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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