IEEE80211_RADIOTAP(9) OpenBSD Kernel Manual IEEE80211_RADIOTAP(9)NAMEieee80211_radiotap - software 802.11 stack packet capture definitions
SYNOPSIS
#include <net80211/ieee80211_var.h>
#include <net80211/ieee80211_ioctl.h>
#include <net80211/ieee80211_radiotap.h>
#include <net/bpf.h>
DESCRIPTION
The ieee80211_radiotap definitions provide a device-independent bpf(4)
attachment for the capture of information about 802.11 traffic which is
not part of the 802.11 frame structure.
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.
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.
The capture fields were packed into the header as compactly as possible,
modulo the requirements that they had to be packed swiftly, with suitable
alignment, in the same order as the bits indicating their presence.
This typically includes information such as signal quality and
timestamps. This information may be used by a variety of user agents,
including tcpdump(8). It is requested by using the bpf(4) data-link type
DLT_IEEE802_11_RADIO.
Each frame using this attachment has the following header prepended to
it:
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 */
} __packed;
A device driver implementing radiotap typically defines a packed
structure embedding an instance of struct ieee80211_radiotap_header at
the beginning, with subsequent fields in the appropriate order, and a
macro to set the bits of the it_present bitmap to indicate which fields
exist and are filled in by the driver.
Radiotap headers are copied to userland via a separate bpf attachment.
It is necessary for the driver to create this attachment after calling
ieee80211_ifattach(9) by calling bpfattach2() with the data-link type set
to DLT_IEEE802_11_RADIO.
When 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 bpf_mtap2() function.
The following extension fields are defined for radiotap, in the order in
which they should appear in the buffer copied to userland:
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.
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.
IEEE80211_RADIOTAP_RATE
This field contains a single unsigned 8-bit value, which is the
data rate in use in units of 500Kbps.
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 <net80211/ieee80211_radiotap.h>.
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.
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.
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.
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 ``Signal Quality'' in some datasheets.
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.
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.
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.
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.
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.
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.
IEEE80211_RADIOTAP_HWQUEUE
This field contains a single unsigned 8-bit value specifying
which hardware queue is being used to transmit the frame.
IEEE80211_RADIOTAP_RSSI
This field contains two unsigned 8-bit values. The first value
is the received signal strength index (RSSI) which indicates the
RF signal power at the antenna. The second value is the relative
maximum RSSI value of the RF interface.
IEEE80211_RADIOTAP_EXT
This bit is reserved for any future extensions to the radiotap
structure. A driver can set 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 IEEE80211_RADIOTAP_EXT in the extensions. The bitmap
ends at the first extension field where IEEE80211_RADIOTAP_EXT is
not set.
EXAMPLES
Radiotap header for the Realtek RTL8180L driver rtw(4):
struct rtw_rx_radiotap_header {
struct ieee80211_radiotap_header rr_ihdr;
u_int64_t rr_tsft;
u_int8_t rr_flags;
u_int8_t rr_rate;
u_int16_t rr_chan_freq;
u_int16_t rr_chan_flags;
u_int16_t rr_barker_lock;
u_int8_t rr_antsignal;
} __packed;
Bitmap indicating which fields are present in the above structure:
#define RTW_RX_RADIOTAP_PRESENT \
((1 << IEEE80211_RADIOTAP_TSFT) | \
(1 << IEEE80211_RADIOTAP_FLAGS) | \
(1 << IEEE80211_RADIOTAP_RATE) | \
(1 << IEEE80211_RADIOTAP_CHANNEL) | \
(1 << IEEE80211_RADIOTAP_LOCK_QUALITY) | \
(1 << IEEE80211_RADIOTAP_DB_ANTSIGNAL) | \
0)
SEE ALSObpf(4), ieee80211(9)HISTORY
The ieee80211_radiotap definitions first appeared in NetBSD 1.5, and were
later ported to FreeBSD 4.6 and OpenBSD 3.6.
AUTHORS
The ieee80211_radiotap interface was designed and implemented by David
Young <dyoung@pobox.com>.
This manual page was written by Bruce M. Simpson <bms@FreeBSD.org> and
Darron Broad <darron@kewl.org>.
OpenBSD 4.9 March 26, 2010 OpenBSD 4.9
