Atheros driver and radiotap reliability

[Sam Leffler]

Sam Pierson wrote:
> On 8/29/05, Sam Leffler <sam at errno.com> wrote:
> 
>>Sam Pierson wrote:
>>
>>>I had some correspondence with the ethereal developers and David Young
>>>and apparently there is a bug in how ethereal handles the radiotap header.
>>
>>News to me; the last time I checked it looked correct.
> 
> 
> I'm not sure.  David told me this:
> FYI, ethereal's radiotap dissector was broken the last time I checked. :-(
> It does not obey the alignment rules for radiotap fields: the radiotap
> producer (usually, the kernel) inserts zeroes to ensure natural
> alignment of all multi-byte fields.  Ethereal does not account for this.
> The tcpdump sources get this right.

David appears to be talking about how netbsd works.  Understand that 
David does not work on FreeBSD; I'm not even sure he uses it.

> 
> 
>>The radiotap header includes the rssi returned by the hardware for rx'd
>>frames.
>>                         sc->sc_rx_th.wr_antsignal = ds->ds_rxstat.rs_rssi;
> 
> 
> I get (slightly) different values for the RSSI displayed in ethereal (if it's 
> correctly being displayed, still looking) than the SS displayed in dB by 
> tcpdump.  Is the SSI displayed by ethereal the sc->sc_rx_th.wr_antsignal
> being passed through?  

tcpdump and ethereal get the same data.  If they display it differently 
given identical data then one is wrong.  If you have an example of where 
things are wrong please present it.

>  
> 
>>Nothing is recorded for tx frames.  You can typically treat it as being
>>in .5dBm units relative to the current noise floor.  
> 
> 
> *it*, referring to the rssi value above?

"it" = ds->ds_rxstat.rs_rssi which is the value returned by the hardware.

	Sam