Re: [Autoconf] aspects of multi-hop wireless communication

Hello Breno,

I have to make this short, but anyway provide some responses to
your comments.

Breno Jacinto wrote:
> Hello Charles,
>
>
>   
>> Suppose we want to broadcast a packet to every node in the system.  We can
>> create a reduced relay set to accomplish this, and it could be constructed
>> using
>> nodes across varying physical media.
>>     
>
>       By system you mean an heterogeneous network? Which involves
> multiple media? That's the only way I can imagine it.
>   

Yes, I meant potentially heterogeneous network.

>       So, supposing that there are multiple media involved. But, it is
> certain that for each media, a different network must be formed.
>   

Well, depending on what you mean by "network", I don't take that as an
article of faith.  However, I reckon we can agree that nodes using different
physical media won't be able to properly manage access to the disparate
media.

> Practical example: nodes having Bluetooth, can only form a Bluetooth
> network. Nodes with 802.11 b/g can only form networks with other nodes
> with 802.11 b/g. And that applies to any technology.
>   

What if you s/networks/links/?

>       Now, if those nodes (which form different networks) wish to
> talk, then there must be at least one node that is able to speak both
> technologies, as in the example, 802.11 AND Bluetooth. Now, if we use
> bridging or any other mechanism (such as Ana4 that I cited before), we
> "glue" these networks at layer two, and they form a single broadcast
> domain (this is in theory, in practice it currently doesn' t seem to
> work).

Well, it does work.

> Then, to IP or any layer 3 protocol, this is just one network
> where it's possible to broadcast and it's about the underlying
> technology how is best to do it. I still dont see how IP can improve
> anything below it in this case?

Here, I am definitely not seeing your point.

>  An IP/ UDP broadcast will, ultimately,
> become a layer 2 broadcast, right?

Not necessarily, especially if one wants to do iterated unicast.

>  If there is any kind of
> optimization, then, it' s up to the underlay network. That' s how I
> see it, if you have a different view about it, I'd really like to know
> :).
>   

One example I mentioned before, was that IP could decide
which nodes handle relay.

>      About the topology information: I was recently talking to the
> guys from AWDS, and they did something interesting. For example, their
> layer 2 protocol is a proactive, link-state one, very much like OLSR.
> So, any information about the network is generated there, at layer 2.
> Now, if, at layer 3, I wish to know any information (such as the
> topology), then, just ask layer 2. They send it to you. That's some
> kind of cross-layering that could be interesting in designing better
> layer 3 protocols in this context.
>
>   
>> It's easier at layer 3, I think.
>>     
>
>     With the example from AWDS, I still think its better at layer 2
> with some layer 3 "talk".
>   

What if your layer-2 signaling eats 30% of the bandwidth and
layer-3 signaling would have only used 5%?  Again, there's no
free lunch.  Furthermore, proactive protocols in any sufficiently
big domain will eventually use 100% of the bandwidth.  I do
understand that many proponents don't believe in huge ad hoc
networks, so from that perspective it's a moot point.

>
>     And that's another point. How can we know the "way" of doing ad
> hoc networking?

Well, I am not a religious person, and especially not religious
about technology choices.  So I don't think there is necessarily
any one "way" to do it.  For me, the bottom line is performance,
including scalability.

>
>   
>> As written, I completely agree with this.  I guess the words are susceptible
>> to varying interpretations, though.  For instance, if IP required symmetric
>> links,
>> some useful communication paths may simply cease to exist since layer 2
>> could not provide them.
>>     
>
>      OK, but I think that a little bit of talking between layer 2 and
> 3 (as the AWDS example mentioned) could solve this. But, of course,
> this not as simple as it sounds. I'm aware that there are challenges
> involved.
>   

It is my belief that the challenges are exacerbated by trying to do multihop
forwarding at layer 2.  But lots of engineers enjoy a good challenge, and if
they can sell the results, even better, except insofar as new challenges are
introduced relating to backwards compatibility to poorer technology choices.

>   
>     No, never.  The non-802.11s (such as 802.11g or whatever) will be
> able to form their own networks. Now, as I said in the beginning,
> someone who is able to talk two (or three or many) must make them
> participate in the world. It's an internetworking world, after all.
> It's just that 802.11s or AWDS approches make it simpler by handling
> multi-hop communication transparently to IP.

Here I disagree.  Complicating one level and reducing proper visibility
at another level is not automatically equal to "simpler".

>  No need of highly
> specialized protocols such as OLSR at layer 3, something similar is
> already at layer 2.
>   

That's an awfully long stretch.

>      The rest is up to IP protocols and internetworking. Symmetric /
> assymetric links can be handled at layer 2 as well... after all, this
> is a layer 2 problem, right? If it is still important in some cases
> for layer 3 to know about them, them a little bit of cross-layering
> sounds good to me.
>   

So, do you want IP to run over asymmetric links, or not?  If layer 2 cannot
create the fiction of symmetry, do we lose the link or not?

Felicitous Festivities!

Regards,
Charlie P.

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