An Investigation into Transport Protocols and Data Transport ...

4.3. Congestion Control 57
excessive transmission of data which will be lost at the receiver.
4.3 Congestion Control
The implications of congestion collapse led to the first proposal for a transport
protocol which will help prevent congestion collapse [Jac88].
TCP utilises another window to determine the dynamic flow control required
of the heterogeneous networks and hence an estimation of the available
capacity in the network.
This congestion window (cwnd) is defined as the number of allowed segments
(or bytes) sent but not yet acknowledged (packets or bytes in transit).
As such, it is analogous to controlling the rate at which packets are allowed
into the network along the bottleneck of a given network path.
Whilst congestion control was not originally part of the original TCP
specification [Pos81b], it has become part of the standard TCP specification
since the introduction of TCP Tahoe (See Section 4.5.1).
The two specific algorithms that control the evolution of cwnd are defined
as follows:
4.3.1 Slow Start
The purpose of the slow start algorithm [Jac88] is to get the ack clock running
and to determine the available capacity in the network. It is a mechanism
to quickly find an operating point at which TCP can work. Therefore it is
invoked at the start of every TCP connection after the initial TCP handshake.
Starting with a conservative cwnd of one segments 2 , the TCP connection
2 Recent suggestions state that TCP may set its initial cwnd to two segments in light
of the increase in link speeds [AFP98].