The PKIX standard specifies the conventions for using the FTP and the HTTP to obtain certificates and certificate revocation lists (CRLs) from PKI repositories. http://www.ietf.org/rfc/rfc2585.txt
The PKIX standard describes the LDAPv2-based protocol to provide access to PKI repositories for the purposes of retrieving and managing PKI information. http://www.ietf.org/rfc/rfc2559.txt
The PKIX document specifies a protocol useful in determining the current status of a digital certificate without requiring CRLs. ftp://ftp.isi.edu/in-notes/rfc2560.txt