]> 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

ops NETMOD immutable read-only NETMOD This document defines a way to formally document existing behavior, implemented by servers in production, on the immutability of some configuration nodes, using a YANG "extension" and a YANG metadata annotation, both called "immutable", which are collectively used to flag which data nodes are immutable. Clients may use "immutable" statements in the YANG, and annotations provided by the server, to know beforehand when 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.

This document defines a way to formally document as a YANG extension or YANG metadata an existing model handling behavior that is already allowed in YANG and has been used by multiple standard organizations and vendors. It is the aim to create one single standard solution for documenting modification restrictions on 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 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; Clients believe that "config true" nodes are modifiable even though the server is allowed to reject such a modification at any time. If the server knows that it will always reject the modification because it internally think it immutable, it should document this towards the clients in a machine-readable way. This document defines a way to formally document existing behavior, implemented by servers in production, on the immutability of some configuration nodes, using a YANG "extension" and a YANG metadata annotation , both called "immutable", which are collectively used to flag which data nodes are immutable. The "immutable" YANG extension is used when the behavior can be described at the schema-level, while the "immutable" metadata annotation is used when the behavior must be described at the YANG "list" or "leaf-list" instance level. Comment: Should the "immutable" metadata annotation also be returned for nodes described as immutable in the YANG schema? Immutability is an existing model handling practice. While in some cases it is needed, it also has disadvantages, therefore it MUST be avoided wherever possible. The following is a list of already implemented and potential use cases. Modeling of server capabilities HW based auto-configuration Predefined Access control Rules Declaring System defined configuration unchangeable Immutable BGP peer type Modeling existing data handling behavior in other standard organizations describes the use cases in detail.

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 following terms are defined in : configuration data The following terms are defined in : data node leaf leaf-list container list anydata anyxml interior node data tree The following terms are defined in : access operation write access The following terms are defined in this document: A read-only state value the server provides to describe data it considers immutable. In schema, the immutability of data nodes is conveyed via a YANG "extension" statement. In instance representations, the immutability of data nodes is conveyed via a YANG metadata annotation. Both the extension statement and the metadata annotation are called "immutable". Together, they are alternative ways to express the same behavior.

The "immutable" concept defined in this document only documents existing write access restrictions to writable datastores. A particular data node or instance has the same immutability in all writable datastores. The immutable annotation information is also visible even in read-only datastores like <system> (if exists), <intended> and <operational> when a "with-immutable" parameter is carried (see ), however this only serves as descriptive information about the instance node itself, but has no effect on the handling of the read-only datastore. The immutability of data nodes is protocol and user independent.

Already some servers handle immutable configuration data and will reject any attempt to the update of such data. This document allows the existing immutable data node or instance to be formally documented by YANG extension or metadata annotation rather than be written as plain text in the description statement. Servers reject client's request for updating configuration data when it internally think it immutable. The error reporting is performed immediately at an <edit-config> operation time, regardless what the target configuration datastore is. For an example of an "invalid-value" error response, see . However, the following operations SHOULD be allowed for immutable nodes: Use a create, update, delete/remove operation on an immutable node if the effective change is null. E.g., if a leaf has a current value of "5" it should be allowed to replace it with a value of "5" Create an immutable data node with a same value that already exists in <operational> or <system> (if exists) in order to e.g., configure a mutable descendant or reference it in a "when", "must" or "leafref" expression. Delete an immutable data node from read-write configuration datastores (i.e., <running>, <startup> and <candidate>) which do not prevent the data node still appearing in <operational> or <system> (if exists) Servers adding the immutable property which does not have any additional semantic meaning is discouraged. For example, a key leaf that is given a value and cannot be modified once a list entry is created. The "immutable" flag is intended to be descriptive.

This section defines what the immutable flag means for each YANG data node. Whilst this section describes immutability at the schema level, it applies equally to when the immutable flag is set via the metadata annotation on data nodes.

When a leaf node is immutable, its value cannot change.

When a leaf-list data node is immutable, its value cannot change. When the "immutable" YANG extension statement is used on a leaf-list data node, or if a leaf-list inherits immutability from an ancestor, it means that the leaf-list as a whole cannot change: entries cannot be added, removed, or reordered, in case the leaf-list is "ordered-by user".

When a container data node is immutable, its instance can neither be created nor removed. Additionally, as with all interior nodes, immutability is recursively applied to descendants (see ).

When a list data node is immutable, its value cannot change, per the description elsewhere in this section. Additionally, as with all interior nodes, immutability is recursively applied to descendants (see ). This statement is applicable only to the "immutable" YANG extension, as the "list" node does not itself appear in data trees.

