Nokia 7705 - 3.7.7.3 Payload-based WRED

QoS and QoS Policies
156
Quality of Service Guide
3HE 11014 AAAC TQZZA Edition: 01
3.7.7.3 Payload-based WRED
The third-generation Ethernet adapter cards and platforms use payload-based
WRED rather than buffer-based WRED (see WRED MinThreshold and
MaxThreshold Computation). Payload-based WRED does not count the unused
overhead space (empty space in the buffer) when making discard decisions,
whereas buffer-based WRED counts the unused overhead space. Payload-based
WRED is also referred to as byte-based WRED.
When a queue on an adapter card that uses payload-based WRED reaches its
maximum fill (that is, the total byte count exceeds the configured maximum
threshold), tail drop begins and operates in the same way as it does on any other
adapter card or platform.
With payload-based WRED, the discard decision is based on the number of bytes in
the queue instead of the number of buffers in the queue. For example, to accumulate
512 bytes of payload in a queue will take four buffers if the frame size is 128 bytes,
but will take one buffer if the frame size is 512 bytes or more. Basing discards on
bytes rather than buffers improves the efficient use of queues. In either case, byte-
or buffer-based WRED, random discards begin at the minimum threshold (minT)
point.
For example, assume a queue has MBS set to 512 kilobytes (converts to 1000
buffers), minT (start-avg) is set to 10% (100 buffers), and maxT (max-avg) is set to
80% (800 buffers). Table 30 shows when discards and tail drop start when
payload-based WRED is used.
For tail drop, if the high-priority-only threshold is set to 10%:
• when any frame size is greater than or equal to 512 bytes, tail drop starts after
900 buffers are in use for low-priority traffic
Table 30 Payload-Based WRED: Discard and Tail Drop Start
Frame Size Discards Start Tail Drop Starts
128 bytes 400 buffers in the queue (100 x 4) 3200 buffers in the queue (800 x 4)
512 bytes 100 buffers in the queue 800 buffers in the queue
1024 bytes 100 buffers in the queue 800 buffers in the queue