TCP Schemes Investigation in Wired and Wireless Hybrid Networks

1. TCP Schemes Investigation in Wired and Wireless Hybrid Networks By Weiwei Hu and Wei Zha Dec. 8, 2004

2. Outline • Introduction • Random Loss Model • TCP-Jersey • TCP-Westwood • Performance Analysis • Conclusion • References

3. Introduction • Transmission Control Protocol (TCP) • reliable • end-to-end • window based • designed for the wired networks (no random packet losses): not fit for wireless network • Application • WWW - HTTP • File transfer - FTP • E-Mail - SMTP • Remote access - Telnet

4. Introduction (Cont.) • Wireless networks: bursty and high channel error rates. • Throughput decreases greatly. • Extending TCP as used over the wired links to the wireless links may bring inefficient usage of bandwidth. • The TCP protocol is still used to transfer data over the wireless link.

5. Introduction (cont.) • A lot of attention is currently being paid to the design of a better TCP scheme over wireless links. • Difficulty in modeling the TCP protocol analytically • Many of these studies are simulation based. • Hard to obtain insight into the effects of particular parameters on the behavior of TCP using simulations with specific settings

6. Random Loss Model • Independent identically distributed (i.i.d ) • The packet transmission time of the packets on both lossless and lossy link are exponentially distributed. • The congestion avoidance phrase is modeled using probability. • Correlated • Hiding the losses from the transport layer and moving the problem to link layer • Applicable on very low bandwidth wireless links • cannot be used to model the fast-transmit and fast-recovery phrases

7. TCP-Jersey • Congestion warning marking function (RED and ECN) • Routers can take part in controlling TCP transmission rate: active queue management (AQM) • Random early detection (RED): AQM scheme implementation • Explicit congestion notification (ECN): an extension to RED • RED and ECN are sensitive to parameter settings Cause unsatisfactory TCP performance

8. TCP-Jersey (Cont.) • TCP-Jersey simplifies the marking function: use only one threshold (thresh) • Marking all the packets with probability 1 if the average queue length exceeds the predefined threshold. • Calculating the average queue length using a larger queue weight than the original ECN.

9. TCP-Jersey (Cont.) • Available Bandwidth Estimation • Estimate the available bandwidth (Rn) for the TCP connection. • ABE estimator: • Calculate the optimum congestion window(ownd) size once every RTT. Ln: size of packet n; t n-1: previous packet arrival time.

10. TCP-Westwood • A sender-side modification of the TCP congestion window algorithm • improves upon the performance of TCP in wired and mixed network • uses an end-to-end bandwidth measurement. • The ACK-based measurement procedure • filtering the ACK reception rate • The effects of delayed and cumulative ACKs on bandwidth measurement

11. TCP-Westwood (Cont.) • Main idea: exploit TCP acknowledgement packets to derive rather complicated measurements. • A source perform an end-to-end estimate of the bandwidth available along a TCP connection by measuring and averaging the rate of returning ACKs.

12. TCP-Westwood (Cont.) Three types of filters actually used in the simulation: • Original filter: current_bwe_ = current_bwe * fr_alpha_ + sample_bwe * (1 – fr_alpha_) • Filter 1: current_bwe_ = current_bwe_ * .9047 + (sample_bwe + last_bwe_sample_) * .0476 • Filter 2: (LPF) current_bwe_ = cureent_bwe_ * .93548 + (sample_bwe+ last_bwe_sample_) * .03225

13. TCP-Westwood (Cont.) • TCP-Westwood controls the window using end-to-end rate transmission to continuously estimate the packet rate of the connection at the TCP sender. • It can reset the window to match available bandwidth, which makes it more robust to the random loss in wireless link. • TCP-Westwood has many superior features which can handle losses caused by link errors or wireless channel conditions more efficiently than most of previous existing TCP schemes.

14. Performance Analysis • We chose TCP-Westwood as a starting point because of its similarity to TCP-Jersey. • We tried to simulate TCP-Jersey in ns2. However, due to the opaque description for detailed mechanism in TCP-Jersey and lack of references, the implementation seems infeasible so far. • Problem: The congestion warning part (how to define the proper thresh of average queue length). • Thus, we simulate TCP-Westwood and all the other conventional TCP schemes in NS-2 network simulator.

15. Simulation Environment • The source connects to a wired based station via a 20 Mb/s error free link with 35 ms one-way delay. • The base station is linked to the destination, a wireless mobile node, via a 2 Mb/s lossy channel with 1 ms delay. • A single TCP connection running a long-live FTP application delivers data from the source to the destination. 2 Mb, 1 ms 20Mb, 35ms BS S D

16. Bandwidth Estimation • An effective way to calculate the congestion window size • Control data traffic efficiently. • Expecting TCP-Westwood can reach a good estimation for bandwidth so that the lossy link can be utilized as much as possible.

17. Bandwidth Estimation (Cont.) • When the simulation time is greater than 7s, the TCP Westwood reaches the max bandwidth utilization.

