Network Modeling D. Fedyk Internet-Draft LabN Consulting, L.L.C. Intended status: Standards Track S. Mansfield Expires: 3 May 2027 Ericsson 30 October 2026 Extending Normalized Forms to String-Derived Types draft-fedyk-netmod-yang-normal-form-01 Abstract YANG models frequently define identifiers using string or string- derived types whose lexical space permits multiple representations of the same underlying value. This can lead to incorrect behavior when semantically equivalent values are compared lexically. This document adds an optional extension to the existing YANG concept of normalized form to string-derived types whose lexical space permits multiple representations of the same underlying value. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 3 May 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components Fedyk & Mansfield Expires 3 May 2027 [Page 1] Internet-Draft YANG String Normalized Form October 2026 extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4 4. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 5 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 5 5.2. YANG Extension Definition . . . . . . . . . . . . . . . . 6 5.3. Semantics . . . . . . . . . . . . . . . . . . . . . . . . 7 5.4. Normalized identity . . . . . . . . . . . . . . . . . . . 8 5.4.1. mac-48 . . . . . . . . . . . . . . . . . . . . . . . 8 6. Design Rationale . . . . . . . . . . . . . . . . . . . . . . 9 7. Example Usage . . . . . . . . . . . . . . . . . . . . . . . . 9 8. IEEE MAC Address example . . . . . . . . . . . . . . . . . . 9 9. IETF MAC Address example . . . . . . . . . . . . . . . . . . 10 10. Comparison Example . . . . . . . . . . . . . . . . . . . . . 10 11. List Key Example . . . . . . . . . . . . . . . . . . . . . . 10 12. Backward Compatibility . . . . . . . . . . . . . . . . . . . 11 13. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 11 14. Security Considerations . . . . . . . . . . . . . . . . . . . 11 15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 16.1. Normative References . . . . . . . . . . . . . . . . . . 11 16.2. Informative References . . . . . . . . . . . . . . . . . 12 Appendix A. MAC Address Representation Background . . . . . . . 12 A.1. Background . . . . . . . . . . . . . . . . . . . . . . . 12 A.2. Current State . . . . . . . . . . . . . . . . . . . . . . 12 A.3. Detail . . . . . . . . . . . . . . . . . . . . . . . . . 13 A.3.1. IETF Format . . . . . . . . . . . . . . . . . . . . . 13 A.3.2. IEEE Format . . . . . . . . . . . . . . . . . . . . . 13 A.4. Why Normalized Form Helps . . . . . . . . . . . . . . . . 13 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction YANG [RFC7950] 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. Fedyk & Mansfield Expires 3 May 2027 [Page 2] Internet-Draft YANG String Normalized Form October 2026 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 ([RFC6991], [RFC9911]) 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 Appendix A. 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-based operations. The normalized form is an abstract comparison value and is not a new lexical representation. This extension does not alter the lexical space or encoding requirements of the underlying YANG type. This mechanism is intentionally non-invasive. This extension defines a normalization procedure for determining semantic equivalence between values. Unless explicitly defined by a future extension, it does not alter the relational comparison semantics of the underlying YANG type. Operations based on equality (such as equality tests and uniqueness evaluation) operate on the normalized form. Relational operators (<, <=, >, >=) continue to use the semantics of the underlying type. 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. Fedyk & Mansfield Expires 3 May 2027 [Page 3] Internet-Draft YANG String Normalized Form October 2026 2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. 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 consider comparison semantics. 4. Design Goals The solution defined in this document: * MUST NOT alter the lexical space or encoding requirements of the underlying type. * 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. Fedyk & Mansfield Expires 3 May 2027 [Page 4] Internet-Draft YANG String Normalized Form October 2026 5. Proposed Solution 5.1. 