TCP Performance Over Wireless Links

1. TCP Performance Over Wireless Links Read: - Hari Balakrishnan et. al , “A Comparison of Mechanisms for Improving TCP Performance over Wireless Links …”, SIGCOMM 1996. Ibrahim Matta et. al, “Open Issues on TCP for Mobile Computing”, WCMB 2002. - S. Biaz et al. “De-Randomizing” Congestion Losses to Improve TCP Performance over Wired-Wireless Networks”, to appear in June 2005 IEEE/ACM Transactions on Networking

2. Impact of the Packet Error Rate on TCP • FromSally Floyd, HighSpeed TCP for Large Congestion Windows. RFC 3649 Packet Drop Rate P Congestion Window W RTTs Between Losses ------------------ ------------------- ------------------- 10^-2 12 8 10^-3 38 25 10^-4 120 80 10^-5 379 252 10^-6 1200 800 10^-7 3795 2530 10^-8 12000 8000 10^-9 37948 25298 10^-10 120000 80000 Table 2: TCP Response Function for Standard TCP. The average congestion window W in MSS-sized segments is given as a function of the packet drop rate P. TCP Wireless

3. Impact of the Bit Error Rate on TCP • AfterSally Floyd, HighSpeed TCP for Large Congestion Windows. RFC 3649 Bit Error Rate BER Congestion Window W RTTs Between Losses ------------------ ------------------- ------------------- 10^-6 12 8 10^-7 38 25 10^-8 120 80 10^-9 379 252 10^-10 1200 800 10^-11 3795 2530 10^-12 12000 8000 10^-13 37948 25298 10^-14 120000 80000 Table 2: TCP Response Function for Standard TCP. The average congestion window W in MSS-sized segments is given as a function of the packet drop rate P. TCP Wireless

4. Impact of the Bit Error Rate on TCP • Consider an MSS of 1 KB and an RTT of 100ms, what should be the BER to reach a throughput of 54 Mbps? TCP Wireless

5. Impact of Wireless Losses on TCP • The congestion window is unnecessarily decreased • Retransmission timer is biased towards less responsiveness (TCP sender wrongly “senses” congestion.). TCP Wireless

6. Strategies to Improve TCP Wireless • Do not use losses as congestion indication (delay/throughput based congestion control such as TCP Westwood) • Differentiate congestion losses from random wireless losses • End to end techniques • Network feedback TCP Wireless

7. Metrics of Accuracy • Accuracy of diagnosing correctly: • Congestion losses (Ac) • Random wireless losses (Aw) • Ngc: Number of congestion losses correctly diagnosed as such • Nc: Total number of congestion losses TCP Wireless

8. Metrics of Accuracy (2) • Example of a TCP connection with: • 60 congestion drops of which 40 were diagnosed correctly  Ac = 40/60 = 0.67 • 20 random wireless losses of which 15 were diagnosed correctly  Aw = 15/20 = 0.75 • Normal TCP • Ac = 1 • Aw = 0 TCP Wireless

9. Expected Improvement TCP Wireless

10. Expected Improvement (RTT) TCP Wireless

11. “Middle man” Techniques

12. Indirect TCP (I-TCP) (Bakre et. al) • TCP connection broken into two TCP connection • Each TCP connection adapts to specific environment: mss, congestion avoidance, timer.. Mobile Wired Network Sender Basestation TCP Wired TCP Wireless TCP Wireless

13. Snoop (Balakrishnan et. al) • One TCP connection • A Snoop agent on basestation: • “Shields” sender from wireless losses (in fact, hides losses) • Retransmit on wireless link in case of random wireless drops. Mobile Wired Network Sender Basestation Snoop TCP Wireless

14. Snoop (Balakrishnan et. al) (2) • Snoop buffers at basestation packets until acked by mobile. Snoop may retransmit packets. • Snoop perfectly diagnoses congestion losses and wireless losses. • When out-of-order packet reach Snoop, Snoop generates duplicate acks (congestion losses). • When dupacks reach Snoop (wireless losses), Snoop filters these dupacks and retransmits packets. Mobile Wired Network Sender Basestation Snoop TCP Wireless

15. Explicit Bad State Notification (EBSN) (Bakshi et. al) • Basestation keeps track of channel state and feeds it back to sender • Sender diagnoses losses using ESBN and reacts appropriately Mobile Wired Network Sender Basestation EBSN TCP Wireless

16. Explicit Transport Error Notification (ETEN) (Krishnan et. al) • Link layer (at any hop) would notify endpoint of losses other than congestion • Endpoint takes appropriate action Mobile Wired Network Sender Basestation TCP Wireless

17. Explicit Congestion Notification (ECN) (Floyd and Jacobson) • Can ECN be used to infer/diagnose the cause of a loss? Random or congestion • Bad especially when TCP sender is ECN responsive. TCP Wireless

18. “De-randomize” Congestion Losses (Biaz and Vaidya):Biased Queue Management • Packets of the same flow are marked with different discard priorities: 1 every k packets is marked “out”, all others marked “in” • When congestion occurs: drop FIRST packet marked “out”. • Good results: Ac close to 1 and Aw close to 0.75 TCP Wireless

19. Congested router Biased Queue Management TCP Wireless

20. At the Receiver:How to Interpret Losses? • Consider there were S packets in flight and r packets are lost. • Let X be a random variable = number of packets marked “out” within the r lost packets • What is the probability P(X=x) that x packet among the r packets are marked “out”? TCP Wireless

21. At the Receiver:How to Interpret Losses? (2) • P(X=x) has a hypergeometric distribution • Examples: • Case 1: S = 24, k = 8, P (X = 3) = 1/2024 (Random ?) • Case 2: S = 24, k = 8, P (X = 0) = 0.65 (Random?) TCP Wireless