but that we should be able to leverage the
work already done there.
Any interesting metric (packet loss, delay, etc.) will cause a
negative
feedback loop, which will lead to oscillations. In Babel, there are
three
mechanisms that cope with the oscillations caused by feedback loops:
1. the protocol is loop-avoiding, which means that even when
oscillations
happen they don't normally cause packets to be lost;
2. the protocol uses delayed updates, which means that even when a
metric
is varying the amount of network traffic remains controlled;
3. the protocol uses a hysteresis mechanism which limits the frequency
of
oscillations.
Points 2 and 3 are, I believe, the work we should be able to fairly
easily leverage.
IS-IS is fundamentally based on the notion that a topology change is
propagated throughout the network in a timely manner and SPF is
recomputed
by all nodes -- it has no loop-avoidance mechanism other then timely
reconvergence. If implemented naively, a dynamically computed metric
will
cause repeated flooding, repeated SPF computation, and repeated
transient
loops.
In fact much work has been done on this in the routing area. For example
please see:
https://tools.ietf.org/html/rfc6976
Thanks,
Chris.
I'm sure these issues can be solved, and I'm pretty confident that
Henning
can tell you how; at any rate, it would be a very interesting research
project. However, it is a difficult one, and the three techniques
used in
Babel do not apply directly to a link-state protocol.
-- Juliusz
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