An Investigation into Transport Protocols and Data Transport ...

4.1. Overview 50
4.1 Overview
The Transmission Control Protocol was originally devised by Postel in 1981
[Pos81b]. Designed to enable inter-communication between disparate systems,
it is encapsulated above the stateless Internet Protocol [Pos81a].
IP routers connected on multiple network paths are shared resources
which may suffer from congestion when the aggregated input rate at any
networking node exceeds the output capacity. As there is no global signalling
mechanism to control the rate at which end-nodes send data into the
network, congestion can occur. Consequently, flows tend to experience varying
network conditions that affect the original traffic profile from the source.
This is known as Best-Effort servicing as each individual node tries its best
to forward packets without any service guarantees upon its performance.
Should all Internet users be using UDP as their primary transport protocol,
the aggregated input rate at any router would be greater than the
maximum output rate and overloading of Internet resources would take place
and potential Congestion Collapse [Nag84] would occur.
When congestion occurs, the amount of useful work done is diminished
to such a degree that the throughput declines and the network is termed
as undergoing congestion collapse. [FF99] and [MR91] outline the need for
mechanisms to prevent congestion collapse.
Congestion collapse is shown in Figure 4.1. It shows that as the sending
rate increases the useful work done increases linearly up to a point where
it reaches the network bandwidth. Past this point the throughput saturates
and the link pipes are filled and packets get queued at the routers. Similarly,
the delays experienced by the traffic increase due to the buildup of routers’
queues, and soon after the flows starts to lose packets. The loss can be re-