User Manual - Configuration Guide (Volume 3)
Versatile Routing Platform Chapter 3
Congestion Management
3-2
resend the data packets due to timeout, and congestion happens again, and then the
packets are sent again, a vicious circle happens. To manage the congestion of network,
people adopt some policies. When congestion occurs, the router can adopt a specif ic
policy to dispatch the data packets, and decide which packets to be sent with priority,
and which packets to be discarded. The policy adopted by the router to deal with
congestion is called congestion management policy. Usually Queuing technology is
adopted to manage the congestion. When congestion occurs, data packets queu e up
at the sending interface of the router following a certain policy. When the packets are
dispatched, the sending sequence of the packets is determined following a certain
policy.
I. FIFO Queuing
In FIFO (First-In First-Out) queuing, communication priority and classificati on is not
concerned. In FIFO application, the sequence of sending out data packets from the
interface depends on the arriving sequence of the data packets at this int erface.
FIFO provides basic capability of storage and forwarding. In some cas es, FIFO is the
default queuing algorithm, which should not be modified.
II. PQ (Priority Queuing)
In PQ (Priority Queuing), packets with a specific communication priority will be
forwarded earlier than all other packets with lower priorities, thus ensur ing that the
packets with higher priority can be sent out in time.
PQ is used to provide strict priority for important network data. PQ can flexibly specify
priority sequence according to network protocol (IP protocol for exam ple), data inf low
interface, length of the packet, source address/destination address, etc. It ensur es that
the most important data in the network unit using PQ can be processed as fast as
possible.
III. CQ (Custom Queuing)
In CQ (Custom Queuing), a specific proportion of the available bandwidth of the
interface is reserved for each specified traffic type. When the reserved bandwidth is not
used by its type of traffic, it can be used by other types of traffic. T hat is, in CQ mode,
bandwidth is allocated in proportion, and CQ permits to specif y the total number of
byte/packet to be extracted from the queue.
For interfaces with a low rate, the data flow passing through this interface can also
receive network service to some extent when the queue is customized for the interface.
IV. WFQ (Weighted Fair Queuing)
WFQ (Weighted Fair Queuing) provides a dynamic and fair queuing mode, in which the
traffic is distinguished based on priority/weight, and according to the different sessions.
The occupation of bandwidth is determined for each session, thus ensuring a fair
treatment to all communications according to their weight. The traff ic is classified by
WFQ in the light of source address, destination address, source port number,
destination port number and protocol type, etc.