System update spec proposal

[Alexander Larsson]

On Tue, 2007-06-26 at 15:03 -0400, Mike C. Fletcher wrote:

> My understanding here is that Alex's system is currently point-to-point, 
> but that we don't yet have any way to distribute it on the mesh?  That 
> is, that we are looking at a research project to determine how to make 
> it work in-the-field using some form of mesh-based discovery protocol 
> that's going to try to optimise for connecting to local laptops.  I 
> personally don't care which way we distribute, but I'm wary of having to 
> have some mesh-network-level hacking implemented to provide discovery of 
> an update server.

I'm not sure what you mean here exactly. Discovery is done using avahi,
a well known protocol which we are already using in many places on the
laptop. The actual downloading of file uses http, which is a well known
protocol with many implementations.

The only thing i'm missing atm is a way to tell which ip addresses to
prefer downloading from since they are close. This information is
already availible in the mesh routing tables in the network driver (and
possibly the arp cache), and its just a question of getting this info
and using it to drive what servers to pick for downloading.

> > Basically aside from the vserver bits, which no one has seen, I don't
> > see a particular advantage to using rsync.  In fact, I see serious
> > downsides since it misses some of the key critical advantages of using
> > our own tool not the least of which is that we can make our tool do what
> > we want and with rsync you're talking about changing the protocols.
> >   
> Hmm, interestingly I see "using our own tool" as a disadvantage, not a 
> huge one, but a disadvantage nonetheless, in that we have more untested 
> code on the system (and we already have a lot), and in this case, in a 
> critical must-never-fail system.  For instance, what happens if the user 
> is never connected to another XO or school server, but only connects to 
> a (non-mesh) WiFi network?  Does the mesh-broadcast upgrade discovery 
> protocol work in that case?

Avahi works fine for these cases too. Of course, since it was originally
created for normal networks. However, if you never come close to another
OLPC machine, then we won't find a machine to upgrade against. Its quite
trivial to make it pull from any http server on the net, but that has to
be either polled (which I don't like) or initiated manually (which might
be fine).