[ntp:questions] Query about NTP accuracy
Richard B. Gilbert
rgilbert88 at comcast.net
Fri May 22 00:59:32 UTC 2009
Andy Yates wrote:
> Does anybody have any figures that shows the effect on accuracy of an
> NTP v3 client using a stratum 1 server rather than a stratum 2 or 3
> server? It's all in a GE LAN based scenario, commercial stratum 1
> servers connected to GPS and stratum 2 and 3 servers are typically
> dedicated Linux boxes.
Stratum has very little to do with accuracy! It's simply a count of the
number of servers between you and the root of the tree; e.g. the atomic
If you have low latency connections a stratum three server can give you
millisecond or maybe microsecond accuracy. Let those connections get
really busy and your accuracy goes down the toilet even at stratum two!
> The reasons is that I would rather scale by adding strata - its a very
> big data center with thousands of clients and has several "zones" that
> are isolated. However some opinion is suggesting we run IRIG-B between
> the GPS receiver and a bunch of stratum 1 servers and clients access
> these directly. Much more expensive and any increase in accuracy from a
> client experience may be negligible.
> However I'm been pressed to supply an SLA for accuracy. My argument is
> that although you can get your stratum one server to synchronize to
> microseconds of UTP, as soon as the client uses NTP v3 over the LAN,
> even a GE LAN, then the accuracy degrades and putting well designed well
> specified stratum between the boxes is not going to decrease accuracy
> sufficiently to warrant purchasing many stratum one appliances.
NTP v4 over the LAN would be just as bad as V3. Think about broadcast or
multicast! That way you don't clutter up the net with queries and
response. Instead, every N seconds your server announces: "at the chime
the correct time. . . ." and your clients can adjust their clocks as
necessary. Your broadcast and/or multicast clients will exchange
packets initially in order to establish the round trip delay. After
that initial volley, the clients just sit and listen.
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