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Preliminary work on URI documentation to address usnistgov#1249.
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| --- | ||
| title: URI Usage | ||
| description: Provides information on the use of URIs in OSCAL. | ||
| weight: 40 | ||
| --- | ||
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| According to [RFC 3986](https://www.rfc-editor.org/rfc/rfc3986) a Uniform Resource Identifier (URI) "is a compact sequence of characters that identifies an abstract or physical resource." URIs are an important concept, which is used extensively in OSCAL. | ||
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| ## Uniform Resource Identifier Overview | ||
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| According to RFC 3986, a URI has the following syntax as represented in [Augmented Backus-Naur Form (ABNF)](https://www.rfc-editor.org/rfc/rfc5234.html). | ||
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| > ``` | ||
| > URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ] | ||
| > hier-part = "//" authority path-abempty | ||
| > / path-absolute | ||
| > / path-rootless | ||
| > / path-empty | ||
| > ``` | ||
| > | ||
| > The scheme and path components are required, though the path may be empty (no characters). When authority is present, the path must either be empty or begin with a slash ("/") character. When authority is not present, the path cannot begin with two slash characters ("//"). These restrictions result in five different ABNF rules for a path ([Section 3.3](https://www.rfc-editor.org/rfc/rfc3986#section-3.3)), only one of which will match any given URI reference. | ||
| > | ||
| > The following are two example URIs and their component parts: | ||
| > | ||
| > ``` | ||
| > foo://example.com:8042/over/there?name=ferret#nose | ||
| > \_/ \______________/\_________/ \_________/ \__/ | ||
| > | | | | | | ||
| > scheme authority path query fragment | ||
| > | _____________________|__ | ||
| > / \ / \ | ||
| > urn:example:animal:ferret:nose | ||
| > ``` | ||
| According to RFC 3986, a URI can be used as: | ||
| - **A URI with a scheme:** As indicated above with the required scheme and path components. | ||
| - **A relative reference:** Which references a resource relative to another *[base URI](https://www.rfc-editor.org/rfc/rfc3986#section-5.1)*. Such a URI is resolved using [reference resolution](https://www.rfc-editor.org/rfc/rfc3986#section-5). | ||
| According to RFC 3986, the [syntax of a relative reference](https://www.rfc-editor.org/rfc/rfc3986#section-4.2) is: | ||
| > ``` | ||
| > relative-ref = relative-part [ "?" query ] [ "#" fragment ] | ||
| > | ||
| > relative-part = "//" authority path-abempty | ||
| > / path-absolute | ||
| > / path-noscheme | ||
| > / path-empty | ||
| > ``` | ||
| - **A URI reference:** Which is a typical use of a URI, allowing a URI with a scheme or a relative reference to be used. | ||
| According to RFC 3986, the [syntax of a relative reference](https://www.rfc-editor.org/rfc/rfc3986#section-4.1) is: | ||
| > URI-reference = URI / relative-ref | ||
| - **An absolute URI:** | ||
| According to RFC 3986, the [syntax of an absolute URI](https://www.rfc-editor.org/rfc/rfc3986#section-4.3) is: | ||
| > ``` | ||
| > absolute-URI = scheme ":" hier-part [ "?" query ] | ||
| > ``` | ||
| Recognizing these URI forms is important in understanding how URIs are used in OSCAL. | ||
| ## URIs in OSCAL | ||
| The following sections discuss how URIs are used in OSCAL. | ||
| ### OSCAL URI Data Types | ||
| OSCAL uses two data types for representing URIs. | ||
| 1. [`uri`](/reference/datatypes/#uri) - A URI which must provided the required scheme and path components. A *relative-ref* is not allowed for this data type. | ||
| 1. [`uri-reference`](/reference/datatypes/#uri-reference) - A URI reference, which may be a URI or a *relative-ref*. This allows all forms of URIs. | ||
| Data elements within OSCAL expecting a URI will use one of these two data types. | ||
| Examples of `uri` use include: | ||