When an anydata data node is immutable, its instance can neither be created nor removed. Additionally, as with all interior nodes, immutability is recursively applied to descendants (see ). Descendants for anydata data node is unknown at module design time, they cannot reset the immutability state with "immutable" YANG extension.

When a "anyxml" data node is immutable, its instance can neither be created nor removed. Additionally, as with all interior nodes, immutability is recursively applied to descendants (see ). Descendants for anyxml data node is unknown at module design time, they cannot reset the immutability state with "immutable" YANG extension.

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. For example, given the following application configuration XML snippets:

<application im:immutable="true"> <name>predefined-ftp</name> <protocol>ftp</protocol> <port-number im:immutable="false">69</port-number> </application>

The list entry named "predefined-ftp" is immutable=true, but its child node "port-number" has the immutable=false (thus the client can change this value). The other child node "protocol" not specifying the immutability explicitly inherits immutability from its parent node thus also immutable=true.

If servers always reject client modification attempts to some data node that they internally think immutable regardless of how it is instantiated, an "immutable" YANG extension can be used to formally indicate to the clients. The "immutable" YANG extension can be a substatement to a "config true" leaf, leaf-list, container, list, anydata or anyxml statement. It has no effect if used as a substatement to a "config false" node, but can be allowed anyway. The "immutable" YANG extension defines an argument statement named "value" which is a boolean type to indicate that whether the node is immutable or not. If the "immutable" YANG extension is not specified for a particular data node, the default immutability is the same as that of its parent node. The immutability for a top-level data node is false by default.

If servers always reject clients modification to some particular instance that they internally think immutable, an "immutable" metadata annotation can be used to formally indicate to the clients. The "immutable" metadata annotation takes as an value which is a boolean type, it is not returned unless a client explicitly requests through a "with-immutable" parameter (see ). If the "immutable" metadata annotation for data node instances 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 annotation is used to annotate data node instances. A list may have multiple entries/instances in the data tree, "immutable" can annotate some of the instances as read-only, while others are read-write.

The YANG model defined in this document (see ) augments the <get-config>, <get> operation defined in RFC 6241, and the <get-data> operation defined in RFC 8526 with a new parameter named "with-immutable". When this parameter is present, it requests that the server includes "immutable" metadata annotations in its response. This parameter may be used for read-only configuration datastores, e.g., <system> (if exists), <intended> and <operational>, but the "immutable" metadata annotation returned indicates the immutability towards read-write configuration datastores, e.g., <startup>, <candidate> and <running>. If the "immutable" metadata annotation for returned child nodes are omitted, it has the same immutability as its parent node. The immutability of top hierarchy of returned nodes is false by default. Note that "immutable" metadata annotation is not included in a response unless a client explicitly requests them with a "with- immutable" parameter.

If a data node or particular data node instance is considered as immutable, the servers always reject any operation that attempts to update them. Servers Rejecting an operation due to immutability SHALL be done independent of any access control settings.

<CODE BEGINS> file="ietf-immutable@2023-05-25.yang" //RFC Ed.: replace XXXX with RFC number and remove this note module ietf-immutable { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-immutable"; prefix im; import ietf-yang-metadata { prefix md; } import ietf-netconf { prefix nc; reference "RFC 6241: Network Configuration Protocol (NETCONF)"; } import ietf-netconf-nmda { prefix ncds; reference "RFC 8526: NETCONF Extensions to Support the Network Management Datastore Architecture"; } organization "IETF Network Modeling (NETMOD) Working Group"; contact "WG Web: <https://datatracker.ietf.org/wg/netmod/> WG List: <mailto:netmod@ietf.org> Author: Qiufang Ma <mailto:maqiufang1@huawei.com> Author: Qin Wu <mailto:bill.wu@huawei.com> Author: Balazs Lengyel <mailto:balazs.lengyel@ericsson.com> Author: Hongwei Li <mailto:flycoolman@gmail.com>"; description "This module defines a YANG extension and a metadata annotation both called 'immutable', to allow the server to formally document existing behavior on the mutability of some configuration nodes. Clients may use 'immutable' extension statements in the YANG, and annotations provided by the server to know beforehand when certain otherwise valid configuration requests will cause the server to return an error. Copyright (c) 2023 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 2023-05-25 { description "Initial revision."; // RFC Ed.: replace XXXX and remove this comment reference "RFC XXXX: YANG Extension and Metadata Annotation for Immutable Flag"; } extension immutable { argument value; description "If servers always reject client modification attempts to some data node that can only be created, modified and deleted by the device itself, an 'immutable' YANG extension can be used to formally indicate to the client. The statement MUST only be a substatement to a 'config true' leaf, leaf-list, container, list, anydata or anyxml statement. Zero or one immutable statement per parent statement is allowed. No substatements are allowed. The argument of the 'immutable' statement defines the value, indicating whether the node is immutable or not. Adding immutable of an existing immutable statement is non-backwards compatible changes. Other changes to immutable are backwards compatible."; } md:annotation immutable { type boolean; description "If servers always reject clients modification to some particular instance that can only be created, modified and deleted by the device itself, an 'immutable' metadata annotation can be used to formally indicate to the clients. The 'immutable' annotation indicates the immutability of an instantiated data node. The 'immutable' metadata annotation takes as a value 'true' or 'false'. If the 'immutable' metadata annotation for data node instances is not specified, the default value is false. Explicitly annotating instances as immutable=true has the same effect as not specifying this value."; } grouping with-immutable-grouping { description "define the with-immutable grouping."; leaf with-immutable { type empty; description "If this parameter is present, the server will return the 'immutable' annotation for configuration that it internally thinks it immutable. When present, this parameter allows the server to formally document existing behavior on the mutability of some configuration nodes."; } } augment "/ncds:get-data/ncds:input" { 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" { 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" { description "Allows the server to include 'immutable' metadata annotations in its response to get operation."; uses with-immutable-grouping; } } <CODE ENDS>

