DESCRIPTION

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. If the most significant bit is set, the remaining bits contain an MCS index instead of a data rate.

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.

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;

#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)