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Data Transport Challenges for e-VLBI

Data Transport Challenges for e-VLBI. Julianne S.O. Sansa*. * With Arpad Szomoru, Thijs van der Hulst & Mike Garret. Outline. Network performance tests Simulation results conclusion. Network Performance Measurements.

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Data Transport Challenges for e-VLBI

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  1. Data Transport Challenges for e-VLBI Julianne S.O. Sansa* * With Arpad Szomoru, Thijs van der Hulst & Mike Garret

  2. Outline • Network performance tests • Simulation results • conclusion 27 October 2005

  3. Network Performance Measurements • Investigate critically several connections established. Wire speeds suggests much higher throughput than what application data realises. • TCP Congestion Control algorithm (AIMD) • SS ACK:Cwnd  Cwnd +1 • CA ACK:Cwnd  Cwnd + 1/Cwnd • DROP: Cwnd  Cwnd -1/2*Cwnd • Cwnd = max. # packets that TCP injects into network before receiving ACK. • Cwndoptimal ~ Throughput *RTT • Cwndaverage = 1.22*MSS/sqrt (p) [Floyd & Fall (1999), Padhya et.al (1998)] 27 October 2005

  4. Specific Questions • How much bandwidth is available to the these TCP connections? Is it what is seen by the app? • If it is less than the theoretic available b/w, what is the bottleneck? • How do we minimise this bottleneck? • How do multiple TCP connections share available bandwidth? • What is the stability of these TCP connection (repeatability /predectability)? 27 October 2005

  5. Results with web100 • File transfer of 10 GB & 1GB file • Modified file transfer (app socket buffers) • Memory-memory with iperf 27 October 2005

  6. Cwnd, RwinRcvd & for a file transfer / memory-memory 27 October 2005

  7. Achieved/Available throughput 27 October 2005

  8. Summary Test results 27 October 2005

  9. NIC RTT/loss discrepancies 27 October 2005

  10. The bottlenecks • Application socket buffers • Hardware (PCI bus limit, NICs) • The OS (more or less tuned optimally) • The transport protocol (TCP) • Window limits • Retransmissions • Interface stalls • Vendor specific implementations (Other Reductions) 27 October 2005

  11. Transport Protocol Analysis • Already many proposals to alter this behaviour: HighSpeed TCP, scalable TCP, Westwood TCP, HTCP, Vegas, FAST, BIC, C-TCP 27 October 2005

  12. Loss-based, delay-based,or equation-based? • Which way do we go? • Consider getting the best out each world/Allow the application to dynamically detect network conditions & decide which algorithm to use. 27 October 2005

  13. Preliminary Simulation results 27 October 2005

  14. Cwnd for the simulated protocols 27 October 2005

  15. Achieved Throughput for the simulated protocols 27 October 2005

  16. Conclusions & further work • Hardware (PCI bus, NICs,) on end systems as well as the application (buffers) need to be optimised. • Model TCP data flows & relate flow analysis with correlation. • More simulation work on Transport Protocol analysis (response function) 27 October 2005

  17. References • Floyd & Fall (1999) “Promoting the use of end-to-end congestion control in the internet”, IEEE/ ACM Trans. on Networking, August 1999. • Padhya et.al (1998) “Modeling TCP throughput: A Simple model and its empirical validation” in Proc ACM SigCOMM 1998 • Antony et.al(2004) “Exploring Practical Limitations of TCP over Transatlantic Networks” submitted Elsevier Science(2004) 27 October 2005

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