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-based operations, including equality comparisons, list-key identity, leaf-list uniqueness, and evaluation of the YANG unique statement. The normalized representation is not required to appear in an encoding or retrieval operation. This extension does not modify the lexical space of an existing YANG type 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. A leafref whose target type has a normalized form MUST use that normalized form when determining whether its value identifies an existing target instance. When evaluating an instance-identifier, predicate values corresponding to list keys or leaf-list values having a normalized form MUST be validated according to the corresponding schema node's type and compared using normalized equality. The normalized representation is not required to appear in the instance-identifier lexical value. For NETCONF insertion before or after an existing user-ordered list or leaf-list entry, values carried in yang:key or yang:value MUST conform to the lexical space of the corresponding schema node. When the server resolves the target entry, it MUST use normalized equality if the corresponding schema node has a normalized form. The normalized representation itself is not required to appear in yang:key or yang:value. Fedyk & Mansfield Expires 3 May 2027 [Page 5] Internet-Draft YANG String Normalized Form October 2026 When a NETCONF subtree-filter content-match node corresponds to a schema node having a normalized form, the filter value MUST first be valid according to the target node's YANG type. The server MUST compare the filter value and datastore value using their normalized forms. Normalization does not expand the lexical space accepted by the type, and a retrieved value continues to use the encoding rules of the underlying type. 5.2. YANG Extension Definition Fedyk & Mansfield Expires 3 May 2027 [Page 6] Internet-Draft YANG String Normalized Form October 2026 module ietf-yang-normalized-form { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-yang:normalized-form"; prefix iynf; organization "IETF NETMOD Working Group"; contact "WG Web: 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 a 48-bit MAC address. The input value MUST first be valid according to the lexical space of the type to which this normalized form is applied. All separators are removed and hexadecimal digits 'a' through 'f' are converted to uppercase. The resulting normalized form consists of 12 uppercase hexadecimal digits."; } extension normalized-form { argument form; description "Specifies a deterministic normalization form used to derive the normalized form of a value."; } } 5.3. Semantics If a type includes the normalized-form extension: 1. A normalized form MUST be derived using the specified identity. Fedyk & Mansfield Expires 3 May 2027 [Page 7] Internet-Draft YANG String Normalized Form October 2026 2. Equality comparisons, including = and !=, MUST use the normalized form. 3. List-key identity and list-key uniqueness MUST be evaluated using the normalized form. 4. Leaf-list uniqueness MUST be enforced using the normalized form. 5. Evaluation of the YANG unique statement MUST use the normalized form for each referenced descendant schema node that has a normalized form. 6. A leafref whose target type has a normalized form MUST use that normalized form when determining whether its value identifies an existing target instance. 7. When evaluating an instance-identifier, predicates that identify list-key or leaf-list values having a normalized form MUST use normalized equality after validating the predicate value against the corresponding schema node's lexical space. 8. Relational operators (<, <=, >, >=) are not changed by this extension and continue to use the semantics of the underlying YANG type. 9. The lexical representation MUST NOT be modified solely to satisfy this extension. 5.4. Normalized identity Normalized forms are identified by identity and are associated with a deterministic algorithm. 5.4.1. 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: 1. Validate the input against the lexical space of the type to which the normalized form is applied. 2. Remove the separator characters permitted by the lexical representation of the underlying type. 3. Convert hexadecimal digits to uppercase. Fedyk & Mansfield Expires 3 May 2027 [Page 8] Internet-Draft YANG String Normalized Form October 2026 4. Use the resulting 12 hexadecimal digits as the normalized form of the MAC address. Equivalent inputs include: aa:bb:cc:dd:ee:ff AA:BB:CC:DD:EE:FF aa-bb-cc-dd-ee-ff AA-BB-CC-DD-EE-FF These values yield the same normalized form: AABBCCDDEEFF 6. Design Rationale A normalized-form identity names a normalization algorithm rather than a regular-expression pattern. Pattern restrictions can constrain lexical syntax, but they cannot in general express normalization procedures such as case folding, separator removal, IPv6 textual normalization, or other procedural transformations. The identity therefore provides a stable name for a fully specified normalization procedure. A pattern may be useful as part of the description of a simple normalization, but it is not sufficient as the general mechanism defined by this document. 7. 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. 