A Next-Generation Firewall (NGFW) uses SYN flood protection to determine the legitimacy of new session setups. This is a crucial component of Denial-of-Service (DoS) protection. The firewall monitors the rate of incoming TCP SYN packets (the first step in the three-way handshake). If the rate surpasses a predefined threshold, the firewall suspects a SYN flood attack, which consists of illegitimate session requests. At this point, it can employ countermeasures, such as SYN cookies, to validate the source and ensure that only legitimate clients, which properly complete the handshake, are allowed to establish a session.

B. SYN bit: The SYN bit is a flag within the TCP header used to initiate a connection; it is a component of the protocol, not a security functionality that determines legitimacy.

C. Random Early Detection (RED): RED is a queue management algorithm for network congestion avoidance. It drops packets randomly to signal senders to reduce their rate, but it does not specifically distinguish between legitimate and illegitimate traffic.

D. SYN cookies: SYN cookies are a specific method or technique used as a countermeasure within the broader SYN flood protection functionality. The overall functionality itself is "SYN flood protection."

1. Palo Alto Networks PAN-OS® Administrator’s Guide 10.2, "DoS Protection." In the section "Packet-Based Attack Protection," the guide details how the firewall protects against flood attacks. It states, "For a SYN flood, the firewall determines that an attack is in progress when the number of SYN packets per second (sps) from a single source or to a single destination exceeds a threshold... The firewall can then use the SYN cookie method to protect itself and the end host." This confirms "SYN flood protection" is the functionality that uses methods like SYN cookies to validate sessions.

2. Palo Alto Networks PAN-OS® Administrator’s Guide 10.2, "Zone Protection Profile." This section describes how to configure flood protection. It lists "SYN Flood" as a specific type of flood to protect against, with configurable thresholds (Alarm Rate, Activate Rate, Max Rate) and actions (SYN Cookies, Random Early Drop). This clearly identifies SYN flood protection as the overarching NGFW functionality.

3. Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. In Chapter 3, Section 3.5.3, the text describes the TCP SYN flood attack. It explains that a defense mechanism is for the gateway router (or firewall) to deploy "SYN cookies," which is a primary technique to thwart this type of DoS attack by validating the client's reachability before allocating resources. This academic source frames SYN cookies as a solution to the problem handled by SYN flood protection functionality.