path: root/Documentation/networking/radiotap-headers.txt
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authorAndy Green <andy@warmcat.com>2007-07-10 19:29:37 +0200
committerJohn W. Linville <linville@tuxdriver.com>2007-07-12 16:07:24 -0400
commit08d1f2155cd5b21bb3848f46d9747afb1ccd249d (patch)
treec477c1566a427028f8faf5d857f62faa8638af29 /Documentation/networking/radiotap-headers.txt
parent15028aad00ddf241581fbe74a02ec89cbb28d35d (diff)
[PATCH] mac80211: Monitor mode radiotap injection docs
Add monitor mode radiotap injection docs. Signed-off-by: Andy Green <andy@warmcat.com> Signed-off-by: Jiri Benc <jbenc@suse.cz> Signed-off-by: John W. Linville <linville@tuxdriver.com>
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+How to use radiotap headers
+Pointer to the radiotap include file
+Radiotap headers are variable-length and extensible, you can get most of the
+information you need to know on them from:
+This document gives an overview and warns on some corner cases.
+Structure of the header
+There is a fixed portion at the start which contains a u32 bitmap that defines
+if the possible argument associated with that bit is present or not. So if b0
+of the it_present member of ieee80211_radiotap_header is set, it means that
+the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the
+argument area.
+ < 8-byte ieee80211_radiotap_header >
+ [ <possible argument bitmap extensions ... > ]
+ [ <argument> ... ]
+At the moment there are only 13 possible argument indexes defined, but in case
+we run out of space in the u32 it_present member, it is defined that b31 set
+indicates that there is another u32 bitmap following (shown as "possible
+argument bitmap extensions..." above), and the start of the arguments is moved
+forward 4 bytes each time.
+Note also that the it_len member __le16 is set to the total number of bytes
+covered by the ieee80211_radiotap_header and any arguments following.
+Requirements for arguments
+After the fixed part of the header, the arguments follow for each argument
+index whose matching bit is set in the it_present member of
+ - the arguments are all stored little-endian!
+ - the argument payload for a given argument index has a fixed size. So
+ IEEE80211_RADIOTAP_TSFT being present always indicates an 8-byte argument is
+ present. See the comments in ./include/net/ieee80211_radiotap.h for a nice
+ breakdown of all the argument sizes
+ - the arguments must be aligned to a boundary of the argument size using
+ padding. So a u16 argument must start on the next u16 boundary if it isn't
+ already on one, a u32 must start on the next u32 boundary and so on.
+ - "alignment" is relative to the start of the ieee80211_radiotap_header, ie,
+ the first byte of the radiotap header. The absolute alignment of that first
+ byte isn't defined. So even if the whole radiotap header is starting at, eg,
+ address 0x00000003, still the first byte of the radiotap header is treated as
+ 0 for alignment purposes.
+ - the above point that there may be no absolute alignment for multibyte
+ entities in the fixed radiotap header or the argument region means that you
+ have to take special evasive action when trying to access these multibyte
+ entities. Some arches like Blackfin cannot deal with an attempt to
+ dereference, eg, a u16 pointer that is pointing to an odd address. Instead
+ you have to use a kernel API get_unaligned() to dereference the pointer,
+ which will do it bytewise on the arches that require that.
+ - The arguments for a given argument index can be a compound of multiple types
+ together. For example IEEE80211_RADIOTAP_CHANNEL has an argument payload
+ consisting of two u16s of total length 4. When this happens, the padding
+ rule is applied dealing with a u16, NOT dealing with a 4-byte single entity.
+Example valid radiotap header
+ 0x00, 0x00, // <-- radiotap version + pad byte
+ 0x0b, 0x00, // <- radiotap header length
+ 0x04, 0x0c, 0x00, 0x00, // <-- bitmap
+ 0x6c, // <-- rate (in 500kHz units)
+ 0x0c, //<-- tx power
+ 0x01 //<-- antenna
+Andy Green <andy@warmcat.com>