An Investigation into Transport Protocols and Data Transport ...

8.4. Discussion of Results 187
Asymmetric
New-TCP Symmetric Small Buffer Large Buffer Friendliness
Standard TCP 0.52±0.01 2.09±0.15 1.57±0.25 0.53±0.01
BicTCP 5.08±0.00 6.44±0.70 5.05±0.67 3.25±0.40
FAST 20.2±10.9 29.2±0.61 0.36±0.04 0.01±0.00
HSTCP 2.95±0.01 7.63±0.65 6.53±0.77 3.98±0.21
H-TCP 3.11±0.04 4.99±0.09 4.47±1.00 3.99±0.13
ScalableTCP 9.64±0.15 12.3±0.64 11.45±0.83 22.4±0.08
Table 8.4: Summary overhead (Bytes/Mbit/sec × 1e-7) of two competing New-
TCP flows at 250Mbit/sec bottleneck and 82ms RTT.
convergence time is almost constant.
8.4.4 Overhead
H-TCP is good at being fair and switching into aggressiveness to utilise
spare capacity as shown in Table 8.4. However, the polynomial increase with
respect to time means that a lot of packets can be lost in one window, and
efficiency decreases as a result. This is most evident for network environments
which result in large periods of congestion epochs, whilst the overhead is
similar to Standard TCP under environments with short congestion epoch
times.
FAST, with its delay based approach to congestion control, is also very
capable of low loss rates, but requires that the bottleneck queue sizes are
provisioned such that they are at least equal to the number of flows multiplied
by their α values. Under such environments, the overhead of FAST can be
as much as two orders of magnitude better than Standard TCP. However,
under environments with low levels of queue provisioning along the path,
the loss rates of FAST are much higher resulting in two orders of magnitude
less efficiency compared to StandardTCP. This is caused primarily by the