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

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 .

name: ietf-immutable prefix: im namespace: urn:ietf:params:xml:ns:yang:ietf-immutable RFC: XXXX // RFC Ed.: replace XXXX and remove this comment

The YANG module specified in this document defines a YANG extension and a metadata Annotation. These can be used to further restrict write access but cannot be used to extend access rights. This document does not define any protocol-accessible data nodes. Since immutable information is tied to applied configuration values, it is only accessible to clients that have the permissions to read the applied configuration values. 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.

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

3GPP 3GPP ONF

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. E.g., 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" extension making it unchangable. After this the 'interface-timer' shall be defined as a leaf-ref pointing at the 'supported-timer-values'.

This section shows how to use immutable YANG extension to mark some data node as immutable. When an interface is physically present, the system will create an interface entry automatically with valid name and type values in <system> (if exists, see ). The system-generated data is dependent on and must represent the HW present, and as a consequence must not be changed by the client. The data is modelled as "config true" and should be marked 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. The immutability of the data is the same for all interface instances, thus following fragment of a fictional interface module including an "immutable" YANG extension can be used:

container interfaces { list interface { key "name"; leaf name { type string; } leaf type { im:immutable; type identityref { base ianaift:iana-interface-type; } mandatory true; } leaf mtu { type uint16; } leaf-list ip-address { type inet:ip-address; } } }

Note that the "name" leaf is defined as a list key which can never been modified for a particular list entry, there is no need to mark "name" as immutable.

This section shows an example of an error response due to the client modifying an immutable configuration. Assume the system creates an interface entry named "eth0" given that an inerface is inserted into the device. If a client tries to change the type of an interface to a value that doesn't match the real type of the interface used by the system, the request will be rejected by the server:

<rpc message-id="101" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0"> <edit-config> <target> <running/> </target> <config> <interface xc:operation="merge" xmlns:ianaift="urn:ietf:params:xml:ns:yang:iana-if-type"> <name>eth0</name> <type>ianaift:tunnel</type> </interface> </config> </edit-config> </rpc> <rpc-reply message-id="101" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" xmlns:xc="urn:ietf:params:xml:ns:netconf:base:1.0"> <rpc-error> <error-type>application</error-type> <error-tag>invalid-value</error-tag> <error-severity>error</error-severity> <error-path xmlns:t="http://example.com/schema/1.2/config"> /interfaces/interface[name="eth0"]/type </error-path> <error-message xml:lang="en"> Invalid type for interface eth0 </error-message> </rpc-error> </rpc-reply>

Setting up detailed rules for access control is a complex task. (see ) A vendor may provide an initial, predefined set of groups and related access control rules so that the customer can use access control out-of-the-box. The customer may continue using these predefined rules or may add his own groups and rules. The predefined groups shall not be removed or altered guaranteeing that access control remains usable and basic functions e.g., a system-security-administrator are always available. The system needs to protect the predefined groups and rules, however, the list "groups" or the list "rule-list" cannot be marked as config=false or with the "immutable" extension in the YANG model because that would prevent the customer adding new entries. Still it would be good to notify the client in a machine readable way that the predefined entries cannot be modified. When the client retrieves access control data the immutable="true" metadata annotation should be used to indicate to the client that the predefined groups and rules cannot be modified.

