Alcatel-Lucent 7950, Quality Of Service Manual

Page: 47 / 612

QoS Policies
7950 XRS Quality of Service Guide Page 47
RED Slopes
Operation and Configuration
Each buffer pool supports a high-priority RED slope, a non-TCP RED slope, and a low-priority
RED slope. The high-priority RED slope manages access to the shared portion of the buffer pool
for high-priority or in-profile packets. The low-priority RED slope manages access to the shared
portion of the buffer pool for low-priority or out-of-profile packets.
For access buffer pools, the percentage of the buffers that are to be reserved for CBS buffers is
configured by the user software (cannot be changed by user). This setting indirectly assigns the
amount of shared buffers on the pool. This is an important function that controls the ultimate
average and total shared buffer utilization value calculation used for RED slope operation. The
CBS setting can be used to dynamically maintain the buffer space on which the RED slopes
operate.
For network buffer pools, the CBS setting does not exist; instead, the configured CBS values for
each network forwarding class queue inversely defines the shared buffer size. If the total CBS for
each queue equals or exceeds 100% of the buffer pool size, the shared buffer size is equal to 0
(zero) and a queue cannot exceed its CBS.
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.
The following is a simplified overview of how a RED slope determines shared buffer availability
on a packet basis:
1. The RED function keeps track of shared buffer utilization and shared buffer average utiliza-
tion.
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 deter-
mine 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.