Re: [PATCH v2] mac80211: Jitter HWMP MPATH reply frames to reduce collision on dense networks.

On 06/03/17 13:38, Johannes Berg wrote:

quoted

Well it certainly attempts to via stuff like carrier sense. But that
is not fool proof and any time two routers hear a frame and both
decide to forward it immediately there is a chance that they will
both sense the air at the same time, decide that it is clear, and
lose both their forwarded frames due to a collision. How often that
happens is hard to say but we have observed that exact behavior a few
years ago with an 802.11 multicast routing protocol and adding jitter
significantly improved reliability.

I'm really surprised by this since they both should jitter their
transmissions already between CWmin and CWmax. Is that window somehow really super small for what you're doing?

johannes

Isn't CWmin and CWmax only used for retries?
We recently had the problem that on 5MHz channels probe-responses of APs
which can't hear each other (hidden node problem) always collide.
See [1] for a trace showing the problem.
Yellow is the probe-request (and ack on success), the other colours are
3 APs.
Putting probe-responses on their own queue with it's own timing results
in [2] and seems to make the problem less worse.
However the first frame still always collides, and only subsequent
retries have the randomness of cwmin/cwmax added.
5MHz channels make the problem worse since frames are 4 times longer.

I'm currently trying to find a way to add some randomness to the initial
response, which it seems this patchset attempts to solve as well
(different context though).

[1] http://may.nu/images/problem.png
[2] http://may.nu/images/jittered.png