]> Huawei

101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China maqiufang1@huawei.com

Huawei

101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China bill.wu@huawei.com

Ericsson

balazs.lengyel@ericsson.com

HPE

flycoolman@gmail.com

Operations and Management netmod immutable flag system configuration This document defines a way to formally document existing behavior, implemented by servers in production, on the immutability of some system-provided nodes, using a YANG metadata annotation called "immutable" to flag which nodes are immutable. Clients may use "immutable" annotations provided by the server, to know beforehand why certain otherwise valid configuration requests will cause the server to return an error. The immutable flag is descriptive, documenting existing behavior, not proscriptive, dictating server behavior. Discussion Venues Discussion of this document takes place on the Network Modeling Working Group mailing list (netmod@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction This document defines a way to formally document as a YANG metadata annotation an existing model handling behavior that has been used by multiple standard organizations and vendors. It is the aim to create one single standard solution for documenting non-modifiable system data declared as configuration, instead of the multiple existing vendor and organization specific solutions. See for existing implementations. YANG is a data modeling language used to model both state and configuration data, based on the "config" statement. However, there exists some system configuration data that cannot be modified by the client (it is immutable), but still needs to be declared as "config true" to:

• allow configuration of data nodes under immutable lists or containers;
• place "when", "must" and "leafref" constraints between configuration and immutable data nodes;
• ensure the existence of specific list entries that are provided and needed by the system, while additional list entries can be created, modified or deleted.

If the server always rejects the client attempts to override some system-provided data because it internally thinks immutable, it should document it towards the clients in a machine-readable way rather than writing as plain text in the "description" statement. This document defines a way to formally document existing behavior, implemented by servers in production, on the immutability of some system-provided nodes, using a YANG metadata annotation called "immutable" to flag which nodes are immutable. This document does not apply to the server not having any immutable system configuration. While in some cases immutability may be needed, it also has disadvantages, therefore it SHOULD be avoided wherever possible. The following is a list of already implemented and potential use cases:

• UC1 Modeling of server capabilities
• UC2 HW based auto-configuration
• UC3 Predefined administrator roles
• UC4 Declaring immutable system configuration from an LNE's perspective

describes the use cases in detail.

Updates to RFC 6241 and RFC 8526 This document updates and . The NETCONF <get> and <get-config> operations defined in , and <get-data> operation defined in are augmented with an additional input parameter named "with-immutable", as specified in .

Updates to RFC 8040 This document updates Sections and of to add an additional input parameter named "with-immutable", as specified in .

Editorial Note (To be removed by RFC Editor) Note to the RFC Editor: This section is to be removed prior to publication. This document contains placeholder values that need to be replaced with finalized values at the time of publication. This note summarizes all of the substitutions that are needed. No other RFC Editor instructions are specified elsewhere in this document. Please apply the following replacements:

• XXXX --> the assigned RFC number for this draft
• 2024-06-04 --> the actual date of the publication of this document

Conventions and Definitions 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. The document uses the following definition in :

• configuration data

The document uses the following definition in :

• data node
• leaf
• leaf-list
• container
• list
• anydata
• anyxml
• interior node
• data tree

The document uses the following definition in :

• access operation

This document defines the following term:

immutable flag:

A read-only state value the server provides to describe immutability of the data, which is conveyed via a YANG metadata annotation called "immutable" with a boolean value.

Applicability While immutable flag applies to all configuration nodes, its value "true" can only be used for system configuration. The immutable flag is also visible in read-only datastores like <system> (if implemented, see ), <intended> and <operational> when a "with-immutable" parameter is carried (), however this only serves as descriptive information about the instance node itself, but has no effect on the handling of the read-only datastore. Configuration data must have the same immutability in different writable datastores. The immutability of data nodes is protocol and user independent. The immutability and configured value of an existing node MUST only change via software upgrade, hardware resources change, or license change.

