An Investigation into Transport Protocols and Data Transport ...

4.6. Summary 76
into slow start. However, this will result in the breaking of the ack clock
which is required for stable TCP connections and therefore would take a
few RTTs to get back into congestion avoidance. Therefore, TCP Reno was
devised to be able to differentiate moderate congestion and react by only
halving cwnd. This is called AIMD in reference to increase of cwnd by one
segment per RTT when probing and the halving of cwnd upon loss.
However, as connection bandwidths and end-to-end latencies of the Internet
increase, cwnd has to increase to retain enough state to enable the sliding
window mechanism required for reliable delivery of data. As this occurs, the
chance of multiple losses of segments in a single window increases.
Unfortunately, TCP Reno and TCP Tahoe do not handle such situations
well and each will successively call its corresponding loss detection mechanism.
The effect of this is the stalling of the TCP connection as it attempts
to retransmit what it thinks is lost in the network.
Two primary proposals were developed to facilitate the recovery of TCP
under multiple packet losses per window. Of the two, Selective Acknowledgments
(SACKs) allow the TCP sender to recover from the losses in the
order of just one RTT. However, it requires that both the sender and receiver
specially implement SACKs in order to operate. TCP NewReno, however,
only requires a sender side modification, but it will take an amount of time
related to the number of lost packets in order to recover. Otherwise, it will
require that a RT O timeout occur at the sender.
Overall, it can be seen that many changes have been made over time in
order to improve TCP’s behaviour with respect to congestion. In the next
Chapter, more recent modifications are considered with the aim of improving
performance in high-speed networks.