I am new to ODL. Although I have read MD-SAL FAQ, Ping Example, Toaster step-by-step. However, I still cannot make up my mind which kind of SAL should I use when I design a new application. Can anyone tell me when do we use AD-SAL, when do we use MD-SAL? Take Topology Manager for example, why it makes use of both AD-SAL and MD-SAL?