As stated in the device itself might supply some configuration. As defined in that document in section "5.4. Modifying (overriding) System Configuration" the server may allow some parts of system configuration to be modified while other parts of the system configuration are non-modifiable. The immutable extension or metadata annotation can be used to define which parts are non-modifiable and to inform the client about this fact.

Another example is the type attribute of BGP neighbors. The peer type of the BGP neighbor is closely related to the network topology: external BGP (EBGP) peer type relationships are established between BGP routers running in different ASs; while internal BGP (IBGP) peer type relationships are established between BGP routers running in the same AS. Thus BGP peer type cannot be changed to the value which does not match the actual one. Since there are EBGP/IBGP-specific configurations which need to reference the "peer-type" node (e.g., in "when" statement) and be conditionally available, it can only be modelled as "config true" but immutable. Following is the fragment of a simplified BGP module with the /bgp/neighbor/peer-type defined as immutable:

container bgp { leaf as { type inet:as-number; mandatory true; description "Local autonomous system number of the router."; } list neighbor { key "remote-address"; leaf remote-address { type inet:ip-address; description "The remote IP address of this entry's BGP peer."; } leaf peer-type { im:immutable; type enumeration { enum ebgp { description "External (EBGP) peer."; } enum ibgp { description "Internal (IBGP) peer."; } } mandatory true; description "Specify the type of peering session associated with this neighbor. The value can be IBGP or EBGP."; } leaf ebgp-max-hop { when "../peer-type='ebgp'"; type uint32 { range "1..255"; } description "The maximum number of hops when establishing an EBGP peer relationship with a peer on an indirectly-connected network. By default, an EBGP connection can be set up only on a directly-connected physical link."; } } }

A number of standard organizations and industry groups (ITU-T, 3GPP, ORAN) already use concepts similar to immutability. These modeling concepts sometimes go back to more than 10 years and cannot be and will not be changed irrespective of the YANG RFCs. Some of these organizations are introducing YANG modelling. Without a formal YANG statement to define data nodes immutable the property is only defined in plain English text in the description statement. The immutable flag can be used to define these existing model properties in a machine-readable way.

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

Note to RFC Editor (To be removed by RFC Editor) v06 - v07 Use a Boolean type for the immutable value in YANG extension and metadata annotation Define a "with-immutable" parameter and state that immutable metadata annotation is not included in a response unless a client explicitly requests them with a "with-immutable" parameter reword the abstract and related introduction section to highlight immutable flag is descriptive Add a new section to define immutability of interior nodes, and merge with "Inheritance of Immutable configuration" section Add a new section to define what the immutable flag means for each YANG data node Define the "immutable flag" term. Add an item in the open issues tracking: Should the "immutable" metadata annotation also be returned for nodes described as immutable in the YANG schema so that there is a single source of truth? v05 - v06 Remove immutable BGP AS number case Fix nits v04 - v05 Emphasized that the proposal tries to formally document existing allowed behavior Reword the abstract and introduction sections; Restructure the document; Simplified the interface example in Appendix; Add immutable BGP AS number and peer-type configuration example. Added temporary section in Appendix B about list of existing non-standard solutions Clarified inheritance of immutability Clarified that this draft is not dependent on the existence of the <system> datastore. v03 - v04 Clarify how immutable flag interacts with NACM mechanism. v02 - v03 rephrase and avoid using "server MUST reject" statement, and try to clarify that this documents aims to provide visibility into existing immutable behavior; Add a new section to discuss the inheritance of immutability; Clarify that deletion to an immutable node in <running> which is instantiated in <system> and copied into <running> should always be allowed; Clarify that write access restriction due to general YANG rules has no need to be marked as immutable. Add an new section named "Acknowledgements"; editoral changes. v01 - v02 clarify the relation between the creation/deletion of the immutable data node with its parent data node; Add a "TODO" comment about the inheritance of the immutable property; Define that the server should reject write attempt to the immutable data node at an <edit-config> operation time, rather than waiting until a <commit> or <validate> operation takes place; v00 - v01 Added immutable extension Added new use-cases for immutable extension and annotation Added requirement that an update that means no effective change should always be allowed Added clarification that immutable is only applied to read-write datastore Narrowed the applied scope of metadata annotation to list/leaf-list instances

Should the "immutable" metadata annotation also be returned for nodes described as immutable in the YANG schema so that there is a single source of truth?