TCP Westwood: Experiments over Large Pipes

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TCP Westwood Experiments over Large Pipes

• Cesar Marcondes
• Anders Persson
• Prof. M.Y. Sanadidi
• Prof. Mario Gerla
• NRL Network Research Lab
• UCLA

2
Background

• TCP NewReno is challenged on large pipes
• Slow convergence to full utilization
• Not intended to handle non-congestion packet loss
• Large Pipes performance criteria
• Utilization
• Stability
• Fast Ramp Up to Cruising Speed from Slow start
• Fairness under differing RTTs
• Friendliness to NewReno
• Alternatives include HS TCP, FAST, TCPW
• Goal of this study Measurements of TCPW, FAST
  and HS TCP over large pipes

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TCPW

• Goal high utilization, fairness, and
  friendliness over large leaky dynamic pipes
• Sender side only estimation of Eligible Rate
  Estimate (ERE)
• Estimation takes into account congestion level,
  capacity of the bottleneck, achieved rate
• Exponential filtering to time average estimates
  and avoid network conditions instability
• ERE is used to
• (1) set congestion window after packet loss
• (2) repeatedly reset ssthresh to reach cruising
  speed fast from slow start

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TCPW ABSE
RE Sampling Packet train, fair estimate
under congestion, underestimates under random
loss

• BE Sampling
• Packet pair,
• effective under random loss, overestimates
  under congestion

Under Congestion
Under No Congestion

• To obtain ERE adapt the sample interval Tk
  according to congestion level
• Congestion level is similar to that in Vegas
  Expected Rate-Achieved Rate

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Experiments Environment
NewReno Sender
Gigabit link
Internet2
NewReno Receiver (Alabama)
Gigabit link
UCLA Gigabit Switch
Advanced TCP Sender

• (Powerful Machines)
• CPU Xeon 3.06GHz
• Cache 512 L2/ 1MB L3
• Intel 1000PRO
• PCI-X BUS 133MHz

NewReno Receiver (Caltech)
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UCLA Internet2 Link Traffic
Other UCLA Users in Background
Our Experiments Traffic
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Test Methodology

• Automated Scripts
• Scheduled by Unix crontab
• Automatically reinitiate the O.S. with each
  protocol and conduct new measurements
• Linux FAST, HS-TCP and NewReno
• FreeBSD TCPW
• Sender/Receiver buffer is set to 2 MB to enable
  high utilization of Gbps links
• Iperf traffic generation, TCPdump, Nistnet
  emulator

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Benchmark Tests

• Case Study I
• UCLA-Alabama (155 Mbps, 64 msec)
• Case Study II
• UCLA-CalTech (1 Gbps, 4msec)
• Group of 10 successive night time runs for each
  test
• Throughput, fairness, friendliness
• Artificial non-congestion loss (PER 0.1 to 0.5)

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Case Study I UCLAAlabama
NewReno Sender
Internet2 (Gigabit)
ATM Atlanta Alabama
NewReno Receiver (Alabama)
Advanced TCP Sender
155Mbps ATM Link Bottleneck Link as measured by
PathRate And confirmed later by the network admin
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Throughput
UCLA-Alabama

• Convergence to cruising speed varies among
  protocols
• High deviation among multiple runs in HSTCP and
  NewReno
• HSTCP deviations decrease over time (as the AIMD
  behavior changes)

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UCLA-Alabama
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Transfer Completion Times
UCLA-Alabama

• On average
• TCPW and FAST 0 to 100 MB in 5.8 Sec!
• HSTCP 0 to 100 MB in 7.5 Sec!
• NewReno 0 to 100 MB in 11 Sec!

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Friendliness
UCLA-Alabama
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TCP FAST Preliminary Analysis
UCLA-Alabama
RTT Variation over Time as Observed by TCPdump
Outstanding Window as Observed by TCPdump
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Random Loss Emulation
UCLA-Alabama
UCLA Alabama
NewReno Receiver (Alabama)
Advanced TCP Sender
Nistnet Network Emulator

• Induced non-congestion packet loss in emulator
  (PER 0.1 up to 0.5)
• TCPW throughput much higher than all other schemes

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Random Loss Emulation (Results)
UCLA-Alabama
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Case Study II UCLACalTech
NewReno Sender (UCLA)
Internet2 (Gigabit)
Advanced TCP Sender (UCLA)
TCP Receiver (CalTech)
1 Gbps 4 ms
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Throughput
UCLA-CalTech

• TCP NewReno starts-up really high since it relies
  in the cached threshold and the feedback is
  really fast
• Cached Slow Start Threshold versus Adaptive
  Start-Up (Pros and Cons)
• Westwood is delayed by its own Stability Filter
• Stability-based Filter dampens estimates in
  proportion to the variance of observation

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UCLA-CalTech
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TCP Westwood Stability Filter versus Fixed Gain
Filter
UCLA-CalTech

• Sample Estimations vary a lot due to NIC
  coalescing and OS issues at Gigabit/s.
• As variability increases, stability filter relies
  on a more stable moving average filter
• Solution Use a fixed gain instead of an adaptive
  when we know we are dealing with Gbps range
  speeds
• TCPW ramp up as HS-TCP and FAST

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TCPW Start-Up using Fixed Exponential Average
UCLA-CalTech
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Friendliness
UCLA-CalTech

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Conclusions

• TCPW and FAST performed equally well in terms of
  average throughput
• All Advanced TCP protocols have an excellent
  intra-protocol fairness
• Friendliness
• FAST appears to suffer a synchronization problem

• Under non-congestion error scenario, TCPW shows
  greater robustness
• At Gigabit speed, measurements could be messed up
  by Interrupt Coalescing and other HW/Kernel
  bottlenecks, affecting moving average filters

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Future Work

• New algorithm that is Interrupt Coalescence-Aware
  for Gbps environment
• New Agile and Stable Filter
• Improve the Automated TCP Test Tool (Benchmark
  and New Tests)

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Thanks

• Netlab CalTech
• Xiaoyan Hong CS / Alabama Univ.