| - `ns` - ([JSON/YAML](https://pages.nist.gov/OSCAL/reference/latest/complete/json-index/#/ns)) ([XML](https://pages.nist.gov/OSCAL/reference/latest/complete/xml-index/#/@ns)) | ||
| - `system` - ([JSON/YAML](https://pages.nist.gov/OSCAL/reference/latest/complete/json-index/#/system)) ([XML](https://pages.nist.gov/OSCAL/reference/latest/complete/xml-index/#/@system)) | ||
| - `scheme` - ([JSON/YAML](https://pages.nist.gov/OSCAL/reference/latest/complete/json-index/#/scheme)) ([XML](https://pages.nist.gov/OSCAL/reference/latest/complete/xml-index/#/@scheme)) | ||
| - `threat-id` - ([JSON/YAML](https://pages.nist.gov/OSCAL/reference/latest/complete/json-index/#/threat-id)) ([XML](https://pages.nist.gov/OSCAL/reference/latest/complete/xml-index/#/@threat-id)) | ||
| - `url` - ([JSON/YAML](https://pages.nist.gov/OSCAL/reference/latest/complete/json-index/#/url)) ([XML](https://pages.nist.gov/OSCAL/reference/latest/complete/xml-index/#/urls)) | ||
| Examples of `uri-reference` use include: | ||
| - `href` - ([JSON/YAML](https://pages.nist.gov/OSCAL/reference/latest/complete/json-index/#/href)) ([XML](https://pages.nist.gov/OSCAL/reference/latest/complete/xml-index/#/@href)) | ||
| - `source` - ([JSON/YAML](https://pages.nist.gov/OSCAL/reference/latest/complete/json-index/#/source)) ([XML](https://pages.nist.gov/OSCAL/reference/latest/complete/xml-index/#/@source)) | ||
| - `filename` - ([JSON/YAML](https://pages.nist.gov/OSCAL/reference/latest/complete/json-index/#/filename)) ([XML](https://pages.nist.gov/OSCAL/reference/latest/complete/xml-index/#/@filename)) | ||
| ### Common OSCAL URI Use Cases | ||
| URIs are used in OSCAL to provide pointers to resources in the following ways. | ||
| 1. Link to a network resolvable resource. | ||
| URIs are used to point directly to a network resolvable resource. In such cases, the URI may be: | ||
| - A URI with a scheme, where the scheme will likely be `https` indicating the resource can be accessed using the [Hypertext Transfer Protocol](https://www.rfc-editor.org/rfc/rfc2616.html) (HTTP) using [Transport Layer Security](https://www.rfc-editor.org/rfc/rfc8446) (TLS). | ||
| - A relative reference, pointing to a resource that can resolved using the current document resource as the *base URI*. | ||
| 1. Link to an OSCAL back-matter resource. | ||
| A pointer to an OSCAL resource defined in the either the document's `back-matter` or an imported document's `back-matter`. | ||
| This approach uses a relative URI consisting of only a URI fragment containing the UUID of the referenced resource. | ||
| For example, the resource identified by the UUID `f5a2bdb3-55ad-431e-a7ea-c0fd28fc08a0` can be referenced as follows. | ||
| ``` | ||
| <link rel="related" href="#f5a2bdb3-55ad-431e-a7ea-c0fd28fc08a0"/> | ||
| ``` | ||
| More information about the use of links to reference back-matter resources can be found in the [*Referencing Back-Matter Resources*](https://pages.nist.gov/OSCAL/learn/tutorials/general/extension/#referencing-back-matter-resources) section of the [*Extending OSCAL Models with Props and Links*](/learn/tutorials/general/extension/) tutorial. | ||
| 1. As a naming system identifier | ||
| An absolute URI that identifies the naming system. | ||
| OSCAL supports a number of name/value and other controlled value collections. To allow independent organization to organize these value collections, namespaces are used to partition the value spaces on an organization-by-organization basis. An absolute URI is used as the namespace identifier for these situations. | ||
| A key example of this approach is how property names are managed. | ||
| For example, the namespace `http://example.com/ns/oscal` is used in an OSCAL property as follows. | ||
| ``` | ||
| <prop ns="http://example.com/ns/oscal" name="example-name" value="example-value"/> | ||
| ``` | ||
| To learn more about the namespacing of properties, refer to the [*Extending Existing Prop Values*](/learn/tutorials/general/extension/#extending-existing-prop-values) section of the [*Extending OSCAL Models with Props and Links*](/learn/tutorials/general/extension/) tutorial. | ||
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