If metadata is used, a SASL service's role MUST contain a corresponding whose Location attribute contains the appropriate service name, as described above. The Binding attribute MUST be one of "urn:ietf:params:xml:ns:samlec" (RECOMMENDED) or "urn:oasis:names:tc:SAML:2.0:bindings:PAOS" (for compatibility with older implementations of the ECP profile in existing identity provider software).
https://tools.ietf.org/html/draft-ietf-kitten-sasl-saml-ec-13 specifies:
It appears that the SP does not support ECP requests using the
samlec
ACS binding. Seeshibsp/handler/impl/SAML2SessionInitiator.cpp#L282
Part of #13