ProxySG Content Policy Language Guide
24
Layers
A policy layer is a CPL construct used to evaluate a set of rules and reach one decision. Separating
decisions helps control policy complexity, and is done through writing each decision in a separate
layer. Each layer has the form:
<layer_type [label]> [layer_condition][layer_properties] ...
layer_content
where:
• The
layer_type defines the transactions evaluated against this policy, and restricts the triggers
and properties allowed in the rules used in the layer. See the following Layer Types section.
• The optional
label, separated from the layer type by space, is a CPL User-defined Identifier (see
section Chapter 2), basically an alphabetic character followed by alphanumeric or underscore
characters.
• The optional
layer_condition is a list of triggers, all of which must evaluate to true before the
layer content is evaluated.
• The optional
layer_properties is a list of properties that will become the default settings for
those properties for any rule matched in the layer. These can be overridden by explicitly setting a
different value for that property in a specific rule within the layer.
• The
layer_content is a list of rules, possibly organized in sections. (see following). A layer must
contain at least one rule.
Collectively, the layer_condition and layer_properties are often referred to as a layer guard expression.
If a rule has the logical form “if (condition is true) then set properties”, a layer has the form:
if (layer_condition is true) then
{
if (rule1_condition is true) then
set layer_properties then set rule1 properties
else if (rule2_condition is true) then
set layer_properties then set rule2 properties
else if (rule3_condition is true) then
set layer_properties then set rule3 properties
...
}
Within a layer, the first rule that matches terminates evaluation of that layer.
Layers within a policy are evaluated from top to bottom, with rules in later layers taking precedence
over rules in earlier layers.
In CPL, all policy rules are written in a layer. A rule cannot appear in policy preceding any layer
header.
Sections
The rules in layers can optionally be organized in one or more sections, which is a way of grouping
rules together. A section consists of a section header followed by a list of rules.
A section has the form:
