Drop precedence ranks packets within an assured-forwarding class by their likelihood of being discarded during congestion. Higher precedence means a greater likelihood of drop. section 10.3.3
AF encodes drop precedence as follows:
- Uses four traffic classes, each containing three drop-precedence levels. section 10.3.3
- Uses the five most significant DSCP bits: three identify the class and two identify drop precedence; the least significant bit is zero. section 10.3.3
- Does not make a numerically higher AF value inherently better; AF41 has lower drop precedence than AF43 despite both belonging to class 4. section 10.3.3
Calculating AF42
Multiply the class by 8 and the drop-precedence value by 2, then add: (4 × 8) + (2 × 2) = 36, so AF42 has decimal DSCP value 36. section 10.3.3
Figure 10.14 The AF marking system. The five most significant bits are divided into the class (3 bits) and drop precedence (2 bits). The sixth bit is always 0.
Visualizes the five-bit AF layout, including its class and drop-precedence fields and the fixed sixth bit. section 10.3.3
