[chrony-users] TestC definition in chrony's internal test suite

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We seem to notice in our chrony-based time infrastructure that whenever we fail over to a backup network link (having higher latency), over time chronyd stops accepting return packets from the preferred time server. `chronyc ntpdata` says that we're failing testC for the preferred source. We started noticing this as the LastRx field in `chronyc sources` was in the order of ~100 minutes. I have enabled debugging by starting chronyd with `-dd` and I do see debug messages from inside the check_delay_dev_ratio() function, but it's sort of difficult to understand what those metrics mean and to interpret the debug messages.

Could you explain in very simple terms as to what happens in testC? If there are documentations for this, please do update them here. Also, do you have any recommended chrony.conf tuning parameters (possibly "maxdelayratio" or "maxdelaydevratio") in cases where we have multiple paths to reach the same source, one being longer than the other so that this can be avoided?


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