Can we please stop talking about pings? Bob Camp <[email protected]> wrote on Fri, 13 Jan 2017 at 15:12:38 -0500 in <[email protected]>:
> 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. [email protected] John Hawkinson _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
