Alcatel-Lucent 7450 Quality Of Service Manual page 60

QoS Policies
When a queue depth exceeds the queue's CBS, packets received on that queue must contend
with other queues exceeding their CBS for shared buffers. To resolve this contention, the
buffer pool uses two RED slopes to determine buffer availability on a packet by packet basis.
A packet that was either classified as high priority or considered in-profile is handled by the
high-priority RED slope. This slope should be configured with RED parameters that
prioritize buffer availability over packets associated with the low-priority RED slope. Packets
that had been classified as low priority or out-of-profile are handled by this low-priority RED
slope. At egress, the additional exceed-slope should be configured with RED parameters that
prioritize the high-priority and low-priority traffic above the exceed-profile traffic.
The following is a simplified overview of how a RED slope determines shared buffer
availability on a packet basis:
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1. The RED function keeps track of shared buffer utilization and shared buffer average
utilization.
2. At initialization, the utilization is 0 (zero) and the average utilization is 0 (zero).
3. When each packet is received, the current average utilization is plotted on the slope
to determine the packet's discard probability.
4. A random number is generated associated with the packet and is compared to the
discard probability.
5. The lower the discard probability, the lower the chances are that the random number
is within the discard range.
6. If the random number is within the range, the packet is discarded which results in no
change to the utilization or average utilization of the shared buffers.
7. A packet is discarded if the utilization variable is equal to the shared buffer size or if
the utilized CBS (actually in use by queues, not just defined by the CBS) is
oversubscribed and has stolen buffers from the shared size, lowering the effective
shared buffer size equal to the shared buffer utilization size.
8. If the packet is queued, a new shared buffer average utilization is calculated using the
time-average-factor (TAF) for the buffer pool. The TAF describes the weighting
between the previous shared buffer average utilization result and the new shared
buffer utilization in determining the new shared buffer average utilization. (See
Tuning the Shared Buffer Utilization
9. The new shared buffer average utilization is used as the shared buffer average
utilization next time a packet's probability is plotted on the RED slope.
10. When a packet is removed from a queue (if the buffers returned to the buffer pool are
from the shared buffers), the shared buffer utilization is reduced by the amount of
buffers returned. If the buffers are from the CBS portion of the queue, the returned
buffers do not result in a change in the shared buffer utilization.
Calculation.)
Quality of Service Guide