DSCP value range aaaddO

- Where "aaa" is a binary value of the class

- Where "dd" is drop probability

The AF PHB is identified based on the following:

■ The AF PHB guarantees a certain amount of bandwidth to an AF class.

■ The AF PHB allows access to extra bandwidth, if available.

■ Packets requiring AF PHB should be marked with DSCP value "aaaddO" where "aaa" is the number of the class and "dd" is the drop probability.

There are four standard-defined AF classes. Each class should be treated independently and should have allocated bandwidth that is based on the QoS policy.

Each AF class uses three DSCP values.

Each AF class is independently forwarded with its guaranteed bandwidth.

Each AF class uses three DSCP values.

Each AF class is independently forwarded with its guaranteed bandwidth.

Congestion avoidance is used within each class to prevent congestion within the class.

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Each AF class is assigned an IP Precedence and has three drop probabilities: low, medium, and high.

AFxy: Assured Forwarding (RFC 2597), where x corresponds to the IP Precedence value (only 1-4 are used for AF classes), and y corresponds to the Drop Preference value (either 1, 2, or 3)

An AF implementation must attempt to minimize long-term congestion within each class, while allowing short-term congestion resulting from bursts. This requires an active queue management algorithm. An example of such an algorithm is weighted random early detection (WRED), which is described in detail in the "Congestion Avoidance" module in this course.

The AF specification does not define the use of a particular algorithm, but it does have certain requirements and properties.

An AF implementation must detect and respond to long-term congestion within each class by dropping packets, while handling short-term congestion (packet bursts) by queuing packets. This implies the presence of a smoothing or filtering function that monitors the instantaneous congestion level and computes a smoothed congestion level. The dropping algorithm uses this smoothed congestion level to determine when packets should be discarded.

The dropping algorithm must treat all packets within a single class and precedence level identically. Therefore, within a single traffic class, the discard rate of a particular packet flow will be proportional to the percentage of the total amount of traffic.

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