---
name: "ieee80211_process_callback"
section: "9"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/M_SEQNO_GET"
---

# ieee80211\_process\_callback(9)

software 802.11 stack output functions

Part of [M\_SEQNO\_GET(9)](/M_SEQNO_GET/9.md).

## 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)](/ifnet/9.md) 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)](/bpf/4.md) 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.

## SEE ALSO

[bpf(4)](/bpf/4.md), [ieee80211(9)](/ieee80211/9.md), [ifnet(9)](/ifnet/9.md)