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NAME

     ieee80211_scan -- 802.11 scanning support

SYNOPSIS

     #include <net80211/ieee80211_var.h>

     int
     ieee80211_start_scan(struct ieee80211vap *, int flags, u_int duration,
         u_int mindwell, u_int maxdwell, u_int nssid,
         const struct ieee80211_scan_ssid ssids[]);

     int
     ieee80211_check_scan(struct ieee80211vap *, int flags, u_int duration,
         u_int mindwell, u_int maxdwell, u_int nssid,
         const struct ieee80211_scan_ssid ssids[]);

     int
     ieee80211_check_scan_current(struct ieee80211vap *);

     int
     ieee80211_bg_scan(struct ieee80211vap *, int);

     int
     ieee80211_cancel_scan(struct ieee80211vap *);

     int
     ieee80211_cancel_scan_any(struct ieee80211vap *);

     int
     ieee80211_scan_next(struct ieee80211vap *);

     int
     ieee80211_scan_done(struct ieee80211vap *);

     int
     ieee80211_probe_curchan(struct ieee80211vap *, int);

     void
     ieee80211_add_scan(struct ieee80211vap *,
         const struct ieee80211_scanparams *, const struct ieee80211_frame *,
         int subtype, int rssi, int noise);

     void
     ieee80211_scan_timeout(struct ieee80211com *);

     void
     ieee80211_scan_assoc_fail(struct ieee80211vap *,
         const uint8_t mac[IEEE80211_ADDR_LEN], int reason);

     void
     ieee80211_scan_flush(struct ieee80211vap *);

     void
     ieee80211_scan_iterate(struct ieee80211vap *, ieee80211_scan_iter_func,
         void *);

     void
     ieee80211_scan_dump_channels(const struct ieee80211_scan_state *);

     void
     ieee80211_scanner_register(enum ieee80211_opmode,
         const struct ieee80211_scanner *);

     void
     ieee80211_scanner_unregister(enum ieee80211_opmode,
         const struct ieee80211_scanner *);

     void
     ieee80211_scanner_unregister_all(const struct ieee80211_scanner *);

     const struct ieee80211_scanner *
     ieee80211_scanner_get(enum ieee80211_opmode);

DESCRIPTION

     The net80211 software layer provides an extensible framework for
     scanning.  Scanning is the procedure by which a station locates a BSS to
     join (in infrastructure and IBSS mode), or a channel to use (when
     operating as an AP or an IBSS master).  Scans are either ``active'' or
     ``passive''.  An active scan causes one or more ProbeRequest frames to be
     sent on visiting each channel.  A passive request causes each channel in
     the scan set to be visited but no frames to be transmitted; the station
     only listens for traffic.  Note that active scanning may still need to
     listen for traffic before sending ProbeRequest frames depending on
     regulatory constraints.

     A scan operation involves constructing a set of channels to inspect (the
     scan set), visiting each channel and collecting information (e.g. what
     BSS are present), and then analyzing the results to make decisions such
     as which BSS to join.  This process needs to be as fast as possible so
     net80211 does things like intelligently construct scan sets and dwell on
     a channel only as long as necessary.  Scan results are cached and the
     scan cache is used to avoid scanning when possible and to enable roaming
     between access points when operating in infrastructure mode.

     Scanning is handled by pluggable modules that implement policy per-
     operating mode.  The core scanning support provides an infrastructure to
     support these modules and exports a common API to the rest of the
     net80211 layer.  Policy modules decide what channels to visit, what state
     to record to make decisions, and selects the final station/channel to
     return as the result of a scan.

     Scanning is done synchronously when initially bringing a vap to an
     operational state and optionally in the background to maintain the scan
     cache for doing roaming and rogue AP monitoring.  Scanning is not tied to
     the net80211 state machine that governs vaps except for linkage to the
     IEEE80211_S_SCAN state.  Only one vap at a time may be scanning; this
     scheduling policy is handled in ieee80211_new_state() and is transparent
     to scanning code.

     Scanning is controlled by a set of parameters that (potentially)
     constrains the channel set and any desired SSID's and BSSID's.  net80211
     comes with a standard scanner module that works with all available
     operating modes and supports ``background scanning'' and ``roaming''
     operation.

SCANNER MODULES

     Scanning modules use a registration mechanism to hook into the net80211
     layer.  Use ieee80211_scanner_register() to register a scan module for a
     particular operating mode and ieee80211_scanner_unregister() or
     ieee80211_scanner_unregister_all() to clear entries (typically on module
     unload).  Only one scanner module can be registered at any time for an
     operating mode.

