H3C Technologies H3C SecPath F1000-E - Manual (Page 513)

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a device, resources are assigned for packets depending on their arrival order and load status of the
device. The best-effort service model uses FIFO queuing.
FIFO does not address congestion problems. If there is only one FIFO output/input queue on a port, you
can hardly ensure timely delivery of mission-critical or delay-sensitive traffic or smooth traffic jitter. The
situation gets worsened if malicious traffic is present to occupy bandwidth aggressively. To control
congestion and prioritize forwarding of critical traffic, you need to use other queue scheduling
mechanisms, where multiple queues can be configured. Within each queue, however, FIFO is still used.
By default, FIFO queuing is used on interfaces.
Weighted fair queuing
Figure 4 Weighted fair queuing (WFQ)
Before WFQ is introduced, you need to understand fair queuing (FQ). FQ is designed for fairly
allocating network resources to reduce delay and jitter of each traffic flow as possible. In an attempt to
balance the interests of all parties, FQ follows these principles:
• Different queues have fair dispatching opportunities for delay balancing among streams.
• Short packets and long packets are fairly scheduled: if there are long packets and short packets in
queues, statistically the short packets are scheduled preferentially to reduce the jitter between
packets on the whole.
Compared with FQ, WFQ takes weights into account when determining the queue scheduling order.
Statistically, WFQ gives high priority traffic more scheduling opportunities than low priority traffic. WFQ
can automatically classify traffic according to the “session” information of traffic (protocol type, TCP or
UDP source/destination port numbers, source/destination IP addresses, IP precedence bits in the ToS
field, and so on), and try to provide as many queues as possible so that each traffic flow can be put into
these queues to balance the delay of every traffic flow on a whole. When dequeuing packets, WFQ
assigns the outgoing interface bandwidth to each traffic flow by precedence. The higher precedence
value a traffic flow has, the more bandwidth it gets.
For example, assume that there are five flows in the current interface with precedence 0, 1, 2, 3, and 4
respectively. The total bandwidth quota is the sum of all the (precedence value + 1)s, that is, 1 + 2 + 3
+ 4 + 5 = 15.
The bandwidth percentage assigned to each flow is (precedence value of the flow + 1)/total bandwidth
quota. The bandwidth percentages for flows are 1/15, 2/15, 3/15, 4/15, and 5/15 respectively.