title: "YANG Metadata Annotation for Immutable Flag" abbrev: "immutable flag" category: std updates: 6241, 8040, 8526
docname: draft-ietf-netmod-immutable-flag-latest submissiontype: IETF number: date: consensus: true v: 3 area: "Operations and Management" workgroup: "netmod" keyword:
- immutable flag
- system configuration
fullname: Qiufang Ma
organization: Huawei
role: editor
street: 101 Software Avenue, Yuhua District
city: Nanjing, Jiangsu
code: 210012
country: China
email: [email protected]
- fullname: Qin Wu organization: Huawei street: 101 Software Avenue, Yuhua District city: Nanjing, Jiangsu code: 210012 country: China email: [email protected]
- fullname: Balazs Lengyel organization: Ericsson role: editor email: [email protected]
- fullname: Hongwei Li organization: HPE email: [email protected]
normative:
informative:
TR-531: title: UML to YANG Mapping Guidelines author: - organization: "ONF" target: https://wiki.opennetworking.org/download/attachments/376340494/Draft_TR-531_UML-YANG_Mapping_Gdls_v1.1.03.docx?version=5&modificationDate=1675432243513&api=v2 date: February 2023
TS28.623: title: "Telecommunication management; Generic Network Resource Model (NRM) Integration Reference Point (IRP); Solution Set (SS) definitions" author: - organization: "3GPP" target: https://www.3gpp.org/ftp/Specs/archive/28_series/28.623/28623-i02.zip date: false
TS32.156: title: "Telecommunication management; Fixed Mobile Convergence (FMC) Model repertoire" author: - organization: "3GPP" target: https://www.3gpp.org/ftp/Specs/archive/32_series/32.156/32156-h10.zip date: false
--- abstract
This document defines a way to formally document an 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 an existing behavior, not proscriptive, dictating server behaviors.
This document updates {{!RFC6241}}, {{!RFC8040}}, and {{!RFC8526}}.
--- middle
This document defines a YANG metadata annotation {{!RFC7952}} to formally document 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.
YANG {{!RFC7950}} 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 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 a client's attempt 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 the existing behavior, implemented by servers in production, on the immutability of some system-provided nodes, using a YANG metadata annotation {{!RFC7952}} 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 Hardware based auto-configuration
- UC3 Predefined administrator roles
- UC4 Declaring immutable system configuration from an LNE's perspective
{{use-cases}} describes the use cases in detail.
This document updates {{!RFC6241}} and {{!RFC8526}}. The NETCONF <get> and <get-config> operations defined in {{!RFC6241}}, and <get-data> operation defined in {{!RFC8526}} are augmented with an additional input parameter named "with-immutable", as specified in {{NETCONF-ext}}.
This document updates {{Sections 4.8 and 9.1.1 of !RFC8040}} to add an additional input parameter named "with-immutable", as specified in {{RESTCONF-ext}}.
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
{::boilerplate bcp14-tagged}
The document uses the following definition in {{!RFC6241}}:
- configuration data
The document uses the following definition in {{!RFC7950}}:
- data node
- leaf
- leaf-list
- container
- list
- anydata
- anyxml
- interior node
- data tree
The document uses the following definition in {{!RFC8341}}:
- access operation
This document defines the following term:
immutable flag: : A read-only state value the server provides to describe immutability of the configuration, which is conveyed via a YANG metadata annotation called "immutable" with a boolean value.
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 {{?I-D.ietf-netmod-system-config}}), <intended> and <operational> when a "with-immutable" parameter is carried ({{with-immutable}}), however this only serves as descriptive information about the instance node itself, but has no effect on the handling of the read-only datastore.
An instance has the same immutability if it appears in different writable datastores, the immutability of data object is also 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.
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 ({{with-immutable}}). If the "immutable" metadata annotation for a configuration node is not specified, the default "immutable" value is the same as the value of its parent node in the data tree ({{interior}}). 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.
This section specifies the NETCONF {{!RFC6241}} and RESTCONF {{!RFC8040}} protocol extensions to support the "with-immutable" parameter. The "immutable" metadata annotations are not returned in a response unless explicitly requested by the client using this parameter.
This doument updates {{!RFC6241}} to augment the <get-config> and <get> operations with an additional parameter named "with-immutable". The <get-data> operation defined in {{!RFC8526}} is also updated to support this parameter. If present, this parameter requests that the server includes the "immutable" metadata annotations in its response.
