[ntp:questions] A proposal to use NIC launch time support to improve NTP

Brian Utterback brian.utterback at oracle.com
Thu Dec 20 22:17:38 UTC 2012


On 12/20/2012 6:25 AM, Ulf Samuelsson wrote:
>
>> Yes there is. The ntpd program has to set a timestamp in the outgoing
>> packet and then specify the launchtime when it writes the packet. The
>> goal here is to have the timestamp written in the packet exactly match
>> the time the packet actually hits the wire. So, the timestamp in the
>> packet must be a little in the future when it is written so that by the
>> time the controller gets it the packet can be delayed until the right
>> time. Since ntpd cannot access the clock in the controller,
>
> The Network controller will timestamp the message in H/W as it arrives
> and will add a CMESG with the timestamp, and this is supported in
> ntpd since some time.
>
> You add a small delay to the receive timestamp to make your launchtime.
>
> systime does not get involved at any stage, and can be way off.
>
> (Note that I am only interested in Stratum 1 server functionality)
>

You have to guarantee that the actual processing delay from controller 
to controller is never more than .5 seconds minus your "small delay", 
otherwise the timestamp will be off.  It would be a good idea to 
synchronize the system clock so that you can detect that kind of situation.

Also, if you are only going to be a stratum 1 server, from where are you 
going to get the actual time? If you are only going to use the 
controller timestamps then the controller better be disciplined to some 
source of time. Some how something needs to be setting the time on the 
controller.

Brian.


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