What are the advantages and disadvantages of the RP-centred federation model?

One relying party accepts identities from several IdPs, which gives it a larger reach, but it must communicate with several IdPs that may use different identity formats and different protocols, creating a need for standardisation.

* RP-centred: one relying party, several identity providers to integrate. *

In the diagram one RP sits in the middle, connected to IdP1 to IdP4. The subject authenticates at one of the IdPs and then accesses the RP.

• Advantage for the RP: every user who already has an account at any of the connected IdPs can use the service without a new registration, so the potential audience is larger.
• Disadvantage for the RP: it has to integrate and maintain several IdP connections. The IdPs may represent identities differently (attribute names, identifier formats, assurance levels) and may speak different protocols (SAML here, OpenID Connect there). This is why standards matter: a common protocol and attribute vocabulary keep the integration cost from growing with every additional IdP.