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Data Transport Challenges for e-VLBI

This presentation discusses network performance tests, simulation results, and the conclusions drawn regarding the challenges in e-VLBI data transport. It explores the available bandwidth, bottleneck minimization, stability of TCP connections, and bottleneck factors like application socket buffers, hardware limitations, the OS, transport protocol, window limits, and retransmissions. Additionally, it analyzes different transport protocol proposals and the need for optimization of hardware and TCP data flows.

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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 e-VLBI meeting 12 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)] e-VLBI meeting 12 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? • What is the stability of these TCP connection (repeatability /predectability)? e-VLBI meeting 12 October 2005

  5. Results with web100 • File transfer of 10 GB & 1GB file • Modified file transfer (app socket buffers) • Memory-memory with bwctl e-VLBI meeting 12 October 2005

  6. Cwnd, RwinRcvd & for a file transfer / memory-memory e-VLBI meeting 12 October 2005

  7. Achieved/Available throughput e-VLBI meeting 12 October 2005

  8. Summary Test results e-VLBI meeting 12 October 2005

  9. NIC RTT/loss discrepancies e-VLBI meeting 12 October 2005

  10. TCP /Application throughput e-VLBI meeting 12 October 2005

  11. The bottlenecks • Application socket buffers • Hardware (PCI bus limit, NICs) • The OS (more or less tuned optimally) • The transport protocol (TCP) • Window limits • Retransmissions e-VLBI meeting 12 October 2005

  12. Transport Protocol Analysis • Already many proposals to alter this behaviour: HighSpeed TCP, scalable TCP, Westwood TCP, HTCP, Vegas, FAST, BIC, C-TCP e-VLBI meeting 12 October 2005

  13. 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. e-VLBI meeting 12 October 2005

  14. Preliminary Simulation results e-VLBI meeting 12 October 2005

  15. Cwnd for the simulated protocols e-VLBI meeting 12 October 2005

  16. Achieved Throughput for the simulated protocols e-VLBI meeting 12 October 2005

  17. 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) e-VLBI meeting 12 October 2005

  18. 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) e-VLBI meeting 12 October 2005

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