Re: NTp sources that work in a datacenter (was Re: Is latency equivalent to RTT?)
In message <Pine.LNX.4.44.0305140802170.7544-100000@twin.uoregon.edu>, Joel Jae ggli writes:
Also if you just need a high level of syncronization between the time on all your hosts you can just deploy one standalone ntp server, sync it against public time sources and get everything synced against that. its probably a 95% solution to most people's timeing needs.
If I recall correctly, NTP assumes that latency = RTT/2. You might make it work well for his application *if* you set up your tree so that your paths are each one hop, or at least symmetric over your network. --Steve Bellovin, http://www.research.att.com/~smb (me) http://www.wilyhacker.com (2nd edition of "Firewalls" book)
On Wed, 14 May 2003 13:38:32 EDT, "N. Richard Solis" said:
IIRC, NTP actually sends a packet with a local timestamp, sends it to a remote location for a remote timestamp, and then checks the local timestamp when the packet is received to calculate RTT.
Chicken-and-egg problem - even WITH the timestamps, you can't calculate the actual offset of clocks without making some assumptions regarding RTT. The assumption NTP makes is that the remote timestamp happened exactly halfway between your own send and receive timestamps (i.e. that the RTT is symmetric) I've personally seen our 2 stratum-2 servers diverge *wildly* - through sheer dumb luck and routing updates, ntp-1 had 6 peers all of which were routed via our NetworkVirginia link, and ntp-2 had 6 peers all of which went via our Abeline link. So of *course* one night I get a phone call about the fact that the two were about 7.5 entire seconds out of sync. Run 'ntpq -p' on each of them, and see that each is reporting millisecond offsets, but ntp-2 reported huge delay values. Seems that this particular night, we were seeing high packet drops and a consistent 15-second (yes, second) delay *outbound* through some ATM fabric. I never did get a better explanation than "poltergeists"...
participants (3)
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N. Richard Solis -
Steven M. Bellovin -
Valdis.Kletnieks＠vt.edu