An Investigation into Transport Protocols and Data Transport ...

6.1. Survey of New-TCP Algorithms 117
∆ B is a design parameter.
By increasing the value of α(∆) such that it slowly increases as a function
of time, H-TCP is able to switch in to an aggressive mode which under heavy
congestion is fair with Standard TCP flows, but is able to utilise available
bandwidth quickly. This transition into H-TCP’s high-speed mode is effected
a certain time ∆ L after the last congestion event such that the Standard
TCP update algorithm is used while ∆ ≤ ∆ L . The suggested value of ∆ L is
1 second.
A quadratic increase function ᾱ is suggested in [LS04a], such that:
ᾱ(∆) = 1 + 10(∆ − ∆ L ) + 0.25(∆ − ∆ L ) 2 (6.11)
Therefore, the growth of cwnd is such that H-TCP operates similarly to
Standard TCP in conventional networks where the period between congestion
events is small. Under high speed long distance networks, H-TCP switches
to a polynomial increase in cwnd to take advantage of available capacity
where Standard TCP is incapable of high performance. As the mode switch
is based on time rather than on the current value of cwnd, sources already in
a high speed mode do not gain a long term advantage over new flows starting
up and therefore guarantee fairness.
The calculated increase parameter ᾱ(∆) is multiplied by the minimum
experienced latency, T min , of the flow in order to facilitate fairness between
competing flows with different latencies [LS04b]. This feature is called RTT
Scaling.
In order to utilise the capacity effectively, H-TCP defines a back-off factor
β(k) derived from the desire to maintain a queue occupancy after congestion
such that the queue is not empty for too long. Under the assumption of the