8. IEEE MAC Address example The following example show that the normalized form can be added to any definition. Below is an IEEE example. leaf address { type ieee:mac-address; iynf:normalized-form "iynf:mac-48"; mandatory true; description "A sample IEEE MAC address format."; } Fedyk & Mansfield Expires 3 May 2027 [Page 9] Internet-Draft YANG String Normalized Form October 2026 9. IETF MAC Address example The following example show that the normalized form can be added to any definition. Below is an IETF example. leaf address { type ietf:mac-address; iynf:normalized-form "iynf:mac-48"; mandatory true; description "A sample IETF MAC address format."; } 10. Comparison Example The following values use different lexical representations but identify the same underlying 48-bit MAC address: aa:bb:cc:dd:ee:ff AA-BB-CC-DD-EE-FF Because both types declare the same mac-48 normalized form, both values yield the same normalized form: AABBCCDDEEFF Implementations that support this extension compare the values using the normalized form for equality and uniqueness operations, while preserving each type's lexical requirements. 11. List Key Example list fdb-entry { key "mac vlan"; leaf mac { type mac-address; iynf:normalized-form "iynf:mac-48"; } leaf vlan { type uint16; } leaf port { type string; } } Fedyk & Mansfield Expires 3 May 2027 [Page 10] Internet-Draft YANG String Normalized Form October 2026 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. 12. Backward Compatibility Existing YANG models are unaffected unless the extension is used. The lexical space and encoding requirements of the underlying type are unchanged. Behavior changes only for operations whose semantics depend on equality or identity. 13. 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. 14. Security Considerations Normalization reduces ambiguity and helps prevent duplicate or conflicting configuration entries. Implementations MUST ensure that normalization algorithms are deterministic and unambiguous. 15. IANA Considerations This document has no IANA actions. 16. References 16.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types", RFC 6991, DOI 10.17487/RFC6991, July 2013, . [RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016, . Fedyk & Mansfield Expires 3 May 2027 [Page 11] Internet-Draft YANG String Normalized Form October 2026 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC9911] Schönwälder, J., Ed., "Common YANG Data Types", RFC 9911, DOI 10.17487/RFC9911, December 2025, . 16.2. Informative References [IEEE-802-1Qcw] IEEE, "IEEE Standard for Local and Metropolitan Area Networks--Bridges and Bridged Networks Amendment 36: YANG Data Models for Scheduled Traffic, Frame Preemption, and Per-Stream Filtering and Policing", IEEE 802.1Qcw-2023, 17 November 2023, . Appendix A. MAC Address Representation Background A.1. Background MAC Address Formats in the IETF and IEEE YANG modules are different. A.2. Current State The IETF and IEEE YANG modules define MAC-address types derived from the YANG string type, but use different lexical patterns and canonical representations. The issue is that the IETF and IEEE use different patterns and have different canonical forms, which leads to a situation where equivalent MAC Addresses will not match. Both organizations have a long history of supporting these formats. This appendix is meant to document the issue, and support the solution proposed in this document. For example, the following MAC Address is in IETF Canonical Format 90:10:00:01:02:aa For example, the following MAC Address is in IEEE Canonical Format 90-10-00-01-02-AA The MAC addresses are equivalent, but will not match if used in an XPath, or as a key, or any string comparison. Fedyk & Mansfield Expires 3 May 2027 [Page 12] Internet-Draft YANG String Normalized Form October 2026 There are several potential trouble spots in published IETF YANG modules. A.3. Detail A.3.1. IETF Format The IETF Format (from ietf-yang-types@2013-07-15.yang) [RFC9911] used in the mac-address typedef is found below. typedef mac-address { type string { pattern '[0-9a-fA-F]{2}(:[0-9a-fA-F]{2}){5}'; } description "The mac-address type represents an IEEE 802 MAC address. The canonical representation uses lowercase characters. In the value set and its semantics, this type is equivalent to the MacAddress textual convention of the SMIv2."; reference "IEEE 802: IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture RFC 2579: Textual Conventions for SMIv2"; } A.3.2. IEEE Format The IEEE Format used in the mac-address typedef [IEEE-802-1Qcw] is found below. typedef mac-address { type string { pattern "[0-9a-fA-F]{2}(-[0-9a-fA-F]{2}){5}"; } description "The mac-address type represents a MAC address in the canonical format and hexadecimal format specified by IEEE Std 802. The hexadecimal representation uses uppercase characters."; reference "3.1, 8.1 of IEEE Std 802"; } A.4. Why Normalized Form Helps Normal form allows these definitions to remain. The YANG extension described in this document enables comparison of MAC addresses regardless of the format. Fedyk & Mansfield Expires 3 May 2027 [Page 13] Internet-Draft YANG String Normalized Form October 2026 Acknowledgments The authors would like to thank Balazs Lengyel and Joe Clarke for their comments. Authors' Addresses Don Fedyk LabN Consulting, L.L.C. Email: dfedyk@labn.net Scott Mansfield Ericsson Email: scott.mansfield@ericsson.com Fedyk & Mansfield Expires 3 May 2027 [Page 14]