A label range is the set of potentially usable labels at which users can operate. Both users and resources both have label ranges. Resources that can be protected by label ranges include such things as allocatable devices, networks, interfaces, frame buffers, and commands or actions. A label range is defined by a clearance at the top of the range and a minimum label at the bottom.
A range does not necessarily include all combinations of labels that fall between a maximum and minimum label. Rules in the label_encodings file can disqualify certain combinations. A label must be well-formed, that is, permitted by all applicable rules in the label encodings file, in order to be included in a range.
However, a clearance does not have to be well-formed. Suppose, for example, that a label_encodings file prohibits any combination of compartments Eng, Mkt, and Fin in a label. INTERNAL Eng Mkt Fin would be a valid clearance but not a valid label. As a clearance, this combination would let a user access files that are labeled INTERNAL Eng, INTERNAL Mkt, and INTERNAL Fin.
When you assign a clearance and a minimum label to a user, you define the upper and lower boundaries of the account label range in which that user is permitted to operate. The following equation describes the account label range, using ≤ to indicate “dominated by or the same as”:
minimum label ≤ permitted label ≤ clearance
Thus, the user is permitted to operate at any label that is dominated by the clearance as long as that label dominates the minimum label. When a user's clearance or minimum label is not expressly set, the defaults that are defined in the label_encodings file take effect.
Users can be assigned a clearance and a minimum label that enable them to operate at more than one label, or at a single label. When a user's clearance and minimum label are equal, the user can operate at only one label.
The session range is the set of labels that is available to a user during a Trusted Extensions session. The session range must be within the user's account label range and the label range set for the system. At login, if the user selects single-label session mode, the session range is limited to that label. If the user selects multilabel session mode, then the label that the user selects becomes the session clearance. The session clearance defines the upper boundary of the session range. The user's minimum label defines the lower bound. The user begins the session in a workspace at the minimum label. During the session, the user can switch to a workspace at any label within the session range.