An Investigation into Transport Protocols and Data Transport ...

5.4. Analysis of AIMD Congestion Control 102
For example, in real life systems, a packet loss rate of about 10 −7 is
comparable to the random losses that occur in long haul fibres, routers,
switches and end-systems. This physical limit imposes an absolute limit
on the throughput that a TCP connection can achieve - even without
competing traffic on the end-to-end path (see Section 5.4.2).
Bandwidth The higher throughput that the TCP connection has to sustain,
the larger its cwnd for the connection. With the implementation of the
current Additive Increase of 1 segment per round-trip time, TCP probes
for extra network capacity slowly, and therefore is unable to effectively
use available network resources that are present on high bandwidth
networks that are not fully utilised.
Also, the detection of packet loss through congestion or network corruption
in TCP results in a halving of its cwnd. When high speeds are
reached this mechanism of Multiplicative Decrease by half leads to a
significant decrease in the throughput of the TCP connection.
As TCP congestion control is cyclic (i.e. it will continue to slowly
increase its sending rate until congestion/loss is detected, where upon
it will halve the rate) the overall utilisation of the link by a single TCP
flow could be low compared to the capacity of the bottleneck link on
the path.
Round-Trip Time TCP utilises acknowledgments to update its window
based algorithms. This provides a feedback loop that drives the linear
increase of cwnd to increase its throughput. As the round-trip latency
increases, the time required for this feedback also increases. This results
in a slower rate of increase (in absolute time) of cwnd and therefore a