< draft-ietf-netmod-yang-versioning-reqs-04.txt		draft-ietf-netmod-yang-versioning-reqs-05.txt >
	Network Working Group J. Clarke, Ed.		Network Working Group J. Clarke, Ed.
	Internet-Draft Cisco Systems, Inc.		Internet-Draft Cisco Systems, Inc.
	Intended status: Informational January 5, 2021		Intended status: Informational July 6, 2021
	Expires: July 9, 2021		Expires: January 7, 2022
	YANG Module Versioning Requirements		YANG Module Versioning Requirements
	draft-ietf-netmod-yang-versioning-reqs-04		draft-ietf-netmod-yang-versioning-reqs-05
	Abstract		Abstract
	This document describes the problems that can arise because of the		This document describes the problems that can arise because of the
	YANG language module update rules, that require all updates to YANG		YANG language module update rules, that require all updates to YANG
	module preserve strict backwards compatibility. It also defines the		module preserve strict backwards compatibility. It also defines the
	requirements on any solution designed to solve the stated problems.		requirements on any solution designed to solve the stated problems.
	This document does not consider possible solutions, nor endorse any		This document does not consider possible solutions, nor endorse any
	particular solution.		particular solution.
	skipping to change at page 1, line 35 ¶		skipping to change at page 1, line 35 ¶
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at https://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on July 9, 2021.		This Internet-Draft will expire on January 7, 2022.
	Copyright Notice		Copyright Notice
	Copyright (c) 2021 IETF Trust and the persons identified as the		Copyright (c) 2021 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 3, line 36 ¶		skipping to change at page 3, line 36 ¶
	standardized YANG modules have to be perfect on day one (at least the		standardized YANG modules have to be perfect on day one (at least the
	structure and meaning), which in turn might explain why IETF YANG		structure and meaning), which in turn might explain why IETF YANG
	modules take so long to standardize. Shooting for perfection is		modules take so long to standardize. Shooting for perfection is
	obviously a noble goal, but if the perfect standard comes too late,		obviously a noble goal, but if the perfect standard comes too late,
	it doesn't help the industry.		it doesn't help the industry.
	2.2. Some YANG Modules Are Not Backwards-Compatible		2.2. Some YANG Modules Are Not Backwards-Compatible
	As we learn from our mistakes, we're going to face more and more non-		As we learn from our mistakes, we're going to face more and more non-
	backwards-compatible YANG modules. An example is the YANG data model		backwards-compatible YANG modules. An example is the YANG data model
	for L3VPN service delivery [RFC8049], which, based on implementation		for L3VPN service delivery [RFC8049] , which, based on implementation
	experience, has been updated in a non-backwards-compatible way by		experience, has been updated in a non-backwards-compatible way by
	[RFC8299].		[RFC8299] .
	While Standards Development Organization (SDO) YANG modules are		While Standards Development Organization (SDO) YANG modules are
	obviously better for the industry, we must recognize that many YANG		obviously better for the industry, we must recognize that many YANG
	modules are actually generated YANG modules (for example, from		modules are actually generated YANG modules (for example, from
	internal databases), which is sometimes the case for vendor modules		internal databases), which is sometimes the case for vendor modules
	[RFC8199]. From time to time, the new YANG modules are not		[RFC8199] . From time to time, the new YANG modules are not
	backwards-compatible.		backwards-compatible.
	Old module parts that are no longer needed, no longer supported, or		Old module parts that are no longer needed, no longer supported, or
	are not used by consumers need to be removed from modules. It is		are not used by consumers need to be removed from modules. It is
	often hard to decide which parts are no longer needed/used; still the		often hard to decide which parts are no longer needed/used; still the
	need and practice of removing old parts exist. While it is rare in		need and practice of removing old parts exist. While it is rare in
	standard modules it is more common in vendor YANG modules where the		standard modules it is more common in vendor YANG modules where the
	usage of modules is more controlled.		usage of modules is more controlled.
	The problems described in Section 2.7 may also result in incompatible		The problems described in Section 2.7 may also result in incompatible
	skipping to change at page 7, line 11 ¶		skipping to change at page 7, line 11 ¶
	is not a trivial thing to do. This "may or may not" is unacceptable		is not a trivial thing to do. This "may or may not" is unacceptable
	in a contract. In effect, this works as if there would be an if-		in a contract. In effect, this works as if there would be an if-
	feature statement on each deprecated schema node where the server		feature statement on each deprecated schema node where the server
	does not advertise whether the feature is supported or not. Why is		does not advertise whether the feature is supported or not. Why is
	it not advertised?		it not advertised?
	3. Terminology and Conventions		3. Terminology and Conventions
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in [RFC2119].		document are to be interpreted as described in [RFC2119] .
	In addition, this document uses the following terminology:		In addition, this document uses the following terminology:
	o YANG module revision: An instance of a YANG module, with no		o YANG module revision: An instance of a YANG module, with no
	implied ordering or backwards compatibility between different		implied ordering or backwards compatibility between different
	revisions of the same module."		revisions of the same module."
	o YANG module version: A YANG module revision, but also with an		o YANG module version: A YANG module revision, but also with an
	implied partial ordering relationship between other versions of		implied partial ordering relationship between other versions of
	the same module. Each module version must be uniquely		the same module. Each module version must be uniquely
	identifiable.		identifiable.
	o Non-backwards-compatible (NBC): In the context of this document,		o Non-backwards-compatible (NBC): In the context of this document,
	the term 'non-backwards-compatible' refers to a change or set of		the term 'non-backwards-compatible' refers to a change or set of
	changes between two YANG module revisions that do not adhere to		changes between two YANG module revisions that do not adhere to
	the list of allowable changes specified in Section 11 "Updating a		the list of allowable changes specified in Section 11 "Updating a
	Module" of [RFC7950], with the following additional clarification:		Module" of [RFC7950] , with the following additional
			clarification:
	* Any addition of, or change to, a "status" statement that allows		* Any addition of, or change to, a "status" statement that allows
	a server to remove support for a schema node is considered a		a server to remove support for a schema node is considered a
	non-backwards-compatible change		non-backwards-compatible change
	4. The Problem Statement		4. The Problem Statement
	Considering the issues described in the background, the problem		Considering the issues described in the background, the problem
	definition can be summarized as follows.		definition can be summarized as follows.
End of changes. 8 change blocks.
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