H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 532)

46-3
Countermeasures
A simple solution for congestion is to increase network bandwidth. However, it cannot solve all the
problems that cause congestion.
A more effective solution is to provide differentiated services for different applications through traffic
control and resource allocation. In this way, resources can be used more properly. During resources
allocation and traffic control, the direct or indirect factors that might cause network congestion should be
controlled to reduce the probability of congestion. Once congestion occurs, resource allocation should
be performed according to the characteristics and demands of applications to minimize the effects of
congestion on QoS.
CBQ
In general, congestion management adopts queuing technology. The system uses a certain queuing
algorithm for traffic classification, and then uses a certain precedence algorithm to send the traffic. Each
queuing algorithm is used to handle a particular network traffic problem and has significant impacts on
bandwidth resource assignment, delay, and jitter.
Class-based queuing (CBQ) assigns an independent reserved FIFO queue for each user-defined class
to buffer data of the class. In the case of network congestion, CBQ assigns packets to queues by
user-defined traffic classification rules. It is necessary to perform the congestion avoidance mechanism
(tail drop or weighted random early detection (WRED)) and bandwidth restriction check before packets
are enqueued. When being dequeued, packets are scheduled by WFQ.
CBQ provides an emergency queue to enqueue emergent packets. The emergency queue is a FIFO
queue without bandwidth restriction. However, delay sensitive flows like voice packets may not be
transmitted timely in CBQ since packets are fairly treated. To solve this issue, Low Latency Queuing
(LLQ) was introduced to combine PQ (Priority Queuing) and CBQ to transmit delay sensitive flows like
voice packets preferentially.
When defining traffic classes for LLQ, you can configure a class of packets to be transmitted
preferentially. Such a class is called a priority class. The packets of all priority classes are assigned to
the same priority queue. It is necessary to check bandwidth restriction of each class of packets before
the packets are enqueued. During the dequeuing operation, packets in the priority queue are
transmitted first. WFQ is used to dequeue packets in the other queues.
In order to reduce the delay of the other queues except the priority queue, LLQ assigns the maximum
available bandwidth for each priority class. The bandwidth value is used to police traffic in the case of
congestion. In the case of no congestion, a priority class can use more than the bandwidth assigned to
it. In the case of congestion, the packets of each priority class exceeding the assigned bandwidth are
discarded. LLQ can also specify burst-size.
The system matches packets with classification rules in the following order:
Match packets with priority classes and then the other classes.
Match packets with priority classes in the order configured.
Match packets with other classes in the order configured.
Match packets with classification rules in a class in the order configured.