An Investigation into Transport Protocols and Data Transport ...

5.5. Summary 109
5.5 Summary
Even though TCP has been a fundamental part of the success of the Internet,
enabling reliable data transfers and preventing congestion collapse, it was
never designed to be scalable in terms of throughput. As such TCP has
problems with long distance, fast networks which will form the backbone of
future grids in order for computers to interoperate and share data files.
There has been a recent interest in utilising the new backbone speeds
available in today’s Academic and Research networks, especially in terms of
large scale data replication that is required for projects such as the LHC
[CFK + 01, SRGC00]. As such, network researchers have pushed the boundaries
on tuning host hardware and software to facilitate high utilisation of
network resources. Many of the techniques used have been studied in detail
in this Chapter.
However, [ST] states that 90% of bulk transports (i.e. those over 10MB)
typically achieve transfer speeds of less than 5Mbit/sec. Furthermore, Equation
5.6 shows that the maximum utilisation of a single TCP flow has a
theoretical limit of 75% when there is zero buffering. Full utilisation is only
achieved when the queue size is equal to the BDP of the TCP flow.
Loss is also a fundamental limit on the throughput achievable with Standard
TCP. It was shown that the maximum throughput of a typical trans-
Atlantic transfer without competing flows sharing the network path is only
450Mb/sec - because of physical limits upon bit error rates.
Therefore, it has been demonstrated both experimentally in Section 3.2.2
and theoretically that TCP is simply incapable of high throughput in long
distance, high capacity paths - regardless of the amount of tuning performed
outside of the AIMD algorithms.