An Investigation into Transport Protocols and Data Transport ...

4.3. Congestion Control 58
sends out as many segments as it can ensuring that the number of packets
in flight does not exceed cwnd. After approximately one RT T , the TCP
sender should receive an acknowledgment informing receipt of the data from
the receiver. For every non-duplicate ack received,
cwnd ← cwnd + 1 (4.3)
During slow start, this algorithm is equivalent to an exponential increase
in the cwnd size as it effectively doubles every RT T as one more packet is sent
out for every non-dupack received. This increase of cwnd is quite aggressive.
However, it enables the TCP connection to quickly reach a stable point for
congestion avoidance (See Section 4.3.2) to take place.
Slow start ends when a segment loss is detected or when the congestion
window reaches the slow start threshold, ssthresh (See Section 4.3.3).
4.3.2 Congestion Avoidance
Under the packet conversation principle, a TCP connection at equilibrium
will send new packets with the rate at which it receives acks. Under ideal
circumstances, this rate should reflect the rate packets are served by the
slowest link of the entire path, also known as the ‘bottleneck link’.
When a sender is operating at equilibrium, i.e. in steady state, it should
be able to adapt to changes in the condition of the path. More importantly, if
more connections (say from other users) start to use part of the path, any one
TCP flow should ‘back-off’ its transmission rate and be fair to other flows.
Similarly, should a connection terminate then the sender should attempt
to utilise the extra bandwidth. This is exactly what congestion avoidance
[Jac88] does, and is implemented through a simple algorithm such that it will