[chrony-users] meaning of +/- error in sources cmd?

[Matthew Eshleman <matthew@xxxxxxxxxxxxxxxxxxxxxxxx>]

Regarding the following chronyc sources output:

MS Name/IP address         Stratum Poll Reach LastRx Last sample               

===============================================================================

^* ntp1.local                    1   0   377     0   -785ns[ -991ns] +/- 2931us

^? time1.facebook.com            1   6     3    23  -1677us[-1659us] +/-   11ms

I am curious about the source of the +/- error, which is described in the docs as:

"The number following the +/- indicator shows the margin of error in the measurement."

How is the value determined and what impacts it? How does one make this smaller? I'm happy to review source code or such if necessary, I'm just not sure where to look beyond the chronyc doc.

Ultimately the goal is to have multiple embedded linux devices time synchronized to less than 1ms of error across units, using a local LAN NTP server. 

Thank you very much,

Matthew Eshleman