1. Improving TCP Performance over MANETs by Exploiting Cross-Layer Information Awareness Xin Yu NYU Presented by: David Choffnes

2. 2 Outline Intro to MANETs and DSR Problems with TCP over MANETs EPLN and BEAD Results Conclusion

3. 3 Mobile Ad-Hoc Networks (MANETs) Differences from wired networks –Wireless link much less predictable Pathloss Interference –Mobility leads to rapidly changing topology –Every host is a router Routing in MANETs –Approaches Link state –Requires every node to know about all other nodes –Too much wireless transmission overhead in MANETS Distance vector (e.g., DSDV, AODV) Source routing (e.g., DSR)

4. 4 Routing in MANETs (cont) Key observation –Interference/contention can significantly reduce performance –To reduce overhead, discover routes only when they are needed; otherwise, aggressively cache overhead/previously heard routes Ad-hoc On-Demand Distance Vector (AODV) –DV algorithm, floods route request to find path Dynamic Source Routing (DSR) –Similar to AODV, but returns the entire path –Establishes bi-directional paths –Route failure: send ROUTE ERROR messages, try to use cached route

5. 5 Mobility and TCP 038939131 TCP connection (0,1) Link failure Node 31 drops all in-flight packets to Node 1

6. 6 ELFN: A Solution to Mobile TCP Explicit Link Failure Notification (ELFN) –Sends ICMP message to TCP Retransmission timer disabled Sets thaw timer to 2s Sends a probe data packet to determine if a new route was established Issues –How do we set RTO and cwnd after thaw? Small receive window can cause idle state after thaw Smaller RTO leads to quicker recovery from freeze –When do we freeze TCP? ELFN does not distinguish packet loss from link failures Try to notify TCP of lost data and lost acks

7. 7 EPLN and BEAD Overview EPLN (Early Packet Loss Notification) –Intermediate nodes notify TCP senders about lost data packets BEAD (Best-Effort ACK Delivery) –Intermediate nodes retransmit ACKs by extensively using cached routes when links fail Rules when dropping packets –First link failure: notify TCP sender –After link failure recovery Data: notify intermediate node, which tries to resend and notifies sender ACK: notify intermediate node, try to resend using cached route; if no luck, notify TCP receiver –Similar for notification packet losses

8. 8 Examples: EPLN ABCGED Data packets (A,E) Link failure F Node C uses cached route: F-G-E Link failure Data packets dropped at Node F Notification to intermediate node (F,C) Node C has no other cached route, sends packet loss notification to sender (A) Notification (C,A)

9. 9 Examples: BEAD ABCED Link failure HIJK ACK (E, A) Node C retransmits using cached route C-I-H-A Link failure Node I drops ACK Node I sends notification to intermediate node (C) Node C sends notification to sender (A) Node D hears notification, retransmits ACK using cached path D-K-J-B-A ACK received at A

10. 10 Cross Layer Interactions TCP Sender –Notified about lost packets, not simply broken links ICMP message contains seq number, packet status –Freeze TCP even if packet is salvaged, but retransmit packet only if this is the first packet lost (serves as probe packet) –Otherwise, wait for ACK to resume TCP –ACK received: restore TCP state to values before freezing

11. 11 Evaluation Setup Simulator: NS-2 Mobility: RWP (boo!) Speed: 1± v, where v is the mean speed Field: 1500x1000m (50 nodes), 2200x600m (100 nodes) –Why? MAC: 802.11, 2Mbps Transmission radius: 250m (boo!) TCP: Reno, 1460B packets, rwnd: 8 App: FTP

12. 12 Setting RTO and cwnd Using “old” values is better than resetting them –Reducing cwnd can cause TCP to enter idle state, so TCP throughput becomes dependent on RTO –Lower RTO due to “old” value improves throughput (reduces slow starts) –Improvement not as much for higher traffic load; due to fresher routes –Can lead to lower performance in heavily congested scenarios (MAC contention due to increased route discoveries)

13. 13 Evaluation: Throughput 27%-210% higher throughput over EPLN Improvement increases as –Traffic load increases No reason given –Number of nodes increase More cached routes leads to improved delivery DSR-Update, a distributed cache update algorithm for DSR, further improves performance

14. 14 Evaluation: Number of Slow Starts Reduces timeouts by as much as 90% Mostly due to cache update algorithm Diminishes for larger numbers of nodes, larger traffic load –Probably due to contention

15. 15 Packet Overhead Increases overhead for small #’s of connections and low speed Reduces overhead otherwise Distributed DSR route update algorithm generally reduces overhead compared to standard DSR –Fair? How big is DSR’s FIFO cache compared to the cache for the dist. cache update algo?

16. 16 Conclusion Cross-layer information awareness is key to improving TCP performance Efficient route updates significantly improve performance Should we ditch TCP? See ATP.