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.

A critical problem faced by feedback-merger mechanisms is the lack of information that is discarded due to the hidden nodes in multicast trees. A node is said to be hidden from another if it is located in a sub tree that is a result from a fork in any upstream node. We propose in this paper the virtual layering scheme to avoid the problem caused by hidden-nodes in multi-layered multicast video environments. The virtual layering scheme induces intermediate nodes to keep extra states of the multicast session, which reduces the video degradation for the whole set of receivers. Furthermore, this scheme is coupled with the Direct Algorithm in order to improve the degree of satisfaction at heterogeneous receivers. The algorithm relies on a mechanism that dynamically controls the rates of the video layers and addresses scalability issues by implementing a merging procedure at intermediate nodes in order to avoid packet implosion at the source. The Virtual Layering scheme combined with the Direct Algorithm is optimized to achieve high global video quality and reduced bandwidth requirements. The results show that the proposed scheme leads to improved global video quality at heterogeneous receivers with no cost of extra bandwidth.