"Axonn" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] [...] > 1. I know this is a stupid question probably, but I'm still gonna ask > it: synchronizing via SNTP should discard network latencies right? ...
People will shortly fall over each other to impress upon you that anything to do with SNTP can never "synchronise" any clock. They reserve that term for what the full package does, based on its communication with reference servers, and insist an SNTP client can only _set_ the time, as often as you like. The very word SNTP means you never look at more than one packet at a time, more or less. Actually two - a request and a reply. That gives you four timestamps, and a way to estimate the total link delay. No way to estimate the two links delays separately, because you mustn't assume the clock in the request tells the same time as the clock in the reply. So SNP only gives you a means to peek at another machine's clock, at an instant you can only guess, because you receive it one network latency period later, and you don't know the exact latency. You only know the sum of it and the latency in the opposite direction. It's not unlike gazing at different stars through a telescope. The distance to each star is different, and with that the time it takes its light to reach you. If you're reading clocks on their surfaces that _should_ all be running synchronously, of course you should compensate for the observation delay or their combined times will never make sense. > 2. Why do some servers have "stratum" = 1 and other = 2? Stratum goes from zero to sixteen. Reference clocks are stratum zero by definition, and every synchronisation hop adds one. In practice, few if any paths to a reference clock are longer than four or five hops. That leaves room for tricks such as fudging reference clocks that don't really know what time it is to stratum 8 or 10 (for backup purposes), and interpreting 16 as infinity and unsynchronised. So a stratum-1 server is synchronising its own clock to an attached (hardware) reference clock, and a stratum-2 server is synchronising its clock to a stratum-1 server. My Internet gateway is currently stratum 3, synchronised to one of my ISP's stratum-2 servers, and the machine I'm typing this on is synchronised to the gateway, making it stratum 4. If the gateway loses its Internet connection, it will start using its own internal clock at stratum 8 and become stratum 9 itself, and all the other machines in my attic will drop to stratum 10. Groetjes, Maarten Wiltink _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
