Hierarchical Distance - Vector

The exponential growth of the Multicast Backbone (MBone) has resulted in increased routing overhead and processing costs. In this paper we propose a two-level hierarchical routing model as a solution to this problem. This approach involves partitioning the MBone into non-overlapping regions using DVMRP as the inter-region routing protocol; intra-region routing may be accomplished by any of a number of existing multicast protocols. Our design is exible enough to accommodate additional levels of hierarchy , and protocols other than DVMRP at the higher levels. The unique feature of this approach is the independence of the higher level routing protocol from the subnet addresses, which allows for easy incremental deployment with small changes to existing intra-region protocols. 1 Introduction The Internet Multicast Backbone or MBone Cas94] is an interconnected set of routers and subnets that provides IP multicast delivery in the Internet RFC1112]. The MBone was established in early 1992 Cas92], initially interconnecting 40 subnets (mostly Ethernets) in four diierent countries. In the three years since then, the MBone has grown exponentially to reach more than 1500 subnets in 25 countries (see Figure 1). This growth has been driven by the availability of popular new multicast applications, particularly those providing real-time audio and video teleconferencing capabilities to Internet hosts. With new multicast applications and multicast-based services appearing regularly, with multicast support becoming available on more types of host and router platforms, with suucient bandwidth for typical MBone applications becoming more plentiful, and with the population of potential IP multicast users increasing as the Internet itself grows, it appears likely that the MBone will continue to grow at an ever increasing rate. In order to decide where to forward IP multicast packets , the MBone routers run a multicast routing protocol. The protocol that is used in most of the MBone routers today Figure 1: Growth of the MBone|Number of Subnets is DVMRP, the Distance-Vector Multicast Routing Protocol Dee91]. DVMRP routers treat the MBone topology as a single , \\at" routing domain. That means that each DVMRP router maintains a separate entry in its routing table for every subnet in the MBone, exchanges periodic routing messages with each of its neighbors identifying every subnet, and is susceptible to additional processing costs whenever there is a topological change anywhere in the MBone. As the number of subnets grows exponentially, those routing costs grow exponentially. If nothing is done about it, the memory and/or …