Differences between SP initiated SSO and IDP initiated SSO

IDP Initiated SSO

From PingFederate documentation :- https://docs.pingidentity.com/bundle/pf_sm_supportedStandards_pf82/page/task/idpInitiatedSsoPOST.html

In this scenario, a user is logged on to the IdP and attempts to access a resource on a remote SP server. The SAML assertion is transported to the SP via HTTP POST.

Processing Steps:

1. A user has logged on to the IdP.
2. The user requests access to a protected SP resource. The user is not logged on to the SP site.
3. Optionally, the IdP retrieves attributes from the user data store.
4. The IdP’s SSO service returns an HTML form to the browser with a SAML response containing the authentication assertion and any additional attributes. The browser automatically posts the HTML form back to the SP.

SP Initiated SSO

From PingFederate documentation:- http://documentation.pingidentity.com/display/PF610/SP-Initiated+SSO--POST-POST

In this scenario a user attempts to access a protected resource directly on an SP Web site without being logged on. The user does not have an account on the SP site, but does have a federated account managed by a third-party IdP. The SP sends an authentication request to the IdP. Both the request and the returned SAML assertion are sent through the user’s browser via HTTP POST.

Processing Steps:

1. The user requests access to a protected SP resource. The request is redirected to the federation server to handle authentication.
2. The federation server sends an HTML form back to the browser with a SAML request for authentication from the IdP. The HTML form is automatically posted to the IdP’s SSO service.
3. If the user is not already logged on to the IdP site or if re-authentication is required, the IdP asks for credentials (e.g., ID and password) and the user logs on.

4. Additional information about the user may be retrieved from the user data store for inclusion in the SAML response. (These attributes are predetermined as part of the federation agreement between the IdP and the SP)

5. The IdP’s SSO service returns an HTML form to the browser with a SAML response containing the authentication assertion and any additional attributes. The browser automatically posts the HTML form back to the SP. NOTE: SAML specifications require that POST responses be digitally signed.

6. (Not shown) If the signature and assertion are valid, the SP establishes a session for the user and redirects the browser to the target resource.

In IDP Init SSO (Unsolicited Web SSO) the Federation process is initiated by the IDP sending an unsolicited SAML Response to the SP. In SP-Init, the SP generates an AuthnRequest that is sent to the IDP as the first step in the Federation process and the IDP then responds with a SAML Response. IMHO ADFSv2 support for SAML2.0 Web SSO SP-Init is stronger than its IDP-Init support re: integration with 3rd Party Fed products (mostly revolving around support for RelayState) so if you have a choice you'll want to use SP-Init as it'll probably make life easier with ADFSv2.

Here are some simple SSO descriptions from the PingFederate 8.0 Getting Started Guide that you can poke through that may help as well -- https://documentation.pingidentity.com/pingfederate/pf80/index.shtml#gettingStartedGuide/task/idpInitiatedSsoPOST.html