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Configuring congestion avoidance
Overview
Avoiding congestion before it occurs is a proactive approach to improving network performance. As
a flow control mechanism, congestion avoidance actively monitors network resources (such as
queues and memory buffers), and drops packets when congestion is expected to occur or
deteriorate.
Compared with end-to-end flow control, this flow control mechanism controls the load of more flows
in a device. 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.
Tail drop
Congestion management techniques drop all packets arriving at a full queue. This tail drop
mechanism 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 rates of some TCP sessions slow down after their packets are dropped, other TCP sessions
remain at high sending rates. Link bandwidth is efficiently used because TCP sessions at high
sending rates always exist.
The RED or WRED algorithm sets an upper threshold and lower threshold for each queue, and
processes the packets in a queue as follows:
•
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.
WRED uses differentiated drop policies for different IP precedence values. Packets with a lower IP
precedence are more likely to be dropped.
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 bursty traffic can be fairly treated. The average queue size is calculated using the
following formula: average queue size = previous average queue size × (1-2
-n
) + current queue size
× 2
-n
, where n can be configured with the
qos wred weighting-constant
command.
With WFQ queuing used, you can set the exponent for average queue size calculation, upper
threshold, lower threshold, and drop probability for packets with different precedence values to
provide differentiated drop policies.
With FIFO queuing, PQ, or CQ used, you can set the exponent for average queue size calculation,
upper threshold, lower threshold, and drop probability for each queue to provide differentiated drop
policies for different classes of packets.
