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

e-VLBI meeting 12 October Outline Network performance tests Simulation results conclusion

e-VLBI meeting 12 October Network Performance Measurements Investigate critically several connections established. Wire speeds suggests much higher throughput than what application data realises. TCP Congestion Control algorithm (AIMD) –SSACK:Cwnd  Cwnd +1 –CAACK:Cwnd  Cwnd + 1/Cwnd DROP: Cwnd  Cwnd -1/2*Cwnd Cwnd = max. # packets that TCP injects into network before receiving ACK. Cwnd optimal ~ Throughput *RTT Cwnd average = 1.22*MSS/sqrt (p) [Floyd & Fall (1999), Padhya et.al (1998)]

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

e-VLBI meeting 12 October Achieved/Available throughput

e-VLBI meeting 12 October Summary Test results Memory –MemoryFile transferModified file transfer Disk2net-net2file (yet to be done) Cwnd (bytes) TCP (Mbps) UDP (Mbps) Cwnd (bytes) TCP (Mbps) Cwnd (bytes) TCP (Mbps) CwndTCPUDP Bench via Amste rdam

e-VLBI meeting 12 October NIC RTT/loss discrepancies

e-VLBI meeting 12 October TCP /Application throughput

e-VLBI meeting 12 October 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 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 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 Preliminary Simulation results Simulated file transfer of bench via Amsterdam scenario TCP UDPHSTCPFAST Cwnd (bytes) 1, n/a4, T/put (bps) 61,600,000960,000,00061,600,000310,870,560

e-VLBI meeting 12 October Cwnd for the simulated protocols

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

e-VLBI meeting 12 October 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 References Floyd & Fall (1999) “Promoting the use of end-to- end congestion control in the internet”, IEEE/ ACM Trans. on Networking, August 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)