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Assured Forwarding and Expedited Forwarding

Assured Forwarding and Expedited Forwarding

The Assured Forwarding (AF) PHB contains two separate QoS functions. The first function is Queueing. Each router classifies the packets into 4 different classes, and packets from each classes are placed in seperate queues. The AF PHB does not guarantee that each packet will be delivered and it also does not reserve any minimum bandwidth for each class. The second function defines Congestion Avoidance.

RFC 2597 has AF PHB concepts summarized as follows-

  • Use upto 4 different queues
  • Use three different congestion thresholds inside each queue to determine when to begin discarding different types of packets
  • to mark these packets, 12 DSCP values are needed

 

Low Drop Probability within class Medium Drop Probability within class High Drop Probability within class
Class 1  AF11 AF12 AF13
 Class 2  AF21  AF22  AF23
Class 3  AF31  AF32  AF33
Class 4  AF41  AF42  AF43

To convert from the AF name to the decimal equivalent, use the formula 8x + 2y, if the AF value is considered AFxy.

For example, the decimal value for AF43 is 8(4) + 2(3) = 38.

The¬†Expedited Forwarding (EF)¬†PHB also suggests two QoS functions. The first function is¬†Queuing¬†which must be used to minimize the time the EF packet spend in a queue. Therefore, EF’s goal of reducing delay, jitter and loss can be achieved with a queueing method such as Priority Queueing. The second component of EF PHB is¬†Policing.

The EF PHB uses a DSCP name of EF, binary value is 101110 and decimal value 46.

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