Quality of Service Guide QoS and QoS Policies
Edition: 01 3HE 11014 AAAC TQZZA 153
3.7.7 Slope Policies (WRED and RED)
As part of 7705 SAR queue management, policies for WRED and/or RED queue
management (also known as congestion management or buffer management) to
manage the queue depths can be enabled at both access and network ports and
associated with both ingress and egress queues. WRED policies can also be
enabled on bridged domain (ring) ports.
Without WRED and RED, once a queue reaches its maximum fill size, the queue
discards any new packets arriving at the queue (tail drop).
WRED and RED policies prevent a queue from reaching its maximum size by starting
random discards once the queue reaches a user-configured threshold value. This
avoids the impact of discarding all the new incoming packets. By starting random
discards at this threshold, customer devices at an end-system may be adjusted to
the available bandwidth.
As an example, TCP has built-in mechanisms to adjust for packet drops. TCP-based
flows lower the transmission rate when some of the packets fail to reach the far end.
This mode of operation provides a much better way of dealing with congestion than
dropping all the packets after the whole queue space is depleted.
The WRED and RED curve algorithms are based on two user-configurable
thresholds (minThreshold and maxThreshold) and a discard probability factor
(maxDProbability) (see Figure 24). The minThreshold (minT) indicates the level
when where discards start and the discard probability is zero. The maxThreshold
(maxT) is indicates the level where the discard probability reaches its maximum
value. Beyond this the maxT level, all newly arriving packets are discarded. The
steepness of the slope between minT and maxT is derived from the maxDProbability
(maxDP). Thus, the maxDP indicates the random discard probability at the maxT
level.
The main difference between WRED and RED is that with WRED, there can be more
than one curve managing the fill rate of the same queue.
WRED slope curves can run against high-priority and low-priority traffic separately
for ingress and egress queues. This allows the flexibility to treat low-priority and
high-priority traffic differently. WRED slope policies are used to configure the minT,
maxT and maxDP values, instead of configuring these thresholds against every
queue. It is the slope policies that are then applied to individual queues. Thus, WRED
slope policies affect how and when the high-priority and low-priority traffic is
discarded within the same queue.
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