A multicast system is described in which a mechanism for suppressing feedback involves the receivers using an optimized timer function for selecting a backoff delay time before sending feedback, and canceling their feedback if in the meantime they detect that another receiver has sent a duplicate feedback message. The timer function is optimised, taking into account both the expected number of feedback messages and the extra latency which is introduced by the feedback mechanism, by minimizing an objective function expressed in terms of both these two aspects.