Data Transport Challenges for e-VLBI

Julianne S.O. Sansa*

* With Arpad Szomoru, Thijs van der Hulst & Mike Garret

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Outline

• Network performance tests

• Simulation results

• conclusion

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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*CwndCwnd = 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)]

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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)?

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Results with web100

• File transfer of 10 GB & 1GB file

• Modified file transfer (app socket buffers)

• Memory-memory with iperf

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Cwnd, RwinRcvd & for a file transfer / memory-memory

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Achieved/Available throughput

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Summary Test results

Memory –Memory File transfer Modified file transfer

Disk2net-net2file

(yet to be done)

Cwnd(bytes)

TCP (Mbps)

UDP(Mbps)

Cwnd (bytes)

TCP (Mbps)

Cwnd(bytes)

TCP

(Mbps)

Cwnd TCP UDP

Bench via Amsterdam

2251640 624.1 955 65160 56.6 220000 191.1

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NIC RTT/loss discrepancies

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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)

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Transport Protocol Analysis

• Already many proposals to alter this behaviour: HighSpeed TCP, scalable TCP, Westwood TCP, HTCP, Vegas, FAST, BIC, C-TCP

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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.

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Preliminary Simulation results

Simulated file transfer of bench via Amsterdam scenario

TCP

UDP HSTCP FAST

Cwnd (bytes) 1,220.86 n/a 4,543.42 205

T/put (bps) 61,600,000 960,000,000 61,600,000 310,870,560

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Cwnd for the simulated protocols

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Achieved Throughput for the simulated protocols

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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)

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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)