ALMI: Application-Level Middleware Infrastructure

ALMI is motivated by the need to support group communication among small groups of hosts without reliance on IP multicasting. A significant number of multicast applications involve small group sizes; several tens or less than 100. Such applications include videoconferrencing, multiparty games, private chat rooms, web cache replication and database/directory replication. For such small and possibly sparse groups, the benefits offered by IP multicast in terms of bandwidth efficiency and scalability are minimal and can be outweighed by the control complexity associated with group set-up and maintenance.

In this work we propose the use of application level multicast to alleviate some of the limitations of the IP multicast model and to provide the benefits of group communication even in the absence of network infrastructure support. ALMI offers accelerated deployment, simplified configuration and better access control at the cost of relatively small traffic load overhead. It also allows more flexibility in plug-in modules such as transcoding, error recovery, flow control and security in an application specific way.

In our scheme, participants of a multicast session are connected via a virtual multicast tree, i.e. a tree that consists of unicast connections between end hosts. The tree is formed as a Minimum Spanning Tree (MST), where the cost of each link is an application specific metric. We have implemented ALMI as a java based middleware package and conducted initial performance experiments on a LAN. We plan to conduct experiments in a WAN environment and to compare the cost of ALMI with IP multicast.