"Immutable" Metadata Annotation

Definition The immutable flag which is defined as the metadata annotation takes a boolean value, and it is returned as requested by the client using a "with-immutable" parameter (). If the "immutable" metadata annotation for configuration is not specified, the default "immutable" value is the same as the immutability of its parent node in the data tree (). The immutable metadata annotation value for a top-level instance node is "false" if not specified. Note that "immutable" metadata annotations are used to annotate data node instances. A list may have multiple instances in the data tree, servers may annotate some of the instances as immutable, while others as mutable. Servers MUST ignore any immutable annotations sent from the client.

"with-immutable" Parameter This section specifies the NETCONF and RESTCONF protocol extensions to support "with-immutable" parameter. The "immutable" metadata annotations are not returned in a response unless explicitly requested by the client using this parameter.

NETCONF Extensions to Support "with-immutable" This doument updates to augment the <get-config> and <get> operations with an additional parameter named "with-immutable". The <get-data> operation defined in is also updated to support this parameter. If present, this parameter requests that the server includes the "immutable" metadata annotations in its response. provides the tree structure of augmentations to NETCONF operations, as defined in the "ietf-immutable" module ().

Augmentations to NETCONF Operations

Servers' support for accepting "with-immutable" parameter and returning "immutable" annotations is identified with the feature "immutable".

RESTCONF Extensions to Support "with-immutable" This document extends Sections and of to add a query parameter named "with-immutable" to the GET operation. If present, this parameter requests that the server includes the "immutable" metadata annotations in its response. This parameter is only allowed with no values carried. If it has any unexpected value, then a "404 Bad Request" status-line is returned. To enable a RESTCONF client to discover if the "with-immutable" query parameter is supported by the server, the following capability URI is defined:

Use of Immutable Flag for Different Statements This section defines what the immutable flag means to the client for each instance of YANG data node statement. Throughout this section, the word "change" refers to create, update, and delete.

The "leaf" Statement When a leaf node instance is immutable, its value cannot change.

The "leaf-list" Statement When a leaf-list node instance is immutable, its value cannot change. The immutable annotation attached to the individual leaf-list instance provides immutability with respect to the instance itself. If leaf-list inherits immutability from an ancestor (e.g., container), it is identical to each individual leaf-list entry being annotated without any bearing on the entry ordering and addition of new entries.

The "container" Statement When a container node instance is immutable, it cannot change, unless the immutability of its descendant node is toggled. By default, as with all interior nodes, immutability is recursively applied to descendants ().

The "list" Statement When a list node instance is immutable, it cannot change, unless the immutability of its descendant node is toggled. By default, as with all interior nodes, immutability is recursively applied to descendants (). The immutable annotation attached to the individual list instance provides immutability with respect to the instance itself. If list inherits immutability from an ancestor (e.g., container), it is identical to each individual list entry being annotated without any bearing on the entry ordering and addition of new entries.

The "anydata" Statement When an anydata node instance is immutable, it cannot change. Additionally, as with all interior nodes, immutability is recursively applied to descendants ().

The "anyxml" Statement When an "anyxml" node instance is immutable, it cannot change. Additionally, as with all interior nodes, immutability is recursively applied to descendants ().

Immutability of Interior Nodes Immutability is a conceptual operational state value that is recursively applied to descendants, which may reset the immutability state as needed, thereby affecting their descendants. There is no limit to the number of times the immutability state may change in a data tree. If the "immutable" metadata annotation for returned child node is omitted, it has the same immutability as its parent node. The immutability of top hierarchy of returned nodes is false by default. Servers may suppress the annotation if it is inherited from its parent node or uses the default value as the top-level node, but are not precluded from returning the annotation on every single element. For example, the following XML snippets shows applications configuration a server might return: ssh tcp 22 my-ssh tcp 10022 ]]> In the example, there are two "application" list entries inside "applications" container node. The "immutable" metadata attribute for applications container instance is "false", which is also its default value as the top-level element, and thus can be omitted. The "application" list entry named "ssh" is immutable with the immutability of its child node "port-number" being explicitly toggled. The other child nodes inside "ssh" application instance inherit immutability from their parent node thus are also immutable. The "immutable" metadata attribute for application list entry named "my-ssh" is "false", which is also the same value as its parent node, and thus can be omitted.

