projects / deliverable / linux.git / blame
| Commit | Line | Data |
|---|---|---|
| 08d1f215 AG |
1 | How to use radiotap headers |
| 2 | =========================== | |
| 3 | ||
| 4 | Pointer to the radiotap include file | |
| 5 | ------------------------------------ | |
| 6 | ||
| 7 | Radiotap headers are variable-length and extensible, you can get most of the | |
| 8 | information you need to know on them from: | |
| 9 | ||
| 10 | ./include/net/ieee80211_radiotap.h | |
| 11 | ||
| 12 | This document gives an overview and warns on some corner cases. | |
| 13 | ||
| 14 | ||
| 15 | Structure of the header | |
| 16 | ----------------------- | |
| 17 | ||
| 18 | There is a fixed portion at the start which contains a u32 bitmap that defines | |
| 19 | if the possible argument associated with that bit is present or not. So if b0 | |
| 20 | of the it_present member of ieee80211_radiotap_header is set, it means that | |
| 21 | the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the | |
| 22 | argument area. | |
| 23 | ||
| 24 | < 8-byte ieee80211_radiotap_header > | |
| 25 | [ <possible argument bitmap extensions ... > ] | |
| 26 | [ <argument> ... ] | |
| 27 | ||
| 28 | At the moment there are only 13 possible argument indexes defined, but in case | |
| 29 | we run out of space in the u32 it_present member, it is defined that b31 set | |
| 30 | indicates that there is another u32 bitmap following (shown as "possible | |
| 31 | argument bitmap extensions..." above), and the start of the arguments is moved | |
| 32 | forward 4 bytes each time. | |
| 33 | ||
| 34 | Note also that the it_len member __le16 is set to the total number of bytes | |
| 35 | covered by the ieee80211_radiotap_header and any arguments following. | |
| 36 | ||
| 37 | ||
| 38 | Requirements for arguments | |
| 39 | -------------------------- | |
| 40 | ||
| 41 | After the fixed part of the header, the arguments follow for each argument | |
| 42 | index whose matching bit is set in the it_present member of | |
| 43 | ieee80211_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 | ||
| 76 | Example 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 | ||
| 87 | Andy Green <andy@warmcat.com> |