An Investigation into Transport Protocols and Data Transport ...

4.1. Overview 51
Throughput
Delay
Load
(a) Throughput
Load
(b) Delay
Figure 4.1: Congestion Collapse
covered by the sending hosts retransmitting them, but part of the bandwidth
will be wasted due to retransmissions.
At the turning point where the flow starts losing packets the amount of
useful work done by the network decreases dramatically and connectivity
diminishes completely; congestion collapse has occurred.
There are many types of congestion collapse [FF99]. Two of the most
common forms is congestion from the retransmission of packets already in
transit or that have been received (classical collapse), and that arising from
the transmission of packets that are subsequently dropped by the network
(congestion collapse from undelivered packets).
Originally devised to simply transport data reliably across networks [Pos81b],
TCP was adapted in 1988 by Van Jacobson [Jac88] to avoid network congestion
collapse and to provide network fairness between network users. Much
research, development and standardisation since its incarnation has led TCP
to be widely used in the Internet and it is considered as the de-facto standard
transport-layer protocol.
The achievements of TCP are most evident by the fact that the protocol
has changed little over the last two decades. With the advent of fibre optics