An Investigation into Transport Protocols and Data Transport ...

5.3. Network Aid in Congestion Detection 100
1
Drop Probability
max p
0
min th
max th
Average Queue Length
Figure 5.8: Random Early Detection Parameters.
net [BCC98] to be accommodated without any bias. The above parameters
are combined with maxp which sets the maximum probability that a packet is
to be dropped at the router. It is defined as a function of the average queue
size and is set to a linear dropping probability from 0 to maxp,whereby arriving
packets are marked depending on q. Should q be less than minth
upon an arriving packet, the packet will be processed normally. However,
if q is greater than maxth, then the packet will be dropped. Otherwise, the
incoming packet will be dropped according to the probability related to the
average queue occupancy. This is shown in Figure 5.8.
Therefore, the router selectively drops packets from the queue based on
the various parameters and occupancy of the queue. RED is able to achieve
fairness between flows as higher throughput connections will have a greater
occupancy in the queue, and hence a greater chance of packet drops.
The use of RED can also reduce the possibility of TCP synchronisation
whereby all flows experience drops at the same time and then simultaneously
increases their rates, only to cyclically have all flows experience packet loss.
The occurrence of synchronisation results in poor throughput and low link
utilisation [ASA00].