Olenjan
commented
3 years ago All of the required information is described in NTP specification rfc5905. Chrony is just an implementation of given specification.
5.1.2.1. How is Time synchronized? (http://www.ntp.org/ntpfaq/NTP-s-algo.htm)?
Synchronizing a client to a network server consists of several packet exchanges where each exchange is a pair of request and reply.
When sending out a request, the client stores its own time (originate timestamp) into the packet being sent.
When a server receives such a packet, it will in turn store its own time (receive timestamp) into the packet,
and the packet will be returned after putting a transmit timestamp into the packet. When receiving the reply,
the receiver will once more log its own receipt time to estimate the travelling time of the packet.
The travelling time (delay) is estimated to be half of "the total delay minus remote processing time",
assuming symmetrical delays.There seem to be 2 main algorithms
1: on-wire algorithm
calculates packet time offset and round trip
Lets say
T1 = local clock, time request sent by client;
T2 = server clock, time request received by server;
T3 = server clock, time reply sent by server;
T4 = local clock, time reply received by client
Then offset = [(T2 - T1) + (T3 - T4)] / 2 delay = (T4 - T1) - (T3 - T2).
2: Clock Filter Algorithm Chooses the best samples from series of latest on-wire algorithm offsets/round trips
Lets say theta = T(B) - T(A) = 1/2 * [(T2-T1) + (T3-T4)] = on-wire offset delta = T(ABA) = (T4-T1) - (T3-T2) = on-wire delay
The dispersion statistic epsilon(t) represents the maximum error due to the frequency tolerance and time since the last packet was sent. It is initialized epsilon(t_0) = r.rho + s.rho + PHI * (T4-T1)
In the next step, the shift register stages are copied to a temporary
list and the list sorted by increasing delta. Let i index the stages
starting with the lowest delta. If the first tuple epoch t_0 is not
later than the last valid sample epoch tp, the routine exits without
affecting the current peer variables. Otherwise, let epsilon_i be
the dispersion of the ith entry, then
Let the first stage offset in the sorted list be theta_0; then, for the other stages in any order, the jitter is the RMS average
where n is the number of valid tuples in the filter (n > 1). In order to ensure consistency and avoid divide exceptions in other
[... Work in progress ...]
Is your feature request related to a problem? Please describe. Currently we don't know how time sync offset error is calculated
Describe the solution you'd like Description on how offset error is retrieved (i.e. how many samples) and calculated (can be pseudocode or mathematical formulation) in our current time sync implementation. This should be added to wiki.
Describe alternatives you've considered Creation of our own estimation but this would also mean re-implementing whole time synchronization.
Use cases In order to implement time sync over BLE and serial, we should have same calculation principles so we can compare time synchronization quality.
Additional context https://github.com/mlichvar/chrony/blob/master/ntp_core.c#L1467 - This is where it seems that it is implemented in code.