TamTam roundup.

[Dan Williams]

On Sun, 2007-05-20 at 02:20 -0400, John Watlington wrote:
> On May 20, 2007, at 2:01 AM, Hal Murray wrote:
> 
> >
> >> Yes, NTP is one of the services provided by the school servers.
> >> Ideally, a laptop's NTP server would be determined by a protocol
> >> similar to the one used for internet portal and DNS server ---
> >> identifying the "closest" server in the mesh.  In any case, network
> >> propagation time is taken into account by the NTP (although I believe
> >>  Hal is far more knowledgable than I in such matters...)
> >
> > I'm pretty sure there us a slot allocated in DHCP to specify NTP  
> > servers.
> > That requires some script to get the DHCP info and then rewrite  
> > ntp.conf.
> >
> > I don't know much about that area.  It's not as common as it should  
> > be,
> > mostly because ISPs have their head in the sand about providing NTP  
> > services
> > for their customers which is partly a chicken/egg tangle.
> >
> > One obvious alternative for olpc is to have a magic DNS name that  
> > resolves to
> > the local NTP server.
> 
> Maigc DNS names at your service.  And since the DNS server used by a  
> laptop in a school setting will also be the closest mesh portal, the  
> NTP server can be the closest on the mesh.

It's also fairly easy to do with NM if you push the info through DHCP.

Dan

> > How do the backup scripts find their local server?  If they use a  
> > magic name,
> > ntpd should be able to do something similar.
> >
> > ntpd assumes that the network delays between client and server are  
> > symmetric.
> >  It fudges the response by half the round trip delay and then  
> > filters that.
> > That works pretty well except for several cases which may not be  
> > uncommon:
> 
> > If you have an asymmetric link like ADSL, you will be off by the  
> > bandwidth
> > differences multiplied by the packet size, even if the link is  
> > unloaded.
> > That gives you a constant offset which is often OK.
> >
> > The filter gets confused by queuing delays if you have a link with  
> > asymmetric
> > loads for extended periods of time, like when downloading a CD over  
> > SDSL.
> >
> >
> >> Anybody on the XO side know if NTP is part of the build yet ?
> >
> > I think it is included in 406.  The config file is setup to use  
> > several pool
> > servers.  That's typical.  It's a lot better than nothing but  
> > generally picks
> > servers at random from a set scattered around the world.  What you  
> > really
> > want is sane nearby servers where nearby means low network delays  
> > rather than
> > few miles/kilometers.
> >
> > We'll know a lot more when somebody gets some ping data on a  
> > typical mesh.
> > (I realize that "typical" probably doesn't exist, but we have to start
> > somewhere.)
> >
> > [I've been using my own ntpd setup in order to collect some  
> > statistics.  I
> > noticed the default setup after updating to 406 before I smashed it  
> > with my
> > stuff.  I didn't investigate.]
> 
> _______________________________________________
> Devel mailing list
> Devel at laptop.org
> http://mailman.laptop.org/mailman/listinfo/devel