[ntp:questions] Query about NTP accuracy
andyy1234 at gmail.com
Fri May 22 10:40:38 UTC 2009
Richard B. Gilbert wrote:
> 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
Many thanks Richard - exactly my argument too.
> 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.
Agreed - we already use multicast clients by default however my point is
that having your stratum 1 server synchronised to microseconds does not
guarantee a microsecond NTP client experience and adding additional
strata does not materially effect this. However I need evidence to back
this up to support the SLA - I don't want to get into having to get
testers in to prove the obvious however if anybody has already looked
into this and has evidence then this would be useful.
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