Behavior of Uncoordinated TCP Flows

Figure 7: TCP flows competing for bandwidth during congestion.

To better see the problem addressed by CP, we first examine how several TCP connections behave without coordination. In Figure 7, we see the throughput plot of three TCP connections as network congestion occurs between time 8.0 and 13.0 seconds. Flow 0 belongs to an application process with higher bandwidth requirements than processes associated with flows 1 and 2. This can be seen clearly at the right and left edges of the plot when flow 0 takes its full share of the bandwidth under congestion-free circumstances.

We note the following observations:

• During the congestion interval, all three flows compete with one another and receive a roughly similar portion of the available bandwidth.
• The flows continue to compete in a similar fashion during the period directly afterward (time 13.0 through 22.0) as each struggles to send accumulated data and regain its requisite level of bandwidth.
• The bandwidth used by each flow is characterized by jagged edges, often criss-crossing one another. This makes sense since each flow operates independently, searching the bandwidth space by repeatedly ramping up and backing off.