Group communication primitives have broad utility as a building block
for distributed applications. However, creating and maintaining the
distributed structures that support these primitives is challenging,
particularly when network and node characteristics are transient.

We propose a solution for creating and maintaining multicast
structures that uses an unstructured overlay and a novel approach to
extract source-specific multicast trees. The strength of our solution
comes from the following properties: First, it decouples the overlay
construction and maintenance mechanism from the tree-extraction
mechanism, allowing for separate component optimization. Second, it
relies on soft-state and passive data collection to adapt to the
dynamics of the physical network, resulting in low protocol
complexity.

We implemented our solution and evaluated it on a 65-node PlanetLab
testbed and on up to 1024-node emulated ModelNet networks.
Experimental and analytical evaluations demonstrate low communication
overheads (up to 1%), efficient network usage (low stress and stretch)
compared to alternative solutions, and ability to quickly adapt to
network changes and recover from node failures. In addition, we built
a replica location service on top of this multicast layer to prove the
practicality of our solution.