HP 6125G HP 6125G & 6125G/XG Blade Switches ACL and QoS Configuration - Page 55

Configuring congestion avoidance, Overview, Tail drop, RED and WRED

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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 that are 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 the user-configured drop probability. Introduction to WRED configuration On the HP Blade 6125 Switch Series, WRED is implemented with WRED tables. WRED tables are created globally in system view and then applied to interfaces. Before configuring WRED, determine the following parameters: • Lower threshold and upper threshold-When the average queue length is below the lower threshold, no packet is dropped. When the average queue length exceeds the lower threshold and 49

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49
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 that are 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 the user-configured drop probability.
Introduction to WRED configuration
On the HP Blade 6125 Switch Series, WRED is implemented with WRED tables. WRED tables are
created globally in system view and then applied to interfaces.
Before configuring WRED, determine the following parameters:
•
Lower threshold and upper threshold
—When the average queue length is below the lower
threshold, no packet is dropped. When the average queue length exceeds the lower threshold and