Alcatel-Lucent 7210 SAS M OS Quality Of Service Manual page 60

QoS Overview
RED Slopes In Network and Access-uplink Mode
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. The non-TCP slope manages
access to the shared portion of the buffer pool for non-TCP packets (such as MPLS packets
received on network ingress).
By default, the high-priority, low-priority , and non-TCP RED slopes are disabled.
In the 7210 SAS M, WRED is supported. WRED uses average queue lengths, queue thresholds
provisioned, and drop probablility to calculate the packet's eligibility to be enqueued. The
committed portion of the buffer pool is exclusively used by a queue to enqueue traffic within
committed rate.
In the 7210 SAS M, WRED is supported. WRED uses average queue lengths, queue thresholds
provisioned, and drop probablility to calculate the packet's eligibility to be enqueued. The
committed portion of the buffer pool is exclusively used by a queue to enqueue traffic within
committed rate.
For the queues within a buffer pool, packets are either queued using committed burst size (CBS)
buffers or shared buffers. The CBS buffers are simply buffer memory that has been allocated to the
queue while the queue depth is at or below its CBS threshold. The amount of CBS assigned to all
queues is dependent upon the number of queues created, the setting of the default CBS as defined
in the policy, and any CBS values set per queue within a QoS policy. However, from a functional
perspective, the buffer pool does not keep track of the total of the CBS assigned to queues serviced
by the pool. CBS subscription on the pool is an administrative function that must be monitored by
the queue provisioner.
For access and network buffer pools, the percentage of the buffers that are to be reserved for CBS
buffers is configured by the usersoftware (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.
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
Page 60 7210 SAS M OS Quality of Service Guide