--- src/share/man/man9/ieee80211_radiotap.9	2006/06/28 19:41:59	1.4
+++ src/share/man/man9/ieee80211_radiotap.9	2006/12/25 14:45:33	1.5
@@ -25,11 +25,11 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\" $FreeBSD$
+.\" $FreeBSD: src/share/man/man9/ieee80211_radiotap.9,v 1.3 2004/07/07 12:59:39 ru Exp $
+.\" $NetBSD: ieee80211_radiotap.9,v 1.9 2006/03/12 03:22:02 dyoung Exp $
 .\" $DragonFly$
-.\" $Id: ieee80211_radiotap.9,v 1.4 2006/06/28 19:41:59 swildner Exp $
 .\"
-.Dd June 25, 2006
+.Dd December 25, 2006
 .Dt IEEE80211_RADIOTAP 9
 .Os
 .Sh NAME
@@ -54,8 +54,7 @@ Radiotap was designed to balance the des
 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.
+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
@@ -63,17 +62,17 @@ 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 suitable alignment, in the same order as the bits indicating
-their presence.
+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 1 .
+.Xr tcpdump 8 .
 It is requested by using the
 .Xr bpf 4
 data-link type
-.Dv DLT_IEEE_80211_RADIO .
+.Dv DLT_IEEE802_11_RADIO .
 .Pp
 .\"
 Each frame using this attachment has the following header prepended to it:
@@ -89,15 +88,35 @@ struct ieee80211_radiotap_header {
 .\"
 A device driver implementing
 .Vt radiotap
-typically defines a packed structure embedding an instance of
+typically defines a 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
+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 are in little-endian byte order.
+.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
@@ -107,7 +126,7 @@ with the data-link type set to
 .Dv DLT_IEEE_80211_RADIO .
 .Pp
 .\"
-When the the information is available,
+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
 .Fn bpf_ptap
@@ -187,7 +206,17 @@ arbitrary, fixed reference.
 This bit is reserved for any future extensions to the
 .Vt radiotap
 structure.
-It should not be used at this time.
+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 Intel(R) PRO/Wireless 2200BG/2225BG/2915ABG driver
@@ -229,6 +258,9 @@ 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