About the TSFT field

About the TSFT field

[Roberto Riggio]

Hi,

I'm planning to use the TSFT field present in the radiotap
header in order to reconstruct a timeline of packet
reception at different AP.

As far as I've understood the TSFT is generated from the MAC,
however I do not know to which instant the TSFT is relative,
the first bit of the MAC payload, the beginning of the PLCP,
etc.

Also I do not know from which reference the TSFT is generated,
I mean two AP synchronized using NTP will have consistent
TSFT, which kind of error should I expect?

R.

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Re: About the TSFT field

[Johannes Berg]

On Wed, 2014-08-13 at 11:52 +0200, Roberto Riggio wrote:

> As far as I've understood the TSFT is generated from the MAC,
> however I do not know to which instant the TSFT is relative,
> the first bit of the MAC payload, the beginning of the PLCP,
> etc.

It's documented in the radiotap spec, but not all drivers are really
doing the right thing here.

> Also I do not know from which reference the TSFT is generated,
> I mean two AP synchronized using NTP will have consistent
> TSFT, which kind of error should I expect?

It's purely local time - no synchronisation happens.

johannes

Re: About the TSFT field

[Roberto Riggio]

Inline...

On 13/08/14 12:05, Johannes Berg wrote:
> It's documented in the radiotap spec, but not all drivers are really
> doing the right thing here.

Found it. Thanks.

>> Also I do not know from which reference the TSFT is generated,
>> I mean two AP synchronized using NTP will have consistent
>> TSFT, which kind of error should I expect?
> 
> It's purely local time - no synchronisation happens.

Meaning that is the system time. right? What if I create an adhoc
interface together with the AP? According to the standard devices
in IBSS shall synch with each other using the tsf info in the beacon
frames. Is this behavior implemented by mac80211?

Thanks
R.

Re: About the TSFT field

[Johannes Berg]

On Wed, 2014-08-13 at 19:33 +0200, Roberto Riggio wrote:

> > It's purely local time - no synchronisation happens.
> 
> Meaning that is the system time. right?

No, it's wifi time. TSF in TUs. Whatever that means. It may or may not
even have a fixed reference, and it's not even guaranteed to sync with
the network.

> What if I create an adhoc
> interface together with the AP? According to the standard devices
> in IBSS shall synch with each other using the tsf info in the beacon
> frames. Is this behavior implemented by mac80211?

That behaviour is implemented, but it's not guaranteed that the TSF will
be used for the radiotap timestamp field. In fact radiotap isn't really
specified to be used while operating, so it seems to me that any
free-running clock with the right frequency (TUs) would be acceptable.

In that sense, the only real use for the timestamp field (barring
external information, like knowing what your driver does) would be for
relative time inside a single capture file.

johannes

Re: About the TSFT field

[Roberto Riggio]

On 13/08/14 20:29, Johannes Berg wrote:
> On Wed, 2014-08-13 at 19:33 +0200, Roberto Riggio wrote:
> That behaviour is implemented, but it's not guaranteed that the TSF will
> be used for the radiotap timestamp field. In fact radiotap isn't really
> specified to be used while operating, so it seems to me that any
> free-running clock with the right frequency (TUs) would be acceptable.

Do you (or somebody else on this ML) happen to know if TSF is used for the
radiotap TS for the ath9k driver?

> In that sense, the only real use for the timestamp field (barring
> external information, like knowing what your driver does) would be for
> relative time inside a single capture file.

Like above, does anybody know if the ath9k driver synchronizes with other
devices in ad hoc mode in the network.

> johannes
r.