System Configuration Datastore Interactions Immutable configuration can only be created, updated and deleted by the server, and it is present in <system>, if implemented. That said, the existence of immutable configuration is independent of whether <system> is implemented or not. Not all system configuration data is immutable. Immutable configuration does not appear in <running> unless it is explicitly provided by the client or copied by the server via "resolve-system" parameter defined in . A client may create/delete immutable nodes with same values as found in <system> (if implemented) in read-write configuration datastore (e.g., <candidate>, <running>), which merely mean making immutable nodes visible/invisible in the datastore.

NACM Interactions The server rejects an operation request due to immutability when it tries to perform the operation on the request data. It happens after any access control processing, if the Network Configuration Access Control Model (NACM) is implemented on a server. For example, if an operation requests to override an immutable configuration data, but the server checks the user is not authorized to perform the requested access operation on the request data, the request is rejected with an "access-denied" error.

YANG Module This module imports definitions from and . WG List: Author: Qiufang Ma Author: Qin Wu Author: Balazs Lengyel Author: Hongwei Li "; description "This module defines a metadata annotation called 'immutable' to allow the server to formally document existing behavior on the mutability of some system configuration. Clients may use 'immutable' metadata annotation provided by the server to know beforehand why certain otherwise valid configuration requests will cause the server to return an error. Copyright (c) 2024 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Revised BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This version of this YANG module is part of RFC HHHH (https://www.rfc-editor.org/info/rfcHHHH); see the RFC itself for full legal notices. 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 (RFC 2119) (RFC 8174) when, and only when, they appear in all capitals, as shown here."; revision 2024-06-04 { description "Initial revision."; // RFC Ed.: replace XXXX and remove this comment reference "RFC XXXX: YANG Metadata Annotation for Immutable Flag"; } md:annotation immutable { type boolean; description "The 'immutable' metadata annotation indicates the immutability of an instantiated data node. It takes as a value 'true' or 'false'. If the 'immutable' metadata annotation is not specified, the default value is the same as the value of its parent node in the data tree. The default value for a top-level instance node is false if not specified."; } feature immutable { description "Indicates that the server supports the 'immutable' metadata annotation."; } grouping with-immutable-grouping { description "Grouping for the with-immutable parameter that augments the RPC operations."; leaf with-immutable { type empty; description "If this parameter is present, the server returns the 'immutable' annotation for configuration that it internally thinks immutable."; } } augment "/ncds:get-data/ncds:input" { if-feature "immutable"; description "Allows the server to include 'immutable' metadata annotations in its response to get-data operation."; uses with-immutable-grouping; } augment "/nc:get-config/nc:input" { if-feature "immutable"; description "Allows the server to include 'immutable' metadata annotations in its response to get-config operation."; uses with-immutable-grouping; } augment "/nc:get/nc:input" { if-feature "immutable"; description "Allows the server to include 'immutable' metadata annotations in its response to get operation."; uses with-immutable-grouping; } } ]]>

Security Considerations This section uses the template described in . The "ietf-immutable" YANG module specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF or RESTCONF . The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) . The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS . The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. The YANG module specified in this document defines a metadata annotation, it also extends the RPC operations of the NETCONF protocol in and . The security considerations for the Defining and Using Metadata with YANG (see ) apply to the metadata annotation defined in this document. The security considerations for the NETCONF protocol operations (see and ) still apply to the operations extended in this document.

IANA Considerations

The "IETF XML" Registry This document registers one XML namespace URN in the 'IETF XML registry', following the format defined in .

The "YANG Module Names" Registry This document registers one module name in the 'YANG Module Names' registry, defined in .

