R3166-R3206-HP High-End Firewalls Network Management Configuration Guide-6PW101

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Causes, impacts, and countermeasures of congestion
Congestion occurs on a link or node when traffic size exceeds the processing capability of the link or
node. It is typical of a statistical multiplexing network and can be caused by link failures, insufficient
resources, and various other causes. Figure 114 sh
ows some common congestion scenarios:
Figure 114 Traffic congestion causes
Congestion can bring the following negative results:
• Increased delay and jitter during packet transmission
• Decreased network throughput and resource use efficiency
• Network resource (memory in particular) exhaustion and system breakdown
Congestion is unavoidable in switched networks or multi-user application environments. To improve the
service performance of your network, you must take measures to manage and control it.
One major issue that congestion management deals with is how to define a resource dispatching policy
to prioritize packets for forwarding when congestion occurs.
Queuing is a common congestion management technique. It classifies traffic into queues and picks out
packets from each queue by using a certain algorithm. Various queuing algorithms are available, and
each addresses a particular network traffic problem. Your choice of algorithm affects bandwidth
assignment, delay, and jitter significantly.
Congestion management involves queue creating, traffic classification, packet enqueuing, and queue
scheduling. Queue scheduling treats packets with different priorities differently to transmit high-priority
packets preferentially.
CBQ
In general, congestion management uses 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 addresses a particular network traffic problem and which algorithm is used affects
bandwidth resource assignment, delay, and jitter significantly.
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 match criteria
(also known as 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)