Paper review: End-to-End Packet Delay and Loss Behavior in the Internet

Reviewer: Mike Liu

1. State the problem the paper is trying to solve.
The main problem the paper is trying to solve is: "How to measure and analyze the end-to-end packet delay and loss behavior in the Internet? "
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 the paper is that it presented the idea of measuring round trip delays of small UDP probe packets sent at regular time intervals to analyze the Internet's end-to-end packet delay and loss behavior. They observed such interest phenomena as compression of the probe packets, rapid fluctuations of queing delays over small intervals, etc. They showed in the paper that the UDP echo is not only useful for studying network problems but that it can also be useful in analyzing end-to-end characteristics over different time scales, of connections over the internets. This is a useful problem because it gets at the beginning of analysis of the behavior of load on the Internet. The UDP probe packets are somewhat like ping and may have led to the development of ping in the current day. In understanding packet delay and loss behavior, one can apply this knowledge in the proper design of network algorithms such as routing and flow control algorithms, in dimensioning of buffers and link capacity, and in choosing parameters in simulations and analytics studies. This understanding is also essential for designing the emerging audio and video applications for the Internet. For this last reason also, the problem still is rather relevant today.
3. Summarize the (at most) 3 key main ideas (each in 1 sentence.) 
The first main idea in the analysis of the data presented in the paper is that the transatlantic link between France and the United States with a bandwidth equal to 128 kb/s is the bottleneck link on the path from INRIA to UMd. The second main idea presented in the analysis was that clp, conditional loss probability, is greater than ulp, unconditional loss probability, for all values of sigma(in ms), the interval between the send times of two successive packets. Third main idea that was presented in the analysis is that the loss gap stays close to 1 even for small values of sigma, which has important consequences for the design of audio and video applications.
4. Critique the main contribution
   The main contributions seem to be the advancement of work in the area of analysis of the Internet. The contributions to bottlenecks, loss probabilty, and loss gap are important, especially in the currently growing area of audio and video applications of the internet. These applications demand high and consistent bandwidth for their proper functioning and understanding how to navigate the load of the Internet is very important when trying to develop them. The methods used by the author, however, seemed to be have some room for improvement. Packet delays and losses on the INRIA-UMd connection were obtained using NetDyn, a mesurement tool. UDP packets were sent from the source host to an intermediate host and to a destination host. Unfortunately, source, intermediate, and destination host are geographically distant and their local clocks may not have been synchronized, and hence the timestamps in the UDP probe packets would have been difficult to interpret. The author solved this by setting the source and destination host to be the same host. This was a good compromise for a solution but it would have been much better if he could used two actually geographically distinct hosts but synchronized both their time clocks to some common atomic clock. This would have given more accurate results and made the tests closer to real internet traffic.
   • 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 believe this paper make a modest contribution because it presents a good rudimentary study of load on the internet using relatively simple and common mechanisms and does a good job of incorporating past research. The paper also made three discoveries in its data analysis and explained several practical ways the information could be utilized, such as in audio and video applications.
   • 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 author used an analytic approach to studying the Internet's packet delay and loss behavior. He cites previous work as being of three types: analytic, simulation, and experimental and compares his analytic approach to the other two as well as previous analytic approaches. Previous analytic approaches use queueing network models to analyze packet delay in computer networks. Simulation approaches examine the impact of routing and flow control mechanisms on end-to-end delay. Experimental approaches used systematic measurements of packet delay and loss on the ARPANET. The nice thing about the paper is that the analytic observations that were made agree with the results obtained using simulation and experimental approaches. The paper is printing convincing in that it used the actual Internet to obtain its results. Unfortunately, the paper analyzed the Internet of 1993 and the results may be different for the Internet of today. Even so, the paper obtained pretty nice data and so that analyses that followed from this data were pretty convincing in terms of considerations that should be kept in mind when designing future applications for the Internet that would be affected by load.
   • What is the most important limitation of the approach?
   The most important limitation of the approach is that it only provides analysis for the system at its time and the analysis would have to be repeated at a later time for the conclusions to still hold relevant. This is seen by the fact that when the analysis was performed there were approximately 1 million hosts. Today, the number of hosts (and routers) is much greater.
5. What lessons should researchers and builders take away from this work. What (if any) questions does this work leave open?
Researchers should take away lessons on how to construct an analysis of packet delay and loss behavior using the UDP echo tool probe packets. The work leaves open questions of what this analysis would look like today as well as what the implications of the discovery that probe packets are lost randomly except when Internet traffic intensity is very high are.