1. Leveraging Multiple Network Interfaces for Improved TCP Throughput Sridhar Machiraju, Prof. Randy Katz

2. Motivation Proliferation of Wireless LANs, rollout of 3G and availability of devices with multiple network interfaces (MNIs) Can endhosts leverage multiple network interfaces for – improved throughput (our current focus) masking packet losses improving connectivity …

3. Problem Statement (Sender) N 1 N 2 (Receiver) N4N4 N3N3 2 Mbps, 50ms 5 Mbps, 100ms 5 Mbps, 50ms FTP Source N 1 has 2 NIs The 2 paths to N 2 are using N 3 (2Mbps, 100ms) and N 4 (5Mbps, 150ms) Can N 1 obtain a throughput of 7Mbps?

4. A Simple RR Scheme Assume that the ratio of bottlenecks on both paths is known (2:5 in the figure) Route 2 of every 7 packets through N 4 and the rest through N 3 Throughput Time Notice that the throughput is less than what is obtained from even one path

5. Why it fails… The RR scheme fails - different delays on paths-> packet reordering-> duplicate ACKs -> false congestion signals Reordering can be solved by – Increasing DupACK threshold Buffering packets/ACKs Our solution – use multiple paths to the source in an intelligent fashion

6. DupACK routing policy (Sender) N 1 N 2 (Receiver) Assume 2 paths are (delay FP1 < delay FP2 ) such that (delay FP2 +delay RP1 ) < (delay FP1 +delay RP2 ) (delay property) DupACKs received only after subsequent ACKs and are discarded FP1 FP2 RP1 RP2 If DupACK, route through RP1 else use RP2

7. DupACK routing policy (cont.) Single TCP flow is able to obtain better throughput – 7Mbps Pros – better throughput simple policy packets not buffered or dropped Cons – 2 paths need to obey the delay property slower increase of congestion window Throughput Time

8. ACK Routing Policy What if DupACK policy cannot be used? Throughput can still be improved if delay(P1) < delay(P2) capacity (P1) > ACK traffic Use high capacity path (P2) for data; low delay path (P1) for ACKs (Sender) N 1 N 2 (Receiver) P1 P2 Route ACKs through P1

9. ACK Routing Policy (cont.) Among competing TCP flows, flows with smaller RTT fare better Using our policy, RTT can be reduced Competing TCP Multipath TCP Time Throughput Time

10. Future Work Investigate the effect of competing TCP flows on such policies Devise more mechanisms esp. for other transport protocols Infer path properties dynamically Investigate approaches to deployment – receiver side modification use mobility servers such as home agents Finally, implement these