An Investigation into Transport Protocols and Data Transport ...

6.2. Discussion and Deployment Considerations of New-TCP Algorithms132
algorithms should be friendly. It implements the idea that upon congestion
the H-TCP flow should always revert back to Standard TCP and hence maintain
at least temporary fairness with Standard TCP. Only after a period of
time, after congestion, should the high speed mode be applied.
However, the balance between being able to co-exist with legacy traffic,
and being able to achieve high throughput seems difficult. On one hand, it is
a necessity to increase the aggressiveness of the congestion control algorithm
in order to utilise spare bandwidth. On the other hand, this potentially
means starving less aggressive flows of network resources.
This also applies to flows of the same algorithm as it is unlikely that
two flows on the Internet sharing a bottleneck will also have the same endto-end
latency. Therefore, a question arises over how fair these algorithms
are in sharing bandwidth when competing flows have different end-to-end
latencies. The problem of this RTT unfairness stems from the fact that low
latency flows are more responsive, and hence more aggressive, than flows on
longer latency links. Therefore, it is possible that the flows on a low latency
path completely starve the less responsive flow of throughput.
H-TCP implements a ‘RTT scaling’ function in order to maintain fairness
between competing flows of different RTT’s. This is implemented by altering
the value of its increase parameter α such that it is scaled by the ratio of the
experienced minimum latency and the average latency for a flow [LS04b].
It is also possible to help impose fairness between flows using AQM (See
Section 5.3.2). Using RED, for example, would drop a higher proportion of
packets from a more intensive flow; and therefore reduce its throughput and
equalise fairness [Has89]. This also has the beneficial effect of preventing
lock-out [FJ92] by aggressive flows that prevent less aggressive flows from
getting any throughput.