Configuring Congestion Avoidance; Overview; Tail Drop; Red And Wred - HP FlexFabric 5700 series Configuration Manual

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).
Drops packets when congestion is expected to occur or deteriorate.
•
When dropping packets from a source end, congestion avoidance cooperates with the flow control
mechanism 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 limit and lower limit for each queue, and processes the
packets in a queue as follows:
When the queue size is shorter than the lower limit, no packet is dropped.
•
When the queue size reaches the upper limit, all subsequent packets are dropped.
•
When the queue size is between the lower limit and the upper limit, the received packets are
•
dropped based on the user-configured drop probability.
If the current queue size is compared with the upper limit and lower limit to determine the drop policy,
burst traffic is not fairly treated. To solve this problem, WRED compares the average queue size with the
upper limit and lower limit to determine the drop probability.
The average queue size reflects the queue size change trend but is not sensitive to burst queue size
changes, and burst traffic can be fairly treated.
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