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RFC 8178

Proposed STD
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Rules for NFSv4 Extensions and Minor Versions

Updates:    5661    7862


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Internet Engineering Task Force (IETF)                         D. Noveck
Request for Comments: 8178                                        NetApp
Updates: 5661, 7862                                            July 2017
Category: Standards Track
ISSN: 2070-1721


             Rules for NFSv4 Extensions and Minor Versions

Abstract

   This document describes the rules relating to the extension of the
   NFSv4 family of protocols.  It covers the creation of minor versions,
   the addition of optional features to existing minor versions, and the
   correction of flaws in features already published as Proposed
   Standards.  The rules relating to the construction of minor versions
   and the interaction of minor version implementations that appear in
   this document supersede the minor versioning rules in RFC 5661 and
   other RFCs defining minor versions.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8178.

Copyright Notice

   Copyright (c) 2017 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
   (http://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 extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Use of Keywords Defined in RFCs 2119 and 8174 . . . . . .   4
     2.2.  Use of Feature Statuses . . . . . . . . . . . . . . . . .   4
     2.3.  NFSv4 Versions  . . . . . . . . . . . . . . . . . . . . .   5
   3.  Consolidation of Extension Rules  . . . . . . . . . . . . . .   6
   4.  XDR Considerations  . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  XDR Extension . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Rules for XDR Extension within NFSv4  . . . . . . . . . .   8
     4.3.  Handling of Protocol Elements by Responders . . . . . . .   9
     4.4.  Inter-version Interoperability  . . . . . . . . . . . . .  11
       4.4.1.  Requirements for Knowledge of Protocol Elements . . .  11
       4.4.2.  Establishing Interoperability . . . . . . . . . . . .  12
       4.4.3.  Determining Knowledge of Protocol Elements  . . . . .  14
     4.5.  XDR Overlay . . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Other NFSv4 Protocol Changes  . . . . . . . . . . . . . . . .  15
     5.1.  Field Interpretation and Use  . . . . . . . . . . . . . .  15
     5.2.  Behavioral Changes  . . . . . . . . . . . . . . . . . . .  16
   6.  Extending Existing Minor Versions . . . . . . . . . . . . . .  17
   7.  Minor Versions  . . . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Creation of New Minor Versions  . . . . . . . . . . . . .  18
   8.  Minor Version Interaction Rules . . . . . . . . . . . . . . .  18
     8.1.  Minor Version Identifier Transfer Issues  . . . . . . . .  19
     8.2.  Minor Version Compatibility . . . . . . . . . . . . . . .  19
   9.  Correction of Existing Minor Versions and Features  . . . . .  20
     9.1.  XDR Changes to Implement Protocol Corrections . . . . . .  21
     9.2.  XDR Corrections to OPTIONAL Features  . . . . . . . . . .  21
     9.3.  XDR Corrections to REQUIRED Features  . . . . . . . . . .  22
     9.4.  Addressing XDR Corrections in Later Minor Versions  . . .  24
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     12.2.  Informative References . . . . . . . . . . . . . . . . .  25
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  25
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  26

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1.  Introduction

   To address the requirement for an NFS protocol that can evolve as the
   need arises, the Network File System (NFS) version 4 (NFSv4) protocol
   provides a framework to allow for future changes via the creation of
   new protocol versions, including minor versions and certain forms of
   modification of existing minor versions.  The extension rules
   contained in this document allow extensions and other changes to be
   implemented in a way that maintains compatibility with existing
   clients and servers.

   Previously, all protocol changes had been part of new minor versions.
   The COMPOUND procedure (see Section 14.2 of [RFC7530]) specifies the
   minor version being used by the client in making requests.  The
   CB_COMPOUND procedure (see Section 15.2 of [RFC7530]) specifies the
   minor version being used by the server on callback requests.

   Creation of a new minor version is no longer the only way in which
   protocol changes may be made.  Optional features may be added as
   extensions and protocol corrections can be proposed, specified, and
   implemented within the context of a single minor version.  Creation
   of new minor versions remains available when needed.

   The goal of allowing extensions within the context of a minor version
   is to provide more implementation flexibility while preserving
   interoperability on protocol upgrade.  As described in Section 4.4, a
   client and server may each choose a subset of available extensions.
   Each party can successfully use a subset of protocol elements that
   are known to and supported by both the client and server.  Support
   for this common subset is not affected by the fact that extensions
   outside this common subset may be supported by the server or
   potentially used by the client.

2.  Terminology

   A basic familiarity with NFSv4 terminology is assumed in this
   document and the reader is pointed to [RFC7530].

   In this document, the term "version" is not limited to minor
   versions.  When minor versions are meant, the term "minor version" is
   used explicitly.  For more discussion of this and related terms, see
   Section 2.3.

