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....@nanog.org] On > Behalf Of Rafael Possamai > Sent: Thursday, July 16, 2015 8:53 AM > To: Matthew Huff > Cc: nanog@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 <mh...@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 > > > >