Of the choices provided, the DiffServ class AF23 is most likely to be dropped. AF23 is a Differentiated Services Code Point (DSCP) value, which is a 6bit header value that identifies the Quality of Service (QoS) traffic class that is assigned to the packet. DSCP values beginning with AF are called Assured Forwarding (AF) perhop behaviors (PHBs), which are defined in Request for Comments (RFC) 2597. AF separates packets into four queue classes and three drop probabilities. The AF values are specified in the format AFxy, where x is the queue class and y is the drop probability.

The first three DSCP bits correspond to the queue class, the fourth and fifth DSCP bits correspond to the drop probability, and the sixth bit is always set to 0. To quickly convert AF values to decimal values, you should use the formula 8x + 2y. For example, AF42 converts to a decimal value of 36, because (8 x 4) + (2 x 2) = 32 + 4 = 36.

Packets with higher AF values are not necessarily given preference over packets with lower AF values. Packets with a higher queue class value are given queuing priority over packetswith a lower queue class, but packets with a higher drop rate value are dropped more often than packets with a lower drop rate value. Packets with DSCP values of AF13, AF23, AF33, and AF43 all have high drop probabilities; packets with DSCP values of AF11, AF21, AF31, and AF41 all have low drop probabilities.