The INSPECT engine inserts itself into the kernel between which two OSI model layers?

Physical and Data Link are the two lowest layers; the INSPECT engine does not sit below the Data Link layer because it is a software kernel module, not a physical driver.

Session and Transport are Layers 5 and 4 respectively; placing inspection there would allow packets to already be partially processed by the network stack before being evaluated.

The INSPECT engine operates at the kernel level by positioning itself between Layer 2 (Data Link) and Layer 3 (Network). This placement ensures every packet is inspected before the IP stack processes it, allowing Check Point to enforce policy before any routing or higher-layer handling occurs. This is a fundamental architectural feature of the Stateful Inspection technology.

Presentation and Application are the topmost layers; inspection at that level would allow traffic to traverse the entire TCP/IP stack unchecked, defeating the purpose of a kernel-level firewall.