22 CHAPTER 2 About Industrial Networks FIGURE 2.6 Topological Defense in Depth Provides Additional Layers of Protection. through the careful creation and management of policies that implicitly define which servers can connect over a given protocol or port. This is covered in detail in Chapter 7, "Establishing Secure Enclaves." CAUTION Don't forget to control communications in both directions through a firewall. Not all threats originate from outside. Open, outbound traffic policies can facilitate an insider attack, enable the internal spread of malware, enable outbound command and control capabilities, or allow for data leakage or information theft.