NAME
ieee80211_output —
software 802.11 stack output
functions
SYNOPSIS
#include
<net80211/ieee80211_var.h>
int
M_WME_GETAC(struct
mbuf *);
int
M_SEQNO_GET(struct
mbuf *);
struct ieee80211_key *
ieee80211_crypto_encap(struct
ieee80211_node *, struct
mbuf *);
void
ieee80211_process_callback(struct
ieee80211_node *, struct mbuf *,
int);
DESCRIPTION
The net80211 layer that supports 802.11
device drivers handles most of the work required to transmit frames. Drivers
usually receive fully-encapsulated 802.11 frames that have been classified
and assigned a transmit priority; all that is left is to do crypto
encapsulation, prepare any hardware-specific state, and push the packet out
to the device. Outbound frames are either generated by the
net80211 layer (e.g. management frames) or are
passed down from upper layers through the
ifnet(9) transmit queue. Data frames passed down for transmit flow
through net80211 which handles aggregation, 802.11
encapsulation, and then dispatches the frames to the driver through it's
transmit queue.
There are two control paths by which frames reach a driver for
transmit. Data packets are queued to the device's
if_snd queue and the driver's
if_start method is called. Other frames are passed
down using the ic_raw_xmit method without queueing
(unless done by the driver). The raw transmit path may include data frames
from user applications that inject them through
bpf(4) and NullData frames generated by
net80211 to probe for idle stations (when operating
as an access point).
net80211 handles all state-related
bookkeeping and management for the handling of data frames. Data frames are
only transmit for a vap in the IEEE80211_S_RUN
state; there is no need, for example, to check for frames sent down when CAC
or CSA is active. Similarly, net80211 handles
activities such as background scanning and power save mode, frames will not
be sent to a driver unless it is operating on the BSS channel with
“full power”.
All frames passed to a driver for
transmit hold a reference to a node table entry in the
m_pkthdr.rcvif field. The node is associated with the
frame destination. Typically it is the receiver's entry but in some
situations it may be a placeholder entry or the “next hop
station” (such as in a mesh network). In all cases the reference must
be reclaimed with
ieee80211_free_node()
when the transmit work is completed. The rule to remember is:
net80211 passes responsibility for the
mbuf and “node reference” to the driver
with each frame it hands off for transmit.
PACKET CLASSIFICATION
All frames passed by net80211 for transmit
are assigned a priority based on any vlan tag assigned to the receiving
station and/or any Diffserv setting in an IP or IPv6 header. If both vlan
and Diffserv priority are present the higher of the two is used. If WME/WMM
is being used then any ACM policy (in station mode) is also enforced. The
resulting AC is attached to the mbuf and may be read back using the
M_WME_GETAC()
macro.
PAE/EAPOL frames are tagged with an
M_EAPOL mbuf flag; drivers should transmit them with
care, usually by using the transmit rate for management frames.
Multicast/broadcast frames are marked with the
M_MCAST mbuf flag. Frames coming out of a station's
power save queue and that have more frames immediately following are marked
with the M_MORE_DATA mbuf flag. Such frames will be
queued consecutively in the driver's if_snd queue and
drivers should preserve the ordering when passing them to the device.
FRAGMENTED FRAMES
The net80211 layer will fragment data
frames according to the setting of iv_fragthreshold if
a driver marks the IEEE80211_C_TXFRAG capability.
Fragmented frames are placed in the devices transmit queue with the
fragments chained together with m_nextpkt. Each frame
is marked with the M_FRAG mbuf flag, and the first
and last are marked with M_FIRSTFRAG and
M_LASTFRAG, respectively. Drivers are expected to
process all fragments or none.
TRANSMIT CALLBACKS
Frames sent by net80211 may be tagged with
the M_TXCB mbuf flag to indicate a callback should
be done when their transmission completes. The callback is done using
ieee80211_process_callback()
with the last parameter set to a non-zero value if an error occurred and
zero otherwise. Note net80211 understands that
drivers may be incapable of determining status; a device may not report if
an ACK frame is received and/or a device may queue transmit requests in its
hardware and only report status on whether the frame was successfully
queued.