Paper review: IP Multicast Channels: EXPRESS Support for Large-scale Single-source Applications

Reviewer: Mike Liu

1. State the problem the paper is trying to solve.
The main problem the paper is trying to design a system for extending IP multicast to support a channel mode of multicast.
2. State the main contribution of the paper: solving a new problem, proposing a new algorithm, or presenting a new evaluation (analysis). If a new problem, why was the problem important? Is the problem still important today? Will the problem be important tomorrow?  If a new algorithm or new evaluation (analysis), what are the improvements over previous algorithms or evaluations? How do they come up with the new algorithm or evaluation? 
The main contribution of this paper is that it presents a specific realization of a channel model of multicast called EXPlicitly REquested Single-Source (EXPRESS) multicast. The problem of designing a channel mode of multicast is relatively new extension of the challenges of multicast on the Internet and is important for several new applications on the verge of arriving on the Internet. These new applications include Internet TV, distance learning, file distribution, and other emerging large-scale multicast applications that strain the current model of multicast, which lacks a basis for charging, lacks access control, and is difficult to scale.
3. Summarize the (at most) 3 key main ideas (each in 1 sentence.) 
The three 3 key main ideas are: (1) EXPlicitly REquested Single-Source (EXPRESS) multicast channels can be provided through a series of simple modifications: (a) First, using a small portion of the class D address space in the IP multicast model, EXPRESS provides orders of magnitude more multicast channels per host than available in the group model, and address can be allocated by each host and without global coordination; (b) Second, EXPRESS channels can be implemented using a single simple protocol that requires no change to the current fast-path mechanisms in routers (2) Counting support in EXPRESS can be implemented efficiently using a method of proactive counting, and provides a simple but powerful extension for many-to-one communication. (3) Large-scale multicast applications that are almost single-source can be implemented on top of EXPRESS channels with a modest amount of middleware using application-selected and controlled session relay nodes; scalable multicast applications that we are aware of can be implemented efficiently and robustly using the EXPRESS service model.
4. Critique the main contribution
   • Rate the significance of the paper on a scale of 5 (breakthrough), 4 (significant contribution), 3 (modest contribution), 2 (incremental contribution), 1 (no contribution or negative contribution). Explain your rating in a sentence or two.
   I give this paper a rating of 4 because it presents a great scheme for tackling the problem of how to expand the IP multicast model to provide a basis for charging, access control, and extensive scalability, and gives a possible implementation of this scheme on top of the current IP model.
   • Rate how convincing the methodology is: how do the authors justify the solution approach or evaluation? Do the authors use arguments, analyses, experiments, simulations, or a combination of them? Do the claims and conclusions follow from the arguments, analyses or experiments? Are the assumptions realistic (at the time of the research)? Are the assumptions still valid today? Are the experiments well designed? Are there different experiments that would be more convincing? Are there other alternatives the authors should have considered? (And, of course, is the paper free of methodological errors.)
   The authors' methodology was to present an analysis of the problems of the problems of the current IP multicast model and how their EXPRESS channel model solves these problems. After presenting EXPRESS, they gave further analysis of why EXPRESS would work and is better than the current group model and presented some simulations of the performance of EXPRESS in terms of scalability and costs both for the model itself and its counting mechanism.
   • What is the most important limitation of the approach?
   The biggest limitation of this approach is that since it is only an initial stage analysis of one of the first proposals for this problem, it does not dig out the problems that may be exposed when the EXPRESS protocol is actually implemented and deployed on the real internet. For such potential problems, extensive experimentation would be needed after a decent scale of deployment is acheived. This would be the most surefire way to see if the system actually works for an event like the Superbowl that is mentioned in the paper however, at this stage, their analysis is adequate to allow EXPRESS to be considered for an event like the Superbowl and after running such a large scale experiment, further improvements and modifications could be discovered and would overcome such limitations of preliminary analysis.
5. What lessons should researchers and builders take away from this work. What (if any) questions does this work leave open?
The lessons that researchers should take away is that the EXPRESS channel model seems to be a good solution for solving the limitations of the current IP multicast group model with its provisions for charging, access control, and scalability. The questions it leaves open are whether their are further bugs that need to be worked out in EXPRESS, whether it fares well in its actual implementation, and how well it can be pervasively deployed.