TamTam roundup.

Hal Murray hmurray at megapathdsl.net
Sun May 20 02:01:23 EDT 2007

> 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.

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 

[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.]

These are my opinions, not necessarily my employer's.  I hate spam.

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