An Investigation into Transport Protocols and Data Transport ...

4.3. Congestion Control 60
collapse is a distributed one whereby all transport protocol control algorithms
should react upon indication of congestion by decreasing their sending rates.
The algorithm that governs Congestion Avoidance is referred to as Additive
Increase Multiplicative Decrease, (AIMD) [CJ89], due to the form of
Equations 4.4 and 4.5.
4.3.3 Slow Start Threshold
The general characteristics of the TCP algorithm are an initial, relatively fast
scan of the network capacity using Slow Start, followed by a cyclic adaptive
behaviour of Congestion Avoidance that reacts to congestion by reducing its
sending rate, and then slowly increasing the sending rate in an attempt to
stay within the area of maximal transfer efficiency.
In order to determine the balance between the aggressive algorithm of
Slow Start and the steady-state congestion probing algorithm of Congestion
Avoidance, another variable, ssthresh is introduced and is used as follows:
if cwnd < ssthresh
do slow start
else
do congestion avoidance
This slow start threshold, ssthresh, is typically set to a large initial value
and updated upon loss detection. In other words, after loss, the value of
ssthresh is set to half of the number of packets in-flight.
Through continuing updates of the value of ssthresh, the TCP connection
is able to recover to a stable operating region of congestion avoidance
quickly using slow-start when a RT O occurs and then continue its conges-