
Using DLSw Prioritization
114063 Rev. A 6-5
One goal of weighted dequeuing is to send smaller packets ahead of large packets,
without violating the bandwidth of each queue or depriving large packets. The
algorithm accomplishes this by putting smaller packets ahead of larger packets by
simultaneously considering how long the larger packets have been in the dequeue
list. A large packet accumulates credit (lowering its weighted score) as each
smaller packet gets ahead of it, and eventually the larger packet moves to the front
of the dequeue list.
The Dequeue List
Three factors determine a packet’s weighted score:
• Size of the packet
• Percent of bandwidth allocated to the packet’s queue
• Time spent in the dequeue list
The dequeuing algorithm calculates a packet’s dequeue weight using the
following formula:
Weighted Dequeuing Algorithm
Weighted dequeuing works as follows:
1. Each priority queue enters its first (oldest) entry on a dequeue list.
2. The dequeue list orders the packets according to a weighted score.
3. TCP requests DLSw packets.
4. The router sends the requested number of packets or bytes to TCP from the
top of the dequeue list, up to the configured queue limit.
5. TCP transmits the packets.
6. The sequence repeats at Step 1.
Figure 6-1 illustrates weighted dequeuing.
Weight =
Size of packet
Bandwidth %
- Time in dequeue list
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