lớp mạng tuy nhiên phải xác định đường đi cho gói tin. Đây là công việc của giao thức định tuyến lớp mạng. Tại trung tâm của bất kỳ giao thức định tuyến là thuật toán (thuật toán định tuyến ") để xác định đường dẫn cho một gói tin. Mục đích của một thuật toán định tuyến là đơn giản: | Multicast Routing Having examined the protocol for joining and leaving multicast groups we are now in a better position to reflect on the current Internet multicast service model which is based on the work of Steve Deering RFC 1112 Deering 1991 . In this multicast service model any host can join a multicast group at the network layer. A host simply issues a membership_report IGMP message to its attached router. That router working in concert with other Internet routers will soon begin delivering multicast datagrams to the host. Joining a multicast group is thus receiver-driven. A sender need not be concerned with explicitly adding receivers to the multicast group as is the case with multicast in ATM but neither can it control who joins the group and therefore receives datagrams sent to that group. Indeed recall that it is not possible at the network layer to even know which hosts network-wide have joined a multicast group. Similarly there is no control over who sends to the multicast group. Datagrams sent by different hosts can be arbitrarily interleaved at the various receivers with different interleaving possible at different receivers . A malicious sender can inject datagrams into the multicast group datagram flow. Even with benign senders since there is no network layer coordination of the use of multicast addresses it is possible that two different multicast groups will choose to use the same multicast address. From a multicast application viewpoint this will result in interleaved extraneous multicast traffic. These problems may seem to be insurmountable drawbacks for developing multicast applications. All is not lost however. Although the network layer does not provide for filtering ordering or privacy of multicast datagrams these mechanisms can all be implemented at the application layer. There is also ongoing work aimed at adding some of this functionality into the network layer Cain 1999 . In many ways the current Internet multicast service model reflects
