TCP connections stall when station re-associates.

TCP connections stall when station re-associates.

[Ben Greear]

We're seeing something funny while testing ath9k and ath5k with
multiple VIFs.  We are using send-to-self routing rules
and have a TCP connection going between two station interfaces
connected to the same AP.  Kernel is ~2.6.37 (wireless-testing from
a few days ago, plus some local patches).  We tried commercial
APs and ath9k/ath5k APs running 2.6.37 kernel and hostap with
similar results.

The problem is that when the stations loose association and
re-associate (and maybe other times as well), the TCP connections
often hang.  I see buffers in the sendq in netstat, and retransmits
on the wire, but it seems to make no progress.

Here's a pair of sockets with one side in the state:

tcp        0   7312 7.7.1.54:33513              7.7.1.56:33514              ESTABLISHED
tcp        0      0 7.7.1.56:33514              7.7.1.54:33513              ESTABLISHED

Here's a brief snippet from the interface with 7.7.1.56 on it:

  51.517954     7.7.1.56 -> 7.7.1.54     TCP 33514 > 33513 [PSH, ACK] Seq=64229 Ack=1 Win=63 Len=12 TSV=8209159 TSER=7621148
  51.521719     7.7.1.54 -> 7.7.1.56     TCP 33513 > 33514 [ACK] Seq=13 Ack=64241 Win=377 Len=0 TSV=8209162 TSER=8209159
  52.279788     7.7.1.54 -> 7.7.1.56     TCP [TCP Retransmission] 33513 > 33514 [PSH, ACK] Seq=1 Ack=64241 Win=377 Len=12 TSV=8209920 TSER=8209159

The TCP connection is trying to run at 9.6kbps, sending 1460 bytes
per 'send' call.

If I start a UDP connection on the same pair of ports it
runs fine, even while the TCP connection is hung.

I am curious if anyone has any suggestions for debugging
this further, for instance, some way to see why the
TCP connection is not making forward progress.

Thanks,
Ben

-- 
Ben Greear <greearb-my8/4N5VtI7c+919tysfdA@public.gmane.org>
Candela Technologies Inc  http://www.candelatech.com

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Re: TCP connections stall when station re-associates.

[Ben Greear]

On 01/11/2011 05:02 PM, Ben Greear wrote:
>
> We're seeing something funny while testing ath9k and ath5k with
> multiple VIFs. We are using send-to-self routing rules
> and have a TCP connection going between two station interfaces
> connected to the same AP. Kernel is ~2.6.37 (wireless-testing from
> a few days ago, plus some local patches). We tried commercial
> APs and ath9k/ath5k APs running 2.6.37 kernel and hostap with
> similar results.

When the stations re-connect, we re-run dhcp client, which involves
setting IP to zero and then re-acquiring the IP address.  We
then set up new routing rules, etc to have it back to the way
it was before the station lost contact with the AP.

The TCP/IP connections are specifically bound to local IP and port.

Maybe it is invalid to expect TCP connections to survive this
IP change?

I'll work on reproducing this DHCP behaviour on some regular
wired interfaces and see if I can reproduce in a simpler test
case.

Thanks,
Ben

-- 
Ben Greear <greearb-my8/4N5VtI7c+919tysfdA@public.gmane.org>
Candela Technologies Inc  http://www.candelatech.com
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