Differentiated Services Code Point (DSCP) is a mechanism for classifying and managing network traffic. It is a core component of the Differentiated Services (DiffServ) architecture. The DSCP value is located in the 8-bit Differentiated Services (DS) field within the Layer 3 (IPv4 or IPv6) header, which replaced the original Type of Service (ToS) field. The first 6 bits of this DS field are used for the DSCP, allowing for 64 (2^6) different classification values. These values are used by routers to determine the Per-Hop Behavior (PHB), such as queuing and drop priority, that a packet should receive.

C. DSCP specifies eight different priorities.

Incorrect. DSCP is a 6-bit field allowing for 64 distinct values. While Class Selector PHBs provide backward compatibility with the eight levels of IP Precedence, DSCP itself is not limited to eight priorities.

D. DSCP specifies four precedence levels.

Incorrect. This confuses DSCP with its predecessor, IP Precedence, which used 3 bits to define eight (not four) precedence levels.

E. DSCP is part of the Layer 2 header.

Incorrect. DSCP is a Layer 3 marking within the IP header. Layer 2 QoS marking is typically handled by the 3-bit Priority Code Point (PCP) field in an 802.1Q VLAN tag.

F. DSCP specifies 128 different per-hop behaviors.

Incorrect. As a 6-bit field, DSCP can only represent 2^6, or 64, different code points, and therefore a maximum of 64 different per-hop behaviors.

1. Nokia 7750 SR OS Quality of Service Guide, Release 21.10.R1: In the "QoS Overview" chapter, section "Differentiated Services (DiffServ)", it is stated: "The DiffServ architecture is based on a model where traffic entering a network is classified... and assigned to a behavior aggregate. Each behavior aggregate is identified by a single DSCP code point. The DS code point is carried in a DS field in the header of an IP packet." This confirms DSCP is in the Layer 3 IP header. The guide further details the 64 possible DSCP values used for classification.

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

Section 3, "The DS Field": "This document redefines the IPv4 TOS octet and the IPv6 Traffic Class octet as an 8-bit Differentiated Services (DS) field." This places the field in the Layer 3 header.

Section 3.1, "Layout of the DS Field": "A 6-bit Differentiated Services Codepoint (DSCP) field is defined in the most significant 6 bits of the DS field." This confirms it is a 6-bit field.

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In Chapter 4, "The Network Layer: Data Plane," the section on Differentiated Services explains that the 8-bit ToS field in the IPv4 header was redefined for DiffServ, with the first 6 bits used for the DSCP. This is standard university courseware material.