{{tree}} provides the tree structure {{?RFC8340}} of augmentations to NETCONF operations, as defined in the "ietf-immutable" module ({{module}}).
module: ietf-immutable
augment /ncds:get-data/ncds:input:
+---w with-immutable? empty {immutable}?
augment /nc:get-config/nc:input:
+---w with-immutable? empty {immutable}?
augment /nc:get/nc:input:
+---w with-immutable? empty {immutable}?
{: #tree title="Augmentations to NETCONF Operations" artwork-align="center}
Servers' support for accepting "with-immutable" parameter and returning "immutable" annotations is identified with the feature "immutable".
This document extends {{Sections 4.8 and 9.1.1 of !RFC8040}} 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:
urn:ietf:params:restconf:capability:with-immutable:1.0
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.
When a leaf node instance is immutable, its value cannot change.
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 a 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.
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 ({{interior}}).
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 ({{interior}}).
The immutable annotation attached to the individual list instance provides immutability with respect to the instance itself. If a 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.
When an anydata node instance is immutable, it cannot change. Additionally, as with all interior nodes, immutability is recursively applied to descendants ({{interior}}).
When an "anyxml" node instance is immutable, it cannot change. Additionally, as with all interior nodes, immutability is recursively applied to descendants ({{interior}}).
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 application configuration a server might return:
<applications im:immutable="false">
<application im:immutable="true">
<name>ssh</name>
<protocol>tcp</protocol>
<port-number im:immutable="false">22</port-number>
</application>
<application im:immutable="false">
<name>my-ssh</name>
<protocol>tcp</protocol>
<port-number>10022</port-number>
</application>
</applications>
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.
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 configured.
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.
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) {{!RFC8341}} 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.
This module imports definitions from {{!RFC6241}} and {{!RFC8526}}.
<CODE BEGINS> file "[email protected]"
{::include ietf-immutable.yang}
<CODE ENDS>
This section is modeled after the template described in {{Section 3.7 of ?I-D.ietf-netmod-rfc8407bis}}.
The "ietf-immutable" YANG module defines a data model that is designed to be accessed via YANG-based management protocols, such as NETCONF {{!RFC6241}} or RESTCONF {{!RFC8040}}. These protocols have to use a secure transport layer (e.g., SSH {{?RFC4252}}, TLS {{?RFC8446}}, and QUIC {{?RFC9000}}) and have to use mutual authentication.
The Network Configuration Access Control Model (NACM) {{!RFC8341}} 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 {{!RFC6241}} and {{!RFC8526}}.
The security considerations for the Defining and Using Metadata with YANG (see {{Section 9 of !RFC7952}}) apply to the metadata annotation defined in this document.
The security considerations for the NETCONF protocol operations (see {{Section 9 of !RFC6241}} and {{Section 6 of !RFC8526}}) still apply to the operations extended in this document.
This document registers one XML namespace URN in the 'IETF XML registry', following the format defined in {{!RFC3688}}.
URI: urn:ietf:params:xml:ns:yang:ietf-immutable
Registrant Contact: The IESG.
XML: N/A, the requested URIs are XML namespaces.
This document registers one module name in the 'YANG Module Names' registry, defined in {{!RFC6020}}.
name: ietf-immutable
prefix: im
namespace: urn:ietf:params:xml:ns:yang:ietf-immutable
RFC: XXXX
This document defines the following capability identifier URNs in the "RESTCONF Capability URNs" registry defined in {{!RFC8040}}:
Index Capability Identifier
----------------------------------------------------------------------
:with-immutable urn:ietf:params:restconf:capability:with-immutable:1.0
--- back
System capabilities might be represented as immutable configuration. 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'.
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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'.
{{?RFC8343}} 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 {{?I-D.ietf-netmod-system-config}}).
The system-generated type value is dependent on and represents the hardware 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 modeled as "config true" and thus 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;
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The key leaf (name) cannot be marked as "config false" as the list itself is config true;
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The type cannot be marked "config false", because we MAY need to reference the type to make different configuration nodes conditionally available.
User and group management is fundamental for setting up access control rules (see {{Section 2.5 of !RFC8341}}).
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 a new user/group can be defined granted particular privileges, but the predefined administrator account and its granted access are immutable.
An logical network element (LNE) is an independently managed virtual network device made up of resources allocated to it from its host or parent network device {{?RFC8530}}. 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.
Note to the RFC Editor: Please remove this section prior to publication.
There are already a number of full or partial implementations of immutability:
-
3GPP TS 32.156 {{TS32.156}} and 28.623 {{TS28.623}}: Requirements and a partial solution
-
ITU-T using ONF TR-531 {{TR-531}} concept on information model level but no YANG representation.
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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
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Junos OS provides a hidden and immutable configuration group called junos-defaults
{:numbered="false"}
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.