| [chrony-users] Root delay and Root dispersion values in `chronyc ntpdata' |
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- To: chrony-users@xxxxxxxxxxxxxxxxxxxx
- Subject: [chrony-users] Root delay and Root dispersion values in `chronyc ntpdata'
- From: Chris Perl <cperl@xxxxxxxxxxxxxx>
- Date: Fri, 23 Jun 2017 16:49:49 -0400
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I have a setup that looks like the following:
GPS Receiver -- PTP --> machine_a -- NTP --> machine_b
Where:
These machines are all pretty close together (the switching latency
between them should be something like 10us).
machine_a is running linuxptp and chrony with a shared memory region
via timemaster (similar to what is described in [1]).
machine_b is running chrony, polling machine_a every 1s.
What I'm trying to understand is why on machine_b, I consistently see
a "Root delay" and "Root dispersion" of 15us in the output of `chronyc
ntpdata'. It doesn't vary, its always 15us.
I've verified that the packets themselves flowing from server to
client have a "Root Delay" and a "Root dispersion" of 0 (via tcpdump),
so I'm guessing this must be getting calculated on the client, but I
can't figure out where or how.
[cperl@machine_b ~]$ sudo chronyc ntpdata
Remote address : 192.168.1.10 (C0A8010A)
Remote port : 123
Local address : 192.168.1.20 (C0A80114)
Leap status : Normal
Version : 4
Mode : Server
Stratum : 1
Poll interval : 0 (1 seconds)
Precision : -24 (0.000000060 seconds)
Root delay : 0.000015 seconds
Root dispersion : 0.000015 seconds
Reference ID : 50545030 (PTP0)
Reference time : Fri Jun 23 20:31:30 2017
Offset : +0.000001374 seconds
Peer delay : 0.000114600 seconds
Peer dispersion : 0.000000397 seconds
Response time : 0.000064935 seconds
Jitter asymmetry: +0.00
NTP tests : 111 111 1101
Interleaved : Yes
Authenticated : No
TX timestamping : Kernel
RX timestamping : Kernel
Total TX : 3410
Total RX : 3410
Total valid RX : 3410
[1] http://rhelblog.redhat.com/2016/07/20/combining-ptp-with-ntp-to-get-the-best-of-both-worlds/
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