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802.11 WLAN defines three types of frame formats: management frames (Management Frame), control frames (Control Frame), and data frames (Data Frame). The type field in the frame header identifies which field the frame belongs to. This blog post only introduces management frames, followed by control frames and data frames.

As shown in the figure above, in the 802.11 frame format, the first two bytes are Frame Control fields, and the first two bits are the protocol version. Because there is currently only one version of the 802.11 protocol suite, these two bits have always been 00, the next two bits are frame type, and there are currently three values: 00 for management frame, 01 for control frame, and 10 for data frame.

Manage the structure of the frame:

HT Management frame structure (802.11n):

In the next four bits, which are subtype identification bits, let's introduce the subtypes of the management frame one by one:

0000cr Association request: associate request frame

After the authentication is successful, the STA will enter the association phase, and the purpose of this interaction is to join the BSS and obtain an AID. Through the information carried by Association Request, let AP know the relevant capability information of STA, so that AP can decide how to communicate with the STA.

0001pm Association response: associated request reply frame

When AP receives the Association Request, it checks each 802.11 parameter and matches it with the 802.11 parameter it supports. If there is a mismatch, AP will see whether the current difference belongs to Bloocking Factor. If so, AP will reject the association, otherwise AP will mark the difference and return to Association Response with its own 802.11 parameter.

AP returns a status code, and 0 indicates a successful association. And assign an AID to STA, which is 1byte 2007 (2 bytes long, using only the lowest 14 Bit, and the other bits are 1.)

0010 reassociation request: re-associate the request frame

This frame can only be emitted by STA. The main usage scenario is when a STA has been associated with an ESS and wants to be connected to another AP under the same ESS. It can also be used to re-associate the AP when the STA briefly leaves the currently associated AP. It can also be used for authentication and re-association of Authenticator timer when STA expires. A STA that is already associated with an AP can also renegotiate some parameters through this frame.

0011pr reassociation response: re-associate request reply frame

AP uses this frame to respond to the association request made by STA. Its format is similar to Association Response.

0100penny probe request: probe request frame

Mainly used for active scanning to find AP information. STA can also use Probe Request to discover network-specific information. To support this, the Probe Request can contain an optional Request Information information that can request one or more additional parameters. In addition, Probe Request can also carry some vendor-defined IE information.

0101 probe response: probe request reply frame

Mainly used for active scanning, AP replies to specific requests for finding AP information. Its frame content is equivalent to beacon frame, except that it does not contain the TIM field and the QoS field. If the Probe request contains Request Information information, this frame should contain the corresponding Request Information information.

1000 Beacon: beacon frame

AP sends Beacon frames at regular intervals and notifies the next Beacon frame when it will be sent. If the current media is busy, the transmission of the current Beacon frame will be delayed, but the transmission of the next Beacon frame will not be affected, and the next Beacon frame will still be sent at the scheduled time.

The beacon frame contains all the attributes of the AP in case the STA verifies that it meets its requirements before associating it.

1001 department announcement traffic indication message (ATIM):

This frame applies only to IBSS networks and is used to inform other STA in power save mode whether they should switch sleep mode to active state to receive buffered data.

1010 dint disassociation: disassociate frames:

The frame may be a unicast frame or a multicast frame. STA in the Disassociated state is still in the Authenticated state, and they can be reassociated directly.

1011pr Authentication: authentication frame:

In all networks, STA must complete authentication before it wants to join a particular network. Unlike the associated request and reply, the authentication process uses only one format of the authentication frame.

1100Magi Deauthentication: decrypt authentication frame:

When the communication between the two parties is complete, this frame is sent to end the session. The format of the frame is similar to Disassociation.

1101action: behavior frame:

This frame is used only for special network behaviors.

1110,Action no ack:

The Action frame of ack is not needed.

Management frames usually have a standard MAC header size of 24 bytes and have three address domains. Management frames are always processed by the MAC layer, and they are not passed to the upper layer (for app). In addition, their To DS and From DS values are always 0. 0.

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