If all you have is Alice and Bob, there's no way to determine the time from Alice to Bob, or from Bob to Alice, you can only determine the sum of the two. This is because Alice and Bob have different local time scales.
To put it another way, you can't distinguish being in sync and Bob -> Alice taking 4 ms, and Alice -> Bob taking 6 ms, from being out of sync by 1 ms, and both directions taking 5 ms.
If you've got an external reference, such as GPS or an adjusted for time in transit radio clock, you can figure this out. If you've got a asymmetric first hop, but afterwards a mostly symmetric path (common for residential customers connecting to commercially hosted ntp servers) and a set of servers that are different round trips away, you can get an estimate. But if all the servers are the same/similar round trips, you likely won't have enough information.
To put it another way, you can't distinguish being in sync and Bob -> Alice taking 4 ms, and Alice -> Bob taking 6 ms, from being out of sync by 1 ms, and both directions taking 5 ms.
If you've got an external reference, such as GPS or an adjusted for time in transit radio clock, you can figure this out. If you've got a asymmetric first hop, but afterwards a mostly symmetric path (common for residential customers connecting to commercially hosted ntp servers) and a set of servers that are different round trips away, you can get an estimate. But if all the servers are the same/similar round trips, you likely won't have enough information.