Scalable Application Layer Multicast

Summary

The goal of this project is to provide a low-overhead multicast service as an overlay. Currently, true multicast is generally not supported at the ISP level, however, it can be approximated by application layer multicast. This works by having end nodes forward packets instead of doing forwarding in the network (e.g. at routers or switches). There are two main goals for application layer multicast:

1. Minimize stress. Stress is the number of times an identical packet is sent over a link.
2. Minimize stretch. Stretch is the path length from the source to a member along the overlay divided by the length of the unicast path.

These two goals are often at odds: minimizing stress may require many forwarders, however, this increases stretch. More generally the paper evaluates application layer multicasts protocols based on:

• Quality of the data delivery path. This consists of the stress and stretch metrics.
• Robustness of the overlay. This quantifies the extent of data delivery disruption that occurs when an end node crashes.
• Control overhead. This is the overhead of the multicast protocol.

The proposed solution is to create a NICE tree over the end nodes which defines the data paths. NICE assigns nodes to different layers in a hierarchy. Each layer consists of a set of clusters of size 3k-1. Each cluster is a set of nearby nodes. The center of each cluster is considered the "leader" and placed at the next layer up. The source node sends data to all nodes within its cluster and to its parent node. At each layer this repeats recursively.

Thoughts

I am not convinced by the evaluation in this paper. The authors show their average link stress during the join phase to range from 1.6 to 2.1. This is significantly more than the "stress" of 1 we would expect from network based multicast, which would seem to limit the use cases of their approach. Also, their evaluation only stresses building the NICE tree by having nodes joining and leaving. It seems crucial to have stress and stretch graphs for actually sending data from a variety of end points.