Wifi roaming controller vs controller less

One of the wifi deployments I worked with had an issue of RADIUS pkts received out of sequence. When client roam from AP-1 to AP-2, AP-2 sent accounting start first and then the AP-1 sent accounting stop. The wifi deployment was controller less, when the client roam to AP-2 the AP-2 immediately sent the accounting start, but the AP-1 which the client roamed away from took a little bit longer to realise that the client was disconnected and the the RADIUS accounting stop was sent later. This caused issues with the single sign on system because it depended on RADIUS accounting pkts to determine whether a client was connected or disconnected from the wifi network.

The conclusion about the above behaviour was that this is normal for controller less architecture. This lead me to test the same scenario with a controller based solution and the result is below. You can see that the accounting start and stop is used only when the client is disconnected from the network, when the client was roaming only accounting interim updates are sent. Controller based systems has several disadvantages like the controller being a single point of failure and a bottleneck when it comes to 802.11n/ac but roaming seems to be one of the things controller handles well.