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Sorry, it's just not that clear to me from the man pages, Wikipedia, etc.

I understand the concept of stratum 0 as an authoritative time source and stratum 1 as the connected server. However, I am not sure what to consider for a larger network that may have additional hops between the stratum 1 time server and ultimately the desktop. Do you consider stratum incremented with each hop as you would see in traceroute... or would these hosts be stratum 2 regardless of network "distance."

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2 Answers

up vote 5 down vote accepted

The "official" NTP client software simply increments stratum by one for each connected server. That is all servers connected to a Stratum 0 server are Stratum 1, all servers connected to them are Stratum 2, and so on.

In general this is sensible. You most certainly do not want to increase your stratum by 1 per network hop, as this is:

  1. unreliable, since not all hops will show up in traceroute anyway, and
  2. would result in crazy stratum numbers, it is not uncommon to be 10 or more traceroute hops away from your NTP provider.

In general the NTP protocol is designed to be resilient to "network distance", that is RTT are not terribly relevant. What is much more relevant is the consistency of those RTTs.

Edit: To address one of the comments, the normal "remote" configuration has the local clock as being stratum 12. In general, if a server is being synced up against a stratum 0/1/2 server then it should be considered more reliable than the clock on the computer's motherboard. This means that it should have a stratum less than 12, otherwise "normally configured" clients will believe that they are more reliable.

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This makes sense. Regarding crazy stratum numbers though, I see that up to 256 is supported and that stratum is not indicative of accuracy. This is why I avoided the assumption that stratum 30 or 40 would be too far fetched let alone 5 to 10 as I would see in my own scenario had it been left to hops counting. Thanks for the clarification! –  Aaron Copley Nov 3 '10 at 23:14
    
@Aaron: You are correct, that the spec allows for a large number, however in normal use, you will never see a stratum above 4, unless it is synced to its local clock, at which point it will normally be put on stratum 12. –  Paul Wagland Nov 3 '10 at 23:17
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We run 2 servers syncing with external stratum-1-sources like ptbtime1.ptb.de, ptbtime2.ptb.de, ntps1-0.cs.tu-berlin.de. Since ptb.de is driven by atomic clocks and ntps1-0.cs.tu-berlin.de uses GPS (atomic clock driven as well), those are stratum-1 and our 2 servers are stratum-2. All the other servers inside our network refer to those 2, so, all the others are stratum-3.

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