TCP and UDP Performance Over A Wireless LAN

TCP and UDP Performance Over A Wireless LAN Professor：柯開維老師 Speaker：許家豪 Date：2004/04/16

Outline • Introduction • Experimental Setup • Testing • Analysis Of Test results • Conclude • Reference

Our Goals • In order to ameliorate WLAN performance problems we need a clearer understanding of WLAN behavior and analyzing it. • Our aim was to compile a comprehensive set of data describing the performance of a WaveLAN system in terms of throughput and loss under various realistic conditions.

Network Performance • Be Influenced by：(difficult to perceive) 1、Network and Host Processing Hardware. 2、Interface Device Drivers. 3、Network Protocol Implementation in the OS.

Methods : • We aims to extend published results in many ways 1、System Heterogeneity 2、New Implementations 3、Bidirectional Communications 4、Error Modeling 5、Operating System

Methods Description (1) • We used hosts with varying processing power and different wireless interface implementation. • Use 2.4 GHz version. Also used faster processors that could potentially achieve higher throughputs. • We measured the performance of TCP, in addition to UDP.

Methods Description (2) 4. We present additional measurements and also analyze bidirectional traffic effects. 5. We employed the Linux OS instead of BSD UNIX derivatives used in previous work.

Hardware • Buffer of PCMCIA cards has single buffer. • Buffer of ISA cards have multiple buffers

Equipment • Digital RoamAbout 2.4GHz DSSS system, an OEM version of the Lucent WaveLAN. • IOS (better) and MYKONOS (poor) are desktops（PC）. • SYROS is a laptop , and its processor operates at a lower clock frequency but has more on_chip cache memory. • Use CAMA/CA to handle collision problems.

Software (1) • All hosts ran the Linux OS, in multiuser mode, but with no user tasks executing, testing time is late in the evening. • We made a minor modification to the wireless interface drivers to record and report detailed statistics plus histograms of signal and noise levels.

Software (2) • Two benchmarks were used 1、TTCP： Sends a number of packets of a specified size to a receiver using either TCP or UDP. 2、ETTCP (For UDP test) ： Uses packet sequence number so that the receiver can detect and report packet losses.

Software (3) • Use nstat to gather IP, UDP and TCP statistics aggregated across all interfaces to check for unexpected network activity during the tests. • Use tcpdump to record detailed logs of all packets set and received by the wireless interfaces during each tests.

Location map 45 60 45 feet

Environment • A floor plan of the area (5th floor) where the experiments took place. • All rooms are laboratories and machine rooms containing numerous hardware devices but no direct sources of interference. • Hosts were kept immobile during each test to avoid mobility induced problems.

Testing Scenario

Testing Methods (1) • The main test parameters were transfer direction, peer names, packet size, protocol. • A test script first reset and dumped interface statistics and nstat output, then started tcpdump to record all packets through the wireless interface, and finally started ettcp to transfer 10000 packets.

Testing Methods (2) • All tests were performed in both directions between the peers to reveals any performance asymmetries. • All tests were performed for both TCP and UDP, with four IP datagram sizes：100, 500, 1000 and 1500 bytes to show the effects of varying amounts of overhead and packet error probability on throughput.

Testing Methods (3) • Throughput and loss rate are comparable across all tests since their units are independent of packet size. These can be used to determine the optimal packet size where overhead and loss are best balanced.

Scenario 1 • Two hosts were placed next to each other to avoid signal degradation. • Goal： It was to determine the peak performance of ISA cards and reveal processing power induced asymmetries.

Result (1-1) Receiver view

Result (1-2) • When the slower host (MYKONOS) is sending, packet loss is negligible. In contrast, the faster host (IOS) overwhelms a slower receiver, leading to loss (0.3-0.6%) which grows with packet size.

Result (2-1) Receiver view Mykonos Ios

Result (2-2) • Net UDP throughput increases with packet size since UDP/IP overhead drops. • TCP throughput is not only below UDP, it actually drops with large packet sizes. • Slower sender makes fewer losses.

Result (3-1) Data sent (I) Data received (M) Data sent (M) Data received (I)

Result (3-2) • The gaps between sent and received curves for both packet types show considerable loss on the link, growing with packet size. • Since the gaps are roughly the same, we conclude that their magnitude represents the number of undetected collisions of CSMA/CA.

Result (4-1)

Result (4-2)

Result (4-3) • A collision occurs when one data and one acknowledgment packet are shown on the sender but not on the receiver.

Result (5-1)

Result (5-2) • Both histograms are nearly symmetric since the peer interfaces were exactly the same and host processing power does not influence the radios.

Scenario 2 • It employs one ISA and one PCMCIA host, again placed next to each other, to establish a performance baseline for mixed interface tests. • The processing power of SYROS and MYKONOS is roughly equivalent, thus comparisons with the first scenario are direct.

Result (1-1)

Result (1-2) • When the faster host is sending, implying that both ISA and PCMCIA receivers are overwhelmed by faster senders. • In Syros to Ios direction, the perceived losses are due to packets never leaving the sending interface.

Result (2-1) Ios to Syros (UDP) Ios to Syros (TCP) Syros to Ios (UDP) Syros to Ios (TCP)

Result (2-2) • In the IOS(ISA) to the SYROS(PCMCIA) direction UDP is faster than TCP, due to less header overhead and the absence of TCP retransmissions and acknowledgments. • TCP throughput in the reverse direction is slightly lower, verifying previous claims that PCMCIA cards are slower senders.

Result (3-1)

Result (3-2)

Result (3-3) • Sequence numbers increase faster with an ISA sender despite occasional retransmissions. The PCMCIA sender leaves short gaps between transmission bursts due to the transmit buffer shortages • PCMCIA card has single transmit buffer, and ISA has multiple buffers. So PCMCIA is easy to overrun.

Scenarios 3 & 4 (1) • Scenarios 3 : The same as 1, just indicating again that a faster sender overruns a slower receiver. But signal level are uniformly lower. • Scenarios 4 : The same as 2. But signal level are uniformly lower.

Scenarios 3 & 4 (2) • We conclude that this distance and obstacles do not have measurable effects on performance in both ISA/ISA and ISA/PCMCIA tests.

Scenarios 5 • The main difference with 2 & 4 is a nearly zero loss rate in the ISA to PCMCIA direction. This shows that hosts matched in processing power avoid losses due to receiver overruns.

Scenario 6 • The only difference with scenario 4 is a slightly higher loss rate, both due to the increased distance and obstacles. And the signal level is lower than scenario 4.

Conclude • Fast senders can overwhelm slower receivers (both ISA and PCMCIA), leading to semi-periodic packet loss.

Reference • TCP and UDP Performance over a Wireless LAN ( George Xylomenos and George C. Polyzos )

TCP and UDP Performance Over A Wireless LAN