An Investigation into Transport Protocols and Data Transport ...

9.4. Summary 244
problems in being able to run these congestion control algorithms effectively
as single high bandwidth flows. The problems arise primarily through the
limited resources of hardware and implementation issues with software.
The problems in particular are related to the large number of packets
that need to be processed by both the sender and receiving TCP hosts. In
particular, it was identified that the TCP receiver was not always sending
acknowledgments for at least every other data packet. This results in ack
stretching which increases the likelihood of buffer overflows due to larger
bursts of data packets on the forward path. More evident, however, was that
it causes Linux specific performance penalties due to the way in which cwnd
and the number of packets in flight are calculated.
A more serious problem, especially for larger values of cwnd required for
higher speed, long distance networks, is that the processing of SACK packets
was demonstrated to also be a bottleneck upon TCP transfer rates.
As demonstrated, the former problem can be rectified with modifications
to the Linux kernel. The latter problem can also be resolved with newer
and faster hardware; however, a network-based solution was implemented to
smooth the loss pattern and hence reduce the processing burden required.
A limited study was conducted to show the interaction of multiple flows
of New-TCP traffic. The results demonstrate and corroborate prior results
(in Chapter 8) that ScalableTCP has problems with fair and equal sharing of
network resources. It was also demonstrated that these New-TCP algorithms
can have a serious negative impact upon existing network traffic flows, reducing
the amount of traffic utilised by bulk Standard TCP flows by several
orders of magnitude. However, as a direct consequence of the implementation
problems aforementioned, this impact was reduced due to the inability
for the New-TCP flows to be able to achieve high goodputs.