CWAP – Reassociation Req/Response

Tags

802.11r FT, CWAP, Reassociation Request, Reassociation Response

Reassociation Request frame sent only by a STA to an AP & used when the STA already associated to the ESS & want to associate to another AP connecting to the same ESS. This frame can also be used if the STA left the cell for a short duration & want to rejoin the cell again.

We will use the below topology to look into Reassociation Request/Response frames detail. In this post we will not discuss 802.11r process in detail & refer this post if you require more on that.

If you capture wireless frames on CH149, you should see the Reassociation Request & Response frame exchange as shown below.

Frame format of a Reassociation Request frame is shown below.

When STA move from LAP2 to 3702-2 AP cell area, you will noticed STA roamed to 3702-2 AP. To initiate this process STA will send a Reassociation Request frame to the 3702-2.

Here is the Reassociation Request send by iPhone5. It is a management frame with subtype 2. Note that it will ist the Current AP Address (LAP2- B8:38:61:99:1A:AE) STA is associated to. STA can be associated to ONE AP at any given time where as it can be authenticated to multiple AP. Note that FTE & MDIE information element is there because BSS is configured for 802.11r FT.

Here is the list of information contain in the frame body of a Reassociation Request frame (source IEEE 802.11-2012)

Once AP received the Reassociation Request will acknowledge that by sending an ACK.

Then AP will respond with Reassociation Response frame. It is a management frame with subtype 3. Depend on the information provided in Reassociation Request, AP may or may not be allow STA to join. In given scenario status code=0 implies a successful reassociation & AID is given for the client (AID=1 in my case).

Here is the complete list of information that can be in the frame body of Reassociation Response frame. (source IEEE 802.11-2012)

Finally client STA will send an ACK for the Reassociation Response frame.

References
1. CWAP Official Study Guide – Chapter 4
2. IEEE 802.11-2012 Standard

1. 802.11 Mgmt Frame Types
2. 802.11 Mgmt : Beacon
3. 802.11 Mgmt : Association Request/Response
4. 802.11 Mgmt : Probe Req/Response
5. 802.11 Mgmt : Authentication Frame
6. 802.11 Deauthentication & Disassociation
7. 802.11 Mgmt : Action Frames
8. 802.11 Mgmt : Spectrum & TPC
9. 802.11 Mgmt : Admission Control

1 thought on “CWAP – Reassociation Req/Response”

ramkrish said:

November 28, 2017 at 5:20 pm

In general if I have 2 APs from a different vendor and logically mapped to same vlan and have same SSID and same security / passphrase configured , can I make seamless roaming work? Will the STA send a re-assoc req while moving from one AP to other?