North American Network Operators Group

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Re: Dampening considered harmful? (Was: Re: and other email grief)

  • From: Iljitsch van Beijnum
  • Date: Tue Dec 21 05:20:22 2004

On 21-dec-04, at 9:16, Jerry Pasker wrote:

IF there's a connection problem, or implementation difference that makes a lot of up/down, then dampening could occur close to the "problem" but it will be contained close, and won't spread to the rest of the internet.
Today's AS hierarchy is quite flat, which severely limits the usefulness of dampening. If the link between ASes A and B flaps, then B doesn't get to dampen these flaps. C, connected to D, does, but if C is a small network that doesn't help much as flap dampening brings its own overhead. In a two or three router network there probably isn't any advantage in dampening. Only when you get to protect a larger number of routers from the update, it helps. Now of course D, connected to C, will be isolated from the instability. But in today's internet, there often isn't a D. According to the weekly routing table report the current average AS path length is 4.5. Subtract at least .5 for prepending, and there must be a significant number of 3 or even 2 AS hop paths to get the average at 4.