An Investigation into Transport Protocols and Data Transport ...

7.1. Metrics 137
various New-TCP algorithms should also be investigated.
7.1.2 Stability
In order to gauge the stability of a TCP flow, it must understood that TCP
flows are inherently unstable as they strive to always probe for extra bandwidth
and adapt to network conditions.
However, under static environments, the characteristics of the long term
stability of a TCP flow in steady state can be determined. Define the following
simple metric for the stability, S i , of a flow i for a period s = 1, ..., t:
S i := 1¯x i
√ √√√
1
n − 1
t∑
(x i (s) − ¯x i ) 2 (7.4)
s=1
The stability, S, of n competing flows is therefore:
S := 1 n
n∑
S i (7.5)
i=1
S as defined is the standard deviation of the average goodput of n flows,
each normalised by its mean goodput, and can be used as an indication
of goodput predictability of n flows. It is therefore preferable to have high
average goodput, ¯x i , and low standard deviation, S i , and hence low S.
7.1.3 Convergence Time
On long time-scales, such as with bulk transfer, the performance of TCP
is dependent on the responsiveness of TCP flows to changes in the network
state. In order to take advantage of increases in network capacity or to reduce
sending rates to enable fair share of network traffic, it is important that the