Final answer:
Incomplete application traffic on a Palo Alto NGFW indicates network sessions that start but fail to complete a requisite TCP three-way handshake. Such issues can be due to network timeouts, security policies, or interrupted client attempts, and may suggest network problems or suspicious activities.
Step-by-step explanation:
When one speaks of "incomplete" application traffic in the context of a Palo Alto NGFW (Next-Generation Firewall), it refers to network sessions that start but do not complete a full TCP three-way handshake, which is necessary for establishing a proper TCP/IP connection. These incomplete sessions may occur for several reasons such as network timeouts, traffic being blocked by a security policy, or the client aborting the connection attempt. Incomplete traffic is notable in firewall logs as it could signify network issues or potentially malicious activity such as reconnaissance scans by attackers trying to identify open ports.