[ntp:questions] what happens when sys.peer turns stratum 16?

unruh unruh at wormhole.physics.ubc.ca
Mon May 31 03:46:25 UTC 2010


On 2010-05-30, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> bombjack wrote:
>> Hi,
>> I am fairly new to the ntp arena, but have to learn fast as part of my
>> work is trying to fix ntp problems. Lately I had a customer that
>> complained that there servers went bananas due to time being wrong.
>> (The customer claims that ntp had changed the time 20 years ahead).
>
> That is somewhere between highly improbable and impossible!  NTPD had to 
> have some human "help" to do that.
>
>> Looking at some wire shark logs it seems that sys.peer for some reason
>> had turned into a stratum 16 server (i.e. unsynched) and had "leap"
>> set to 3 -"Alarm condition (clock not synchronized"
>> 
>> the setup: klient syncs to 2 servers, one stratum 1 (sys.peer) and one
>> stratum 2 server. If sys.peer all of a sudden says it's a stratum 16
>> server, how does ntp react to this? instant outvote of the sys.peer?
>> or are these packets filtered for some time as being errornous? or
>> anything else?
>
> With only two servers nobody can outvote anybody else!!  Is it not 
> written that a man with two clocks can never be certain what time it is?

But if one clock tells you that it itself is completly nuts, it is not
hard to tell which the right one is. 

If you ask me and my friend what time it is, and he says it is 12 noone,
and I say it is 5PM, but my watch is broken, is it hard for you to
figure out what the time is?

>
> This is the reason why four servers is the minimum recommended 
> configuration.  One server can fail; either fail to respond at all or 
> respond with an incorrect time and be voted out and you still have a 
> working configuration.

That only requires 3 servers. And 4 has a problem that two can wander
off togetehr ( eg both depend on the same bad timesource). and then the
two groups cannot outvote each other either.

>> 
>> I have told the customer to use more than two servers, but they insist
>> that they want answers to the above scenario.
>> 
>> Any help welcome
>> 
>> //Fredrik
>
> I hope you charge them extra for this!  You can't itemize it as a fine 
> for stupidity but I bet you wish you could!
>
>




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