Requesting users attributes as part of Authentication Request Protocol

Hi all.

 

Even though I have seen in some documentations that it is possible to

request users' attributes from the IdP as part of the authentication request

protocol (i.e. <AuthnRequest> element), I did not manage to find specification

which would indicate how to do it? Can someone forward me to such a 

specification?

 

PS. I know how to do the same, using the assertion query and request protocol.

 

Best Wishes,

Giorgi