Low Delay Marking for TCP in Wireless Ad Hoc Networks

1. Low Delay Marking for TCPin Wireless Ad Hoc Networks Choong-Soo Lee, Mingzhe Li Emmanuel Agu, Mark Claypool, Robert Kinicki Worcester Polytechnic Institute Apr 17, 2004

2. Introduction • Wireless Ad Hoc Network uses TCP • TCP, being designed for wired networks, performs poorly over wireless networks. • Wireless ad hoc network uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and Request-to-Send/Clear-to-Send (RTS/CTS) mechanism to avoid collisions. • TCP performance suffers from the contention delays and drops known as RTS/CTS jamming and RTS/CTS-induced congestion.

3. Introduction • Previous research to improve TCP performance includes • Investigation of link breakage and routing failure problems [4] [5] [6] • link layer/MAC solutions [7] [8] [9] • protocol modifications [10] • Most of these approaches are link layer optimizations tied to the device drivers rather than the operating system.

4. Motivation • Previous research is only concerned with improved throughput. • Emerging applications such as interactive multimedia and network games demand low round-trip times. • End-to-end delays will become increasingly important relative to throughput. • Throughput is important but we project steady increase in maximum wireless network capacity.

5. Proposal • We propose an IP layer solution which modifies the packet queue manager. • Our goal is to improve round-trip times, loss rates and collisions with minimal degradation to throughput. • This facilitates easier deployment since operating system upgrades/patches can be used independently of hardware changes.

6. Outline • Introduction • Background • Proposed Mechanism • Evaluation • Conclusion and Future Work

7. Explicit Congestion Notification • Traditionally, TCP uses dropped packets as an indication of network congestion. • This requires 3 duplicate acknowledgement. • Window size below 4 results in a retransmission timeouts and reduces throughput significantly. • Explicit Congestion Notification (ECN) uses an unused bit (the ECN bit) in IP header to get congestion notification.

8. Link RED and Adaptive Pacing • Link RED has a similar mechanism to Random Early Detection (RED). • It uses an exponentially weighted average of RTS retries to calculate the dropping/marking probability. • Adaptive Pacing is an additional mechanism that Link RED controls. • Adaptive pacing adds extra back-off before trying to send a packet. [8]

9. Outline • Introduction • Background • Proposed Mechanism • Evaluation • Conclusion and Future Work

10. Performance and Window Size • [8] demonstrates that the throughput is not optimal with regular TCP. • Optimal Window Size also provides reasonable delay. • We can adjust the packet marking probability to force TCP to operate around the optimal window size.

11. Low Delay Marking Algorithm • At each node, on receiving packet p • identify flow fi to which p belongs • estimate hi for fi • estimate n • calculate wopt • calculate pmark • mark p with probability pmark p : packet fi : the i-th flow hi : the number of wireless hops n : the total number of flows going through the node wopt : the optimal window size for fi pmark : the marking probability

12. Optimal Window Size • Optimal window size is a function of the number of hops between the source and destination nodes. • Due to the hidden terminal problem, it is derived that there should be only one packet in transit every 4 hops for optimal TCP throughput.

13. Number of Hops • We use Time-To-Live (TTL) values in the data packets. • The default TTL values are typically 128 or 256. • We keep track of source and destination node pairs to identify each flow. • We take the TTL values from the packets going one way and the packets going the other way. • We subtract them from the default TTL values and sum the difference.

14. Number of Flows • We estimate the number of flows using Morris’ calculation. • We use a fixed-length bit v. • A packet is hashed based on source-destination address and port number and the corresponding bit in v is set. • The bits in v are cleared at a certain rate and also the corresponding number of hops. [17]

15. Marking Probability • We use Morris’ formula that links the overall loss rate and the TCP window size. • We consider the overall loss rate as an equivalent of marking probability. • Then we substitute all the previous calculated/estimated values to come up with

16. Marking Probability • However, this is the overall marking probability, NOT per-node. • We distribute the overall marking probability uniformly along all nodes.

17. Outline • Introduction • Background • Proposed Mechanism • Evaluation • Conclusion and Future Work

18. Simulation Setup • We used NS-2 to implement and evaluate LDM. • LRED and Adaptive Pacing implementations • LDM implementations (hard-coded version) • Wireless Multi-hop Chain Network • For h-hop network, we need h+1 nodes (n0 to nh). • All TCP flows go from n0 to nh. • We tested 7-hop, 15-hop and 24-hop networks. • All TCP flows use TCP NewReno.

19. Single Flow Experiment

20. Single Flow Experiment

21. Multiple Flow Experiment

22. Multiple Flow Experiment

23. Summary Performance Comparison to Regular TCP + : better by more than 10% 0 : within 10% – : worse by more than 10%

24. Summary Performance Comparison to Adaptive Pacing + : better by more than 10% 0 : within 10% – : worse by more than 10%

25. Outline • Introduction • Background • Proposed Mechanism • Evaluation • Conclusion and Future Work

26. Conclusion • Low Delay Marking (LDM) • is an IP layer approach. • lowers delay and loss rate without sacrificing throughput. • round-trip time up to 57.6% • loss rate up to 59.5% • reduces MAC layer congestion. • We successfully implemented and evaluated Low Delay Marking (LDM) in NS-2.

27. Future Work • All our evaluation is done over with the number of hops and number of flows known ahead of time at each node. • Implementation and evaluation of hop and flow counting techniques • Investigation of LDM performance over more complex topologies such as crosses and grids to evaluate robustness.