Outlines the rational for using a single sign-on mechanism and then gives technical details (without assuming use of a standard) for how to implement a provider and consumer services.

This is useful an outline of how a centralized service might work (i.e. only relying on one authentication provider). The same concepts could work distributed authentication providers, but an additional step of choosing the user's provider is necessary.

I.e. something akin to this:

(or for a longer list, a dropdown menu could appear)

Other would allow the user to enter the preferred authentication provider.