TamTam roundup.

John Watlington wad at laptop.org
Sun May 20 02:20:50 EDT 2007

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.

> 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  
>> 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.]

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