In a federated single sign-on environment, which protocol exchanges XML-based assertions between an identity provider...

Security Assertion Markup Language (SAML) is the XML-based federation standard used to exchange authentication and authorization assertions between an identity provider and a service provider. In a typical browser SSO flow, the service provider redirects the user to the identity provider. After authentication, the identity provider returns a digitally signed SAML assertion that the service provider validates before establishing the application session. OAuth 2.0 primarily delegates authorization to APIs and does not itself define user authentication. LDAP provides directory access and authentication functions but is not the XML assertion protocol used for browser federation. RADIUS transports centralized network access authentication, authorization, and accounting messages. Because the question explicitly specifies federated SSO, XML assertions, an identity provider, and a service provider, SAML is the exact match. Cisco SAML overview