Langue: en

Version: 365518 (ubuntu - 25/10/10)

Section: 9 (Appels noyau Linux)

BSD mandoc


ieee80211_node - software 802.11 stack node management functions


In net80211/ieee80211_var.h Ft struct ieee80211_node * Fo ieee80211_find_rxnode Fa struct ieee80211com * Fa const struct ieee80211_frame_min * Fc Ft struct ieee80211_node * Fo ieee80211_find_rxnode_withkey Fa struct ieee80211com * Fa const struct ieee80211_frame_min * Fa ieee80211_keyix Fc Ft struct ieee80211_node * Fn ieee80211_ref_node struct ieee80211_node * Ft void Fn ieee80211_unref_node struct ieee80211_node * Ft void Fn ieee80211_free_node struct ieee80211_node * Ft void Fo ieee80211_iterate_nodes Fa struct ieee80211_node_table * Fa ieee80211_iter_func *f Fa void *arg Fc Ft void Fo ieee80211_dump_nodes Fa struct ieee80211_node_table * Fc Ft void Fo ieee80211_dump_node Fa struct ieee80211_node * Fc  


The net80211 layer that supports 802.11 device drivers maintains a database of peer stations called the ``node table '' in the Vt ic_sta entry of the Vt ieee80211com structure. Station mode vaps create an entry for the access point the station is associated to. AP mode vaps create entries for associated stations. Adhoc and mesh mode vaps create entries for neighbor stations. WDS mode vaps create an entry for the peer station. Stations for all vaps reside in the same table; each node entry has a Vt ni_vap field that identifies the vap that created it. In some instances an entry is used by multiple vaps (e.g. for dynamic WDS a station associated to an ap vap may also be the peer of a WDS vap).

Node table entries are reference counted. That is, there is a count of all long term references that determines when an entry may be reclaimed. References are held by every in-flight frame sent to a station to insure the entry is not reclaimed while the frame is queued or otherwise held by a driver. Routines that lookup a table entry return a ``held reference'' (i.e. a pointer to a table entry with the reference count incremented). The Fn ieee80211_ref_node and Fn ieee80211_unref_node calls explicitly increment/decrement the reference count of a node, but are rarely used. Instead most callers use Fn ieee80211_free_node to release a reference and, if the count goes to zero, reclaim the table entry.

The station table and its entries are exposed to drivers in several ways. Each frame transmitted to a station includes a reference to the associated node in the Vt m_pkthdr.rcvif field. This reference must be reclaimed by the driver when transmit processing is done. For each frame received the driver must lookup the table entry to use in dispatching the frame ``up the stack'' This lookup implicitly obtains a reference to the table entry and the driver must reclaim the reference when frame processing is completed. Otherwise drivers frequently inspect the contents of the Vt iv_bss node when handling state machine changes as important information is maintained in the data structure.

The node table is opaque to drivers. Entries may be looked up using one of the pre-defined API's or the Fn ieee80211_iterate_nodes call may be used to iterate through all entries to do per-node processing or implement some non-standard search mechanism. Note that Fn ieee80211_iterate_nodes is single-threaded per-device and the effort processing involved is fairly substantial so it should be used carefully.

Two routines are provided to print the contents of nodes to the console for debugging: Fn ieee80211_dump_node displays the contents of a single node while Fn ieee80211_dump_nodes displays the contents of the specified node table. Nodes may also be displayed using ddb(9) with the ``show node'' directive and the station node table can be displayed with ``show statab''


Node data structures may be extended by the driver to include driver-private state. This is done by overriding the Vt ic_node_alloc method used to allocate a node table entry. The driver method must allocate a structure that is an extension of the Vt ieee80211_node structure. For example the iwi(4) driver defines a private node structure as:
 struct iwi_node {
         struct ieee80211_node   in_node;
         int                     in_station;

and then provides a private allocation routine that does this:

 static struct ieee80211_node *
 iwi_node_alloc(struct ieee80211vap *vap,
     const uint8_t mac[IEEE80211_ADDR_LEN])
         struct iwi_node *in;
         in = malloc(sizeof (struct iwi_node), M_80211_NODE,
                 M_NOWAIT | M_ZERO);
         if (in == NULL)
                 return NULL;
         in->in_station = -1;
         return &in->in_node;

Note that when reclaiming a node allocated by the driver the ``parent method'' must be called to ensure net80211 state is reclaimed; for example:

 static void
 iwi_node_free(struct ieee80211_node *ni)
         struct ieee80211com *ic = ni->ni_ic;
         struct iwi_softc *sc = ic->ic_ifp->if_softc;
         struct iwi_node *in = (struct iwi_node *)ni;
         if (in->in_station != -1)
                 free_unr(sc->sc_unr, in->in_station);
         sc->sc_node_free(ni);   /* invoke net80211 free handler */

Beware that care must be taken to avoid holding references that might cause nodes from being reclaimed. net80211 will reclaim a node when the last reference is reclaimed in its data structures. However if a driver holds additional references then net80211 will not recognize this and table entries will not be reclaimed. Such references should not be needed if the driver overrides the Vt ic_node_cleanup and/or Vt ic_node_free methods.


Node table lookups are typically done using a hash of the stations' mac address. When receiving frames this is sufficient to find the node table entry for the transmitter. But some devices also identify the sending station in the device state received with each frame and this data can be used to optimize lookups on receive using a companion table called the ``keytab'' This table records a separate node table reference that can be fetched without any locking using the table index. This logic is handled with the Fn ieee80211_find_rxnode_withkey call: if a keytab entry is found using the specified index then it is returned directly; otherwise a normal lookup is done and the keytab entry is written using the specified index. If the specified index is IEEE80211_KEYIX_NONE then a normal lookup is done without a table update.


ddb(9) ieee80211(9), ieee80211_proto9