CONTENTS

SYNOPSIS

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(1). It is requested by using the bpf(4) data-link type DLT_IEEE_80211_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 */
} __attribute__((__packed__));

A device driver implementing
.Vt radiotap typically defines a packed structure embedding an instance of
.Vt "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 the 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_IEEE_80211_RADIO.

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 bpf_mtap2 function.

The following extension fields are defined for
.Vt radiotap , in the order in which they should appear in the buffer copied to userland:

EXAMPLES

HISTORY

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