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Updates to OSCAL Metaschema documentation and constraints (usnistgov#…
…1263) * completed partial update of the Metadata object documentation. * adjustments to roles * adjusted the cardinality of location/address to make address optional. * Improved documentation and constraints related to location and parties * addressed the remainder of metadata and control feedback from @Rene2mt. * Improved the introductory remarks for a profile to better describe what a profile is and what it does. * Fixed a broken constraint that was not targeting the right node. * started refining descriptions and adding properties to describe identifier attributes. * Addressed feedback from AJ during 20220718-20220722. (#48) * Week 30 feedback on SSP model. (#49) * Proposed metaschema docs updates (#50) * Addressed feedback based on usnistgov#1392 * Adjustments based on model review feedback on 8/12. * Removed outdated merge phase remarks. Created issue #53 to address this. * Addressed A.J. Stein's Week 32 Feedback for Model Review (#52) * Addressed AJ Stein's week 32 feedback for usnistgov#1331. * Addressed DRAFT: Update catalog & profile metaschema documentation (#51) * Update catalog & profile metaschema documentation * Add props to control identifier * Fixed broken syntax and addressed consistency in wording within the Profile 'merge' construct. * Adjustments to alter, moving to to an inline definition * cleaned up empty remark. * Removed redundant constraints * removed some redundant constraints * Preliminary work on URI documentation to address usnistgov#1249. * More work on document URI use in OSCAL * Updating data types related to usnistgov/metaschema#224. * Improved consistency of how URI concepts are discussed. * Added note about party locations * Updated Metaschema instances of `uri` and `uri-reference` data types to indicate their URI semantics. Resolves usnistgov#1249. * Added identifier props to control layer metaschemas (#55) * Responding to usnistgov#1066: metaschema edits; CSS enhancement (#56) * Whitespace cleanup in metadata metaschema * Apply suggestions from code review Co-authored-by: Alexander Stein <[email protected]> Co-authored-by: Wendell Piez <[email protected]> Co-authored-by: Rene Tshiteya <[email protected]>
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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 are 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, which is represented in [Augmented Backus-Naur Form (ABNF)](https://www.rfc-editor.org/rfc/rfc5234.html) below. | ||
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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 in a few different ways. Recognizing these URI forms is important in understanding how URIs are used in OSCAL. | ||
| ### URI with a Required Scheme | ||
| As indicated above with the required scheme and path components. | ||
| ### Relative Reference | ||
| A URI that is a relative reference, 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). | ||
| 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 | ||
| > ``` | ||
| ### URI Reference | ||
| A typical use of a URI, allowing a [URI with a required scheme](#uri-with-a-required-scheme) or a [relative reference](#relative-reference) to be used. | ||
| The [syntax of a URI reference](https://www.rfc-editor.org/rfc/rfc3986#section-4.1) is: | ||
| > URI-reference = URI / relative-ref | ||
| ### 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 ] | ||
| > ``` | ||
| ## URI vs URL vs URN | ||
| According to RFC 3986 section [1.1.3](https://www.rfc-editor.org/rfc/rfc3986#section-1.1.3), "a URI can be further classified as a *locator*, a *name*, or *both*." A given URI scheme is not limited to being only a name or a locator; both characteristics can be associated. | ||
| - To be a *locator*, the resource pointed to by a URI needs to have persistence. | ||
| - To be a *name*, the URI needs to be used consistently to refer to the thing that is named. A URI used only as a name is not required to resolve to a location. URIs schemes requiring an [*authority*](https://www.rfc-editor.org/rfc/rfc3986#section-3.2) element provide a means to use a registered DNS name to assert organizational control over a naming value space or namespace. | ||
| A *Uniform Resource Locator (URL)* "refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network "location")." | ||
| A URL, when applied consistently, can be used as a *name*. Optionally in such cases, the resource it resolves to can provide information about how to use the URL as a name. | ||
| A *Uniform Resource Name (URN)* "has been used historically to refer to both URIs under the `urn` scheme [RFC2141](https://www.rfc-editor.org/rfc/rfc2141), which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable, and to any other URI with the properties of a name. | ||
| A URN is often not a good fit for use as a *locator*, since it requires a secondary resolution process that maps the URN's *name* to a specific *location*. | ||
