One of the core principles of Zero Trust (ZT) is to "never trust, always verify" every request for access to a resource, regardless of where it originates or what resource it accesses1. This means that ZT does not rely on implicit trust based on network perimeters, device types, or user roles, but rather on explicit verification based on multiple data points, such as user identity, device health, location, service, data classification, and anomalies1.
References =
* Zero Trust Architecture | NIST
* Zero Trust Model - Modern Security Architecture | Microsoft Security
* How To Implement Zero Trust: 5-steps Approach & its challenges - Fortinet

Explanation
The ZT tenet of assume breach is based on the notion that malicious actors reside inside and outside the network, and that any user, device, or service can be compromised at any time. Therefore, ZT requires continuous verification and validation of all entities and transactions, and does not rely on implicit trust or perimeter-based defenses

During the monitoring and analytics phase of ZT transaction flows, organizations should collect statistics and profile the behavior of transactions to support a continuous assessment of all transactions. A continuous assessment of all transactions means that the organization constantly evaluates the security posture, performance, and compliance of each transaction, and detects and responds to any anomalies, deviations, or threats. A continuous assessment of all transactions helps to maintain a high level of protection and resilience in the ZTA, and enables the organization to adjust and improve the policies and controls accordingly.
References =
* Zero Trust Planning - Cloud Security Alliance, section "Monitor & Measure"
* The role of visibility and analytics in zero trust architectures, section "The basic NIST tenets of this approach include"
* Move to the Zero Trust Security Model - Trailhead, section "Monitor and Maintain Your Environment"

Explanation
Testing of ZT is providing evidence of continuous improvement because it helps to measure the effectiveness and efficiency of the ZT and ZTA implementation. Testing of ZT also helps to identify and address any gaps, issues, or risks that may arise during the ZT and ZTA lifecycle. Testing of ZT enables the organization to monitor and evaluate the ZT and ZTA performance and maturity, and to apply feedback and lessons learned to improve the ZT and ZTA processes and outcomes.
References = Certificate of Competence in Zero Trust (CCZT) - Cloud Security Alliance, Zero Trust Training (ZTT) - Module 8: Testing and Validation

Explanation
Different SDP deployment models have different advantages and disadvantages depending on the organization's use case, such as the type of resources to be protected, the location of the clients and servers, the network topology, the scalability, the performance, and the security requirements. Network architects should consider their use case before selecting an SDP model that best suits their needs and goals.
References =
Certificate of Competence in Zero Trust (CCZT) prepkit, page 21, section 3.1.2
6 SDP Deployment Models to Achieve Zero Trust | CSA, section "Deployment Models Explained" Software-Defined Perimeter (SDP) and Zero Trust | CSA, page 7, section 3.1 Why SDP Matters in Zero Trust | SonicWall, section "SDP Deployment Models"