Is an RDS CAL Required for VDI?

Historically, a Remote Desktop Services (RDS) Client Access License (CAL) was needed purely for scenarios that relied on sessions, hosted on the RDS Session Host. As Microsoft began to offer virtual desktops hosted in virtual machines (VMs) running on the Windows Server hypervisor, it chose to use key RDS components for both session- and VM-based virtual desktops. Therefore, whenever any VDI solution uses these components, an RDS CAL is required. Use of RDS components is not a strict technical requirement for VDI, because alternative technologies that do not use the Microsoft RDS components are available from third parties.

All VDI deployments require technology for a number of management and deployment tasks. For example, the VMs running the Windows client OS and applications for the virtual desktops have to be stopped and started as needed, and users must have a way to find the virtual desktops that are designated for their use. Once a user is allocated a virtual desktop, a connection has to be established between the user and the virtual desktop.