The multicast technology is applied and developed initially on the network layer. With development of the Internet, more and more data, voice, and video information is exchanged in the network, and new services such as Internet Protocol Television (IPTV), e-commerce, online conference, online auction, Video On Demand (VOD), and tele-education are emerging. Such services require information security and network bandwidth, and are not free of charge. As regards such single-point-to-multipoint issues, the multicast technology provides an effective solution, implements efficient transfer of data from a single point to multiple points in the IP network, saves the network bandwidth massively, and reduces the network load. New Value-Added Services (VASs) can be provided conveniently by using the multicast features of the network.
IP multicast protocols mostly include the Internet Group Management Protocol (IGMP) applied to host registration and the multicast routing protocol applied to multicast routing and forwarding.
The IGMP is generally applied between a receiving host and a multicast router. It defines how to establish and maintain the multicast membership between the host and the router.
The multicast routing protocol runs between the multicast routers to establish and maintain multicast routes, and forwards the multicast packets correctly and efficiently. The multicast route establishes a loopless data transmission path from the data source to multiple receivers, namely, constructs a distribution tree structure. Like unicast routing protocols, the multicast routing protocols come in two categories: intra-domain multicast routing protocols, which transfer the information to the receiver by discovering the multicast source and constructing the multicast distribution tree, for example, Distance Vector Multicast Routing Protocol (DVMRP), Multicast Open Shortest Path First (MOSPF), Protocol Independent Multicast-Dense Mode (PIM-DM), and Protocol Independent Multicast-Sparse Mode (PIM-SM); and inter-domain multicast routing protocols, which discover the multicast sources in other multicast domains, for example, Multicast Source Discovery Protocol (MSDP), and transfer multicast routing information between the autonomous systems, for example, Multicast Border Gateway Protocol (MBGP).
In Protocol Independent Multicast (PIM), the unicast routing protocol that provides routes for the IP multicast may be a static routing protocol, Routing Information Protocol (RIP), Open the Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS), or Border Gateway Protocol (BGP), and so on; the multicast route is independent of the unicast routing protocol, and is appropriate only if the unicast routing protocol can generate routing table entries. Reverse Path Forwarding (RPF) is a multicast forwarding mode. The RPF checks the source IP and interfaces of the received packets first, and compares them with the IP and interfaces in unicast routing table. If the packet can be returned along the unicast route from the receiving interface to the source site, the RPF check succeeds, and the multicast packet is regarded as coming from the correct path; otherwise, the multicast packet is discarded as a redundant packet. By virtue of the RPF forwarding, the PIM transfers the multicast information in the network. For ease of description, the network composed of the PIM-supported multicast routers is called a “PIM multicast domain”.
As shown in FIG. 1, the same packet is sent repeatedly in the shared network, such as Ethernet. For example, multicast routers A, B, and C in the LAN network segment shown in FIG. 1, and each of the routers has a receiving path to the multicast source S. After receiving a multicast data packet, which is sent by multicast source S, from the upstream, the routers A, B, and C forward the multicast packet to the Ethernet, and the Host A, which is a downstream node, receives three identical multicast packets.
In order to avoid such a circumstance, a unique forwarder, namely, Designated Router (DR) should be selected through Assert messages. Each router in the network selects the best path by sending an Assert message, and the selected router becomes an upstream neighbor of the (S, G) and is responsible for forwarding the (S, G) multicast packet. Regarding the remaining routers that are not selected, their corresponding interfaces are cut so that the information forwarding at the interfaces is forbidden. The comparative items may be the metric value, namely, the cost value of routing from routers to the multicast source. If the metric value is the same, the IP address on the interface is compared, namely, greater IP addresses are of higher priority.
The period of the Assert message of the PIM is 60 seconds by default. If the upstream neighboring router fails, the routers re-converge through the Assert messages of the PIM, which takes at least 180 seconds (three periods), during which the multicast traffic is interrupted.
With the fault detection such as Bidirectional Forwarding Detection (BFD) protocol in the prior art, the fault of the PIM router on the shared network segment may be discovered as early as possible. Routers are triggered to reselect a new upstream router in the shared network segment through Assert messages of the PIM.
However, after the fault is discovered through the BFD packets, routers are triggered to re-converge and select a new DR through the Assert messages of the PIM. The switching delay still exists, and the multicast traffic is still interrupted transiently.
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