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

Tue Dec 21 10:18:51 UTC 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.