Wednesday, August 14, 2013

SIGCOMM2013: TCP ex Machina: Computer-Generated Congestion Control

Presenter: Keith Winstein

Introduction/Motivation
The talk was about the new approach to control end-to-end congestion on a multi-user network. The main motivation behind the system built by the authors “is it possible for a computer to discover the right rules for congestion control in heterogeneous and dynamic networks? Should computers, rather than humans, be tasked with congestion control methods? And how well can computers perform this task?”

System 
They presented the system “Remy”, a program that generates congestion control schemes offline. 
Input to the system:
1.  Prior assumptions depicting what the networks may do.
2. Goal to highlight what applications want.
Output: CC algorithm for a TCP sender (RemyCC)
Time: A few hours
Cost: $5-$10 on Amazon EC2
Remy searches for the best congestion-control algorithm, optimizes expected objective over prior assumptions and makes tractable by limiting available state.  Remy finds the piecewise-continuous RULE() that optimizes expected value of objective function.

Evaluation
The authors have done the evaluation of their system in ns-2 and have compared the results with end-to-end (NewReno, Cubic, Compound and Vegas) and in-network solutions (Cubic-over-sfqCoDel, XCP).  The evaluation results showed that with Remy algorithm one can achieve maximum throughput and least queuing latency for fixed rate networks. But the algorithm is not so good for variable rate networks.

Conclusion
Remy provides complex rules with consistent behavior as compared to simple rules and complex behavior in case of traditional policies. Computer-designed and end-to-end solution is better than human-designed and in-network congestion control solutions.

Q: Kelly also solves this problem. So how is your system different than it?
Ans: Remy is targeted for dynamic case real networks
Q: Why were you surprised that your(computer generated) scheme is better? [Unable to understand the complete question clearly]
Ans: I am not surprised that computer generated end-to-end scheme can beat the in-network end-to-end scheme. I am surprised that end-to-end scheme can beat the in-network schemes. I didn’t imagine that how computer-generated schemes can outperform XCP. 
Q: What about heterogeneous systems with different RemyCC vs TCP?
Ans: We did talk about it in a paper with RemyCC vs existing TCP.