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Data Transport Challenges for e-VLBI
Julianne S.O. Sansa*
* With Arpad Szomoru, Thijs van der Hulst & Mike Garret
27 October 2005 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*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)]
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
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
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
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
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)