[ntp:questions] The libntp resumee...

Danny Mayer mayer at ntp.isc.org
Mon Oct 13 03:43:02 UTC 2008


Kay Hayen wrote:
> What worried me more was how often we can query the local ntpd before it will 
> have an adverse effect. Meantime I somehow I sought to convince me I should 
> be able to convince myself that ntpq requests are served at a different 
> priority (other socket) than ntpd requests are. I didn't find 2 sockets 
> though.
> 

They aren't. It's the same socket and each packet is responded to in
turn irrespective of the content. It's also not possible to create a
separate socket unless we have a separate command channel and that does
not currently exist and is nowhere defined in the protocol.

>> Briefly, you use the defaults for MINPOLL and MAXPOLL.  You may use the
>> "iburst" keyword in a server statement for fast startup.  You may use
>> the "burst" keyword ONLY with the permission of the the server's owner.
>> 99.99% of NTP installations will work very well using these rules".  If
>> yours does not, ask here for help!
> 
> Now speaking about our system, not the middleware, with connections as 
> follows: 
> 
> External NTPs <-> 2 entry hosts <-> 8 other hosts.
> 
> And iburst and minpoll=maxpoll=5 to improve the results.

This indicates that you don't understand NTP. You should never ever
change the minpoll and maxpoll values unless you understand the NTP
algorithms in detail and understand the consequences of changing them.
The default values were very carefully chosen to provide a balance
between various conflicting requirements to provide the most stable
clock discipline over a wide range of environments. You are
undersampling at the start of NTP and then oversampling as it starts to
stabilize the discipline loop.

> 
> Currently we observe that both entry hosts can both become restricted due to 
> large offsets on other hosts, so they become restricted and that will make 
> the software refuse to go on. Ideally that would not happen.
> 

If the servers that it uses become divergent it will be unable to pick
the "best" one and it will become unsynchronized.

> I will try to formulate questions:
> 
> When the other hosts synchronize to the entry hosts of our system, don't the 
> other hosts ntpd know when and how much these entry hosts changed their time 
> due to input? 
> 
> Would NTP would be more robust if we would configure routing on the entry 
> hosts, so that they can all speak directly with the external NTPs on their 
> own?
> 

Yes since the stratum will be lower so that the error budget will also
be lower.

> Is the use of ntpdate before starting ntpd recommended and/or does the iburst 
> option replace it?
> 

ntpdate is deprecated and is not normally needed. Make sure you start
ntpd with the -g option to step the clock initially to close to the
correct tick.

Danny
> Best regards,
> Kay Hayen



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