[time-nuts] wifi with time sync
jhawk at MIT.EDU
Fri Jan 13 15:25:23 EST 2017
Can we please stop talking about pings?
Bob Camp <kb8tq at n1k.org> wrote on Fri, 13 Jan 2017
at 15:12:38 -0500 in <C88C78A6-A015-4DCC-9E23-394DC33A3470 at n1k.org>:
> I’m sure you are right about the response time. Right now the
> variation is running almost 3 ms at one sigma on a ping so there is
> a lot to do simply to get the accuracy anywhere near 1 us.
Using ping remains deeply problematic as a measure of what is possible.
The underlying layer does retransmissions and inserts delays. And ping
measures around those things, not within them. But any real tool
that was using the wireless frames for timing would be measuring
the raw layer 2 frame timing.
(It's not apparent that this would require hardware timestamping support,
although that can certainly help.)
This sort of thing is doable with today's hardware and software
technology. Although I'm not aware of a tool that does this today
(but building one is not hard).
Denny Page's comment about hosts not prioritizing responding to ICMP
is...IMO misleading. That's not really what happens. When you ping a
switch, think of the switch as a network switch with a small computer
(host) attached that handles mangement functions, like responding to
pings. They are entirely decoupled, and the load on the management
computer is not related to the load on the network, and sometimes it's
really slow. That makes the claim about priority effectively true for
network equipment, but it's not generally true for *hosts*. Most hosts
will indeed respond to ICMP rapidly (in the kernel) without
prioritizing it any differently from other packets. Except the other
kinds of packets often require leaving kernel space (e.g. application
processes) which ncessarily induces more delay.
But anyhow, the upshot is correct: don't trust pings to switches, but
pings to idle hosts will be much more meaningful. But still not meaningful
on wireless networks.
So...can we please stop talking about pings? Thanks.
--jhawk at mit.edu
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