[Cerowrt-devel] perverse powersave bug with sta/ap mode

Dave Taht dave.taht at gmail.com
Tue Apr 26 19:18:54 EDT 2016

Pain shared, reduced, joy shared increased...

for weeks now I've been puzzling over why a variety of links flapped
the way they did,
routes coming and going, failing over to weird paths, and I think I
have finally isolated one
part of the problem.

In an age where adhoc does not work particularly well, and AP/sta mode
does (as in 6mbit vs 500 in one case), I've had a tendency to nail up
links in ap/sta mode.

Well, at least one ( probably several) of the devices I have in the
new lab has *very aggressive* power save, to where babel ipv6
multicast traffic either doesn't sync up to the AP's request for
multicast (or the sta's), or it is merely completely suppressed by the
stack. (or lost due to a bug!)...


So long as there is unicast traffic on the local part of the link, you
don't see a problem. And there's almost always a bit of traffic on the
link. So, perversely... like when I'm looking at it... ssh'd through
to the other side, running something like "watch tc", or like, pinging
from one side of the link to the other... it works. When I go away for
a bit... it fails. Eventually.

If I run a test, after getting everything all setup and verified the
network looks correct... it works.

If I walk away and run a test that has a few minutes :grump: between
runs to let things "settle down", things actually deteriorate.

Babel misses multicast traffic and gradually increases the metric on
that interface due to the loss - causing a given route, in my case, to
eventually fall over to an adhoc wifi radio elsewhere on the network,
which reduces the probability of unicast traffic through that link
still more, until ultimately the local link, otherwise nailed up,
drops off the network completely.

to "fix" this:

iw dev wlp4s0 set power_save off

worked beautifully on the ath10k driver I'm using. The babel metric
stayed stable, the route stayed stable, life was good, throughput
increased, latency dropped...

That said, I know how hard wifi device driver writers are hammering at
trying to reduce multicast effects, and save power... and I haven't
exactly found the root cause of this problem, in this driver... (or in
the ath9k on the AP side) but I think I've seen it elsewhere also,
while chasing this -l failover issue.

multicast beacons are supposed to say "hey, chips, wake up, you need
to hear this".

Dave Täht
Let's go make home routers and wifi faster! With better software!

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