[time-nuts] NIST time services

Magnus Danielson magnus at rubidium.dyndns.org
Sun Mar 23 20:07:26 EDT 2014


On 23/03/14 17:26, Jason Rabel wrote:
>> NTP is best used over the Internet. It was designed for unreliable data links.
> In the quest for expansion of NTP over the internet, one thing has always nagged me.
> You can find lists of servers and they will give a physical location along with other info about them...
> Big whoop... Often these servers tend to be tied to one backbone, so even if they are physically located in the same city as me, the
> packets still might have to travel thousands of miles just to switch networks. So what should be a 2ms delay has now become 20-40ms
> (or more)... Even if they have multiple backbones, packets coming in are not guaranteed to leave on the same network. The more a
> packet has to travel, the more uncertainty you build up... Yes NTP should still get you a reasonable time, but our quest is always
> for something better.
> If there was some sort of feature in NTP (maybe there already is???), or even a separate program that could "test" a list of NTP
> servers to try and pick the lowest latency, I think that could have a positive benefit on better time transfer.

This hits straight into one of the problems with NTP. It tries to use 
the highest stratum clock rather than best quality clock. A known trick 
is to use a set of stratum 2 servers locally and only let local users 
connect to those, and them have them peer between each other and to the 
same stratum 1 clocks. This gives much better performance then letting 
the clients to use the stratum 1 servers directly.

The hop-count is good to avoid routing loops, but it is not a good 
indicator of achieved quality.

If we have decent intermediary, this would provide much better 
performance. As fewer would query the top servers, the second level 
could query them much more often, and better filter the result for the 
benefit of performance.

But that would break the basic assumptions of NTP, and you can't do 
that, not that the protocol would object.

Your general idea is however sound, and surely you can do stuff with 


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