Assured Forwarding Model

The assured forwarding model uses the 6-bit DSCP field to accommodate a model that lends itself well to backward compatibility with the IP Precedence model. Although a 6-bit field could accommodate 64 markings (2 to the 6th power), the assured forwarding model only uses 12 different markings. The assured forwarding model breaks down the DSCP field into three sections. The three highest order bits are the assured forwarding class indicators, while the next two bits are the drop preference within the class. The last bit of the six bit field is always zero in this model. The bits are as follows: AAA DD 0 AAA = Assured Forwarding class DD = Drop Preference 0 = Reserved (if this bit is set to 1, the DSCP is a user defined value) There are four classes defined in the Assured Forwarding model. They are as follows: Class 1 = 001 Class 2 = 010 Class 3 = 011 Class 4 = 100 In defining the class to which the packet belongs, the three highest order bits of the ToS field are considered. Since we are only factoring three bits, the decimal values are based on 2 to the 0, 1, and 2 values. Binary values double as each bit indicator is added. The class selector has values of 1, 2, 4 (lowest to highest order bit). I have used an overlay in the following binary examples to clearly identify this: 4 2 1 0 0 1 = 1 0 1 0 = 2 0 1 1 = 3 1 0 0 = 4 Each class has three drop preferences (low, medium, and high). The drop preferences are indicated by the next two bits of the DSCP value in the ToS byte. 00 = No drop preference (class selector or user defined) 01 = Low Drop Preference 10 = Medium Drop Preference 11 = High Drop Preference Since we are only factoring two bits in the drop preference (DP) field, there are four decimal values as follows: 2 1 0 0 = 0 0 1 = 1 (Low) 1 0 = 2 (Medium) 1 1 = 3 (High) When there is congestion in an output queue, the router’s congestion avoidance (WRED) mechanism will be able to discard traffic based on the drop preference of the packet within the application class. We will continue our coverage of the assured forwarding model in the next blog.