A method includes measuring behavioral characteristics of a plurality of communication sessions in a communication network. A subset of the sessions is identified using the behavioral characteristics, such that the sessions in the subset are suspected of using a target communication protocol. A candidate session is selected from the subset. A trial communication session is initiated with a node of the communication network, which participates in the candidate session, using the target communication protocol. Responsively to receiving a positive response from the node to initiation of the trial communication session, it is determined that one or more further sessions in the plurality are using the target communication protocol. The one or more further sessions are controlled responsively to a predetermined control criterion that is applicable to the target communication protocol.