Leveraging Multiple Network Interfaces for Improved TCP Throughput

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 N3 2 Mbps, 50ms 2 Mbps, 50ms (Sender) N1 N2 (Receiver) 5 Mbps, 50ms 5 Mbps, 100ms N4 • FTP Source N1 has 2 NIs • The 2 paths to N2 are using N3 (2Mbps, 100ms) and N4 (5Mbps, 150ms) • Can N1 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 N4 and the rest through N3 • Notice that the throughput is less than what is obtained from even one path Throughput Time

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 RP1 FP1 (Sender) N1 N2(Receiver) If DupACK, route through RP1 else use RP2 FP2 RP2 • Assume 2 paths are (delayFP1 < delayFP2) such that (delayFP2+delayRP1) < (delayFP1+delayRP2) (delay property) • DupACKs received only after subsequent ACKs and are discarded

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

8. ACK Routing Policy P1 (Sender) N1 N2(Receiver) Route ACKs through P1 P2 • 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

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

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