Low Delay Marking for TCP in 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 f i to which p belongs estimate h i for f i estimate n calculate w opt calculate p mark mark p with probability p mark p : packet f i : the i-th flow h i : the number of wireless hops n : the total number of flows going through the node w opt : the optimal window size for f i p mark : 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 (n 0 to n h ). All TCP flows go from n 0 to n h. 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 CategorySingle FlowMultiple Flows Hops Throughput Round-Trip Time Loss Rate RTS Collisions Performance Comparison to Regular TCP + : better by more than 10% 0 : within 10% – : worse by more than 10%

24 Summary CategorySingle FlowMultiple Flows Hops Throughput0––0–– Round-Trip Time Loss Rate RTS Collisions 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.