<br><br><div class="gmail_quote">On Tue, May 13, 2008 at 1:23 PM, C. Scott Ananian <<a href="mailto:cscott@cscott.net">cscott@cscott.net</a>> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div class="Ih2E3d">On Mon, May 12, 2008 at 5:47 PM, Ricardo Carrano <<a href="mailto:carrano@laptop.org">carrano@laptop.org</a>> wrote:<br>
> I just performed another test to demonstrate the relation of costs for each<br>
> PREQ and the occurrence of multihop paths. More importantly, maybe, the<br>
> test also confirms that even with hop count as metrics, the multhop paths<br>
> will still form.<br>
> <a href="http://wiki.laptop.org/go/Path_Discovery_Mechanism:Sanity#Hop_count_will_indeed_reduce_it.2C_but_not_eliminate" target="_blank">http://wiki.laptop.org/go/Path_Discovery_Mechanism:Sanity#Hop_count_will_indeed_reduce_it.2C_but_not_eliminate</a><br>
><br>
> More on this soon.<br>
<br>
</div>Perhaps we should seriously consider properly implementing the HWMP<br>
protocol, including the proactive routing components; a proper<br>
spanning tree would prevent these transient multihop paths in most<br>
cases. It may also be worth comparing to a pure proactive approach<br>
such as OLSR, which is also permitted under 802.11s and may scale<br>
further than HWMP. OLSR-based networks have been demonstrated at ~600<br>
nodes, although the nodes were not very mobile:<br>
<a href="http://www.olsr.org/?q=background" target="_blank">http://www.olsr.org/?q=background</a>. In contrast, the author of the<br>
HWMP protocol only expects it to scale to 50 nodes (see attached).<br>
<br>
A reactive approach is much more sensitive to the loss of a single<br>
packet during route-finding; in a proactive approach we have a longer<br>
time period to converge on stable routes before we need to actually<br>
send a packet. Hybrid approaches (such as the one specified in<br>
802.11s but not actually implemented by OLPC) may combine the<br>
strengths of both.</blockquote><div><br><br>I fear that in a proactive protocol, the overhead in a dense network will also grow very fast. It is believed that the virtue of the proactive approach is that routes are there when they are needed, but this implies that the nodes are not moving a lot. That's why it is a successful choice for WMNs. I am not sure if we have examples of this approach over MANETs (I really don't know).<br>
<br>The algorithms to process the tables are also more expensive to implement, I guess.<br><br>Also, as Pol pointed out somewhere, the proactive part of the hybrid approach of HWMP is not exactly a complete table-driven algorithm like OLSR. Though I feel that the Portal/Root Announcements would be really useful.<br>
<br>But, what I really believe is that we haven't reach the moment to judge our design choices. We are just starting to do some tests that are not contaminated by horrid bugs, that render everything inconclusive. I could give you a lot of examples. One recent is #6589. Could we say that the reactive approach was not working for us based on a failed implementation?<br>
<br>I feel that before we remove this bugs and optimize what we can, we should stick to the plan. If we continue to test and optimize we'll soon know our limits and mistakes. Please don't get me wrong, I am not suggesting that we stoically resist to some bad decisions. I am just saying that we still don't know if they were bad. ;-)<br>
<br>Cheers!<br>Ricardo<br></div></div>