Single Packet Authorization (SPA) is a method used in Zero Trust networks to securely request access to a service. A key concept of SPA is that the SPA packet must be self-contained, carrying all necessary information for the authorization decision within a single, encrypted packet. This ensures that the packet alone can provide enough context for the receiving server to authenticate the request and make an authorization decision, without needing additional information exchanges. This self-contained nature of SPA packets aligns with the principle of minimizing the movement and exposure of sensitive credentials, thus enhancing the security of the authentication process.

Network access control (NAC) typically operates at layer 2, the data link layer, of the open systems interconnection (OSI) model. The data link layer is responsible for transferring data between adjacent nodes on a network, such as switches and endpoints. NAC operates at this layer by inspecting and controlling the access of devices to the network based on their MAC addresses, device profiles, security posture, and compliance status.
References = Certificate of Competence in Zero Trust (CCZT) - Cloud Security Alliance, Zero Trust Training (ZTT) - Module 6: Micro-segmentation

The third step in the SDP onboarding process is to onboard and authenticate the initiating hosts, which are the clients that request access to the protected resources. The initiating hosts connect to and authenticate with the SDP gateway, which acts as an accepting host and a proxy for the protected resources. The SDP gateway verifies the identity and posture of the initiating hosts and grants them access to the resources based on the policies defined by the SDP controller.
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

Asset owners are the ones who will determine valid users, roles, and privileges for accessing data as part of data governance. Asset owners are responsible for defining the data classification, sensitivity, and ownership of the data assets they own. They also have the authority to grant or revoke access to the data assets based on the business needs and the Zero Trust policies.
References = Certificate of Competence in Zero Trust (CCZT) - Cloud Security Alliance, Zero Trust Training (ZTT) - Module 2: Data and Asset Classification

Explanation
To ensure an acceptable user experience when implementing SDP, a security architect should collaborate with IT to model and plan the user experience, client software distribution, and device onboarding processes. This is because SDP requires users to install and use client software to access the protected resources, and the user experience may vary depending on the device type, operating system, network conditions, and security policies. By modeling and planning the user experience, the security architect and IT can ensure that the SDP implementation is user-friendly, consistent, and secure.
References = Certificate of Competence in Zero Trust (CCZT) - Cloud Security Alliance, Zero Trust Training (ZTT) - Module 7: Network Infrastructure and SDP