Overview Of Congestion Avoidance And Congestion Management; Congestion Avoidance - Huawei Quidway S7700 Configuration Manual - Qos

Quidway S7700 Smart Routing Switch
Configuration Guide - QoS
3.1 Overview of Congestion Avoidance and Congestion
Management
This section describes the basic concepts of congestion avoidance and congestion management.

3.1.1 Congestion Avoidance

Congestion avoidance is a flow control mechanism. A system configured with congestion
avoidance monitors network resource usage such as queues and memory buffers. When
congestion occurs or aggravates, the system discards packets.
Congestion avoidance mechanisms include tail drop, Random Early Detection (RED), and
Weighted Random Early Detection (WRED). The S7700 performs congestion avoidance based
on WRED.
Congestion avoidance mechanisms include tail drop, Random Early Detection (RED), and
Weighted Random Early Detection (WRED). The S7700 performs congestion avoidance based
on WRED.
Tail Drop
The traditional packet drop policy uses tail drop. The tail drop policy processes all the packets
uniformly, regardless of their class of service (CoS). When congestion occurs, packets at the
end of a queue are discarded until the congestion problem is solved.
The tail drop policy causes global TCP synchronization. When packets from multiple TCP
connections are discarded in a queue, these TCP connections enter the congestion avoidance
and slow start state simultaneously, which is called global TCP synchronization. This causes
traffic reduction and will lead to traffic peak. Such a process repeats, which causes the volume
of network traffic to change from heavy to light and affects the link usage.
RED
The RED mechanism randomly discards packets so that the S7700 reduces the transmission
speeds of multiple TCP connections at different periods of time. This prevents global TCP
synchronization.
RED sets the upper threshold and lower threshold for the length of each queue and processes
packets as follows:
l
l
l
WRED
The WRED mechanism also prevents global TCP synchronization by randomly discarding
packets. The random number generated by WRED is based on the priority. WRED distinguishes
Issue 01 (2011-07-15)
When the queue length is shorter than the lower threshold, no packet is discarded.
When the queue length exceeds the upper threshold, all the received packets are discarded.
When the queue length ranges from the lower threshold to the upper threshold, incoming
packets are dropped randomly. The system sets a random number for each incoming packet,
and compares it with the packet drop probability of the current queue. If the random number
is larger than the drop probability, the packet is dropped. The longer the queue, the higher
the drop probability.
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3 Congestion Avoidance and Congestion Management
Configuration
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