Existing Egress Port Based Virtual Scheduling - Alcatel-Lucent 7950 Quality Of Service Manual

Frame-Based Accounting

Existing Egress Port Based Virtual Scheduling

The port based virtual scheduling mechanism takes the native packet based accounting results
from the queue and adds 20 bytes to each packet to derive the queue's frame based offered load.
The ratio between the frame based offered load and the packet based offered load is then used to
determine the effective frame based CIR and frame based PIR thresholds for the queue. Once the
port virtual scheduler computes the amount of bandwidth allowed to the queue (in a frame based
fashion), the bandwidth is converted back to a packet based value and used as the queue's
operational PIR. The queue's native packet based mechanisms continue to function, but the
maximum operational rate is governed by frame based decisions.
Queue Behavior Modifications for Frame Based Accounting
The frame based accounting feature extends this capability to allow the queue CIR and PIR
thresholds to be defined as frame based values as opposed to packet based values. The queue
continues to internally use its packet based mechanisms, but the provisioned frame based CIR and
PIR values are continuously revalued based on the ratio between the calculated frame based
offered load and actual packet based offered load. As a result, the queue's operational packet based
CIR and PIR are accurately modified during each iteration of the virtual scheduler to represent the
provisioned frame based CIR and PIR.
Virtual Scheduler Rate and Queue Rate Parameter Interpretation
Normally, a scheduler policy contains rates that indicate packet based accounting values. When
the children queues associated with the policy are operating in frame based accounting mode, the
parent schedulers must also be governed by frame based rates. Since either port based or service
based virtual scheduling is required for queue frame based operation, enabling frame based
operation is configured at either the scheduling policy or aggregate rate limit command level. All
queues associated with the policy or the aggregate rate limit command will inherit the frame based
accounting setting from the scheduling context.
When frame based accounting is enabled, the queues CIR and PIR settings are automatically
interpreted as frame based values. If a SAP ingress QoS policy is applied with a queue PIR set to
100Mbps on two different SAPs, one associated with a policy with frame based accounting
enabled and the other without frame based accounting enabled, the 100Mbps rate will be
interpreted differently for each queue. The frame based accounting queue will add 20 bytes to each
packet received by the queue and limit the rate based on the extra overhead. The packet based
accounting queue will not add the 20 bytes per packet and thus allow more packets through per
second.
Similarly, the rates defined in the scheduling policy with frame based accounting enabled will
automatically be interpreted as frame based rates.
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