Unfortunately I don't have the book handy. May be I am wrong too. Just checked and 4 looks to be a valid solution for 1 falseticker according to Byzantine Generals' Problem.
On Sun, Feb 9, 2014 at 10:03 PM, Saku Ytti <s...@ytti.fi> wrote: > On (2014-02-09 21:08 +0100), Andriy Bilous wrote: > > > Best practice is five. =) I don't remember if it's in FAQ on ntp.org or > in > > David Mills' book. Your local clock is kind of gullible "push-over" which > > will "vote" for the "party" providing most reasonable data. The algorithm > > would filter out insane sources which run too far from the rest and then > > group sane sources into 2 "parties" - your clock will follow the one > where > > runners are closer to each other. That is why uneven number of > trustworthy > > sources at least at start is required. With 2 sources you will blindly > > follow the one which is closer to your own clock. You're also having the > > the risk to degrade into this situation when you lose 1 out of 3 sources. > > Four is again 2:2 and only with five you have a good chance to start > > disciplining your clock into the right direction at the right pace, so > when > > 1 source is lost you (most probably) won't run into insanity. > > I'm having bit difficulties understanding the issue with 4. > > Is the implication that you have two groups which all agree with each other > reasonably well, but do not agree between the groups. Which would mean > that 4 > cannot handle situation where 2 develop problem where they agree with each > other but are wrong. > But even in that case, you'd still recover from 1 of them being wrong. So > > 3 = correct time, no redundancy > 4 = correct time, 1 can fail > 5 = correct time, 2 can fail > and so forth? > > But not sure here, just stabbing in the dark. For the fun of it, threw > email > to Mills, if he replies, I'll patch it back here. > > -- > ++ytti > >