An Investigation into Transport Protocols and Data Transport ...

6.1. Survey of New-TCP Algorithms 123
differ from those with traditional congestion control.
FAST TCP [JWL04] is an adaptation of the Vegas algorithm to enable
high speed data transport. Its primary difference is that it utilises RTT estimation
as its primary feedback from the network. The assumption used
is that the queuing delays conveys the ‘right congestion information’ to
maximise the network utilisation and that it scales with network capacity
[JDXW03, CL03, PWLD03]. Their argument for a delay-based congestion
avoidance algorithm is that at high speeds, loss based algorithms will necessarily
change their windows to maintain stable state. Under such circumstances,
the evolution of the window through time is defined in Equation 5.9.
Using only binary packet loss as an indication of congestion is insufficient
to maintain equality between the utility function u i (t), and the end-to-end
congestion feedback p i (t).
The design of FAST incorporates the utility function such that the window
adjustment is small when close to equilibrium, and largely otherwise
independent of where the equilibrium is.
Therefore, the slow convergence
properties due to the necessary oscillation of loss based algorithms are eliminated.
On a packet level implementation, the evolution of cwnd (w) is restricted
such that the calculated optimal window is defined by:
ACK: w ← min{2w, (1 − γ)w + γ
( )
Tmin
¯T w + α(w, T q) } (6.18)
LOSS: w ← w 2
(6.19)
where T q = ¯T − T min , ¯T is the exponentially averaged instantaneous RTT
of the TCP connection and T min is the minimum experienced latency. To