An Investigation into Transport Protocols and Data Transport ...

7.1. Metrics 138
TCP flows are responsive to these network changes.
In dynamic environments where flows join or leave the system often, the
sending rate of each flow will adapt (i.e. lower) to enable sharing of bandwidth
for new incoming flows. Due to this dynamic, the efficiency and fairness
of the system may not always be optimal.
Define ε-convergence time following start-up of a new flow to be the time
before the short-term average throughput ˆx(t) i of the new flow is within a
factor ε of its long-term average value. Arbitrarily choose ε = 0.8 yielding
the 80% convergence time.
7.1.4 Fairness
As the Internet is a shared resource, congestion avoidance algorithms should
ensure that network users obtain a reasonable allocation of network resources
depending on network conditions and the state and condition of competing
users. This concern is embodied into the metric of fairness and depends upon
the state and conditions of the competing flows.
Define fairness as a ratio of goodputs experienced by competing flows. As
the dynamics of congestion control of competing flows with different RTTs
will result in different growth rates of cwnd, the effect of having users with
different RTTs upon the fairness of each flow should also be considered.
The dynamics of the Standard TCP congestion control algorithm can
be estimated from an analysis of cwnd geometry such that at any time k,
the cwnd value w i (k) is dependent upon the decrease fraction β i (k) and the
increase value α i for the congestion epoch k with round trip time T i (k), and