Four timestamps separate processing and transit time
In a basic two-way exchange, t1 is client transmission, t2 and t3 are server reception and transmission, and t4 is client reception. Estimated round-trip delay is (t4−t1)−(t3−t2), subtracting server processing time.
Estimated offset is [(t2−t1)+(t3−t4)]/2 and relies on approximately symmetric transit delays. Persistent asymmetry biases the result even with fine timestamp resolution. Later interleaved modes can exploit more accurate actual transmit timestamps.
Stratum is not an accuracy ranking
Servers form synchronization relationships leading toward reference clocks. Stratum describes that hierarchy, not geographic distance or a simple accuracy ranking. Root delay and dispersion also matter, so a lower stratum alone does not establish the best source for a given path.
Algorithms use repeated offset, delay and jitter measurements and can combine sources to discipline both time and frequency. This is more than periodically overwriting a clock with the last received number; filtering, selection and clock control affect stability.
References: [1]
Authenticating messages cannot eliminate delay
Network Time Security adds authentication and replay protection to NTP client-server operation. TLS establishes keys, after which NTP extension fields protect exchanges; each time request does not require a fresh TLS connection.
Authentication can detect forged or modified contents but cannot prevent an adversary from delaying intact packets. RFC 8915 treats delay attacks separately, so acceptable latency, multiple sources and path assumptions remain relevant to accuracy.
References: [3]