[ntp:questions] Garmin 18 LVC: whether to fudge

Kevin Oberman oberman at es.net
Tue Feb 17 00:13:33 UTC 2009

> Date: Mon, 16 Feb 2009 09:38:25 -0500
> From: Danny Mayer <mayer at ntp.org>
> Sender: questions-bounces+oberman=es.net at lists.ntp.org
> Uwe Klein wrote:
> > Danny Mayer wrote:
> >> Be warned that ping uses ICMP and not UDP so the costs are different.
> > 
> > 
> > Danny,
> > 
> > we are looking at the physical transport layer i.e.
> > "physical link layer" which does not care about protocol.
> > 
> > Latencies on a DSL line are impacted by packet size,
> > effective datarate, lookahead error correction
> > and some ancilary stuff _per_ direction.
> > 
> > Aditionally depending on configuration up and down stream
> > utilisation can have impact on the opposite path.
> > 
> > uwe
> Yes, I know all that. Nevertheless routers and the like also treat ICMP
> packets differently from UDP and TCP. There are lots of reasons to do so
> but we are getting far afield from the original question at this point.

Routers treat ICMP differently when destined to the router processor,
supervisor or routing engine. (These are similar things in different
routers.) I know of no commercial router with hardware based forwarding
that treats transit ICMP any different from transit UDP or anything

It is possible to use CoS and multiple hardware queues to do such a
thing, but I doubt it is common practice as there is little reason to do
so and it is fairly complex to set up (depending on the particular
router involved).

But we are digressing badly from the topic at hand.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751

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