defines the assured forwarding (AF) PHB and describes it as a means for a provider DS domain to offer different levels of forwarding assurances for IP packets received from a customer DS domain. The Assured Forwarding PHB guarantees a certain amount of bandwidth to an AF class and allows access to extra bandwidth, if available. There are four AF classes, AF1x through AF4x. Within each class, there are three drop probabilities. Depending on a given network's policy, packets can be selected for a PHB based on required throughput, delay, jitter, loss or according to priority of access to network services.Classes 1 to 4 are referred to as AF classes. The following table illustrates the DSCP coding for specifying the AF class with the probability. Bits DS5, DS4 and DS3 define the class; bits DS2 and DS1 specify the drop probability; bit DS0 is always zero.
| Drop | Class 1 | Class 2 | Class 3 | Class 4 |
|---|---|---|---|---|
| Low | 001010 AF11 DSCP 10 | 010010 AF21 DSCP 18 | 011010 AF31 DSCP 26 | 100010 AF41 DSCP 34 |
| Medium | 001100 AF12 DSCP 12 | 010100 AF 22 DSCP 20 | 011100 AF32 DSCP 28 | 100100 AF42 DSCP 36 |
| High | 001110 AF13 DSCP 14 | 010110 AF23 DSCP 22 | 011110 AF33 DSCP 30 | 100110 AF43 DSCP 38 |
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