An Investigation into Transport Protocols and Data Transport ...

6.2. Discussion and Deployment Considerations of New-TCP Algorithms127
of 93.75% 2 , rather than 75% for Standard TCP with zero buffering along the
network path.
However, with an increase in the value of the decrease parameter to maximise
utilisation, comes the problem of interaction between similar flows. As
this decrease applies only every congestion event, a pair of flows with the
same end-to-end latency and sharing the same bottleneck will take a greater
number of congestion events before they equalise to a fair share of the network
capacity.
The matter in which the capacity is shared by the flows is broadly known
as fairness [CJ89, BG87, Kel97, LS04b]. Two generally used fairness criteria
are max-min fairness and proportional fairness.
The max-min fairness concept [BG87, Jaf81] emerged from fair queuing
models whereby routers utilise a separate queue per flow and implement the
round-robin model to serve the flows in turn. Therefore, under this model,
the flows that send less data receive higher priority than flows that send bulk
data. Conversely, proportional fairness favours the flows that have more
packets over those with fewer packets. Proportional fairness is useful for
designing pricing models for the Internet [Kel97].
[XHR04] points out that more aggressive TCP leads invariably to unfairness
between the flows, unless care is taken.
Should the competing flows be synchronised in terms of network congestion
events, then this time variant nature of converging to a fair share is
directly related to the value of β; if β is large, then many congestion events
are required before convergence; if β is small, then convergence is reached
quickly - but at the expense of reduced utilisation.
H-TCP directly considers this convergence to fairness by measuring the
2 Actually less due to the increased growth rate of α with respect to time.