[CUWiN-Dev] ieee802 radiotap and clock synchronization

[Jeongkeun Lee]

Thank you, Dave.
The TSF timestamp is what I want. Using pcap library, I programmed my own
sniffer application which peeks out TSFT, RSS from ieee80211_radiotap_header
and also parses UDP payload data. 

But I have encountered two problems.
1. pcap does not recognize filter expressions when I set pcap's datalink
type as ieee802_11_radio. Actually this also happens with tcpdump. When I
try any filter expressions with '-y ieee802_11_radio' option, tcpdump
captures no packets.
For example, 'tcpdump -ne -y ieee802_11_radio -i ath0' itself works well,
'tcpdump port 9191' works well too. But using them together does not work
properly as follows.
--------------------------------------------------------------------------
# tcpdump -ne -y ieee802_11_radio -i ath0 port 9191
tcpdump: data link type ieee802_11_radio
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ath0, link-type IEEE802_11_RADIO (802.11 plus BSD radio
information header), capture size 96 bytes
^C
0 packets captured
13 packets received by filter
0 packets dropped by kernel
--------------------------------------------------------------------------

2. My second problem is about tsft synchronization. From capture logs, I
have realized that tsf timer of mesh nodes can exhibit asynchronous states
time to time, even though they can hear each other's beacon transmissions.
(I have five nodes in operation.) Papers have reported that 802.11 ad hoc
mode operation does not guarantee tsft synchronization because 1) stations
send beacons on a contention basis 2) stations can only set their clock
times forward and never backward. Slower clocks synchronize with faster
clocks, but faster clocks do not synchronize themselves with slower clocks
learned from neighbor's beacons.
 Do we have any solution which includes driver modifications? 
My current solution is to disable beacon TX of all stations. Then stations
do not change their clocks but the clock time difference between two
stations is almost linearly increasing or decreasing over time according to
my observation. So we can measure the difference changing speed and use it
to compensate the difference. 

Thank you,
-- Jeongkeun


> -----Original Message-----
> From: cu-wireless-dev-bounces at lists.cuwireless.net [mailto:cu-wireless-
> dev-bounces at lists.cuwireless.net] On Behalf Of David Young
> Sent: Thursday, February 08, 2007 2:18 AM
> To: cu-wireless-dev at lists.cuwireless.net
> Subject: Re: [CUWiN-Dev] Monitor mode with Atheros
> 
> On Wed, Feb 07, 2007 at 08:44:52PM +0900, Jeongkeun Lee wrote:
> > By any chance, the IEEE80211_RADIOTAP_TSFT type defined in
> > ieee80211_radiotap header has something to do with the tcpdump time
> stamp?
> 
> It is the 802.11 Rx TSF timestamp.  The MAC adds it.  It is precisely
> the timestamp you are looking for.  That is why I suggest using -y
> ieee802_11_radio.  It adds a radiotap header containing the timestamp.
> 
> Dave
> 
> --
> David Young             OJC Technologies
> dyoung at ojctech.com      Urbana, IL * (217) 278-3933
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