A comparison of multilevel structured query language (SQL) implementations

The current commercial multilevel secure (MLS) database management system (DBMS) products provide extensions to SQL to support multilevel database applications. However, the DBMS vendors have implemented a variety of mechanisms that are both difficult to understand and ineffective in addressing a number of application concerns. The paper documents and compares the SQL extensions for Informix Online/Secure, Trusted Oracle, Trusted Rubix, and Sybase Secure SQL server. Based on the vendors' current implementations, we have developed recommendations for an MLS SQL standard that would support interoperability both among the MLS DBMS products and with standard SQL client applications. We have also analyzed the vendors' approaches to polyinstantiation and signaling channels; our recommendations include improved support for cover stories and better control of inherent signaling channels.