Diagnosing Wireless TCP Performance Problems:

A Case Study

Tianbo Kuang, Fang Xiao, and Carey WilliamsonUniversity of Calgary

Agenda

Motivation Background TCP IEEE 802.11b Wireless LAN (WLAN) Universal Serial Bus (USB)

Experimental Methodology Results

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?

Background - TCP

Widely used on the Internet (e.g. Web)Connection-oriented, reliable byte streamWindow-based flow controlSlow start and congestion avoidanceFast retransmission, fast recoveryOther extensions, including TCP SACKMany different versions in use

Background – IEEE 802.11b

An “Ethernet-like” LAN standard (11 Mbps)Infrastructure mode and ad hoc modeCarrier-sense multiple access with collision avoidance (CSMA/CA) to reduce collisionsMAC-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

Background – USB

Widely used industry standard for connecting a computer to its peripherals (bus topology)Lots of USB-based (wireless) network cardsData transfers managed by Host Controller (HC)Synchronous bus: 1 msec slots for transfersTransfer requests are handled using vertical and horizontal linked-list data structuresTwo processing modes for HC: Breadth-First or Depth-First

High Speed Bandwidth Reclamation (HSBR)

Background – USB (cont’d)

Queued mode (keep HC busy) Transfer size: 64 – 1023 bytes each

DF

BF

Background – USB (cont’d)

Queued mode (keep HC busy) Transfer size: 64 – 1023 bytes each

FSBR

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 APnetperf

Sniffer

data

acks

laptopstcpdump

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)

OSLinux

Windows 2000

1.52 Mbps

5.11 Mbps

Throughput

Results – USB Experiments

Results – USB ExperimentsWith FSBR disabled, USB is the bottleneckWith FSBR enabled (the default in Linux), the wireless network is the bottleneckQueued mode makes no difference with FSBR on, but helps when FSBR is turned offQueued mode (even with FSBR turned on) may be very important when higher speed wireless link is used (e.g. IEEE 802.11a)

Results – TCP ProblemsThe “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

Results – TCP “ack holding”

Results – TCP “ack holding”

(laptop) (wired)

(sniffer) (kernel)

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

Results – TCP “suppressed FR”

This is a deliberate feature to prevent a false fast retransmit after a timeoutThis situation is quite likely to occur in a wireless environmentIt’s not a bug, but a feature! (correct)

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%

Results – Wireless Problems (TCP data/ACK collisions)

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

Results – Wireless Problems (MAC-layer rate adaptation)

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

Questions?