Thanks for the info.
I want to have the previous session as well in the query parameter, is the below formatting correct?
FYI info: upon the previous session, I will have a different authentication tree service mapped at IDP assertion context.
http://sp.com:8991/am655sp/saml2/jsp/spSSOInit.jsp?metaAlias=/sp&binding=HTTP-POST&idpEntityID=http://iam.example.com:8991/am655?ForceAuth=true&IsPassive=true&authnContextClassRef=urn:oasis:names:tc:SAML:2.0:ac:classes:PreviousSession.