| Due to the specific characteristics of a URL or URN, the term URI is often used to refer more broadly to all types of resource identifiers. | ||
| ## 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 provide the required scheme and path components. This means the URI will point directly to a resolvable resource. | ||
| The `uri` data type is used in cases where a [*URI with a required scheme*](#uri-with-a-required-scheme) or an *absolute URI* is required. As a result, a [*relative reference*](#relative-reference) or a [*URI reference*](#uri-reference) is not allowed for use with this data type. | ||
| 2. [`uri-reference`](/reference/datatypes/#uri-reference) - A [*URI reference*](#uri-reference), which may be a [*URI with a required scheme*](#uri-with-a-required-scheme) or a [*relative reference*](#relative-reference). This allows all forms of URIs. | ||
| ### Common OSCAL URI Use Cases | ||
| URIs are used in OSCAL to provide pointers to resources in the following ways. | ||
| #### Linking 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 required scheme*](#uri-with-a-required-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). Data fields supporting only this use case will have the `uri` data type. | ||
| OSCAL examples include: | ||
| - `threat-id` - ([JSON/YAML](/reference/latest/complete/json-index/#/threat-id)) ([XML](/reference/latest/complete/xml-index/#/@threat-id)) | ||
| - `url` - ([JSON/YAML](/reference/latest/complete/json-index/#/url)) ([XML](/reference/latest/complete/xml-index/#/urls)) | ||
| - A [*relative reference*](#relative-reference), pointing to a resource that can resolved using the current document resource as the *base URI*. Data fields supporting this use case will have the `uri-reference` data type. | ||
| OSCAL examples include: | ||
| - `href` - ([JSON/YAML](/reference/latest/complete/json-index/#/href)) ([XML](/reference/latest/complete/xml-index/#/@href)) | ||
| - `source` - ([JSON/YAML](/reference/latest/complete/json-index/#/source)) ([XML](/reference/latest/complete/xml-index/#/@source)) | ||
| - `filename` - ([JSON/YAML](/reference/latest/complete/json-index/#/filename)) ([XML](/reference/latest/complete/xml-index/#/@filename)) | ||
| URIs serving this purpose need to be used as a *locator*. URLs are typically used for this purpose since the URI must resolve to a specific location. | ||
| #### Linking to another OSCAL object | ||
| A pointer to an OSCAL object identified by the referenced identifier, may be a [human-oriented](/concepts/identifier-use/#human-oriented) [`token`](/reference/datatypes/#token) or a [machine-oriented](/concepts/identifier-use/#machine-oriented) [`uuid`](https://pages.nist.gov/OSCAL/reference/datatypes/#uuid). | ||
| This approach uses a [*relative reference*](#relative-reference) consisting of only a URI *fragment* containing the identifier or UUID of the referenced object within the current documents effective data model. The effective data model of a document includes all objects identified with the document and any directly or transitively imported documents. Identifiers with a *cross-instance* [scope](/concepts/identifier-use/#scope) are available to importing documents. | ||
| URIs serving this purpose need to be used as a *locator*. | ||
| Any data fields supporting this use case will have the `uri-reference` data type. | ||
| A typical use of OSCAL object identifier linking is referencing a `resource` in the document's `back-matter` or an imported document's `back-matter`. For example, the back-matter 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*](/learn/tutorials/general/extension/#referencing-back-matter-resources) section of the [*Extending OSCAL Models with Props and Links*](/learn/tutorials/general/extension/) tutorial. | ||
| #### Use as a naming system identifier | ||
| An absolute URI that identifies the naming system. URIs serving this purpose are used as a *name*. Data fields supporting this use case will have the `uri` data type. | ||
| 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*](#absolute-uri) is used as the namespace identifier for these situations. | ||
| When used in this way, the authority component of the URI must use a value that the organization has control over. Typically, a DNS domain name controlled by the organization is used for this purpose. | ||
| OSCAL examples include: | ||
| - `ns` - ([JSON/YAML](/reference/latest/complete/json-index/#/ns)) ([XML](/reference/latest/complete/xml-index/#/@ns)) | ||
| - `system` - ([JSON/YAML](/reference/latest/complete/json-index/#/system)) ([XML](/reference/latest/complete/xml-index/#/@system)) | ||
| - `scheme` - ([JSON/YAML](/reference/latest/complete/json-index/#/scheme)) ([XML](/reference/latest/complete/xml-index/#/@scheme)) | ||
| A key example of this approach is how property names are partitioned using a `ns` data element. | ||
| 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 use of namespaces in 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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