An end-user who wants to participate in group communication on the Internet must first resolve the session information including the contents type, data source and destination addresses. An effective session directory system is therefore the necessary component for the delivery of the maximum group communication value to a large number of world-wide users. In this paper, we formalize a distributed session directory system called "Channel Reflector (CR)." The advantage of CR is that, while an end-user can retrieve interesting session information from CR through a regular web interface, there is no contradiction to the dynamic nature of the session announcement scheme and the scoping technique in this architecture. For the deployment purpose, we show the actual implementation and the measurement results on the session synchronization delay given by the PlanetLab overlay network testbed.

A scalable multicast session announcement system is a key component of a group communication framework over the Internet. It enables the announcement of session parameters (like the {source address; group address} pair) to a potentially large number of users, according to each site administrator's policy. This system should accommodate any flavor of group communication system, like the Any-Source Multicast (ASM) and Source-Specific Multicast (SSM) schemes. In this paper we first highlight the limitations of the current Session Announcement Protocol (SAP) and study several other information distribution protocols. This critical analysis leads us to formulate the requirements of an ideal multicast session announcement system. We then introduce a new session announcement system called "Channel Reflector". It appears as a hierarchical directory system and offers an effective policy and scope control technique. We finally mention some design aspects, like the protocol messages and configuration structures the Channel Reflector uses.