   A "feature package" is a set of features that are defined together,
   either as part of a minor version or as part of the same protocol
   extension.

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2.1.  Use of Keywords Defined in RFCs 2119 and 8174

   The keywords defined by [RFC2119] [RFC8174] have special meanings
   that this document intends to adhere to.  However, due to the nature
   of this document and some special circumstances, there are some
   complexities to take note of:

   o  Where this document does not directly specify implementation
      requirements, use of these capitalized terms is often not
      appropriate since the guidance given in this document does not
      directly affect interoperability.

   o  In this document, what authors of RFCs defining features and minor
      versions need to do is stated without these specialized terms.
      Although it is necessary to follow this guidance to provide
      successful NFSv4 protocol extension, that sort of necessity is not
      of the sort defined as applicable to the use of the keywords
      defined in [RFC2119] [RFC8174].

      The fact that these capitalized terms are not used should not be
      interpreted as indicating that this guidance does not need to be
      followed or is somehow not important.

   o  In speaking of the possible statuses of features and feature
      elements, the terms "OPTIONAL" and "REQUIRED" are used.  For
      further discussion, see Section 2.2.

   o  When one of these upper-case keywords defined in [RFC2119]
      [RFC8174] is used in this document, it is in the context of a rule
      directed to an implementer of NFSv4 minor versions, the status of
      a feature or protocol element, or in a quotation, sometimes
      indirect, from another document.

2.2.  Use of Feature Statuses

   There has been some confusion during the history of NFSv4 about the
   correct use of these terms, and instances in which the keywords
   defined in [RFC2119] [RFC8174] were used in ways that appear to be at
   variance with the definitions in that document.

   o  In [RFC3530], the lower-case terms "optional", "recommended", and
      "required" were used as feature statuses, Later, in [RFC5661] and
      [RFC7530], the corresponding upper-case keywords were used.  It is
      not clear why this change was made.

   o  In the case of "RECOMMENDED", its use as a feature status is
      inconsistent with [RFC2119] [RFC8174] and it will not be used for
      this purpose in this document.

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   o  The word "RECOMMENDED" to denote the status of attributes in
      [RFC7530] and [RFC5661] raises similar issues.  This has been
      recognized in [RFC7530] with regard to NFSV4.0, although the
      situation with regard to NFSv4.1 remains unresolved.

   In this document, the keywords "OPTIONAL" and "REQUIRED" and the
   phrase "mandatory to not implement" are used to denote the status of
   features within a given minor version.  In using these terms, RFCs
   that specify the status of features inform:

   o  client implementations whether they need to deal with the absence
      of support for these features.

   o  server implementations whether they need to provide support for
      these features.

2.3.  NFSv4 Versions

   The term "version" denotes any valid protocol variant constructed
   according to the rules in this document.  It includes minor versions,
   but there are situations that allow multiple variant versions to be
   associated with and coexist within a single minor version:

   o  When there are feature specification documents published as
      Proposed Standards extending a given minor version, then the
      protocol defined by the minor version specification document, when
      combined with any subset (not necessarily a proper subset) of the
      feature specification documents, is a valid NFSv4 version variant
      that is part of the minor version in question.

   o  When there are protocol corrections published that update a given
      minor version, each set of published updates, up to the date of
      publication of the update, is a valid NFSv4 version variant that
      is part of the minor version in question.

   Because of the above, there can be multiple version variants that are
   part of a given minor version.  Two of these are worthy of special
   terms:

   o  The term "base minor version" denotes the version variant that
      corresponds to the minor version as originally defined, including
      all protocol elements specified in the minor version definition
      document but not incorporating any extensions or protocol
      corrections published after that original definition.

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   o  At any given time, the term "current minor version" denotes the
      minor version variant including all extensions of and corrections
      to the minor version made by Standards Track documents published
      up to that time.

   Each client and server that implements a specific minor version will
   implement some particular variant of that minor version.  Each
   variant is a subset of the current minor version and a superset of
   the base minor version.  When the term "minor version" is used
   without either of these qualifiers, it should refer to something that
   is true of all variants within that minor version.  For example, in
   the case of a minor version that has not had a protocol correction,
   one may refer to the set of REQUIRED features for that minor version
   since it is the same for all variants within the minor version.  See
   Section 9 for a discussion of correcting an existing minor version.

3.  Consolidation of Extension Rules

   In the past, the only existing extension rules were the minor
   versioning rules that were being maintained and specified in the
   Standards Track RFCs, which defined the individual minor versions.
   In the past, these minor versioning rules were modified on an ad hoc
   basis for each new minor version.

   More recently, minor versioning rules were specified in [RFC5661]
   while modifications to those rules were allowed in subsequent minor
   versions.

