Which of the following statements regarding DSCP bits are TRUE? (Choose three)

The Differentiated Services Code Point (DSCP) is a 6-bit field within the 8-bit Differentiated Services (DS) field in an IP header.
A. The three most significant bits are the Class Selector bits. These three bits allow for 2^3 = 8 distinct classes, known as forwarding classes (CS0 through CS7).
D. These Class Selector codepoints are designed to be backward compatible with the older IP Precedence standard. Routers that only support IP Precedence will interpret these first three bits, ensuring consistent basic prioritization.
B. For the widely used Assured Forwarding (AF) Per-Hop Behavior (PHB), the drop probability is encoded in the 4th and 5th bits of the DSCP field. The 6th (least significant) bit is set to zero. Therefore, the value of the three least significant bits as a group (yy0) uniquely identifies the drop precedence level.

C. The DSCP field uses only the first 6 bits of the 8-bit DS field (formerly the ToS field). The remaining 2 bits are used for Explicit Congestion Notification (ECN).
E. The Assured Forwarding (AF) standard, which uses drop probability, defines three levels of drop precedence (low, medium, high), not eight.

1. IETF RFC 2474: Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers.

Section 3, "The DS Field": "The DS field structure is presented in Figure 1. Six bits of the DS field are used as the Differentiated Services Codepoint (DSCP) ... the remaining two bits are currently unused (CU)." (This refutes option C).

Section 4.1, "Class Selector Codepoints": "There is a set of DSCP values that are recommended for use as Class Selector Codepoints. These are of the form 'xxx000' (where x may be 0 or 1). The three most-significant bits of the DSCP are the Class Selector bits... These codepoints are backward-compatible with the IP Precedence specification." (This supports options A and D).

2. IETF RFC 2597: Assured Forwarding PHB Group.

Section 2, "Assured Forwarding PHB Group": "An AF class is designated as AFci, where c is the class number and i is the drop precedence... The recommended DSCP value for AFci is 'ccc iio', where 'ccc' is the binary value for class c, and 'ii' is the binary value for the drop precedence i." (This supports the explanation for B, showing drop precedence is encoded in two bits plus a trailing zero, and refutes E, as it defines 3 drop precedences, not 8).

3. Nokia Quality of Service (4A0-107) Student Guide.

Module 3, "Classification and Marking": The courseware details the 6-bit DSCP field, explaining that the first three bits are for class selection and are backward compatible with IP Precedence. It also illustrates the Assured Forwarding structure showing three bits for class, two for drop precedence, and a final bit set to zero. (This corroborates the information from the RFCs).