Hsmda Queue Congestion And Buffer Utilization Controls - Alcatel-Lucent 7450 Quality Of Service Manual

HSMDA QoS Model
less, every packet has access to the remaining pool since the maximum size packet requires
less than 64 buffers. While this scheme allows for fair access to the buffer pool, some buffers
will not be used by the HSMDA. This is considered inconsequential due to the limited
number of buffer pools. The absolute worst case in buffer inefficiency is 12,096 buffers (63
buffers * 192 pools) out of 1 million buffers (about 1.2%).
When the pools are first initialized, each pool's buffer-available register is set to the number
of buffers available on the pool. When the HSMDA buffer pool policy managing a pool is
changed so that the number of buffers managed by the pool increases or decreases, the system
will increment or decrement the current value of the buffer-available register based on the
change in the pool size. The register supports a negative value for the case where the buffer
pool size is decreased and the buffer-available register is currently less than the size of the
decrease. If the buffer pool goes negative (or below 64 available buffers), no buffers are
allocated by the pool until buffers associated with the pool are returned to the free list and the
register increments to a value of 64 or higher.

HSMDA Queue Congestion and Buffer Utilization Controls

Each queue supports a 10 bit index into an HSMDA slope policy table. Each policy in the
table consists of two RED slopes (high priority and low priority) for the purpose of managing
queue congestion. Due to the large number of queues supported on the XMA or MDA, the
ability of a queue to effectively mange a weighted sliding window of queue utilization is not
practical. HSMDA RED slopes operate as on the instantaneous depth of the queue.
Each slope policy within the HSMDA consists of two slope definitions, each represented by
a 14 bit start slope value, a 14 bit end slope value and an 8 bit fixed point inverse slope value.
The fixed point value is represented as a 4 bit whole value (values from 0 to 15) and a 4 bit
fraction (from 0 to 0.9375 in 0.0625 increments).
In operation, a packet attempting to enter a queue triggers a check to see if the packet should
be allowed based on queue congestion conditions. The packet contains a congestion-priority
flag in the shim header telling the HSMDA whether to use the high or low slope. The slope
policy containing the slope is derived from the policy index in the queue configuration
parameters on the HSMDA. The XMA or MDA retrieves the current queue depth in buffers
and the slope's three configuration values (start buffer, end buffer and inverse slope value).
Logically, the following algorithm is used to determine whether the packet should be allowed
in the queue based on discard probability:
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• If the queue depth is greater than or equal to the slope's end-buffer value, the packet
is discarded.
• A random number in the range of 0 to 127 is generated.
Quality of Service Guide