RE: [PATCH v2] mac80211: mesh - always do every discovery retry

[Jesse Jones <jjones@uniumwifi.com>]

> > Yes, that is a real issue. We are planning on doing some further work
> > in this area to  try to minimize the explosions that can be seen with
> > PREQs in larger networks while balancing the need for reliability.
> >
> > Path discovery
> >> should stop once the path is established. By attempting 2nd, 3rd or
> >> 4th doesn't guarantee the next path will be "good".
> >
> > It doesn't guarantee anything of course but it does raise the
> > probability that the right path will be found. For example take four
> > nodes in a ring where the A-B, B-C, C-D links are all good but the A-D
> > link is poor. Poor enough that the higher data rates are hosed for
> > that link but the basic rate used by management frames is relatively
> > unaffected. If we assume that the reliability of management frames is
> > 90% then in order for A to route to D it needs to get a PREQ to D and
> > a PREP back. It has two options 1) for A-D the reliability will be
> > 0.9^2 = 81% 2) for A-B-C-D the reliability will be 0.9^6 = 53%.
>
> What is round trip delay between A-D compared to A-B-C-D? 1 hop away the
> throughput is reduced by half.

Well A-D is going to have a much smaller RTT than A-B-C-D. And, yes, using
multiple hops is going to reduce throughput but I'd much rather use multiple
120 Mbps links than a link that only supports 12 Mbps.

>Also if you have more nodes using longer
> path, you consume more airtime leads to really bad network performance,
> especially when B, C, or even D wants to start initial data transmission.

That's why there are link and path metrics.

> One way of improving the path is either enhancing the airtime link metric
> by
> considering the factor that you mentioned in or else introducing vendor-
> specific path metric. Sending more broadcast frames over the network won't
> be a good solution.

Improving the link metric isn't going to help with PREQ reliability
problems. Sending more PREQs will.

  -- Jesse