Similar requirements can be imposed by other kinds of realtime group communication applications, e.g., text, audio, or video chat in social networks. The envisioned scenarios have in common that they need a many-to-many communication in potentially dynamic groups. We have identified the following challenges: (1) The targeted applications are very latencysensitive. Since latency in the Internet is not predictable, however, overlays can only provide best-effort guarantees. (2) Every participant is an event producer and has an individual set of interested participants. (3) Virtual locations of participants, and therefore their interests, are highly dynamic. (4) Interest among participants often has gradations. While some are of high priority, delay or message loss from others might be tolerable. (5) Some participants may be of particularly high interest, causing heterogeneous fan-outs. (6) The participants’ resources are heterogeneous. (7) Additional infrastructure (e.g., multicast servers) might or might not be available.