Speaking of NTP...

[alh-ietf at tndh.net (Tony Hain)]

I have had a consistent 10ms offset on a set of servers for the last 5 years. After extensive one-way tracing, it turns out there is a 20ms asymmetry "within" the Seattle Westin colo between HE & Comcast, causing all the IPv6 peers appearing over the HE tunnel to be 10ms offset from everything else. There may be other instances of indirect peering causing a static asymmetric path delay, and NTP will report that as an offset of half of the difference. 

Tony


> -----Original Message-----
> From: NANOG [mailto:nanog-bounces+alh-ietf=tndh.net at nanog.org] On
> Behalf Of Rafael Possamai
> Sent: Thursday, July 16, 2015 8:53 AM
> To: Matthew Huff
> Cc: nanog at nanog.org
> Subject: Re: Speaking of NTP...
> 
> Depending on how exactly you have these servers configured with relation
> to one another, small variations from one single source can be augmented
> down the line.
> 
> https://en.wikipedia.org/wiki/Propagation_of_uncertainty
> 
> 
> 
> On Mon, Jul 13, 2015 at 8:17 AM, Matthew Huff <mhuff at ox.com> wrote:
> 
> > We have 5 NTP server:  2 x stratum 1 rubidium oscillator time servers
> > with GPS sync, and 3 servers running NTP 4.2.6p5-3 synced to external
> > internet based NTP stratum 1 servers. We monitor our NTP environment
> > closely, and over the last 10+ years, normally all of our NTP servers
> > are sync'ed within
> > +/- 2 msec. Starting last Friday, we started seeing some remote NTP
> > +servers
> > with GPS reference consistently offset by 10 msec.
> >
> > Any one else seeing this?
> >
> > ----
> > Matthew Huff             | 1 Manhattanville Rd
> > Director of Operations   | Purchase, NY 10577
> > OTA Management LLC       | Phone: 914-460-4039
> > aim: matthewbhuff        | Fax:   914-694-5669
> >
> >