Multiple single-level or multi-security level (MSL) is a means to separate different levels of data by using separate computers or virtual machines for each level. It aims to give some of the benefits of multilevel security without needing special changes to the OS or applications, but at the cost of needing extra hardware.

The drive to develop MLS operating systems was severely hampered by the dramatic fall in data processing costs in the early 1990s. Before the advent of desktop computing, users with classified processing requirements had to either spend a lot of money for a dedicated computer or use one that hosted an MLS operating system. Throughout the 1990s, however, many offices in the defense and intelligence communities took advantage of falling computing costs to deploy desktop systems classified to operate only at the highest classification level used in their organization. These desktop computers operated in system high mode and were connected with LANs that carried traffic at the same level as the computers.

MSL implementations such as these neatly avoided the complexities of MLS but traded off technical simplicity for inefficient use of space. Because most users in classified environments also needed unclassified systems, users often had at least two computers and sometimes more (one for unclassified processing and one for each classification level processed). In addition, each computer was connected to its own LAN at the appropriate classification level, meaning that multiple dedicated cabling plants were incorporated (at considerable cost in terms of both installation and maintenance).

The obvious shortcoming of MSL (as compared to MLS) is that it does not support immixture of various classification levels in any manner. For example, the notion of concatenating a SECRET data stream (taken from a SECRET file) with a TOP SECRET data stream (read from a TOP SECRET file) and directing the resultant TOP SECRET data stream into a TOP SECRET file is unsupported. In essence, an MSL system can be thought of as a set of parallel (and collocated) computer systems, each restricted to operation at one, and only one, security level. Indeed, the individual MSL operating systems may not even understand the concept of security levels, since they operate as single-level systems. For example, while one of a set of collocated MSL OS may be configured to affix the character string "SECRET" to all output, that OS has no understanding of how the data compares in sensitivity and criticality to the data processed by its peer OS that affixes the string "UNCLASSIFIED" to all of its output.

Operating across two or more security levels then, must use methods extraneous to the purview of the MSL "operating systems" per se, and needing human intervention, termed "manual review". For example, an independent monitor (not in Brinch Hansen's sense of the term) may be provided to support migration of data among multiple MSL peers (e.g., copying a data file from the UNCLASSIFIED peer to the SECRET peer). Although no strict requirements by way of federal legislation specifically address the concern, it would be appropriate for such a monitor to be quite small, purpose-built, and supportive of only a small number of very rigidly defined operations, such as importing and exporting files, configuring output labels, and other maintenance/administration tasks that require handling all the collocated MSL peers as a unit rather than as individual, single-level systems. It may also be appropriate to utilize a hypervisor software architecture, such as VMware, to provide a set of peer MSL "OS" in the form of distinct, virtualized environments supported by an underlying OS that is only accessible to administrators cleared for all of the data managed by any of the peers. From the users' perspectives, each peer would present a login or X display manager session logically indistinguishable from the underlying "maintenance OS" user environment.

The cost and complexity involved in maintaining distinct networks for each level of classification led the National Security Agency (NSA) to begin research into ways in which the MSL concept of dedicated system high systems could be preserved while reducing the physical investment demanded by multiple networks and computers. Periods processing was the first advance in this area, establishing protocols by which agencies could connect a computer to a network at one classification, process information, sanitize the system, and connect it to a different network with another classification. The periods processing model offered the promise of a single computer but did nothing to reduce multiple cabling plants and proved enormously inconvenient to users; accordingly, its adoption was limited.

In the 1990s, the rise of virtualization technology changed the playing field for MSL systems. Suddenly, it was possible to create virtual machines (VMs) that behaved as independent computers but ran on a common hardware platform. With virtualization, NSA saw a way to preserve periods processing on a virtual level, no longer needing the physical system to be sanitized by performing all processing within dedicated, system-high VMs. To make MSL work in a virtual environment, however, it was necessary to find a way to securely control the virtual session manager and ensure that no compromising activity directed at one VM could compromise another.

NSA pursued multiple programs aimed at creating viable, secure MSL technologies leveraging virtualization. To date, three major solutions have materialized.