PIM-SM is a widely used multicast protocol in Internet. It has two kinds of packet-delivery tree, RPT and SPT. When a PIM-SM router switches from RPT to SPT for delivering packets, packet-loss may occur, which will get more serious when source rate is higher and packet length is shorter. In this paper, an improved PIM-SM used in the IPv6 network to avoid packet-loss is presented. By postponing the PRUNE message to upstream router towards RP, the router temporarily forwards multicast packets both from RPT and SPT to reduce or even eliminate the packet-loss. The improved PIM-SM only needs a few slight modifications of PIM-SM and is compatible with PIM-SM, i.e., the routers running the modified PIM-SM can interact normally with other standard PIM-SM routers.
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