Extending Normalized Forms to String-Derived Types

Introduction YANG treats values of type string as lexically distinct; equality and uniqueness are therefore determined by exact string comparison. YANG defines normalized representations for many built-in data types. Canonical or normalized forms provide a unique representation of a value independent of how it may have been entered or encoded. For string-derived types, YANG currently provides no mechanism to define a normalized form distinct from the lexical representation. As a result, semantically equivalent values that have multiple valid lexical representations may not compare equal and may not be detected as duplicates. This issue has been observed in both IETF and IEEE YANG modules, leading to interoperability problems and incorrect duplicate detection. Existing YANG typedefs such as mac-address (, ) define syntax but do not define normalized comparison semantics. A prominent example is the representation of MAC addresses in YANG. IETF and IEEE modules define different lexical forms for MAC addresses, and equivalent values may not compare equal when represented using different valid formats. This problem is described in . While MAC addresses provide a clear motivating example, the underlying issue is more general. YANG lacks a mechanism for schema authors to define normalized forms for string-derived types whose lexical space permits multiple representations of the same underlying value. The resulting normalized form is used for equality and uniqueness operations. The original lexical representation is preserved for encoding and retrieval. This mechanism is intentionally non-invasive. Existing YANG modules remain valid and unchanged. The extension is applied only where normalized forms are needed and has no effect on implementations that do not support it.

Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Problem Statement When string-derived types permit multiple lexical representations of the same value, the following issues can arise:

Semantically equivalent values may not compare equal.
Duplicate entries may not be detected in keyed lists.
Leaf-list uniqueness constraints may be violated.
XPath equality comparisons may yield incorrect results.

Pattern restrictions alone do not solve this problem, because they validate syntax but do not affect comparison semantics.

Design Goals The solution defined in this document:

MUST preserve the lexical representation of values.
MUST provide deterministic comparison semantics.
MUST apply to equality and uniqueness operations.
MUST be minimally invasive to existing YANG modules.
MUST be opt-in and backward compatible.
MUST be general across string-derived types.
SHOULD be simple to implement.
SHOULD avoid complex transformation languages.

Proposed Solution

Overview This document defines a YANG extension that allows a string-derived type to specify a normalization algorithm. The resulting normalized form is used for equality comparisons, list key uniqueness, and leaf-list uniqueness. The original lexical representation is preserved for encoding and retrieval. This extension does not modify the YANG language and does not require changes to existing typedef definitions. The extension may be attached to existing types, derived types, or schema nodes where normalized form behavior is desired. Implementations that do not understand the extension continue to process the lexical type normally. Implementations that advertise support for the extension apply the specified normalization algorithm for equality and uniqueness operations. Use of this extension does not, by itself, change the behavior of implementations that do not support it.

YANG Extension Definition WG List: "; description "Defines an extension for declaring a normalized form for string-derived types."; revision 2026-07-01 { description "Initial revision."; reference "TBD: This document."; } identity normalized-form { description "Base identity for normalized forms."; } identity mac-48 { base normalized-form; description "Normalized form for 48-bit MAC addresses."; } extension normalized-form { argument form; description "Specifies a deterministic normalization form used to derive the normalized form of a value."; } } ]]>

Semantics If a type includes the normalized-form extension:

A normalized form MUST be derived using the specified identity.
Equality comparisons, including = and != , MUST use the normalized form.
List key uniqueness MUST be enforced using the normalized form.
Leaf-list uniqueness MUST be enforced using the normalized form.
The lexical representation MUST NOT be modified solely to satisfy this extension.

Normalized identity Normalized forms are identified by identity and are associated with a deterministic algorithm.

mac-48 The mac-48 identity applies to 48-bit MAC addresses represented as six hexadecimal octets. The lexical representation is defined by the type that uses the extension. The normalization procedure is:

Validate the input against the lexical pattern.
Remove all separators.
Convert hexadecimal digits to uppercase.
Use the resulting 12 hexadecimal digits as the normalized form of the MAC address.

Equivalent inputs include: These values yield the same normalized form:

Example Usage The definition should be in a top level YANG module. While new types with the normalized form could be created it is also valid to just modify in place IEEE mac-address to support the normalized form.

IEEE MAC Address example The following example show that the normalized form can be added to any definition. Below is an IEEE example.

IETF MAC Address example The following example show that the normalized form can be added to any definition. Below is an IETF example.

Comparison Example The following values use different lexical representations but identify the same underlying 48-bit MAC address: Because both types declare the same mac-48 normalized form, both values yield the same normalized form: Implementations that support this extension compare the values using the normalized form for equality and uniqueness operations, while preserving each type's lexical requirements.

List Key Example A list key using the IETF typedef continues to accept only the IETF colon-separated lexical form. A corresponding IEEE model can use the IEEE typedef and preserve the IEEE dash-separated lexical form. In both cases, the shared mac-48 normalized form enables consistent equality and duplicate detection across representations.

Backward Compatibility Existing YANG models are unaffected unless the extension is used. Lexical representation is preserved. Behavior changes only affect comparison and uniqueness.

Applicability While motivated by MAC addresses, this mechanism can apply to any string-derived type with multiple equivalent representations, including case-insensitive identifiers, formatted identifiers, and normalized encodings.

Security Considerations Normalization reduces ambiguity and helps prevent duplicate or conflicting configuration entries. Implementations MUST ensure that normalization algorithms are deterministic and unambiguous.

IANA Considerations This document has no IANA actions.