At which layer is deep packet inspection investigated on a firewall?

Deep packet inspection (DPI) is a sophisticated method of examining the content of data packets as they pass through a network checkpoint, including both the header and the data payload. DPI is typically performed at the application layer of the Open Systems Interconnection (OSI) model. This allows the inspection process to evaluate the actual content of the packets, not just the header information, enabling the identification of various types of threats and the enforcement of network policies1.

References := The explanation aligns with the information provided by Digital Guardian, which details how DPI works and at which layer it is applied1.