I have an RTL8188CUS dongle. My test setup is a Beaglebone black w/
wireless-testing of a few days ago on a debian rootfs. It is set up as
an unencrypted station. The AP uses a Ubiquiti SR71-A (AR9160), has been
in service for some time, is running compat-wireless-2014-05-22 and
isn't suspect.
After scp'ing several 10MB files in either direction, traffic stops.
Well, not all the traffic. If I ping the AP, Wireshark on a separate
computer and monitor interface shows that the ping from the station gets
on the air and so does the AP's ack, but no ping reply is sent by the
AP. ARP broadcasts from the station are ack'ed and responded to, just
not the pings. If I run tcpdump on the station's interface, it shows the
ARP packets and their response and any other broadcast traffic, just not
the ping or reply. Lastly, this only occurs in HT mode.
I've compared the Wireshark dumps of the pings from before the interface
hangs up and after and can't see any difference.
I repeated this using an X86_64 machine and the same usb dongle. I have
two of the dongles and they behave identically. Further, I've compiled
the Realtek driver (8192cu) and it doesn't exhibit this behavior.
Thanks,
Steve
On Sun, 2014-06-22 at 18:22 -0600, Steve Brown wrote:
> I have an RTL8188CUS dongle. My test setup is a Beaglebone black w/
> wireless-testing of a few days ago on a debian rootfs. It is set up as
> an unencrypted station. The AP uses a Ubiquiti SR71-A (AR9160), has been
> in service for some time, is running compat-wireless-2014-05-22 and
> isn't suspect.
>
> After scp'ing several 10MB files in either direction, traffic stops.
> Well, not all the traffic. If I ping the AP, Wireshark on a separate
> computer and monitor interface shows that the ping from the station gets
> on the air and so does the AP's ack, but no ping reply is sent by the
> AP. ARP broadcasts from the station are ack'ed and responded to, just
> not the pings. If I run tcpdump on the station's interface, it shows the
> ARP packets and their response and any other broadcast traffic, just not
> the ping or reply. Lastly, this only occurs in HT mode.
>
> I've compared the Wireshark dumps of the pings from before the interface
> hangs up and after and can't see any difference.
>
After looking at some more wireshark output, the problem seems to be
that the aggregation sessions deadlock when a second one starts before
the first one completes. That would explain why it only occurs in HT
mode.
The sequence on the air before the deadlock is:
STA->AP block ack req
STA->AP ping req
AP->STA block ack resp
AP->STA ping resp
AP->STA block ack req
STA->AP block ack resp
The sequence on the air after the deadlock is:
AP->STA block ack req
STA->AP block ack req
AP->STA block ack resp
STA->AP block ack resp
STA->AP ping req
...
STA->AP ping req (repeats)
I noticed a fix for a similar problem with ath9k in the description of patch:
ath9k: prevent aggregation session deadlocks
08c96abd611beadf2af414a306fe0fb02ba706ff
Steve