NetBSD/share/man/man9/ieee80211_radiotap.9

.\" $NetBSD: ieee80211_radiotap.9,v 1.7 2005/07/12 21:26:33 dyoung Exp $
.\"
.\" Copyright (c) 2004	Bruce M. Simpson <bms@spc.org>,
.\"			Darron Broad <darron@kewl.org>,
.\"			David Young <dyoung@pobox.com>.
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.\" $FreeBSD: src/share/man/man9/ieee80211_radiotap.9,v 1.3 2004/07/07 12:59:39 ru Exp $
.\"
.Dd July 11, 2005
.Dt IEEE80211_RADIOTAP 9
.Os
.Sh NAME
.Nm ieee80211_radiotap
.Nd software 802.11 stack packet capture definitions
.Sh SYNOPSIS
.In net80211/ieee80211_var.h
.In net80211/ieee80211_ioctl.h
.In net80211/ieee80211_radiotap.h
.In net/bpf.h
.\"
.Sh DESCRIPTION
The
.Nm
definitions provide a device-independent
.Xr bpf 4
attachment for the
capture of information about 802.11 traffic which is not part of
the 802.11 frame structure.
.Pp
Radiotap was designed to balance the desire for a capture format
that conserved CPU and memory bandwidth on embedded systems,
with the desire for a hardware-independent, extensible format
that would support the diverse capabilities of virtually all
802.11 radios.
.Pp
These considerations led radiotap to settle on a format consisting of
a standard preamble followed by an extensible bitmap indicating the
presence of optional capture fields.
.Pp
The capture fields were packed into the header as compactly as possible,
modulo the requirements that they had to be packed swiftly,
with their natural alignment,
in the same order as the bits indicating their presence.
.Pp
This typically includes information such as signal quality and
timestamps.
This information may be used by a variety of user agents, including
.Xr tcpdump 8 .
It is requested by using the
.Xr bpf 4
data-link type
.Dv DLT_IEEE_80211_RADIO .
.Pp
.\"
Each frame using this attachment has the following header prepended to it:
.Bd -literal -offset indent
struct ieee80211_radiotap_header {
	u_int8_t	it_version;	/* set to 0 */
	u_int8_t	it_pad;
	u_int16_t	it_len;		/* entire length */
	u_int32_t	it_present;	/* fields present */
} __attribute__((__packed__));
.Ed
.Pp
.\"
A device driver implementing
.Vt radiotap
typically defines a structure embedding an instance of
.Vt "struct ieee80211_radiotap_header"
at the beginning,
with subsequent fields naturally aligned,
and in the appropriate order.  Also, a driver defines
a macro to set the bits of the
.Va it_present
bitmap to indicate which fields exist and are filled in by the driver.
.\"
.Pp
Radiotap capture fields
.Em must be naturally aligned .
That is, 16-, 32-, and 64-bit fields must begin on 16-, 32-, and
64-bit boundaries, respectively.
In this way, drivers can avoid unaligned accesses to radiotap
capture fields.
radiotap-compliant drivers must insert padding before a capture
field to ensure its natural alignment.
radiotap-compliant packet dissectors, such as
.Xr tcpdump 8 ,
expect the padding.
.Pp
Developers beware: all compilers may not pack structs alike.
If a driver developer constructs their radiotap header with a packed
structure, in order to ensure natural alignment, then it is important
that they insert padding bytes by themselves.
.Pp
Radiotap headers are copied to the userland via a separate bpf attachment.
It is necessary for the driver to create this attachment after calling
.Xr ieee80211_ifattach 9
by calling
.Fn bpfattach2
with the data-link type set to
.Dv DLT_IEEE_80211_RADIO .
.Pp
.\"
When the the information is available,
usually immediately before a link-layer transmission or after a receive,
the driver copies it to the bpf layer using the
.Fn bpf_mtap2
function.
.Pp
.\"
The following extension fields are defined for
.Vt radiotap ,
in the order in which they should appear in the buffer copied to userland:
.Bl -tag -width indent
.It Dv IEEE80211_RADIOTAP_TSFT
This field contains the unsigned 64-bit value, in microseconds,
of the MAC's 802.11 Time Synchronization Function timer,
when the first bit of the MPDU arrived at the MAC.
This field should be present for received frames only.
.It Dv IEEE80211_RADIOTAP_FLAGS
This field contains a single unsigned 8-bit value, containing a bitmap
of flags specifying properties of the frame being transmitted or received.
.It Dv IEEE80211_RADIOTAP_RATE
This field contains a single unsigned 8-bit value, which is the data rate in
use in units of 500Kbps.
.It Dv IEEE80211_RADIOTAP_CHANNEL
This field contains two unsigned 16-bit values.
The first value is the frequency upon which this PDU was transmitted
or received.
The second value is a bitmap containing flags which specify properties of
the channel in use.
