An Investigation into Transport Protocols and Data Transport ...

4.6. Summary 75
traffic for a particular connection. Depending on the network state determined
from the inference of packet loss through the heuristics of acks, TCP
adapts its cwnd. By increasing cwnd such that the rate at which new data
is put into the network is increased TCP is able to take advantage of extra
bandwidth available. And upon indications of network congestion, TCP is
able to quickly free up network resources.
However, in order to reach this ‘steady state’, TCP requires a mechanism
to quickly adapt to network conditions from which Additive Increase Multiplicative
Decrease (AIMD) can take place. This is implemented via a slow
start regime where the sending rate is doubled every RTT (as long as there
is no loss). In conjunction with a ssthresh variable, TCP is able to switch
between the aggressive slow start and congestion avoidance algorithms.
As TCP is a reliable protocol, TCP needs to deal with potential loss of
packets.
It also assumes that loss due to packet corruption is rare 9 TCP
infers the difference between the types of congestion by the arrival (or absence)
of acks from the receiver. Under heavy congestion, it is likely that no
data will get delivered, and therefore there will be no acks in response. To
detect packet loss because of this, TCP implements a RT O soft-timer that
will initiate the retransmission of the packet that it believes is lost. Under
moderate congestion, it is likely that only some segments will be lost, and
this is represented in the arrival of dupacks from the receiver.
Much of the developments of TCP in the past have been related to the way
in which it responds to packet loss detection. TCP Tahoe reacts the same to
both heavy and moderate congestion by reverting the TCP connection back
9 TCP performance suffers in environments where there exists high Bit Error Rates
(BER) (e.g. wireless/radio). However, the kind of BER considered in this dissertation for
the HEP community typically include high quality links optical interconnects which have
low raw BER of 10 −12 .