Information Technology Reference
In-Depth Information
Classification
For a flow to have priorit y, it first must be identified and marked. B oth of these tasks are
referred to as classification. The following technologies are a couple of ways that have fea-
tures that support classification:
Network-Based Application Recognition (NBAR)
is a technology that uses deep
packet content inspection to identify network applications. An advantage of NBAR is
that it can recognize applications even when they do not use standard network ports.
Furthermore, it matches fields at the application layer. Before NBAR, classification
was limited to Layer 4 TCP and User Datagram Protocol (UDP) port numbers.
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Committed access rate (CAR)
uses an ACL to set precedence and allows cus-
tomization of the precedence assignment by user, source or destination IP address,
and application type.
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Congestion Management
Two t y pes of output queues are available on routers: the hardware queue and the soft ware
queue. The hardware queue uses the strategy of first in, first out (FIFO). The software
queue schedules packets first and then places them in the hardware queue. Keep in mind
that the software queue is used only during periods of congestion. The software queue
uses QoS techniques such as priority queuing, custom queuing, weighted fair queuing,
class-based weighted fair queuing, low-latency queuing, and traffic shaping and policing.
Priority Queuing
Priority queuing (PQ) is a queuing method that establishes four interface output queues
that serve different priority levels: high, medium, default, and low. Unfortunately, PQ can
starve other queues if too much data is in one queue because higher-priority queues
must
be emptied before lower-priority queues.
Custom Queuing
Custom queuing (CQ) uses up to 16 individual output queues. Byte size limits are as-
signed to each queue so that when the limit is reached, it proceeds to the next queue. The
network operator can customize these byte size limits. CQ is fairer than PQ because it al-
lows some level of service to all traffic. This queuing method is considering legacy due to
the improvements in the queuing methods.
Weighted Fair Queuing
We i g h t e d f a i r q u e u i n g ( W F Q ) e n s u r e s t h a t t r a f f i c i s s e p a r a t e d i n t o i n d i v i d u a l f l o w s o r s e s -
sions without requiring that you define ACLs. WFQ uses two categories to group ses-
sions: high bandwidth and low bandwidth. Low-bandwidth traffic has priority over
high-bandwidth traffic. High-bandwidth traffic shares the service according to assigned
weight values. WFQ is the default QoS mechanism on interfaces below 2.0 Mbps.
Class-Based Weighted Fair Queuing
