Modbus is a protocol used in SCADA networks which is typically on TCP port
502 (aka Modbus TCP).

The Modbus inspector decodes the Modbus protocol and provides rule options
to access certain protocol fields. This allows a user to write rules for
Modbus packets without decoding the protocol with a series of ”content” and
”byte test” options.

The inspector only evaluates PAF-flushed PDUs. If the rule options don't
check for this, they'll fire on stale session data when the original packet
goes through before flushing.

