An Investigation into Transport Protocols and Data Transport ...

4.3. Congestion Control 59
probe for extra bandwidth by increasing its sending rate slowly, falling back
to a ‘safe’ sending rate should loss or excessive delay of an ack (an implicit
indication congestion) occur.
The implementation of such an algorithm is given below:
ACK : cwnd ← cwnd + 1
cwnd
(4.4)
LOSS : cwnd ← cwnd − 1 × cwnd (4.5)
2
Upon receipt of an acknowledgment, when in congestion avoidance, the
sender increases the cwnd by
1
cwnd .
Therefore, after about cwnd of acks,
the sender will be able to send one more packet due to this increase. This
mechanism allows TCP to ‘probe’ the network to determine if it has more
resources; if it does, then it will make use of the extra bandwidth by continuing
to increase cwnd; if not, after approximately RT T the sender should
either be able to determine packet loss from dupacks sent by the receiver or
through a RT O soft timer expiration (See Section 4.5.1).
Should a loss be detected, then the congestion avoidance algorithm of
TCP reduces cwnd by half 3 and hence reducing the number of packets inflight
from the connection. This will reduce the resources used by this TCP
connection and helps to prevent congestion collapse from occurring.
The value of cwnd is never allowed to be less than 1 packet.
It must be noted that the requirement to prevent congestion collapse is
that all end-to-end connections should have the same form of reactive congestion
control behaviour. Therefore, the requirement of preventing congestion
3 [APS99, Pos81b] actually specify that cwnd ← F lightSize/2, where F lightSize is
the amount of data sent, but yet unacknowledged. For simplicity, it is assumed that the
F lightSize is the same as the cwnd. This assumption is valid only when TCPs ack clock
is not broken.