Controller Based WLANs

Prior to AOS version 6.3.0.0, the delay in association/re-association between a client and the APs was minimized by mechanisms like PMK caching and Opportunistic Key Caching (OKC).
Below is a brief description of PMK Caching and OKC before we proceed with understanding IEEE 802.11r standards-

• PMK Caching is defined by 802.11i and is a technique available for authentication between a single AP and a station. If a station has authenticated to an AP, roams away from that AP, and comes back, it does not need to perform a full authentication exchange. Only the 802.11i 4-way handshake is performed to establish transient encryption keys (Pairwise Transient keys and Group Transient keys).

• Opportunistic Key Caching (OKC) is a similar technique, not defined by 802.11i, available for authentication between multiple APs in a network where those APs are under common administrative control. Using OKC, a station roaming to any AP in the network will not have to complete a full authentication exchange, but will instead just perform the 4-way handshake to establish transient encryption keys (Pairwise Transient keys and Group Transient keys).

 
However, from version 6.3.0.0 onwards, Aruba now implements Fast BSS Transition (IEEE 802.11r) to reduce the delay caused due to re-authentication, every time a client roams from one AP to another. In an 802.11r environment, the FT mechanism allows a client station to establish security and/or QoS state at the target AP prior to or during re-association. This reduces the delay in connecting to the distribution system after transition.
As of AOS version 6.3.0.0, Fast Transition roaming is only supported between APs in the same Mobility Domain on the same controller. We can configure the mobility domain in the 80.211r profile and map it under the SSID profile on the controller. Hence, all the clients connecting to the same SSID roaming across multiple APs on the same controller are a part of the same Mobility Domain.

Difference between Fast BSS Transition and OKC:

1. OKC is not industry standard even though many vendors have implemented it and we may sometimes encounter interoperability issues. On the contrary, 802.11r is industry standard and fully supported by IEEE.
2. The 802.11r amendment fully defines the key hierarchy used when creating the cached keys which is something missing in OKC.

 
Fast Transition Key Hierarchy – WLAN Controller

There are three levels in the FT key hierarchy for controller:

1. Level 1 – The PMK-R0 (Pairwise Master Key-R0) is derived from the MSK (Master Session Key). MSK is the key that is generated at the completion of the 802.1x authentication process. Aruba Controller acts as a key holder to PMK-R0.
2. Level 2 – The PMK-R1 (Pairwise Master Key-R1) is derived from PMK-R0 and sent to the APs if the APs are in decrypt-tunnel or bridge mode. Thus the Access Point is the key holder for PMK-R1. In case of tunnel mode, the controller keeps the PMK-R1 key.
3. Level 3 – PTK (Pairwise Transient Key) and GTK (Group Transient Key) are derived from the PMK-R1 for encrypting/decrypting user data.

 
Fast Transition Key Hierarchy – Supplicant
 
There are three levels in the FT key hierarchy for a client supplicant:

1. Level 1 – The PMK-R0 (Pairwise Master Key-R0) is derived from the MSK (Master Session Key). MSK is the key that is generated at the completion of the 802.1x authentication process. PMK-R0 is cached on the client machine.
2. Level 2 – The PMK-R1 (Pairwise Master Key-R1) is derived from PMK-R0 and is again cached on the client machine. The client machine act as the key holder for both PMK-R0 and PMK-R1 keys.
3. Level 3 – PTK (Pairwise Transient Key) and GTK (Group Transient Key) are derived from the PMK-R1 for encrypting/decrypting user data.