RESTCONF Capability URN Registry This document defines the following capability identifier URNs in the "RESTCONF Capability URNs" registry defined in :

References Normative References YANG is a data modeling language used to model configuration data, state data, Remote Procedure Calls, and notifications for network management protocols. This document describes the syntax and semantics of version 1.1 of the YANG language. YANG version 1.1 is a maintenance release of the YANG language, addressing ambiguities and defects in the original specification. There are a small number of backward incompatibilities from YANG version 1. This document also specifies the YANG mappings to the Network Configuration Protocol (NETCONF). This document defines a YANG extension that allows for defining metadata annotations in YANG modules. The document also specifies XML and JSON encoding of annotations and other rules for annotating instances of YANG data nodes. The Network Configuration Protocol (NETCONF) defined in this document provides mechanisms to install, manipulate, and delete the configuration of network devices. It uses an Extensible Markup Language (XML)-based data encoding for the configuration data as well as the protocol messages. The NETCONF protocol operations are realized as remote procedure calls (RPCs). This document obsoletes RFC 4741. [STANDARDS-TRACK] This document extends the Network Configuration Protocol (NETCONF) defined in RFC 6241 in order to support the Network Management Datastore Architecture (NMDA) defined in RFC 8342. This document updates RFCs 6241 and 7950. The update to RFC 6241 adds new and operations and augments existing,, and operations. The update to RFC 7950 requires the usage of the YANG library (described in RFC 8525) by NETCONF servers implementing the NMDA. This document describes an HTTP-based protocol that provides a programmatic interface for accessing data defined in YANG, using the datastore concepts defined in the Network Configuration Protocol (NETCONF). In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. The standardization of network configuration interfaces for use with the Network Configuration Protocol (NETCONF) or the RESTCONF protocol requires a structured and secure operating environment that promotes human usability and multi-vendor interoperability. There is a need for standard mechanisms to restrict NETCONF or RESTCONF protocol access for particular users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. This document defines such an access control model. This document obsoletes RFC 6536. This document describes a method for invoking and running the Network Configuration Protocol (NETCONF) within a Secure Shell (SSH) session as an SSH subsystem. This document obsoletes RFC 4742. [STANDARDS-TRACK] This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. This document describes an IANA maintained registry for IETF standards which use Extensible Markup Language (XML) related items such as Namespaces, Document Type Declarations (DTDs), Schemas, and Resource Description Framework (RDF) Schemas. YANG is a data modeling language used to model configuration and state data manipulated by the Network Configuration Protocol (NETCONF), NETCONF remote procedure calls, and NETCONF notifications. [STANDARDS-TRACK] Informative References ONF 3GPP 3GPP Huawei Huawei This document defines how a management client and management server handle YANG-modeled configuration data that is instantiated by the server itself. The system-defined configuration can be referenced (e.g., leafref) by configuration explicitly created by a client. The Network Management Datastore Architecture (NMDA) defined in RFC 8342 is updated with a read-only conventional configuration datastore called "system" to expose system-defined configuration. This document updates RFC 6241, RFC 8040, RFC 8342, and RFC 8526. This document captures the current syntax used in YANG module tree diagrams. The purpose of this document is to provide a single location for this definition. This syntax may be updated from time to time based on the evolution of the YANG language. YumaWorks Orange Huawei This memo provides guidelines for authors and reviewers of specifications containing YANG modules, including IANA-maintained modules. Recommendations and procedures are defined, which are intended to increase interoperability and usability of Network Configuration Protocol (NETCONF) and RESTCONF protocol implementations that utilize YANG modules. This document obsoletes RFC 8407. Also, this document updates RFC 8126 by providing additional guidelines for writing the IANA considerations for RFCs that specify IANA-maintained modules. This document defines a YANG data model for the management of network interfaces. It is expected that interface-type-specific data models augment the generic interfaces data model defined in this document. The data model includes definitions for configuration and system state (status information and counters for the collection of statistics). The YANG data model in this document conforms to the Network Management Datastore Architecture (NMDA) defined in RFC 8342. This document obsoletes RFC 7223. This document defines a logical network element (LNE) YANG module that is compliant with the Network Management Datastore Architecture (NMDA). This module can be used to manage the logical resource partitioning that may be present on a network device. Examples of common industry terms for logical resource partitioning are logical systems or logical routers. The YANG model in this document conforms with NMDA as defined in RFC 8342.

