An Investigation into Transport Protocols and Data Transport ...

6.1. Survey of New-TCP Algorithms 118
validity of Equation 4.1, [LS04a] states that this is successful when the maximum
queue size is equal to the maximum RTT measured and the minimum
(zero) queue size occurs when the flow experiences the minimum RTT.
The bandwidth delay products before and after congestion are:
Before congestion: R(k) − = cwnd
RT T max
(6.12)
After congestion: R(k) + = β × cwnd
RT T min
(6.13)
A simple way of ensuring full utilisation of the network by this single
H-TCP flow would be to equate R(k) − and R(k) + . As such, this constraint
can only be achieved by enforcing that:
β(k) ≥ RT T min
RT T max
(6.14)
where β(k) is the value of β at the k’th congestion epoch. The calculation
prevents the queue size from emptying upon loss/congestion detection by the
TCP flow and therefore maintain maximum throughput for the TCP flow.
However, high values of β would cause slow convergence between flows.
Therefore, a parameter β max specifies the largest value of β such that 0.5 ≤
β ≤ β max to enable compatibility with Standard TCP whilst being able to
keep high utilisation of the network path.
To further aid the fair sharing of network bandwidth, H-TCP specifies
that should the measured throughput of a flow at the next congestion epoch
be greater than the parameter ∆ B , then β should default back to 0.5.
∆ B and β max have suggested values of 0.2 and 0.8 respectively.
Figure 6.3 shows the H-TCP algorithm in action. It shows the slow ‘TCPfriendly’
α increase immediately after loss, and then the switch into the high