3com S7906E Instruccion De Instalación
6-1
6
Congestion Avoidance
When configuring congestion avoidance, go to these sections for information you are interested in:
z
z
z
z
Congestion Avoidance Overview
Serious congestion can overwhelm network resources, resulting in network outage. In an effort to
eliminate the risk of congestion before it occurs, congestion avoidance technology was developed.
Congestion avoidance is essentially a flow control mechanism. It actively monitors the usage of
network resources such as queues and memory buffer and drops packets when congestion occurs or
deteriorates to prevent the network from being overwhelmed.
Compared to end-to-end flow control, this flow control mechanism is of broader sense because it can
control the load of more flows in a device. When dropping packets from a source end, it can still
cooperate well with the flow control mechanism (such as TCP flow control) at the source end to better
adjust the network traffic to a reasonable load status. The combination of the packet drop policy of the
local device and the flow control mechanism at the source end can maximize throughput and utilization
rate of the network and minimize packet loss and delay.
Traditional packet drop policy
The traditional packet drop policy is tail drop. When the length of a queue reaches the maximum
threshold, all the subsequent packets are dropped.
Such a policy results in global TCP synchronization. That is, 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. Thus, while
the sending rates of some TCP sessions slow down after their packets are dropped, other TCP
sessions remain at high sending rates. As there are always TCP sessions at high sending rates, link
bandwidth is efficiently utilized.
The RED or WRED algorithm sets an upper threshold and lower threshold for each queue, and
processes the packets in a queue as follows:
z
When the queue size is shorter than the lower threshold, no packet is dropped;
z
When the queue size reaches the upper threshold, all subsequent packets are dropped;