WDS problems observed in today's testing

[Sameer Verma]

Javier Cardona wrote:
> Kim, Michail,
>
> The conclusion to all of this is that we should not use WRT54G in
> deployments, regardless of whether mesh is used or not (in fact, if we
> *only* use mesh we don't have this problem as the AP ignores mesh
> multicast traffic now).  The WRT54G will forward multicast traffic to
> all other APs in the vicinity that it identifies as peers using flawed
> criteria (Lazy-WDS).  Since the xo's generate a lot of multicast
> traffic, that creates a risk of triggering the multicast storms that
> we saw at OLPC.
>
> Javier
>
> PS. If we have no choice but to use that AP, then we should re-image
> the AP with a distribution that allows turning WDS off.  In Tomato
> (http://www.polarcloud.com/tomato) this can be achieved via Basic ->
> Wireless Mode = 'Access Point' and NOT 'Access Point + WDS'
>
>   
dd-wrt (http://www.dd-wrt.com/) has a tab for Wireless -> WDS which 
allows you to disable lazy WDS.

Sameer
> On 3/1/08, Javier Cardona <javier at cozybit.com> wrote:
>   
>> Ricardo,
>>
>>
>>  > - The access point Javier mentions is the one I bought yesterday (Linksys
>>  > WRT54G)
>>
>>
>> Agreed, yes:
>>  35 00:1d:7e:44:ce:6e Broadcast  Beacon frame,SSID: "linksys"
>>
>>
>>  > - Most of this traffic is retransmission (3606):
>>  > (wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4 and wlan.ta[4-5] == ce:6e) &&
>>  > (wlan.fc.retry == 1)
>>
>>
>> Agreed.
>>
>>
>>  > - It is also interesting to detect other wds peers this AP identified (one
>>  > is 00:0b:85:53:27:50 and got 1066 of these frames).
>>  > ((wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4 and wlan.ta[4-5] == ce:6e))
>>  > && (wlan.ra == 00:0b:85:53:27:50)
>>
>>
>> Yes.  Note that none of the WDS peers are xo's, as was the case in the past.
>>
>>
>>  > It seems that the linksys is expecting acks for this wds frames (which btw are mulcast frames). It is amazing.
>>
>>
>> Well, even if the final destination is a multicast address, those WDS
>>  frames are unicast, and therefore acknowledged.  What's troubling is
>>  that the WDS links are not torn down when the link quality is so poor.
>>   But we all know that Lazy-WDS is a flawed protocol.
>>
>>
>>  > I believe we should compare this with the previous capture from the
>>  > Netgear AP, just to confirm that this is (again) specific to WDS issues
>>  > on the Linksys.
>>
>>
>> I don't have access to that capture.  Maybe David?
>>
>>  Javier
>>
>>
>>
>>  > On Sat, Mar 1, 2008 at 5:23 AM, Javier Cardona <javier at cozybit.com> wrote:
>>  >
>>  > > Michail, Chris,
>>  > >
>>  > >
>>  > > This afternoon I captured some traffic while Chris was running tests
>>  > > for Peru.  The test setup consisted on ~25 laptops associated to a
>>  > > WRT54 access point.  When the laptops were on, associated and (not
>>  > > sure about this) idle, we observed a high volume of wireless traffic.
>>  > > The spectrum analyzer showed close to 50% duty cycle utilization of
>>  > > the channel.
>>  > > We also observed that a few xo's could not associate, and some seemed to
>>  > > intermittently lose and recover association.
>>  > >
>>  > > Turning off the WRT54 (and therefore stopping all the infra traffic)
>>  > > freed up most of the bandwidth on that channel.
>>  > >
>>  > > In my 50 second capture (taken before turning off the AP) we observe:
>>  > >
>>  > > Total traffic:                  15081 frames (100%)
>>  > > All WDS traffic (1):             6023 frames ( 40%)
>>  > > WDS, xo is source addr (2):      4343 frames ( 29%)
>>  > >  (96% of the above xmitted at 1 Mbps (3) and 100% sent by a single AP(4))
>>  > >
>>  > > Compare that with
>>  > >
>>  > > xo originated infra frames (5):   401 frames ( 3%)
>>  > >  (77% of the above xmitted at rates higher than 2 Mbps (6))
>>  > >
>>  > > What does all this mean?
>>  > >
>>  > > 1. Multicast traffic gets replicated and retransmitted.
>>  > > 2. The ratio of original frames to AP generated multicast
>>  > > retransmissions is 1:11
>>  > > 3. Taking into account the data rates this means that for 1 airtime
>>  > > unit used to transmit useful traffic, over 200 units are wasted
>>  > > transmitting useless WDS traffic.
>>  > > 4. All this is done by a single Cisco AP, MAC: 00:1e:7e:44:ce:6e
>>  > >
>>  > > Michail, is that one of OLPC APs?
>>  > > Chris, we should see a big improvement if we can disable that
>>  > > "feature" on the AP... or put it under water.
>>  > >
>>  > > I've posted my capture here:
>>  > >
>>  > http://dev.laptop.org/~javier/captures/cisco-wds-traffic-around-xo-testbed.cap
>>  > >  in case someone wants to double check my analysis (Ricardo?).
>>  > >
>>  > > Cheers,
>>  > >
>>  > > Javier
>>  > >
>>  > > (1) wlan.fc.ds == 3
>>  > > (2) wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4
>>  > > (3) wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4 and radiotap.datarate ==
>>  > 0x2
>>  > > (4) wlan.fc.ds == 3 and wlan.sa[0-2] == 00:17:C4 and wlan.ta[4-5] == ce:6e
>>  > > (5) wlan.fc.ds == 1 and wlan.sa[0-2] == 00:17:C4
>>  > > (6) wlan.fc.ds == 1 and wlan.sa[0-2] == 00:17:C4 and radiotap.datarate > 4
>>  > >
>>  > > --
>>  > > Javier Cardona
>>  > > cozybit Inc.
>>  > >
>>  >
>>  >
>>
>>
>>
>> --
>>  Javier Cardona
>>  cozybit Inc.
>>
>>     
>
>
>