What is the fine-grained topic of the following text: The 4Q18 IDC tracker outlines that Fortinet realized 17.7% revenue growth in the total security appliance market year-over-year, out-pacing the industry average of 16.7%, and advancing from the third to second spot in revenue for the combined Firewall, UTM and VPN markets. Only return the topic.
Fortinet's revenue growth and ranking in the security appliance market in 4Q18.