An Investigation into Transport Protocols and Data Transport ...

6.2. Discussion and Deployment Considerations of New-TCP Algorithms126
does not scale to utilise available bandwidth.
As higher capacities and latencies become more common, it is important
the dynamic by which TCP determines that there is spare capacity is sufficiently
scalable. A trivial solution to this problem is to shorten the congestion
epoch time. This can be achieved by increasing the value of α such that the
rate of growth of the cwnd is increased. Therefore, large increases in the
probing rate will facilitate the utilisation of spare capacity over short timescales.
This is most evident with ScalableTCP and H-TCP; ScalableTCP
with its multiplicative increase ensures that the next congestion epoch is
reached quickly.
However, especially for loss-based algorithms, this large increase in cwnd
will result in a higher probability multiple lost segments within the same window
which have to be retransmitted. The result of this retransmission may
result in much burstier flow and could lead to congestion collapse. It is therefore
prudent to minimise the amount of data that needs to be retransmitted
when congestion occurs [KRB05].
FAST minimises this requirement by slowly adjusting its optimal cwnd
value according to multi-bit feedback from network delay. Meanwhile, BicTCP
increases the utilisation of network resources by maintaining a similar plateau
for its cwnd dynamic as it finds its w target value.
However, as defined in Equation 5.6, the actual time independent (i.e.
sufficiently long) utilisation of a flow depends upon not the increase parameter,
but the algorithm’s decrease parameter β assuming a linear increase in
α.
Obviously, the decrease of half used in Standard TCP is too severe at
high speeds. The simple solution would be to minimise this decrease to say,
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as with ScalableTCP. This would therefore result in a network utilisation