Proportional Rate Reduction for TCP

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One Line Summary

A smooth and robust loss recovery for TCP


Proportional Rate Reduction (PPR) improves the accuracy and robustness of TCP’s loss recovery.

Standard Congestion Control requires that TCP and other protocols reduce their congestion window in response to losses. There are two widely deployed algorithms used to implement this window reduction: Fast Recovery and Rate Halving. Both algorithms are needlessly fragile under a number of conditions particularly when there is a burst of losses that such that the number of ACKs delivered is so small that the effective window falls below ssthresh, the target value chosen by the congestion control algorithm.

Proportional Rate Reduction avoids these excess window reductions such that at the end of recovery the actual window size will be as close as possible to the window size determined by the congestion control algorithm. It is patterned after rate halving, but using the fraction that is appropriate for target window chosen by the congestion control algorithm. In addition a second algorithm, Reduction Bound, monitors the total window reduction due to all mechanisms, including application stalls, the losses themselves and inhibits further window reductions when possible.

In this talk, I will describe the design and implementation of PRR in Linux, along with experimental results from Web and datacenter traffic. Details of the PRR design are at:


TCP, loss recovery


  • Biography

    Nandita Dukkipati is a software engineer at Google working on improving the TCP protocol for Web traffic and datacenter applications. She is an active participant of the IETF and activities in networking research. Prior to Google she obtained a PhD in Electrical Engineering, Stanford University.