Re: WDS ping

From: Kristian Hoffmann (
Date: 2002-03-07 21:48:33 UTC

Posting to the list is fine. We have APs connected to APs in a matter of speaking. Instead of a T1 interface we have a wireless interface on some that associates to another AP. WDS is a solution for the problem that arises when you bridge two wireless segments together. One wireless cell is a subset of the other.

When your edge AP forwards frames to the inner AP everything is fine. The problem occurs when frames destined for a station associated with edge AP, reach the inner AP. The inner AP knows, thanks to the bridging code, that the host it is looking for is somewhere on it's wireless interface and transmits the frame. But, the AP driver will drop the frame because the destination mac address doesn't match the mac address of any station directly associated with it. Some "fixes" have been to just broadcast these frames with a mac address of ff:ff:ff:ff:ff:ff which _should_ work but has problems. Our deadbeef code (narf005) lets you specify the mac address of any pseudo-WDS stations. The code adds a seperate interface for each station you add. The driver then looks up the right interface when it receieves a frame from a listed station and "receieves" the frame over that station's special interface. Now the bridging code sees all traffic coming from that WDS station as coming over a different interface than the normal traffic and will send frames for that station and everything behind it back over that special interface. When the bridge sends frames back over that special interface, the driver knows which associated station should receieve the frame even though the ethernet header's destination address doesn't match any known station.

Clear as mud, eh?


On Thu, 7 Mar 2002, Eric Enockson wrote:

> Do you mind if i forward your mail to the list? I had meant
> to send my question there anyway. Essentailly though you do not
> have ap's communicating to eachother via wireless?
> On Thu, Mar 07, 2002 at 09:45:39AM -0800, Kristian Hoffmann wrote:
> > We have a central router that all of our T1s run to. At the other end of
> > each T1 is an AP with a T1 interface. They each get a non-subnetted IP
> > address (host route) over the T1 and then a net route is added on the
> > central router. Basically, we assign subnets to our access points 32 IP
> > addresses at a time. Is that what you were asking? You do lose the
> > ability to roam between access points this way, but all of our antennas
> > are mounted to houses. They aren't going anywhere.
> >
> > -Kristian
> >
> > On Thu, 7 Mar 2002, Eric Enockson wrote:
> >
> > > On Sun, Mar 03, 2002 at 01:10:33AM -0800, Kristian Hoffmann wrote:
> > > >
> > > > Realizing that was a horrible kluge, we decided on a lesser kluge. We
> > > > setup a proc configurable list of mac addresses to check for instead of
> > > > the magic 00:00:de:ad:be:ef. So in that case, you just add the mac
> > > > address for any "WDS" stations. Then when the AP receives a frame from a
> > > > station in the list, it assumes it's an encapsulated frame and ignores the
> > > > dest address on the 802.11 frame and netif_rx's the encapsulated frame
> > > > instead of attempting to reconstruct the header.
> > > >
> > > > The scariest part was that both ways worked. Only we ran into interesting
> > > > problems with broadcast frames but that would take a lot of explaining.
> > > > If anyone isn't scared out of their mind and wants to look at the code,
> > > > I'll post a link. We stopped without working out all the bugs and
> > > > cleaning up the interface because I LEARNED HOW TO ROUTE and stop bridging
> > > > like a wuss. Sorry, I had to. We realized we were going through all this
> > > > trouble to avoid routing. It's not worth it so get over it. :P
> > >
> > > How do you route between 2 access points over the wireless
> > > medium without using some form of 802.11. Either WDS, linksys client
> > > ap, master ap hack, or a special ap that has clients associated in
> > > BSS mode and IBSS between itself and another ap.
> > > So what if you have a routing table on your aps. The pkts
> > > still get between ap's via 802.11 and rf and what do their addresses look
> > > like and how do the ap's know where to send them and that stuff?
> > >
> > >
> > >
> > >
> > >
> > >
> >

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