   This document defines a set of extension rules, including rules for
   minor version construction.  These rules apply to all future changes
   to the NFSv4 protocol.  The rules are subject to change but any such
   change should be part of a Standards Track RFC obsoleting or updating
   this document.

   Rather than a single list of extension rules, as was done in the
   minor versioning rules in [RFC5661], this document defines multiple
   sets of rules that deal with the various forms of protocol change
   provided for in the NFSv4 extension framework.

   o  The kinds of changes in External Data Representation (XDR)
      definitions that may be made to extend NFSv4 are addressed in the
      rules in Section 4.2.

   o  Minor version construction, including rules applicable to changes
      that cannot be made in extensions to existing minor versions are
      addressed in Section 7.1.

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   o  Minor version interaction rules are discussed in Sections 8.1 and
      8.2.

   This document supersedes minor versioning rules appearing in the
   minor version specification RFCs, including those in [RFC5661] and
   also the modification to those rules mentioned in [RFC7862].  As a
   result, potential conflicts among documents should be addressed as
   follows:

   o  The specification of the actual protocols for minor versions
      previously published as Proposed Standards take precedence over
      minor versioning rules in either this document or in the minor
      version specification RFCs.  In other words, if the transition
      from version A to version B violates a minor versioning rule, the
      version B protocol stays as it is.

   o  Since minor versioning rules #11 and #13 from [RFC5661] deal with
      the interactions between multiple minor versions, the situation is
      more complicated.  See Section 8 for a discussion of these issues,
      including how potential conflicts between rules are to be
      resolved.

   o  Otherwise, any conflict between the extension rules in this
      document and those in minor version specification RFCs are to be
      resolved based on the treatment in this document.  In particular,
      corrections may be made as specified in Section 9 for all
      previously specified minor versions, and the extensibility of
      previously specified minor versions is to be handled in accord
      with Section 6.

   Future minor version specification documents should avoid specifying
   rules relating to minor versioning and reference this document in
   connection with rules for NFSv4 extension.

4.  XDR Considerations

   As an extensible XDR-based protocol, NFSv4 has to ensure inter-
   version compatibility in situations in which the client and server
   use different XDR descriptions.  For example, the client and server
   may implement different variants of the same minor version, in that
   they each might add different sets of extensions to the base minor
   version.

   The XDR extension paradigm, discussed in Section 4.1, assures that
   these descriptions are compatible, with clients and servers able to
   determine and use those portions of the protocol that they both share
   according to the method described in Section 4.4.2.

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4.1.  XDR Extension

   When an NFSv4 version change requires a modification to the protocol
   XDR, this is effected within a framework based on the idea of XDR
   extension.  This is in contrast to transitions between major NFS
   versions (including that between NFSv3 and NFSv4.0) in which the XDR
   for one version was replaced by a different XDR for a newer version.

   The XDR extension approach allows an XDR description to be extended
   in a way that retains the structure of all previously valid messages.
   If a base XDR description is extended to create a second XDR
   description, the following will be true for the second description to
   be a valid extension of the first:

   o  The set of valid messages described by the extended definition is
      a superset of that described by the first.

   o  Each message within the set of valid messages described by the
      base definition is recognized as having exactly the same
      structure/interpretation using the extended definition.

   o  Each message within the set of messages described as valid by the
      extended definition but not the base definition must be
      recognized, using the base definition, as part of an unknown
      extension.

   The use of XDR extension can facilitate compatibility between
   different versions of the NFSv4 protocol.  When XDR extension is used
   to implement OPTIONAL features, the greatest degree of inter-version
   compatibility is obtained.  In this case, as long as the rules in
   Section 6 are followed, no change in minor version number is needed
   and the extension may be effected in the context of a single minor
   version.

4.2.  Rules for XDR Extension within NFSv4

   In the context of NFSv4, given the central role of COMPOUND and
   CB_COMPOUND, addition of new RPC procedures is not allowed and the
   enumeration of operations and callback operations have a special
   role.

   The following XDR extensions, by their nature, affect both messages
   sent by requesters (i.e., requests and callbacks), and responders
   (i.e., replies and callback replies).

   o  Addition of previously unspecified operation codes, within the
      framework established by COMPOUND and CB_COMPOUND.  These extend
      the appropriate enumeration and the corresponding switches devoted

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      to requests and responses for the associated direction of
      operation.

   o  Addition of previously unspecified attributes.  These add
      additional numeric constants that define each attribute's bit
      position within the attribute bitmap, together with XDR typedefs
      that specify the attributes' format within the nominally opaque
      arrays specifying sets of attributes.

