ISIS-MTT-Optional: The certificate requested by the client by inserting the RetrieveIfAllowed extension in the request, will be returned in this extension.

ISIS-MTT-SigG: The signature act allows publishing certificates only then, when the certificate owner gives his explicit permission. Accordingly, there may be �nondownloadable� certificates, about which the responder must provide status information, but MUST NOT include them in the response. Clients may get therefore the following three kind of answers on a single request including the RetrieveIfAllowed extension:

• a) the responder supports the extension and is allowed to publish the certificate: RequestedCertificate returned including the requested certificate
• b) the responder supports the extension but is NOT allowed to publish the certificate: RequestedCertificate returned including an empty OCTET STRING
• c) the responder does not support the extension: RequestedCertificate is not included in the response

Clients requesting RetrieveIfAllowed MUST be able to handle these cases. If any of the OCTET STRING options is used, it MUST contain the DER encoding of the requested certificate.

           RequestedCertificate ::= CHOICE {
             Certificate Certificate,
             publicKeyCertificate [0] EXPLICIT OCTET STRING,
             attributeCertificate [1] EXPLICIT OCTET STRING
           }

           RequestedCertificate ::= CHOICE {
             Certificate Certificate,
             publicKeyCertificate [0] EXPLICIT OCTET STRING,
             attributeCertificate [1] EXPLICIT OCTET STRING
           }