Detailed Use Cases

UC1 - Modeling of server capabilities System capabilities might be represented as system-defined data nodes in the model. Configurable data nodes might need constraints specified as "when", "must" or "path" statements to ensure that configuration is set according to the system's capabilities. For example,

• A timer can support the values 1,5,8 seconds. This is defined in the leaf-list 'supported-timer-values'.
• When the configurable 'interface-timer' leaf is set, it should be ensured that one of the supported values is used. The natural solution would be to make the 'interface-timer' a leaf-ref pointing at the 'supported-timer-values'.

However, this is not possible as 'supported-timer-values' must be read-only thus config=false while 'interface-timer' must be writable thus config=true. According to the rules of YANG it is not allowed to put a constraint between config true and false data nodes. The solution is that the supported-timer-values data node in the YANG Model shall be defined as "config true" and shall also be marked with the "immutable" annotation making it unchangeable. After this the 'interface-timer' shall be defined as a leaf-ref pointing at the 'supported-timer-values'.

UC2 - HW based auto-configuration - Interface Example defines a YANG data model for the management of network interfaces. When a system-controlled interface is physically present, the system creates an interface entry with valid name and type values in <system> (if exists, see ). The system-generated type value is dependent on and represents the HW present, and as a consequence cannot be changed by the client. If a client tries to set the type of an interface to a value that can never be used by the system, the request will be rejected by the server. The data is modelled as "config true" and should be annotated as immutable. Seemingly an alternative would be to model the list and these leaves as "config false", but that does not work because:

• The list cannot be marked as "config false", because it needs to contain configurable child nodes, e.g., ip-address or enabled;
• The key leaf (name) cannot be marked as "config false" as the list itself is config true;
• The type cannot be marked "config false", because we MAY need to reference the type to make different configuration nodes conditionally available.

UC3 - Predefined Administrator Roles User and group management is fundamental for setting up access control rules (see ). A device may provide a predefined user account (e.g., a system administrator that is always available and has full privileges) for initial system set up and management of other users/groups. It is possible that clients can define a new user/group and grant it particular privileges, but the predefined administrator account and its granted access cannot be modified.

UC4 - Declaring immutable system configuration from an LNE's perspective An LNE (logical network element) is an independently managed virtual network device made up of resources allocated to it from its host or parent network device . The host device may allocate some resources to an LNE, which from an LNE's perspective is provided by the system and may not be modifiable. For example, a host may allocate an interface to an LNE with a valid MTU value as its management interface, so that the allocated interface should then be accessible as the LNE-specific instance of the interface model. The assigned MTU value is system-created and immutable from the context of the LNE.

Existing Implementations There are already a number of full or partial implementations of immutability:

• 3GPP TS 32.156 and 28.623 : Requirements and a partial solution
• ITU-T using ONF TR-531 concept on information model level but no YANG representation.
• Ericsson: requirements and solution
• YumaPro: requirements and solution
• Nokia: partial requirements and solution
• Huawei: partial requirements and solution
• Cisco using the concept at least in some YANG modules
• Junos OS provides a hidden and immutable configuration group called junos-defaults

Acknowledgments Thanks to Kent Watsen, Jan Lindblad, Jason Sterne, Robert Wilton, Andy Bierman, Juergen Schoenwaelder, Reshad Rahman, Anthony Somerset, Lou Berger, Joe Clarke, and Scott Mansfield for reviewing, and providing important inputs to this document.