[Babel-users] babel and zeroconf

[Juliusz Chroboczek]

> soo... http://wiki.laptop.org/go/Mesh_Network_Details#Experimenting -
> they've ... oh.  they cheated.  they got marvell to do some of the
> low-level mesh networking.

It was my understanding that OLPC and Marvell collaborated on the job.
In particular, I believe that Michail Bletsas invested a lot of work
into the mesh implementation.  I may be wrong, though.

> so, not only am i going to need to merge all the different types of
> communications technology together but also i'm going to need to send
> certain types of traffic (voice, video, jpeg images) over the
> different communications networks, selecting the "best available" one
> for the job. (!)

That's called policy routing, and there's a lot of published papers on
the subject.  Babel doesn't do policy routing right now, but if you
convince me why it's needed, I'm quite willing to add it.

The reason I'm not convinced of the need for policy routing is that
there usually the best link for one type of traffic is also the best
link for all other ones.  There are exceptions, but I'm not sure
they're common enough to justify burdening your routing implementation.

>  yes.  ppp over GSM/GPRS; WIMAX; 802.11; radio-modem; all of it has to
> be sort-of... merged amorphously.

Yes, but.

If I read you right, you're not only asking that routing decisions be
made depending on the available technologies; you're asking that they
be made depending on offered load.  In other words, if both the radio
modem and GPRS are available, you want the radio modem to be used if
the offered load is less than its capacity, but switch to the expensive
GPRS link if the offered load increases, e.g. because the user decided
to start a video conference.

Is that correct, or are you happy with all traffic taking the ``best''
link available at a given time?

(Once again, if you need policy routing, it's your task to convince me.)

> e.g. if you can transfer _some_ routing information - however slowly -
> about the 2-mile-range WIMAX nodes over the 10-mile-range radio-modem,
> then you would, i assume, be able to make better routing decisions
> than you would if you didn't have the long-range radio connection?

Do not worry about the routing traffic -- it's very low rate, on the
order of 1 byte per second per node.  No need to use third-party
announcements (which is the technical term for what you're suggesting).

>> notably what kind of hand-off times are tolerable,

>  walking around inside and outside office buildings, which might cut
> things off randomly, but assume decent antenna (4 or 8-way phased
> ceramic antenna).

Suppose that user walks into a lift, and the WiMAX connection is cut
off.  Is it okay if the mobile node switches to GPRS after one second?
5 seconds?  15 seconds?

>> whether you're interested in multi-hop routing,

> yes.

Good.

> so - like the OLPC thing [except better :) ]  assume that it's got to
> work without centralised infrastructure, but if centralised
> infrastructure is available (e.g. GPRS / 3G or a high-power
> fixed-position WIMAX unit that has better range and coverage than the
> mobile units) then it should be seamlessly taken advantage of.

No problem, we already do that.  But see above about policy routing.

>>  how large is your power budget,

>  luggable rather than portable.  i.e. "big enough" to not have to
>  worry about :)

Good.

>>  and whether you can count on link-layer notifications.
>
> does this mean "inter-layer" notifications?  i.e. like i hinted at
> above, about being able to send notifications over the radio-modem
> about the e.g. WIMAX layer?

No.

Link-layer (or, more generally, cross-layer) notification is about the
link layer informing the routing protocol about the link being
broken.  In other words, you loose WiMAX connectivity, and the WiMAX
driver informs the routing daemon that the WiMAX link is down faster
than the routing daemon would realise that it's no longer receiving
anything over that link.

It allows faster convergence, since the routing daemon does not need
to explicitly probe the broken link.  So it's linked about the point
about hand-off above.

                                        Juliusz