[c-nsp] Dampening

[David Barak]

I seem to be in a minority regarding dampening,
although I would encourage anyone else to read Randy
Bush, Tim Griffin, and Z. Mao's research presentation
regarding how dampening ACTUALLY works in the world - 

http://www.nanog.org/mtg-0210/ppt/flap.pdf

The key takeaway is that dampening is NOT per path,
rather, it's per prefix.

What that means is that the use of dampening can
effectively supress reachability for all of the
customers and downstreams of the provider performing
the dampening.  

Hypothetical example - the customer (cx1) in question
is homed to ISPs 1 and 2, and ISP2 does the dampening.
 Assume that the ISPs are comparable in size and
peering, and are peered with each other.

In the event of a repeated link flap (in my
experience, BGP itself is not what generally flaps,
most BGP flaps are due to link or router flaps), the
transit customers or peers of ISP2 will lose their
route to cx1.  The peers will eventually converge on
the path through ISP1, assuming that they have some
natural way to learn that route.  However, the transit
customers of ISP2 will not ever learn the route to cx1
through ISP1, because any ISP2 wouldn't propogate it.

So in this case, the dampening which ISP2 is
performing will contribute directly to the inability
of ISP2's customers to send traffic to cx1 through
ISP1.

Regarding placing the burden on the customer, the
point I'm trying to make is that most incidents which
cause dampening are one of two things - either some
kind of customer-generated testing (repeated reloading
to test new IOS or some such), or are the result of
some service-provider problem such as a bad circuit.

If my provider chose policies which made outages
WORSE, then they wouldn't be my provider for long. 
But, as has been said before, I encourage my
competitors to do this...

-David Barak

(PS - apologies if anyone was offended by the
signature on my previous email, this is a personal
mailbox, and I had forgotten it was there)


		
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