CEH teaches that certain advanced intrusion evasion techniques rely on understanding how firewalls differentiate between new connections and established traffic. Most perimeter firewalls scrutinize SYN packets and initial HTTP requests but allow ACK packets from established sessions to pass with minimal filtering. ACK tunneling leverages this behavior by embedding malicious payloads inside ACK packets, which appear to be part of a legitimate, pre-established session. Because ACK packets are often considered "safe," they bypass deep inspection engines, intrusion detection systems, and application-layer gateways. This method allows attackers to move data or commands covertly between compromised internal systems and external hosts. CEH references such evasion strategies when discussing bypassing stateful firewalls and making malicious traffic appear legitimate. Port knocking and SYN scans would initiate new connections—precisely what the firewall is heavily inspecting. ICMP flooding is noisy and easily detected. ACK tunneling is specifically designed for stealth and is aligned with red team tradecraft for avoiding packet-level inspection mechanisms.