H3C Technologies H3C SR8800 - Manual (Page 64)

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Configuring congestion avoidance
Congestion avoidance overview
Avoiding congestion before it occurs is a proactive approach to improving network performance. As a
flow control mechanism, congestion avoidance actively drops packets when congestion is expected to
occur or deteriorate by monitoring the utilization of network resources (such as queues or memory buffers)
to alleviate the load on the network.
Compared with end-to-end flow control, this flow control mechanism controls the load of more flows in a
router. When dropping packets from a source end, it cooperates with the flow control mechanism (such
as TCP flow control) at the source end to regulate the network traffic size. The combination of the local
packet drop policy and the source-end flow control mechanism helps maximize throughput and network
use efficiency and minimize packet loss and delay.
Traditional packet drop policy
Tail drop is the traditional approach to congestion avoidance. In this approach, when the size of a queue
reaches the maximum threshold, all the subsequent packets are dropped.
This results in global TCP synchronization. If packets from multiple TCP connections are dropped, these
TCP connections go into the state of congestion avoidance and slow start to reduce traffic, but traffic
peak occurs later. Consequently, the network traffic jitters all the time.
RED and WRED
You can use random early detection (RED) or weighted random early detection (WRED) to avoid global
TCP synchronization.
Both RED and WRED avoid global TCP synchronization by randomly dropping packets. When the
sending rate of a TCP session slows down after its packets are dropped, the other TCP sessions remain
in high packet sending rates. Because always some TCP sessions in high sending rates exist, link
bandwidth is utilized more efficiently.
The RED or WRED algorithm sets an upper threshold and lower threshold for each queue, and processes
the packets in a queue following these rules:
• When the queue size is shorter than the lower threshold, no packet is dropped.
• When the queue size reaches the upper threshold, all subsequent packets are dropped.
• When the queue size is between the lower threshold and the upper threshold, the received packets
are dropped at random. The drop probability in a queue increases along with the queue size under
the maximum drop probability.
Different from RED, WRED determines differentiated drop policies for packets with different IP
precedence values. Packets with a lower IP precedence are more likely to be dropped.
Average queue size
If the current queue size is compared with the upper threshold and lower threshold to determine the drop
policy, bursty traffic is not fairly treated. To solve this problem, WRED compares the average queue size
with the upper threshold and lower threshold to determine the drop probability.
The average queue size reflects the queue size change trend but is not sensitive to bursty queue size
changes, and thus bursty traffic can be fairly treated. The average queue size is calculated using the