1How to use radiotap headers
  2===========================
  3
  4Pointer to the radiotap include file
  5------------------------------------
  6
  7Radiotap headers are variable-length and extensible, you can get most of the
  8information you need to know on them from:
  9
 10./include/net/ieee80211_radiotap.h
 11
 12This document gives an overview and warns on some corner cases.
 13
 14
 15Structure of the header
 16-----------------------
 17
 18There is a fixed portion at the start which contains a u32 bitmap that defines
 19if the possible argument associated with that bit is present or not.  So if b0
 20of the it_present member of ieee80211_radiotap_header is set, it means that
 21the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the
 22argument area.
 23
 24   < 8-byte ieee80211_radiotap_header >
 25   [ <possible argument bitmap extensions ... > ]
 26   [ <argument> ... ]
 27
 28At the moment there are only 13 possible argument indexes defined, but in case
 29we run out of space in the u32 it_present member, it is defined that b31 set
 30indicates that there is another u32 bitmap following (shown as "possible
 31argument bitmap extensions..." above), and the start of the arguments is moved
 32forward 4 bytes each time.
 33
 34Note also that the it_len member __le16 is set to the total number of bytes
 35covered by the ieee80211_radiotap_header and any arguments following.
 36
 37
 38Requirements for arguments
 39--------------------------
 40
 41After the fixed part of the header, the arguments follow for each argument
 42index whose matching bit is set in the it_present member of
 43ieee80211_radiotap_header.
 44
 45 - the arguments are all stored little-endian!
 46
 47 - the argument payload for a given argument index has a fixed size.  So
 48   IEEE80211_RADIOTAP_TSFT being present always indicates an 8-byte argument is
 49   present.  See the comments in ./include/net/ieee80211_radiotap.h for a nice
 50   breakdown of all the argument sizes
 51
 52 - the arguments must be aligned to a boundary of the argument size using
 53   padding.  So a u16 argument must start on the next u16 boundary if it isn't
 54   already on one, a u32 must start on the next u32 boundary and so on.
 55
 56 - "alignment" is relative to the start of the ieee80211_radiotap_header, ie,
 57   the first byte of the radiotap header.  The absolute alignment of that first
 58   byte isn't defined.  So even if the whole radiotap header is starting at, eg,
 59   address 0x00000003, still the first byte of the radiotap header is treated as
 60   0 for alignment purposes.
 61
 62 - the above point that there may be no absolute alignment for multibyte
 63   entities in the fixed radiotap header or the argument region means that you
 64   have to take special evasive action when trying to access these multibyte
 65   entities.  Some arches like Blackfin cannot deal with an attempt to
 66   dereference, eg, a u16 pointer that is pointing to an odd address.  Instead
 67   you have to use a kernel API get_unaligned() to dereference the pointer,
 68   which will do it bytewise on the arches that require that.
 69
 70 - The arguments for a given argument index can be a compound of multiple types
 71   together.  For example IEEE80211_RADIOTAP_CHANNEL has an argument payload
 72   consisting of two u16s of total length 4.  When this happens, the padding
 73   rule is applied dealing with a u16, NOT dealing with a 4-byte single entity.
 74
 75
 76Example valid radiotap header
 77-----------------------------
 78
 79	0x00, 0x00, // <-- radiotap version + pad byte
 80	0x0b, 0x00, // <- radiotap header length
 81	0x04, 0x0c, 0x00, 0x00, // <-- bitmap
 82	0x6c, // <-- rate (in 500kHz units)
 83	0x0c, //<-- tx power
 84	0x01 //<-- antenna
 85
 86
 87Using the Radiotap Parser
 88-------------------------
 89
 90If you are having to parse a radiotap struct, you can radically simplify the
 91job by using the radiotap parser that lives in net/wireless/radiotap.c and has
 92its prototypes available in include/net/cfg80211.h.  You use it like this:
 93
 94#include <net/cfg80211.h>
 95
 96/* buf points to the start of the radiotap header part */
 97
 98int MyFunction(u8 * buf, int buflen)
 99{
100	int pkt_rate_100kHz = 0, antenna = 0, pwr = 0;
101	struct ieee80211_radiotap_iterator iterator;
102	int ret = ieee80211_radiotap_iterator_init(&iterator, buf, buflen);
103
104	while (!ret) {
105
106		ret = ieee80211_radiotap_iterator_next(&iterator);
107
108		if (ret)
109			continue;
110
111		/* see if this argument is something we can use */
112
113		switch (iterator.this_arg_index) {
114		/*
115		 * You must take care when dereferencing iterator.this_arg
116		 * for multibyte types... the pointer is not aligned.  Use
117		 * get_unaligned((type *)iterator.this_arg) to dereference
118		 * iterator.this_arg for type "type" safely on all arches.
119		 */
120		case IEEE80211_RADIOTAP_RATE:
121			/* radiotap "rate" u8 is in
122			 * 500kbps units, eg, 0x02=1Mbps
123			 */
124			pkt_rate_100kHz = (*iterator.this_arg) * 5;
125			break;
126
127		case IEEE80211_RADIOTAP_ANTENNA:
128			/* radiotap uses 0 for 1st ant */
129			antenna = *iterator.this_arg);
130			break;
131
132		case IEEE80211_RADIOTAP_DBM_TX_POWER:
133			pwr = *iterator.this_arg;
134			break;
135
136		default:
137			break;
138		}
139	}  /* while more rt headers */
140
141	if (ret != -ENOENT)
142		return TXRX_DROP;
143
144	/* discard the radiotap header part */
145	buf += iterator.max_length;
146	buflen -= iterator.max_length;
147
148	...
149
150}
151
152Andy Green <andy@warmcat.com>