TCP and UDP Performance Over A Wireless LAN

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TCP and UDP Performance Over A Wireless LAN

• Professor?????
• Speaker???
• Date2004/04/16

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Outline

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

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Outline

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

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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.

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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.

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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

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Methods Description (1)

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

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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.

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Outline

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

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Hardware

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

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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.

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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.

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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.

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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.

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Location map
45
60
45
feet
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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.

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Outline

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

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Testing Scenario
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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.

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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 sizes100, 500, 1000 and
  1500 bytes to show the effects of varying amounts
  of overhead and packet error probability on
  throughput.

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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.

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Outline

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

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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.

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Result (1-1)
Receiver view
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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.

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Result (2-1)
Receiver view
Mykonos
Ios
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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.

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

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Result (4-1)
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Result (4-2)
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Result (4-3)

• A collision occurs when one data and one
  acknowledgment packet are shown on the sender but
  not on the receiver.

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Result (5-1)
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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.

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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.

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Result (1-1)
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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.

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

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Result (3-1)
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Result (3-2)
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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.

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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.

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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.

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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.

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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.

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Outline

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

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Conclude

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

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Outline

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

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Reference

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