These are documented within the header file,
.In net80211/ieee80211_radiotap.h .
.It Dv IEEE80211_RADIOTAP_FHSS
This field contains two 8-bit values.
This field should be present for frequency-hopping radios only.
The first byte is the hop set.
The second byte is the pattern in use.
.It Dv IEEE80211_RADIOTAP_DBM_ANTSIGNAL
This field contains a single signed 8-bit value, which indicates the
RF signal power at the antenna, in decibels difference from 1mW.
.It Dv IEEE80211_RADIOTAP_DBM_ANTNOISE
This field contains a single signed 8-bit value, which indicates the
RF noise power at the antenna, in decibels difference from 1mW.
.It Dv IEEE80211_RADIOTAP_LOCK_QUALITY
This field contains a single unsigned 16-bit value, indicating the
quality of the Barker Code lock.
No unit is specified for this field.
There does not appear to be a standard way of measuring this at this time;
this quantity is often referred to as
.Dq "Signal Quality"
in some datasheets.
.It Dv IEEE80211_RADIOTAP_TX_ATTENUATION
This field contains a single unsigned 16-bit value, expressing transmit
power as unitless distance from maximum power set at factory calibration.
0 indicates maximum transmit power.
Monotonically nondecreasing with lower power levels.
.It Dv IEEE80211_RADIOTAP_DB_TX_ATTENUATION
This field contains a single unsigned 16-bit value, expressing transmit
power as decibel distance from maximum power set at factory calibration.
0 indicates maximum transmit power.
Monotonically nondecreasing with lower power levels.
.It Dv IEEE80211_RADIOTAP_DBM_TX_POWER
Transmit power expressed as decibels from a 1mW reference.
This field is a single signed 8-bit value.
This is the absolute power level measured at the antenna port.
.It Dv IEEE80211_RADIOTAP_ANTENNA
For radios which support antenna diversity, this field contains a single
unsigned 8-bit value specifying which antenna is being used to transmit
or receive this frame.
The first antenna is antenna 0.
.It Dv IEEE80211_RADIOTAP_DB_ANTSIGNAL
This field contains a single unsigned 8-bit value, which indicates the
RF signal power at the antenna, in decibels difference from an
arbitrary, fixed reference.
.It Dv IEEE80211_RADIOTAP_DB_ANTNOISE
This field contains a single unsigned 8-bit value, which indicates the
RF noise power at the antenna, in decibels difference from an
arbitrary, fixed reference.
.It Dv IEEE80211_RADIOTAP_EXT
This bit is reserved for any future extensions to the
.Vt radiotap
structure.
A driver sets
.Dv IEEE80211_RADIOTAP_EXT
to extend the it_present bitmap by another 64 bits.
The bitmap can be extended by multiples of 32 bits to 96, 128, 160
bits or longer, by setting
.Dv IEEE80211_RADIOTAP_EXT
in the extensions.
The bitmap ends at the first extension field where
.Dv IEEE80211_RADIOTAP_EXT
is not set.
.El
.Sh EXAMPLES
Radiotap header for the Cisco Aironet driver:
.Bd -literal -offset indent
struct an_rx_radiotap_header {
	struct ieee80211_radiotap_header	ar_ihdr;
	u_int8_t	ar_flags;
	u_int8_t	ar_rate;
	u_int16_t	ar_chan_freq;
	u_int16_t	ar_chan_flags;
	u_int8_t	ar_antsignal;
	u_int8_t	ar_antnoise;
} __attribute__((__packed__));
.Ed
.Pp
Bitmap indicating which fields are present in the above structure:
.Bd -literal -offset indent
#define AN_RX_RADIOTAP_PRESENT \\
	((1 \*[Gt]\*[Gt] IEEE80211_RADIOTAP_FLAGS) | \\
	 (1 \*[Gt]\*[Gt] IEEE80211_RADIOTAP_RATE) | \\
	 (1 \*[Gt]\*[Gt] IEEE80211_RADIOTAP_CHANNEL) | \\
	 (1 \*[Gt]\*[Gt] IEEE80211_RADIOTAP_DBM_ANTSIGNAL) | \\
	 (1 \*[Gt]\*[Gt] IEEE80211_RADIOTAP_DBM_ANTNOISE))
.Ed
.Sh SEE ALSO
.Xr bpf 4 ,
.Xr ieee80211 9
.Sh HISTORY
The
.Nm
definitions first appeared in
.Nx 1.5 ,
and were later ported to
.Fx 4.6 .
.\"
.Sh AUTHORS
.An -nosplit
The
.Nm
interface was designed and implemented by
.An David Young Aq dyoung@pobox.com .
.An David Young
is the maintainer of the radiotap capture format.
Contact him to add new capture fields.
.Pp
This manual page was written by
.An Bruce M. Simpson Aq bms@FreeBSD.org
and
.An Darron Broad Aq darron@kewl.org .