ath9k / ath9k_htc: strange behavior of the TSF value

[Robert Felten]

Hi all,

in my understanding, the TSF value is used to synchronize the medium
access in a BSS. Also my expectation the TSF value would just increase
monotonic (until rollover).

But the TSF value (provided by the radiotap headers) shows a strange
behavior on my machine:
Sometimes the value increases a lot, and shortly after, in decreases
almost by the same amount.

To understand this behavior, I set up my TL-WN722N (AR9271, ath9k_htc)
device in STA mode (connected it to an AP) and dumped ~200k packets in
Wireshark.
Then I extracted the of TSF the STA and the TSF in the of the beacon
frames via tshark:

 $ tshark -nr dump.pcapng -T fields  -e frame.number -e
radiotap.mactime -e wlan_mgt.fixed.timestamp -R 'wlan.bssid == <AP's
MAC>  and wlan.fc.subtype == 8'"

The AP's TSF shows the expected monotonic behavior, but the local STA
TSF "jumps" somehow, for instance on the frame # 121641 or 224784:

Frame#   STA TSF                AP TSF
121594    2160015947309    0x000000c7ec26f9b2
121614    2160016082477    0x000000c7ec2889b1
121626    2160016152108    0x000000c7ec2a19b2
121641    2160032998968    0x000000c7ec2ba9bf   <-- STA TSF ???
121657    2160016356907    0x000000c7ec2d39b4
121684    2160016459308    0x000000c7ec2ec9b4
121714    2160016594080    0x000000c7ec305828
...
224473    2160099843558    0x000000c7f126a380
224550    2160099913188    0x000000c7f1283380
224683    2160100015591    0x000000c7f129c380
224784    2160368520676    0x000000c7f12b5380  <-- STA TSF ???
224872    2160100222543    0x000000c7f12cebeb
224890    2160100355555    0x000000c7f12e7380
225252    2160100629986    0x000000c7f1332380
...

The STA TSF value only "jumps" on beacon packets (I tested w/o the
filter 'wlan.fc.subtype == 8' ) and the "jumps" differs a lot in
magnitude. I seems that the next beacon set the value back to an
expected value.

I would be very happy if someone could versify / falsify of this
behavior on his/hers machine and/or point me to an explanation. :)

My kernel is 3.19.0-49-generic #55~14.04.1-Ubuntu SMP and an ath9k_htc
based TL-WRN722N WiFi device. Further details of any level could be
provided on request.


Thank you!

BR
Robert