Delay-based Congestion Control for Multipath TCP Yu Cao, Mingwei Xu, Xiaoming Fu Tsinghua University University of Goettingen

Outline Background and problem statement Congestion Equality Principle Weighted Vegas Simulations Conclusions

Multipath Transfer Ever-increasing multihomed hosts Split traffic across multiple paths.  Provide a new opportunity for designers to enhance performance of end-to-end transmission.

Benefits for End-hosts Increase throughput Improve robustness [MPTCP, NSDI 2011] WiFi: high rate unstable low coverage 3G: low rate stable high coverage

Benefits for networks Bandwidth can be more fairly and efficiently shared by flows. S2D2 S1D1 S3D3 6M 9M

Improvement on throughput is constrained by fairness. Traffic engineering at end-systems New Requirements for MPCC 9M S1 S2 S3 D1 D2 D3

Coupling Subflows Together Regard network resources as a whole to compete for bandwidth S2D2 S1D1 S3D3 6M 9M S2D2 S1 D1 S3D3 15M How to determine appropriate rates on each path? How to shift traffic with only local knowledge for sources ?

Congestion Equality Principle A fair and efficient traffic shifting implies that every flow strives to equalize the degree of congestion that it perceives on all its available paths. A knob to control rates A metric to estimate congestion degree

Delay-based vs. Loss-based Packet queuing delay Packet loss events Multi-bit info quantifing congestion degree Single-bit congestion signals Be sensitive to … Perceive changes of congestion in a large timescale RTT fairness Bias against large-RTT flows Low buffer consumption Frequent losses Less aggressively More aggressively -- Linked Increases, CMT/RP

Understanding TCP-Vegas The number of backlogged packets

Bandwidth Sharing 6M 3M 2M 1M

Weighted Vegas 15M 6M 9M 1:4 5M 1:4 ? 5M 1M To equalize congestion degree of the two paths. Core algo.: allocate alpha to each subflow. 4M

Network Utility Maximization Given a fixed budget, invest it in the cheapest paths to maximize the utility. Lowest queuing delay

Iteratively Tweaking Weights The total amount of backlogged packets is fixed at, regardless of the number of subflows. Tweak weights Control rates Update parameters

A summary of weighted Vegas Runs in the same way as TCP-Vegas on each path. is allocated to subflows according to weights. Uses equilibrium rates of subflows to adjust weights. A larger means more packets are backlogged in link queues. A quite small makes wVegas over sensitive to the noise of RTT.

Simulations We implemented wVegas and Linked Increases in NS-3. Focus on the fairness and efficiency Expect wVegas achieves a fine-grained traffic shifting.

Iteratively adjust rate Two bottleneck links

wVegasLinked Increases Transmission rate

Fairness on Bottleneck Links wVegas Linked Increases

Dynamics of Traffic Shifting

The Domino Effect Rate complementation between subflows

Conclusions The Congestion Equality Principle wVegas can achieve fine-grained traffic shifting. wVegas relies on the accurate measurement of RTTs. wVegas and Linked Increases have their own respective advantages and defects.  Combine they two together?

Thanks