Analysis of TCP Latency over Wireless Links Supporting FEC/ARQ-SR for Error Recovery

1. Analysis of TCP Latency over Wireless Links Supporting FEC/ARQ-SR for Error Recovery Raja Abdelmoumen, Mohammad Malli, Chadi BarakatPLANETE group, INRIA Sophia-Antipolis

2. Outline • Introduction • Proposed solutions • Our model for link level error recovery : FEC/ARQ-SR • Simulation scenario & parameters • Simulation results in NS-2 • Conclusions & perspectives June, 2004

3. TCP & wireless links • Wireless links: WLAN, GSM, GPRS, UMTS, satellite, etc. • Characterized by a high bit error rate compared to wired links (non-congestion losses) : • Different sources: Signal attenuation, interference, multi-path fading, shadowing, rain, handoff, etc. • Negative impact on TCP performance: • TCP considers the loss of a packet as a congestion signal and reduces its window unnecessarily • Long term TCP throughput is known to be inversely proportional to the square root of the packet loss rate June, 2004

4. Proposed Solutions • Clean links by correcting non-congestion losses locally : • Use of link-level FEC, ARQ, hybrid FEC/ARQ, etc. • Help TCP to distinguish non-congestion losses : • ELN, loss predictors, etc. • Split the TCP connection, isolate the noisy link, and transmit data over the noisy link using an optimized transport protocol : • I-TCP, STP, etc. Our work focuses on the link-level FEC/ARQ-SR solution … June, 2004

5. FEC : pros & cons • FEC consists in sending redundant information with each data frame • reduces the packet loss rate • Correct packets on the fly, which eliminates any interaction with TCP retransmission timer as in the case of ARQ • FEC is of particular interest on long delay links and at high loss rates but, • The redundant information consumes bandwidth, which may reduce the throughput of TCP if added in large amounts • Processing overhead, frames transmission time What is the amount of FEC that leads to the best TCP throughput ? June, 2004

6. ARQ-SR : pros & cons • ARQ-SR consists in retransmitting the frames corrupted over the wireless link transparently to TCP • ARQ is interesting on short delay links and at low loss rate • Bandwidth is only wasted when packets are lost but, • Introduce jitter, which is harmful for real-time applications (e.g. telnet) • Introduce burstiness when an in-order delivery is supported • Introduce reordering when an out-order delivery is supported • Interfere with TCP timeout mechanism when persistency is high • ARQ Selective Repeat : • complex compared to Stop-And-Wait and Go-Back-N • But, allows higher utilization of the available link capacity June, 2004

7. Hybrid FEC/ARQ-SR • By combining FEC, ARQ-SR and an in-order delivery of packets at the output of the wireless link, better performance can be achieved • Tune FEC amounts and ARQ persistency to obtain a better latency and more throughput June, 2004

8. Our model for FEC/ARQ-SR • FEC: Erasure block code, a frame is recovered if the number of erroneous units is less than (N - K), K/N being the code rate • ARQ-SR : • A TCP/IP packet is divided into X frames • If a frame is not recovered by FEC, it is retransmitted by ARQ-SR • The maximum number of retransmissions is  (persistency of ARQ) • A link-level NACK is sent for each erroneous frame. The frame is quickly retransmitted and given priority over all frames • A packet is discarded when FEC and ARQ-SR fail to recover one of its frames • Packets are delivered in-order at the output of the wireless link June, 2004

9. Simulation Scenario • TCP flows have the same size S in bytes and are launched according to a Poisson process of rate  • The wireless link is the bottleneck for the TCP connections • Frame units are lost according to a Bernoulli process with probability p June, 2004

10. Parameters • Wireless link load :  =  . S / B • is the maximum throughput one can achieve for some tuning of the link layer (in percentage of B) • obtained when the wireless link is saturated • One can obtain : stability condition :  <=  • PT is the probability that a frame is lost one time (trial loss rate) • PF is the probability that a frame and all its retransmissions are lost (frame loss rate) June, 2004

11. FEC alone • There is an optimal amount of FEC to be added • This amount when p & D • For high loss rate : only 50 % of the channel bw can be used for data transmission June, 2004

12. ARQ-SR : alone • For long file transfer : TCP latency improves with  for the different values of D and p June, 2004

13. ARQ-SR : alone • For short file transfer & small p : only when D is short, TCP Latency is improved with  • For high loss rate : only 25 % of the channel bw can be used for data transmission June, 2004

14. FEC/ARQ-SR • For p = 0.1, r = 0.75 : stability condition is satisfied • it is not the case for FEC alone and ARQ-SR alone June, 2004

15. FEC/ARQ-SR • For small p & D : ARQ-SR provides the best TCP latency June, 2004

16. FEC/ARQ-SR • For small p & large D : FEC is the suitable mechanism June, 2004

17. FEC/ARQ-SR D = 20ms, p = 0.1, S = 100KB,  = 0.5 • For large p : hybrid FEC/ARQ-SR provides the lowest TCP latency June, 2004

18. Conclusions • A high persistency of ARQ-SR is almost all time necessary for high performance • FEC must be added in case of high loss rates to avoid ARQ from collapsing • No need for FEC in case of low error rate, EXCEPT in the case of long delay and very short transfers where FEC should be added to disable ARQ (otherwise we will have false Timeouts) June, 2004

19. Perspectives • Model the TCP latencyover wireless link analytically • Study the hybrid FEC/ARQ-SR model in the case of bursty losses • Adapt the hybrid FEC/ARQ-SR model for multimedia traffic : audio & video June, 2004

20. Q & A Thank you Mohammad.Malli@sophia.inria.fr June, 2004