Re: Plans for an online meeting regarding Radiotap

(Removed mailing lists other than radiotap, fixed Damien Bergamini's  
address, and removed the "other" David Young - the right David Young  
is on the radiotap mailing list.)

On Aug 21, 2009, at 7:31 AM, Gábor Stefanik wrote:

> Radiotap is a de-facto standard for 802.11 frame injection and  
> reception.
> Up to field ID 13, it can truly considered a standard (all current  
> implementations
> agree on fields 1-13), but after that, implementations diverge widely.
>
> Here is a map of how current implementations define field IDs 14 and  
> up:

	...

> Wireshark:
> Field 14: RX flags, with option to decode FCS instead
> Fields 15...17 skipped

No:

Fields 15...17: not implemented, so if any of them are present, no  
later fields are dissected

Also:

Tcpdump top-of-tree at tcpdump.org:

Fields 14...17: not implemented, so if any of them are present, no  
later fields are dissected
Field 18: extended channel

> My current proposal for the future standard field ordering beyond  
> field 14:
>
> Field 14: RX flags (as defined by the standard)
> Field 15: TX flags (as used by Linux, NetBSD and aircrack-ng)
> Field 16: RTS retry count (as used by Linux and NetBSD)
> Field 17: Data retry count (as used by Linux and NetBSD)
> Field 18: Extended channel (as used by FreeBSD and Wireshark)

...and top-of-tree tcpdump.org tcpdump (taken from FreeBSD's tcpdump).

> Field 19: RSSI (OpenBSD's field 16 moved to field ID 19 to avoid  
> collisions)

So what does the value of that field mean?  The 802.11 spec just says

	The RSSI is an optional parameter that has a value of 0 through RSSI  
Max. This parameter is a measure by
the PHY of the energy observed at the antenna used to receive the  
current PPDU. RSSI shall be measured
between the beginning of the SFD and the end of the PLCP HEC. RSSI is  
intended to be used in a relative
manner. Absolute accuracy of the RSSI reading is not specified.

so its value doesn't intrinsically mean anything - the only thing you  
can do is say that, if the RSSI value is greater for packet B than for  
packet A, there was higher energy at the antenna when packet B was  
received than when packet A was received.

If a driver supplies IEEE80211_RADIOTAP_DBM_ANTSIGNAL or  
IEEE80211_RADIOTAP_DB_ANTSIGNAL, is the RSSI useful?

Are there any drivers that cannot supply  
IEEE80211_RADIOTAP_DBM_ANTSIGNAL or IEEE80211_RADIOTAP_DB_ANTSIGNAL?