3Com 10014299 manual Congestion Avoidance

698CHAPTER 50: CONGESTION AVOIDANCE

and it causes the sudden increase and decrease of the network traffic, and the line traffic always fluctuates between the states of few or none and full.

RED and WRED RED and WRED can avoid global synchronization of TCP by dropping packets randomly. When the packets of a TCP connection are dropped, and transmission slows down, other TCP connections can still send packets at high rates, thus improving the utilization of the bandwidth.

RED and WRED avoids the TCP global synchronization phenomenon through the random drop packets--when the packet of a TCP connection is dropped and the transmission speed is reduced, other TCP connections still have the higher transmission speeds. Thus, it is always the case that some TCP connection performs the faster transmission, increasing the use ratio of the line bandwidth.

Both RED and WRED compare between the queue length, and minimum and maximum thresholds, to perform the drop (this is to set the absolute length of the queue). It will cause the unfair treatment on the burst data flow and be disadvantageous for the transmission of the data flow. Therefore, when comparing the minimum and maximum thresholds, and when dropping, the average lengths of the queue are adopted (this is to set the relative value of the comparison between the queue threshold and the average length). The average length of the queue is the result of the low pass filtering of the queue length, it reflects the variation trend of the queue, and is not sensitive to the burst change of the queue length, so as to avoid the unfair treatment on the burst data flows.

The relationship between WRED and queue mechanism is shown in Figure 223

Figure 223 Schematic diagram of the relationship between WRED and queue mechanism

	WRED drop	queue1 weight1
incoming packets		outgoing packets

		queue2 weight2
			interface
-¡	-¡	¡- ¡-
classify		queueN-1 weightN-1	transmit
		scheduler	queue
		queueN weightN
Discarded
packets

In the RED class algorithm, a pair of minimum threshold and maximum threshold is set for each queue, and the following specification is set:

■When the length of the queue is less than the minimum threshold, no packet is dropped.

■When the length of the queue is larger than the maximum threshold, all incoming packets are dropped.