how to verify the sender of a SAML authn request

Hello,

I
 have a question regarding the SAML Authentication Request. When an IDP 
receives a SAML authentiocation request, how can they validate the 
sender of the request? The issuer name, provider name, 
AssertionConsumerServiceURL and the signature are all optional. If none 
of these are in the request, how can the sender of the request be 
validated?

Thanks

-Mitu