Figure 3: Red Slope Characteristics - Alcatel-Lucent 7950 Quality Of Service Manual

QoS Policies
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 uti-
lized 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 pre-
vious shared buffer average utilization result and the new shared buffer utilization in deter-
mining the new shared buffer average utilization. (See
Calculation on page
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.
A RED slope itself is a graph with an X (horizontal) and Y (vertical) axis. The X-axis plots the
percentage of shared buffer average utilization, going from 0 to 100 percent. The Y-axis plots the
probability of packet discard marked as 0 to 1. The actual slope can be defined as four sections in
(X, Y) points
1. Section A is (0, 0) to (start-avg, 0). This is the part of the slope that the packet discard value
is always zero, preventing the RED function from discarding packets when the shared buffer
average utilization falls between 0 and start-avg.
2. Section B is (start-avg, 0) to (max-avg, max-prob). This part of the slope describes a linear
slope where packet discard probability increases from zero to max-prob.
Page 48
50.)
1
.75
Probability
.50
Start-Avg
.25
0
0%

Figure 3: RED Slope Characteristics

(Figure 3):
D
C
Max-Avg
B
Max-Prob
A
25% 50% 75% 100%
Average Utilization
OSSG020
7950 XRS Quality of Service Guide
Tuning the Shared Buffer Utilization