1. Diagnosing Wireless TCP Performance Problems: A Case Study Tianbo Kuang, Fang Xiao, and Carey Williamson University of Calgary

2. Agenda Motivation Background TCP IEEE 802.11b Wireless LAN (WLAN) Universal Serial Bus (USB) Experimental Methodology Results

3. Motivation TCP performance often degrades over wireless networks; reasons “well-known” Solutions to improve TCP performance over wireless links exist, but how well do they work in a real wireless LAN environment? How do link-layer mechanisms interact with TCP and affect the overall performance? Where is the bottleneck in the network protocol processing path, and why?

4. Background - TCP Widely used on the Internet (e.g. Web) Connection-oriented, reliable byte stream Window-based flow control Slow start and congestion avoidance Fast retransmission, fast recovery Other extensions, including TCP SACK Many different versions in use

5. Background – IEEE 802.11b An “Ethernet-like” LAN standard (11 Mbps) Infrastructure mode and ad hoc mode Carrier-sense multiple access with collision avoidance (CSMA/CA) to reduce collisions MAC-layer: positive acknowledgment and retransmissions (to recover from channel errors) Dynamic rate adaptation: can choose data transmission rate of 1, 2, 5.5, or 11 Mbps

6. Background – USB Widely used industry standard for connecting a computer to its peripherals (bus topology) Lots of USB-based (wireless) network cards Data transfers managed by Host Controller (HC) Synchronous bus: 1 msec slots for transfers Transfer requests are handled using vertical and horizontal linked-list data structures Two processing modes for HC: Breadth-First or Depth-First High Speed Bandwidth Reclamation (HSBR)

7. Background – USB (cont’d) Queued mode (keep HC busy) Transfer size: 64 – 1023 bytes each DF BF

8. Background – USB (cont’d) Queued mode (keep HC busy) Transfer size: 64 – 1023 bytes each FSBR

9. Experimental Methodology Experimental Setup (HW/SW) Laptop – Compaq Evo 719c with multiport USB wireless card (Linux 2.4) Access point – Lucent RG-1000 Stationary host on Ethernet LAN – SunOS 5.8 Run netperf on laptop and netserver on wired host SnifferPro 4.6 wireless “sniffer” and tcpdump Experimental Factors USB mode, driver settings Wireless channel (distance) between laptop and AP netserver Ethernet AP netperf Sniffer data acks laptops tcpdump

10. Initial Result Windows 2000 implementation of TCP is more than 3 times faster than Linux TCP! Reason: Linux driver bug (2 Mbps vs 11 Mbps) OS Linux Windows 2000 1.52 Mbps 5.11 Mbps Throughput

11. Results – USB Experiments

12. With FSBR disabled, USB is the bottleneck With FSBR enabled (the default in Linux), the wireless network is the bottleneck Queued mode makes no difference with FSBR on, but helps when FSBR is turned off Queued mode (even with FSBR turned on) may be very important when higher speed wireless link is used (e.g. IEEE 802.11a)

13. Results – TCP Problems The “ack holding” problem A bug in the NIC firmware or interrupt driver of Linux OS causes excessive delays (> 100 ms) This leads to a spurious TCP timeout The retransmission of previously acked data! Actually just an artifact of tcpdump observation The lack of a TCP “fast retransmit” after receiving three duplicate Acks A deliberate (but not well-known) feature of TCP

14. Results – TCP “ack holding”

15. (laptop)(wired) (sniffer)(kernel)

16. Results – TCP “repeated data” The spurious TCP timeout was not properly detected Caused by initialization bug in Linux TCP implementation The “repeated data” problem is an artifact induced by presence of link layer buffer

17. Results – TCP “suppressed FR” This is a deliberate feature to prevent a false fast retransmit after a timeout This situation is quite likely to occur in a wireless environment It’s not a bug, but a feature! (correct)

18. Results – Wireless Problems We observed unusually high collision rates on the wireless channel for TCP transfers, which we call the TCP data/ACK collision problem Scenario: laptop and AP are 1 m apart For TCP, MAC-layer retransmit rate: 4.58-4.73% For UDP, MAC-layer retransmit rate: 0.47-0.98% In general, a retransmission rate of 1.75%-7.2% has been seen for other vendor HW/SW (N = 1) For TCP, disabling MAC-layer retransmission degrades throughput by 23%

19. Results – Wireless Problems (TCP data/ACK collisions)

20. Results – Wireless Problems The MAC-layer rate adaptation problem Scenario: laptop and AP are 100 m apart Lousy TCP throughput, lots of retransmits Reason: the multiplicative increase and multiplicative decrease (MIMD) bandwidth probing mechanism causes network thrashing and wastes battery power The small congestion window causes temporary deadlock if the TCP receiver uses delayed Ack

21. Results – Wireless Problems (MAC-layer rate adaptation)

22. Conclusions TCP performance on WLAN can be wacky! (at least for Compaq Multiport 802.11b USB wireless card under Linux 2.4) Several factors can affect overall performance Poorly configured USB bus could be the bottleneck Linux TCP implementation bug makes TCP unable to recognize the first spurious timeout Poor MAC-layer rate adaptation algorithm can cause a “network thrashing” problem TCP’s data/ACK structure may induce excessive collisions at the MAC layer on wireless LANs

23. Questions?