Based on industry standard practice for implementing Quality of Service (QoS), the priority order from highest to lowest is established to protect the most delay-sensitive and critical traffic. Expedited Forwarding (EF) is given the highest priority as it is used for real-time applications like VoIP, which are extremely intolerant of delay and jitter. Network Control (NC) traffic (e.g., routing protocols) is critical for network stability and is prioritized next. Assured Forwarding (AF) provides multiple service levels for important data that is less sensitive than real-time traffic. Default Forwarding (DF), or best-effort, has the lowest priority and is used for all remaining traffic with no service guarantees.

B. EF, AF, NC, DF: This order incorrectly places Network Control (NC) traffic below Assured Forwarding (AF). NC is vital for network operation and is always prioritized higher than AF classes.

C. NC, EF, DF, AF: This order incorrectly places Default Forwarding (DF) above Assured Forwarding (AF). DF is the lowest-priority, best-effort service class by definition.

D. NC, EF, AF, DF: While Network Control traffic is critical, common implementation practice prioritizes user-facing real-time EF traffic in the highest-priority queue to prevent any impact from control plane processing.

1. IETF RFC 4594

"Configuration Guidelines for DiffServ Service Classes": This document provides the foundational recommendations for DiffServ service classes.

Section 3.1 (Network Control): Describes this class for traffic "essential for network operation" and recommends DSCP CS6.

Section 3.2 (Telephony): Recommends the EF PHB for real-time voice traffic

noting its intolerance to delay and jitter.

Section 3.4 (Assured Forwarding Classes) and Section 3.7 (Default Class): Detail the lower-priority AF and DF (Best-Effort) classes

establishing their place below EF and NC.

2. IETF RFC 3246

"An Expedited Forwarding PHB (Per-Hop Behavior)":

Section 1 (Introduction): Defines the EF PHB as a "building block for a low-loss

low-latency

low-jitter

assured-bandwidth service

" justifying its placement in a strict priority queue in network implementations.

3. Stallings

W. (2016). Foundations of Modern Networking: SDN

NFV

and Cloud Computing. Pearson Education.

Chapter 11

Section 11.3 (Differentiated Services): This academic textbook explains that EF is designed to be handled in a priority queue

receiving preferential treatment over other traffic classes like AF and Best-Effort (DF) to create a "virtual leased line." It also notes the high importance of network control traffic.

4. Case Western Reserve University

EECS 425/600: Intro to Computer Networks Courseware

"Differentiated Services" Lecture.

Slide "A Possible Implementation": This university lecture material illustrates a common queuing architecture where EF traffic is directed to a strict priority queue

while AF and Best-Effort (DF) traffic are handled by weighted fair queuing (WFQ) schedulers

demonstrating the top-priority treatment of EF in practice.