Speaking of NTP...

Matthew Huff mhuff at ox.com
Thu Jul 16 19:43:41 UTC 2015


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-ietf at 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.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
>>> 
>>> 
> 




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