[time-nuts] Tracking NTP displacement and correlation between two clients.

[Attila Kinali]

On Thu, 4 Oct 2012 13:04:27 -0400
Bob Bownes <bownes at gmail.com> wrote:

> The problem stems from one of the two (identical) machines drifting off by
> 60-70 seconds per day. So a few ms here and there are ok.

Is it drifting without ntp or with ntp?
1minute drift per day is not unheard of for standard PC RTCs.. i've seen
even worse.. 

> This is all relative to two internal stratum 2 servers on the same command
> and control network. No large xfers allowed over it. There are physically
> separate data, backup, and application networks for that.

On the average LAN (ie one that is connected by a couple of switches.
No routers inbetween), the RTT will be dominated by the minimum packet
times of ethernet. That's where the 200us delay comes from (actually
it's below 1us for Gbit, but for some reason i've never seen it go under
100us). With modern switches that do worm-hole routing (ie just use the
destination address in the first few bytes and then pass the packet on
without further delay, while the "end" of the packet is still on the wire
and not yet received by the switch) you dont even get much added delay
from using multiple switches in line. With such large delays you can ignore
any wire delay completely (which is in the range of 1us for maximum length
cable).

With routers it looks a bit different, there the whole packet is first
stored in memory before being processed (due to more complicated routing
decision). Due to this, RTT is still dominated by delay of the network
hardware and not speed of light. Actually speed of light related delays
will be buried deep in the noise unless you go trans-continental. 
And even then, router delay will still dominate. (eg RTT Europe-Japan
is around 500ms, by which time a packet would be half way to the moon)

So, if you have any significant time difference (>1ms) between two systems
that synchronize to the same NTP server in the same LAN, then the
problem lies somewhere else than the network.


			Attila Kinali
-- 
There is no secret ingredient
         -- Po, Kung Fu Panda