NAP/ISP Saturation WAS: Re: Exchanges that matter...

David Miller david at
Thu Dec 19 20:25:47 UTC 1996

On Tue, 17 Dec 1996, Chris Caputo wrote:

> There is considerable difference between forwarding a packet that happens
> to contain ICMP data (destination not the router) and responding to a
> packet that contains ICMP data (destination is the router).  In the
> former, priority in a Cisco is the same for ICMP as it is for UDP or TCP,
> since this part of the packet is not even being examined.  In the later,
> priority is lower and can be ignored altogether. 
> I treat ignored (link good, but no response received) ICMP echo requests
> as an indicator that a router is too loaded.  If the router has been
> pushed to the point of not being able to respond to an ICMP, how well is
> it going to do when a bunch BGP updates occur?  (rhetorical)  Both are CPU
> intensive operations. 

Would someone please tell me just why icmp echos are "cpu intensive"?

Yes, I know they're in software.  So what? A PC can respond to an 
ethernet loaded with them with a trivial percentage of it's CPU cycles.

This sounds to me a whole lot more like a solution to an imagined 
problem, but I'm prepared to be convinced that responding to pings 
actually takes a great enough percentage of CPU cycles to slow down 
packet delivery.....


		It's *amazing* what one can accomplish when 
		    one doesn't know what one can't do!

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