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|
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.