DRIVER SUPPORT

     Scanning operations are usually managed by the net80211 layer.  Drivers
     must provide ic_scan_start and ic_scan_stop methods that are called at
     the start of a scan and when the work is done; these should handle work
     such as enabling receive of Beacon and ProbeResponse frames and disable
     any BSSID matching.  The ic_set_channel method is used to change channels
     while scanning.  net80211 will generate ProbeRequest frames and transmit
     them using the ic_raw_xmit method.  Frames received while scanning are
     dispatched to net80211 using the normal receive path.  Devices that off-
     load scan work to firmware most easily mesh with net80211 by operating on
     a channel-at-a-time basis as this defers control to net80211's scan
     machine scheduler.  But multi-channel scanning is supported if the driver
     manually dispatches results using ieee80211_add_scan() routine to enter
     results into the scan cache.

SCAN REQUESTS

     Scan requests occur by way of the IEEE80211_SCAN_REQUEST ioctl or through
     a change in a vap's state machine that requires scanning.  In both cases
     the scan cache can be checked first and, if it is deemed suitably
     ``warm'' then it's contents are used without leaving the current channel.
     To start a scan without checking the cache ieee80211_start_scan() can be
     called; otherwise ieee80211_check_scan() can be used to first check the
     scan cache, kicking off a scan if the cache contents are out of date.
     There is also ieee80211_check_scan_current() which is a shorthand for
     using previously set scan parameters for checking the scan cache and then
     scanning.

     Background scanning is done using ieee80211_bg_scan() in a co-routine
     fashion.  The first call to this routine will start a background scan
     that runs for a limited period of time before returning to the BSS
     channel.  Subsequent calls advance through the scan set until all
     channels are visited.  Typically these later calls are timed to allow
     receipt of frames buffered by an access point for the station.

     A scan operation can be canceled using ieee80211_cancel_scan() if it was
     initiated by the specified vap, or ieee80211_cancel_scan_any() to force
     termination regardless which vap started it.  These requests are mostly
     used by net80211 in the transmit path to cancel background scans when
     frames are to be sent.  Drivers should not need to use these calls (or
     most of the calls described on this page).

     The ieee80211_scan_next() and ieee80211_scan_done() routines do explicit
     iteration through the scan set and should not normally be used by
     drivers.  ieee80211_probe_curchan() handles the work of transmitting
     ProbeRequest frames when visiting a channel during an active scan.  When
     the channel attributes are marked with IEEE80211_CHAN_PASSIVE this
     function will arrange that before any frame is transmitted 802.11 traffic
     is first received (in order to comply with regulatory constraints).

     Min/max dwell time parameters are used to constrain time spent visiting a
     channel.  The maximum dwell time constrains the time spent listening for
     traffic.  The minimum dwell time is used to reduce this time--when it is
     reached and one or more frames have been received then an immediate
     channel change will be done.  Drivers can override this behaviour through
     the iv_scan_mindwell method.

SCAN CACHE MANAGEMENT

     The scan cache contents are managed by the scan policy module and are
     opaque outside this module.  The net80211 scan framework defines API's
     for interacting.  The validity of the scan cache contents are controlled
     by iv_scanvalid which is exported to user space through the
     IEEE80211_SCAN_VALID request.

     The cache contents can be explicitly flushed with ieee80211_scan_flush()
     or by setting the IEEE80211_SCAN_FLUSH flag when starting a scan
     operation.

     Scan cache entries are created with the ieee80211_add_scan() routine;
     usually on receipt of Beacon or ProbeResponse frames.  Existing entries
     are typically updated based on the latest information though some
     information such as RSSI and noise floor readings may be combined to
     present an average.

     The cache contents is aged through ieee80211_scan_timeout() calls.
     Typically these happen together with other station table activity; every
     IEEE80211_INACT_WAIT seconds (default 15).

     Individual cache entries are marked usable with
     ieee80211_scan_assoc_success() and faulty with
     ieee80211_scan_assoc_fail() with the latter taking an argument to
     identify if there was no response to Authentication/Association requests
     or if a negative response was received (which might hasten cache eviction
     or blacklist the entry).

     The cache contents can be viewed using the ieee80211_scan_iterate() call.
     Cache entries are exported in a public format that is exported to user
     applications through the IEEE80211_SCAN_RESULTS request.

SEE ALSO

     ioctl(2), ieee80211(9), ieee80211_proto(9)