Thanks. We have always had a few outliers, but we have never had a large number of external NTP have a consistent offset, and not one as big as 10ms. Something changed last Friday, probably at some peering point that caused the issue. Maybe a symmetric path got created to route around some outage. Maybe some MPLS circuit got introduced into the mix that hides the underlying path/latency. Glad to know someone else has seen something like this.
Our 3 NTP servers that sync from external sources have at least 5 upstream stratum 1 servers and are peered to each other . They have settled on a sense of time that is good within +/i 1 msec of our strata 1 clocks, so all is good, but it was a stage occurrence after we had been good for so long. Each of our servers are clients of our 2 x strata 1 servers and 3 x strata 2 NTP servers. They all look good now. > On Jul 16, 2015, at 3:24 PM, Tony Hain <alh-i...@tndh.net> wrote: > > 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 >>> >>> >
