In the near future, RTP/RTCP-based multimedia streaming will become the norm not only in wired networks but also in mobile environments. Presently when a handoff occurs between heterogeneous networks (with different available bandwidths), a RTP sender cannot stream media at a suitable rate over the new network. Furthermore, RTCP fails to precisely adapt to sudden changes in network resources due to handoff. In order to solve these issues, 1) senders should be aware when mobile nodes are about to perform a handoff; 2) senders should then efficiently probe the available bandwidth in the new network and accordingly adjust their streaming rates. In this paper, we propose a scheme that allows mobile nodes to explicitly notify their handoff timing by using newly-defined RTCP packets. In the proposed scheme, senders probe the available bandwidth in the new network using low-priority RTP dummy packets. The performance of the proposed scheme is evaluated and compared with conventional schemes through extensive simulations. The simulation results show that the proposed scheme achieves appropriate bandwidth utilization immediately after a handoff occurrence and lowers packet losses during the handoff. The proposed scheme exhibits also high TCP-friendliness.