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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>
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