Methods, a data source and a router are provided to fragment packets that are to be sent in multicast mode towards a plurality of destinations comprised in a multicast group. Each router along a path leading from the data source towards some of the destinations sends its maximum transmission unit value in an uplink direction. Each router directly connected to the data source calculates a lowest maximum transmission unit based on various maximum transmission unit values received from downstream routers. The data source receives from each of the directly connected routers a maximum transmission unit value, which is certain to be acceptable to all routers and destinations of the multicast group downstream of that directly connected router. The data source selects a lowest one of the received maximum transmission unit values and fragments data contents accordingly.