Operational technology communications occur over serial COM, Ethernet, Wi-Fi, cellular (4G/5G), and satellite mediums. Adversaries may block communications to prevent reporting messages and command messages from reaching their intended target devices disrupting processes, operations, and causing cyber-physical impacts.(Citation: Bonnie Zhu, Anthony Joseph, Shankar Sastry 2011) Adversaries may block communications by either making modifications to software ([System Firmware](https://attack.mitre.org/techniques/T0857), [Module Firmware](https://attack.mitre.org/techniques/T0839), [Hooking](https://attack.mitre.org/techniques/T0874), and [Rootkit](https://attack.mitre.org/techniques/T0851)) and services ([Service Stop](https://attack.mitre.org/techniques/T0881), [Denial of Service](https://attack.mitre.org/techniques/T0814)) on systems and devices or by positioning themselves between systems and devices and intercepting and blocking the communications such as the case with an [Adversary-in-the-Middle](https://attack.mitre.org/techniques/T0830) attack.
Have alternative methods to support communication requirements during communication failures and data integrity attacks. (Citation: National Institute of Standards and Technology April 2013) (Citation: Defense Advanced Research Projects Agency)
M0930Network SegmentationArchitect sections of the network to isolate critical systems, functions, or resources. Use physical and logical segmentation to prevent access to potentially sensitive systems and information. Use a DMZ to contain any internet-facing services that should not be exposed from the internal network. Restrict network access to only required systems and services. In addition, prevent systems from other networks or business functions (e.g., enterprise) from accessing critical process control systems. For example, in IEC 62443, systems within the same secure level should be grouped into a zone, and access to that zone is restricted by a conduit, or mechanism to restrict data flows between zones by segmenting the network. (Citation: IEC February 2019) (Citation: IEC August 2013)
M0807Network AllowlistsNetwork allowlists can be implemented through either host-based files or system hosts files to specify what connections (e.g., IP address, MAC address, port, protocol) can be made from a device. Allowlist techniques that operate at the application layer (e.g., DNP3, Modbus, HTTP) are addressed in [Filter Network Traffic](https://attack.mitre.org/mitigations/M0937) mitigation.
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