18. Bandwidth Estimation (Cont.) • The slow start threshold matches very well with congestion window settings.

19. Goodput • An explicit characteristic for TCP performance. • The effective amount of data delivered through the network. • Run the simulation for TCP -Tahoe, -Reno, New Reno, -Sack, and -Westwood respectively. • Random link error rate varies from 0.001% to 10%. • The simulation time is 60s.

20. Goodput Performance (Cont.) • The first point at link error rate 0.00001, all TCP schemes are closely match to each other, which means that they perform the same at nearly no loss rate condition. • When the lossy rate increases, TCP Westwood outperforms other TCP schemes explicitly, especially when error rate increases to 0.001.

21. Goodput Performance (Cont.) • The buffer size is kept as a constant, B = 60pkts. • TCP-Westwood varies almost linearly to track thebandwidth variations of the bottleneck. • TCP-Westwood is a slightly better than other schemes.

22. Packet Loss 3rd dup Ack triggles a retransmission of packet #115000: Fast Recovery

23. Packet Loss (cont.) The sending rate is slowed down: Congestion Avoidance phase

24. Packet Loss (cont.) Reenter the Slow Start phase

25. Conclusion • Investigate the performance of the different TCP algorithm in a wired and wireless hybrid networks. • Provide an analytical model for TCP-Jersey and TCP-Westwood. • First, we started with TCP-Westwood to perform the simulation. • Haven’t reproduced the TCP-Jersey so far in results of obscure description and lack of references. • Limitation: • Congestion Warning: restrict the threshold of marking algorithm. • Cooperate ECN to mark the packets.

26. Conclusion (cont.) • Different TCP algorithms, namely, Tahoe, Reno, New-Reno, Sack, and Westwood are studied in the given simulation environment. According to the experiment results, TCP-Westwood operates more robust than other existing TCP schemes, which shows the benefits of the bandwidth estimation in wireless or hybrid networks.

27. References • Kai Xu, Ye Tian, and Nirwan Ansari, “TCP-Jersey for Wireless IP Communications”, IEEE Journal on Selected areas in Communications, vol. 22, no.4, May 2004. • Gerla M., Sanadidi M.Y., Ren Wang Zanella, A. Casetti C., and Mascolo S.,“TCP Westwood: Bandwidth Estimation for Enhanced Transport over Wireless Links”, IEEE Global Telecommunications Conference 2001, vol. 3,  pp. 25-29, Nov. 200. • David D. Clark, and Wenjia Fang, “Explicit Allocation of Best-Effort Packet Delivery Service”, IEEE/ACM Trans. Networking, vol. 6, pp.362-373. • Farooq Anjum and Leandros Tassiulas, “Comparative study of various TCP versions over a wireless link with correlated losses”, IEEE/ACM Transaction on Networking, vol. 11, no. 3, June 2003. • A. Kumar and J. Holltzmann, “Comparitive performance analysis of versions of TCP in local network with a lossy link – Part II: Rayleigh Fading Mobile Radio Link”, Rutgers Univ., New Brunswick, NJ, Tech. Rep. WINLAB-TR 129, Oct. 1996. • M.May, J. Bolot, C. Diot, and B. Lyles, “Reasons not to deploy FED,” Proc. Int. Workshop Quality-of-Service (IWQoS), 1999, pp. 260-262. • A. Chochalingam, M. Zozi, and R. R. Rao, “Performance of TCP on wireless fading links with memory,” in Proc. IEEE Int. Conf. Communications, June 1998, pp. 595-600.

28. References (Cont.) • V. Jacobson, “Congestion avoidance and control,” ACM Computer Communications Review, vol. 4, pp. 314-319, Auguest 1988. • C. Casetti, M. Gerla, S. Lee, S. Mascolo, and M. Sanadidi, “TCP with fast recovery”, MILCOM 2000, Los Angeles, CA, Oct. 2000. • A. Zanella, G. Procissi, M. Gerla, M. Y. Snanaddi, “TCP Westwood: analytic model and performance evaluation”, Technical Report, URL: http://www.cs.ucla.edu/csd/phus/pubs.html. • ns-2 network simulator. LBL, URL: http://www.msh.cs.berkley.edu/ns. • TCP Westwood modules for ns-2. URL: http://www1.tcl.polito.it/casetti/tco-westwood. • K. Fall and S. Floyd, “Simulation-based comparisons of Tahoe, Reno, and SACK TCP,” ACM Computer Communication Review, 16:5-21, 1996. • C. Casetti and M. Meo, “A new approach to model the stationary havavior of TCP connections”, In Proc. of IEEE INFOCOME, pp. 367-375, Mar. 2000. • J. Mo, R. La, V. Anantharam, and J. Walrand, “Analysis and comparision of TCP Reno and Vegas,” In Proc. of IEEE INFOCOM, pp. 1556-1563, Mar. 1999. • J. Padhye, V. Firoiu, D. Towsley, and J. Kurose, “Modeling TCP throughput: a simple model and its empirical validation,” ACM Computer Communication Review, 28:303-314.