An Investigation into Transport Protocols and Data Transport ...

9.3. Internet Transfers 229
can cause stalls in the data transfer which lowers the achievable goodput.
Due to Linux specific implementation TCP features that were designed
to lower the processing overhead on the receiver, many ‘dubious events’ were
observed which causes the TCP sender to recalculate cwnd to that of the
number of packets in flight which caused a reduction in the observed goodput.
In order to reduce the processing burden of SACKs, an AQM solution
(WRED) was implemented. Specifically, a drop probability in the router
AF queue with a gentle gradient was proven to be extremely effective in
maintaining high goodput for ScalableTCP. This validates the belief that the
SACK processing is a dominant reason for having poor goodput performance.
This AQM solution can relax the requirement of any further optimisation
of the SACK code. However, it is believed that the main limitations in using
aggressive TCP protocols under high BDP IP DiffServ-enabled paths lies
in not having enough CPU speed to cope with both the sheer number of
packets both coming in and out and extra burden of multiple packet drops
in the same cwnd that they induce.
9.3 Internet Transfers
The transport of New-TCP protocols across real-life wide area networks is
investigated in this section. The goal is to to determine the real-world performance
of the various New-TCP proposals using single flow transfers across
Transatlantic and pan-European academic networks.