1. A Transport Layer Approach for Improving End-to-End Performance and Robustness Using Redundant Paths Min Zhang, Junwen Lai, Arvind Krishnamurthy, Larry Peterson, Randolph Wang

2. Motivation ● Redundant paths on the Internet. – RON, Detour. ● TCP is an end-to-end (one-to-one)transport protocol. ● For multi-paths, need a one-to-many transport protocol.

3. mTCP ● Modify TCP: – Stripe data packets across different paths. – Treat each flow as an individual TCP sub-flows. – Shared congestion scheme across the sub-flows to ensure fairness. ● Advantage over individual TCP sockets: – Retransmission via a different sub-flow.

4. Path Selection ● Underlying RON overlay routing layer. ● Use RON score to grade paths. ● Pick the 'm' best paths with least overlap. ● Use paths in parallel for sending. ● Single best reverse path for ACK reception. ● Avoid overlapping paths. – Traceroute, Ally for ip address resolution.

5. Shared Congestion ● Assume that routers use drop-tail queues. ● Independence of two sub-flows – Inversely related to shared congestion. – Measure correlation between the fast retransmit times. ● Periodically measure shared congestion.

6. Per Flow State ● For each sub-flow p i – Individual sequence numbers. – Congestion window – cwnd i – Next packet to be sent – sndnxt i. – Next packet to be acked – snduna i – Pkts in flight: pipe i = sndnxt i – snduna i – For fast retransmits - dupack i – Timer i for timeouts and timestamping fast retransmit events.

7. Global State ● Send/receive buffer. ● Scoreboard: map individual sequence numbers to buffer range. ● For sending the next pkt: choose path for which pipe i / cwnd i is least.

8. Path Management ● Suppress sub-flow f i if – f i shares congestion with f i & T(f i ) < T(f j ). – T(f i ) < (Max sub-flow throughput)/w. – Path p i fails. ● Mp d flow – Suppress flow if no of paths > d. – TCP friendliness is d-tunable.

9. Results: Throughput

10. ● Timeout = 2*RTO i. ● Primary failure. ● Auxiliary failure. ● Recovery times – Internet is ~minutes. – RON takes ~18s to recover – mtcp: Transmission continues if path exists (~3 s on primary path failure). Robustness

11. Future Work ● Can we do away with TCP slow-start? – Overlay networks are pro-active, metrics are already known. ● Caching at intermediate nodes. – Reduced latency, increased throughput.