1.\"
2.\" Copyright (c) 2004	Bruce M. Simpson <bms@spc.org>,
3.\"			Darron Broad <darron@kewl.org>,
4.\"			David Young <dyoung@pobox.com>.
5.\" All rights reserved.
6.\"
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8.\" modification, are permitted provided that the following conditions
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28.\" $FreeBSD$
29.\" $DragonFly: src/share/man/man9/ieee80211_radiotap.9,v 1.4 2006/06/28 19:41:59 swildner Exp $
30.\" $Id: ieee80211_radiotap.9,v 1.3 2004/07/07 12:59:39 ru Exp $
31.\"
32.Dd June 25, 2006
33.Dt IEEE80211_RADIOTAP 9
34.Os
35.Sh NAME
36.Nm ieee80211_radiotap
37.Nd software 802.11 stack packet capture definitions
38.Sh SYNOPSIS
39.In net/bpf.h
40.In netproto/802_11/ieee80211_var.h
41.In netproto/802_11/ieee80211_ioctl.h
42.In netproto/802_11/ieee80211_radiotap.h
43.\"
44.Sh DESCRIPTION
45The
46.Nm
47definitions provide a device-independent
48.Xr bpf 4
49attachment for the
50capture of information about 802.11 traffic which is not part of
51the 802.11 frame structure.
52.Pp
53Radiotap was designed to balance the desire for a capture format
54that conserved CPU and memory bandwidth on embedded systems,
55with the desire for a hardware-independent, extensible format
56that would support the diverse capabilities of virtually all
57802.11
58radios.
59.Pp
60These considerations led radiotap to settle on a format consisting of
61a standard preamble followed by an extensible bitmap indicating the
62presence of optional capture fields.
63.Pp
64The capture fields were packed into the header as compactly as possible,
65modulo the requirements that they had to be packed swiftly,
66with suitable alignment, in the same order as the bits indicating
67their presence.
68.Pp
69This typically includes information such as signal quality and
70timestamps.
71This information may be used by a variety of user agents, including
72.Xr tcpdump 1 .
73It is requested by using the
74.Xr bpf 4
75data-link type
76.Dv DLT_IEEE_80211_RADIO .
77.Pp
78.\"
79Each frame using this attachment has the following header prepended to it:
80.Bd -literal -offset indent
81struct ieee80211_radiotap_header {
82	uint8_t		it_version;	/* set to 0 */
83	uint8_t		it_pad;
84	uint16_t	it_len;		/* entire length */
85	uint32_t	it_present;	/* fields present */
86} __packed;
87.Ed
88.Pp
89.\"
90A device driver implementing
91.Vt radiotap
92typically defines a packed structure embedding an instance of
93.Vt "struct ieee80211_radiotap_header"
94at the beginning,
95with subsequent fields in the appropriate order,
96and a macro to set the bits of the
97.Va it_present
98bitmap to indicate which fields exist and are filled in by the driver.
99.\"
100.Pp
101Radiotap headers are copied to the userland via a separate bpf attachment.
102It is necessary for the driver to create this attachment after calling
103.Xr ieee80211_ifattach 9
104by calling
105.Fn bpfattach_dlt
106with the data-link type set to
107.Dv DLT_IEEE_80211_RADIO .
108.Pp
109.\"
110When the the information is available,
111usually immediately before a link-layer transmission or after a receive,
112the driver copies it to the bpf layer using the
113.Fn bpf_ptap
114function.
115.Pp
116.\"
117The following extension fields are defined for
118.Vt radiotap ,
119in the order in which they should appear in the buffer copied to userland:
120.Bl -tag -width indent
121.It Dv IEEE80211_RADIOTAP_TSFT
122This field contains the unsigned 64-bit value, in microseconds,
123of the MAC's 802.11 Time Synchronization Function timer,
124when the first bit of the MPDU arrived at the MAC.
125This field should be present for received frames only.
126.It Dv IEEE80211_RADIOTAP_FLAGS
127This field contains a single unsigned 8-bit value, containing a bitmap
128of flags specifying properties of the frame being transmitted or received.
129.It Dv IEEE80211_RADIOTAP_RATE
130This field contains a single unsigned 8-bit value, which is the data rate in
131use in units of 500Kbps.
132.It Dv IEEE80211_RADIOTAP_CHANNEL
133This field contains two unsigned 16-bit values.
134The first value is the frequency upon which this PDU was transmitted
135or received.
136The second value is a bitmap containing flags which specify properties of
137the channel in use.
