An analyst is investigating why an App-ID for a custom application is showing as "unknown-tcp"...

Comprehensive and Detailed 150 to 250 words of Explanation From Palo Alto Networks Network Security Analyst Knowledge:

When traffic is logged as unknown-tcp or unknown-udp, it indicates that the App-ID engine has inspected the traffic but could not find a matching signature in its database. For proprietary or internal applications, this is the expected behavior unless the analyst has created a Custom Application Signature.

To resolve this, the analyst must capture the packet flow and identify a unique data pattern (signature) within the payload that identifies the application. Once the custom App-ID is created and committed, the firewall will correctly categorize the traffic, allowing the analyst to apply granular security profiles and reporting. Identifying and remediating "unknown" traffic is a key monitoring objective, as it helps eliminate visibility gaps and prevents malicious traffic from "hiding" behind unidentified protocols.