| The traditional IP multicast is a model of Any Source Multicast (ASM).When a client intends to receive multicast data packets; it will issue a report packet indicating the group address which it wants to join. Any sender can send multicast packets to it. With the study and application of IP multicast, the disadvantages of ASM are discovered. It is found that most multicast applications are source-aware, so IETF proposed Source Specific Multicast (SSM).According to the framework of SSM, The network layer service provided by SSM is a channel, identified by an SSM destination IP address (G) and a source IP address S. A source S transmits IP packets to an SSM destination address G A receiver can receive these packets by subscribing to the channel (Source, Group) or (S, G). The tree for forwarding IP multicast packets is shortest path tree rooted at the source S.This article focuses on the study of Source Specific Multicast (IPv4) based on IGMPv3 and PIM-SSM. Firstly it describes the basic concepts related to IP multicast, including the structure of multicast, multicast routing protocols, multicast forward tree, reserve path forward and so on. Secondly, it studies the IGMPv3, compares it with IGMPv1/v2. Then, it studies PIM-SSM and concludes the comparison of PIM-SSM and PIM-SM. It analyses the Source Specific Multicast based on IGMPv3 and PIM-SSM in detail, including the framework of SSM, working mechanism and model. The advantages and disadvantages of SSM are discussed.After describing the model of SSM, it implements the SSM module on router embedded software system, including the division of sub modules, design of data structures, handle of protocol packets, the organization of multicast routing tables, etc. In order to solve the problem that most clients cannot support IGMPv2, the SSM-Mapping mechanism is implemented. The implemented SSM function and SSM-Mapping is tested and verified in the networks environment. Finally, it makes prospect of SSM multicast.The implemented SSM function based on IGMPv3 and PIM-SSM has great practical value, and can be widely used in market. |
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