138These are documented within the header file,
139.In netproto/802_11/ieee80211_radiotap.h .
140.It Dv IEEE80211_RADIOTAP_FHSS
141This field contains two 8-bit values.
142This field should be present for frequency-hopping radios only.
143The first byte is the hop set.
144The second byte is the pattern in use.
145.It Dv IEEE80211_RADIOTAP_DBM_ANTSIGNAL
146This field contains a single signed 8-bit value, which indicates the
147RF signal power at the antenna, in decibels difference from 1mW.
148.It Dv IEEE80211_RADIOTAP_DBM_ANTNOISE
149This field contains a single signed 8-bit value, which indicates the
150RF noise power at the antenna, in decibels difference from 1mW.
151.It Dv IEEE80211_RADIOTAP_LOCK_QUALITY
152This field contains a single unsigned 16-bit value, indicating the
153quality of the Barker Code lock.
154No unit is specified for this field.
155There does not appear to be a standard way of measuring this at this time;
156this quantity is often referred to as
157.Dq "Signal Quality"
158in some datasheets.
159.It Dv IEEE80211_RADIOTAP_TX_ATTENUATION
160This field contains a single unsigned 16-bit value, expressing transmit
161power as unitless distance from maximum power set at factory calibration.
1620 indicates maximum transmit power.
163Monotonically nondecreasing with lower power levels.
164.It Dv IEEE80211_RADIOTAP_DB_TX_ATTENUATION
165This field contains a single unsigned 16-bit value, expressing transmit
166power as decibel distance from maximum power set at factory calibration.
1670 indicates maximum transmit power.
168Monotonically nondecreasing with lower power levels.
169.It Dv IEEE80211_RADIOTAP_DBM_TX_POWER
170Transmit power expressed as decibels from a 1mW reference.
171This field is a single signed 8-bit value.
172This is the absolute power level measured at the antenna port.
173.It Dv IEEE80211_RADIOTAP_ANTENNA
174For radios which support antenna diversity, this field contains a single
175unsigned 8-bit value specifying which antenna is being used to transmit
176or receive this frame.
177The first antenna is antenna 0.
178.It Dv IEEE80211_RADIOTAP_DB_ANTSIGNAL
179This field contains a single unsigned 8-bit value, which indicates the
180RF signal power at the antenna, in decibels difference from an
181arbitrary, fixed reference.
182.It Dv IEEE80211_RADIOTAP_DB_ANTNOISE
183This field contains a single unsigned 8-bit value, which indicates the
184RF noise power at the antenna, in decibels difference from an
185arbitrary, fixed reference.
186.It Dv IEEE80211_RADIOTAP_EXT
187This bit is reserved for any future extensions to the
188.Vt radiotap
189structure.
190It should not be used at this time.
191.El
192.Sh EXAMPLES
193Radiotap header for the Intel(R) PRO/Wireless 2200BG/2225BG/2915ABG driver
194.Bd -literal -offset indent
195struct iwi_rx_radiotap_header {
196	struct ieee80211_radiotap_header wr_ihdr;
197	uint8_t		wr_flags;
198	uint8_t		wr_rate;
199	uint16_t	wr_chan_freq;
200	uint16_t	wr_chan_flags;
201	uint8_t		wr_antsignal;
202	uint8_t		wr_antenna;
203};
204.Ed
205.Pp
206Bitmap indicating which fields are present in the above structure:
207.Bd -literal -offset indent
208#define IWI_RX_RADIOTAP_PRESENT \\
209	((1 << IEEE80211_RADIOTAP_FLAGS) | \\
210	 (1 << IEEE80211_RADIOTAP_RATE) | \\
211	 (1 << IEEE80211_RADIOTAP_CHANNEL) | \\
212	 (1 << IEEE80211_RADIOTAP_DB_ANTSIGNAL) | \\
213	 (1 << IEEE80211_RADIOTAP_ANTENNA))
214.Ed
215.Sh SEE ALSO
216.Xr bpf 4 ,
217.Xr ieee80211 9
218.Sh HISTORY
219The
220.Nm
221definitions first appeared in
222.Nx 1.5 ,
223and were later ported to
224.Fx 4.6 .
225.\"
226.Sh AUTHORS
227.An -nosplit
228The
229.Nm
230interface was designed and implemented by
231.An David Young Aq dyoung@pobox.com .
232.Pp
233This manual page was written by
234.An Bruce M. Simpson Aq bms@FreeBSD.org
235and
236.An Darron Broad Aq darron@kewl.org .
237