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Here are your grades for Assignment 3. You may pick up the assignment from me from my office.
More than half of the class got a grade in the A range (A– to A+). These are students that provide detailed analysis of the traces (e.g., values of b, maximum window sizes, careful sampling of RTT, etc.) and its relation to the TCP equation (e.g., realizing the difference between goodput vs. throughput, packet loss rate vs. loss event rate, sender’s perceived losses vs. actual losses, behaviour of TCP vs. “textbook” Reno, etc.).
If you have questions about your grades, please contact me before end of Monday, 7 December 2009. Continue reading →
Some questions from a discussion in my office this morning.
1. Can you give an example of redundant retransmissions caused by coarse feedback?
During class I explained what coarse feedback means but never gave a real example on how this could lead to redundant retransmissions. Vern Paxson gave a real example in his thesis (Page 317-318, ftp://ftp.ee.lbl.gov/papers/vp-thesis/dis.pdf). I could not explain any better than Vern Paxson so let me just refer you to the thesis for the example.
2. To pessimistically estimate the loss rate on an alternative path consists of default paths with loss rate of p1, p2, .. pk, can we really sum up the loss rate (Slide 64, Lecture 10)??
I stand corrected. We assume that loss rate on the paths are independent. Thus, the loss rate on the alternative path, i.e., probability that a packet is lost on one of these default paths, should be 1 – (1-p1)(1-p2)..(1-pk).
In this lecture, we are going to first look at how DNS works in details through a measurement study, and then see how we can exploit DNS to help us measure latency between any pair of end hosts on the Internet.
J. Jung, E. Sit, H. Balakrishnan, R. Morris “DNS Performance and the Effectiveness of Caching,” IEEE/ACM Transactions on Networking, 2002, 10(5), [ Google Scholar]
KP Gummadi, S. Saroiu, S. Gribble, “King: Estimating latency between arbitrary Internet end hosts“, ACM IMC 2002 [Google Scholar]
This paper uses DNS in an unintended and clever way to measure latency between any pair of hosts on the Internet.
In the measurement studies by Stefan Savage et. al., a graph is constructed to represent a set of hosts and links between them. They then run shortest path algorithm on the graph to determine the best alternate path between any two hosts.
Should the graph be a undirected or directed graph? Explain.
Vern Paxson found that route fluttering does occur in the Internet. What undesirable effect(s), if any, do route fluttering have on TFRC-based transport protocol such as DCCP?
Vern Paxson also found that routes on the Internet are often asymmetric. What effects(s), if any, do route asymmetry have on the TCP’s adherence to the principle of packet conservation?
Vern Paxson uses PBM (“packet bunch modes”) to estimate the bottleneck bandwidth. PBM uses a range of packet-bunch sizes to form receiver-side estimates, and is designed to overcome several limitations of packet pair techniques, including bottleneck bandwidth changes.
Is using more than two packets strictly necessary to detect a change in bottleneck bandwidth? Justify your answer.
Vern Paxson suggests that a TCP receiver should wait for time W before sending a duplicate ACK upon detecting a gap in the sequence number to reduce the number of bad retransmission. A good choice of W depends on the network characteristics and behavior. What are the factors that affect a good choice of W? How do these factors affect the effectiveness of a particular choice of W?
In the paper “End-to-End Internet Packet Dynamics”, Vern Paxson says the following after presenting his findings on packet losses on the Internet.
` .. the patterns of loss bursts we observe might be greatly shaped by use of “drop tail” queueing. In particular, deployment of random early drop (RED) could significantly affect these patterns …”
How would deployment of RED affect Vern Paxson’s findings in terms of (i) the distribution of outage duration, (ii) unconditional loss probability , and conditional loss probability ?
(Recall that is conditioned on the loss of previous packet)