Analyzing the effects of asymmetric unicast routes on multicast routing protocols

Different multicast routing protocols construct their distribution trees based on the information obtained from the unicast routing infrastructure. Nevertheless, the design of most of these protocols do not take into account that unicast routes may be asymmetric. Indeed, unicast routes in the Internet are very asymmetric. The effects on the quality of the multicast trees vary according to the routing protocol. These are particularly important for protocols that use the recursive unicast approach to allow the progressive deployment of the multicast service. This paper analyses the effects of asymmetric unicast routing on different multicast protocols. We concentrate on two approaches that implement the multicast service trough recursive unicast trees, HBH (Hop-By-Hop multicast routing protocol) and REUNITE (Recursive UNIcast TrEes). Both protocols construct source-specific trees exclusively, which simplify address allocation. As data packets have unicast destination addresses, pure unicast routers are transparently supported. The branching-nodes recursively create packet copies to implement the distribution. Nevertheless, the tree construction algorithms implemented by HBH and REUNITE are different. The design of HBH takes into account the unicast routing asymmetries of the network. HBH is able to always construct a Shortest-Path Tree. Consequently, HBH provides shorter delay routes in asymmetric networks, and provides smaller bandwidth consumption because useless data duplication is avoided. The results obtained from simulation show the effects of unicast routing asymmetries in the different multicast protocols.