After a principal authenticates with a particular identity provider, the identity provider redirects the principal's browser to the configured Writer Service URL using a parameter that indicates they are the identity provider for this principal. The Writer Service then writes a cookie using the parameter. Thereafter, all providers configured in this common domain will be able to tell which identity provider is used by this principal. Thus, the next time the principal attempts to access a service hosted by a service provider in the same common domain, the service provider retrieves and reads the common domain cookie, using the configured Reader Service URL, to determine the identity provider.
The Writer Service URL and the Reader Service URL can be defined for use with the Liberty ID-FF or the SAML v2 federation protocol. The URLs are defined when you create a circle of trust for federation. The Common Domain for Identity Provider Discovery for Liberty ID-FF is based on the Identity Provider Introduction Profile detailed in the Liberty ID-FF Bindings and Profiles Specifications. The Common Domain for Identity Provider Discovery for SAML v2 is an implementation of the Identity Provider Discovery Profile as described in the Profiles for the OASIS Security Assertion Markup Language (SAML) V2.0 specification.