WDS problems observed in today's testing
John Watlington
wad at laptop.org
Sat Mar 1 16:32:55 EST 2008
Yeah, but unfortunately, we (me and Ricardo) couldn't get DD-WRT to
work on the WRT-54Gs
on Thursday. It installed fine, but configuring it to just be a
bridged access point (and not
a gateway) wasn't happening. It is going to be a problem in the
field asking "lightly trained"
users to upgrade and configure their access points.
wad
On Mar 1, 2008, at 2:22 PM, Michail Bletsas wrote:
>
> I have associated and run successfully web and collaboration
> sessions between groups of 5 machines with 40 XOs on a WRT54GS
> running DD-WRT
>
>
> M.
>
>
>
>
> "Kim Quirk" <kim at laptop.org>
> Sent by: kim.quirk at gmail.com
> 03/01/2008 02:20 PM
>
> To
> "Javier Cardona" <javier at cozybit.com>, "Kim Quirk"
> <kim at laptop.org>, "Michail Bletsas" <mbletsas at laptop.org>, "Chris
> Ball" <cjb at laptop.org>, "OLPC Developer's List" <devel at laptop.org>,
> "Jim Gettys" <jg at laptop.org>, "Ricardo Carrano"
> <carrano at ricardocarrano.com>
> cc
> Subject
> Re: WDS problems observed in today's testing
>
>
>
>
>
> Was anyone able to get a test with a different AP? We were only able
> to associate something like 20 laptops on Fri. Do we believe it should
> be 30 or more?
>
> Kim
>
>
>
>
> On 3/1/08, Javier Cardona <javier at cozybit.com> 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'
> >
> >
> > 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.
> > >
> >
> >
> > --
> > Javier Cardona
> > cozybit Inc.
> >
>
> --
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>
>
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