   Other sorts of changes will generally affect one of requests,
   replies, callback, or callback replies.  Although all are valid XDR
   extensions, the messages that are affected may determine whether the
   extension requires a new minor version (see Section 7) or can be made
   as an extension within an existing minor version (see Section 6).

   o  Addition of new, previously unused, values to existing enums.

   o  Addition of previously unassigned bit values to a flag word.

   o  Addition of new cases to existing switches, provided that the
      existing switch did not contain a default case.

   None of the following is allowed to happen:

   o  Any change to the structure of existing requests or replies other
      than those listed above.

   o  Addition of previously unspecified RPC procedures for either the
      NFSv4 program or the callback program.

   o  Deletion of existing RPC procedures, operation codes, enum values,
      flag bit values, and switch cases.  Note that changes may be made
      to define use of any of these as causing an error, as long as the
      XDR is unaffected.  Similarly, none of these items may be reused
      for a new purpose.

4.3.  Handling of Protocol Elements by Responders

   Implementations handle protocol elements received in requests and
   callbacks in one of three ways.  Which of the following ways are
   valid depends on the status of the protocol element in the variant
   being implemented:

   o  The protocol element is not a part of definition of the variant in
      question and so is "unknown".  The responder, when it does not
      report an RPC XDR decode error, reports an error indicative of the
      element not being defined in the XDR such as NFS4ERR_OP_ILLEGAL,
      NFS4ERR_BADXDR, or NFS4ERR_INVAL.  See Section 4.4.3 for details.

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   o  The protocol element is a known part of the variant but is not
      supported by the particular implementation.  The responder reports
      an error indicative of the element being recognized as one which
      is not supported such as NFS4ERR_NOTSUPP, NFS4ERR_UNION_NOTSUPP,
      or NFS4ERR_ATTRNOTSUPP.

   o  The protocol element is a known part of the variant that is
      supported by the particular implementation.  The responder reports
      success or an error other than the special ones discussed above.

   Which of these are validly returned by the responder depends on the
   status of the protocol element in the minor version specified in the
   COMPOUND or CB_COMPOUND.  The possibilities that can exist when
   dealing with minor versions that have not been subject to corrections
   are listed below.  See Sections 9.1 and 9.3 for a discussion of the
   effects of protocol correction.

   o  The protocol element is not known in the minor version.  In this
      case, all implementations of the minor version MUST indicate that
      the protocol element is not known.

   o  The protocol element is part of a feature specified as mandatory
      to not implement in the minor version.  In this case as well, all
      implementations of the minor version MUST indicate that the
      protocol element is not known.

   o  The protocol element is defined as part of the current variant of
      the minor version but is not part of the corresponding base
      variant.  In this case, the requester can encounter situations in
      which the protocol element is either not known to the responder,
      is known to but not supported by the responder, or is both known
      to and supported by the responder.

   o  The protocol element is defined as an OPTIONAL part of the base
      minor version.  In this case, the requester can expect the
      protocol element to be known but must deal with cases in which it
      is supported or is not supported.

   o  The protocol element is defined as a REQUIRED part of the base
      minor version.  In this case, the requester can expect the
      protocol element to be both known and supported by the responder.

   The listing of possibilities above does not mean that a requester
   always needs to be prepared for all such possibilities.  Often,
   depending on the scope of the feature of which the protocol element
   is a part, handling of a previous request using the same or related
   protocol elements will allow the requester to be sure that certain of
   these possibilities cannot occur.

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   Requesters, typically clients, may test for knowledge of, or support
   for, protocol elements as part of connection establishment.  This may
   allow the requester to be aware of a responder's lack of knowledge of
   or support for problematic requests before they are actually used to
   affect user requests.

4.4.  Inter-version Interoperability

   Because of NFSv4's use of XDR extension, any communicating client and
   server versions have XDR definitions such that each is a valid
   extension of a third version.  Once that version is determined, it
   may be used by both client and server to communicate.  Each party can
   successfully use a subset of protocol elements that are both known to
   and supported by both parties.

4.4.1.  Requirements for Knowledge of Protocol Elements

   With regard to requirements for knowledge of protocol elements, the
   following rules apply.  These rules are the result of the use of the
   XDR extension paradigm combined with the way in which extensions are
   incorporated in existing minor versions (for details, see Section 6).

   o  Any protocol element defined as part of the base variant of a
      particular minor version is required to be known by that minor
      version.  This occurs whether the specification happens in the
      body of the minor definition document or is in a feature
      definition document that is made part of the minor version by
      being normatively referenced by the minor version definition
      document.

   o  Any protocol element required to be known in a given minor version
      is required to be known in subsequent minor versions, unless and
      until a minor version has made that protocol element as mandatory
      to not implement.

   o  When a protocol element is defined as part of an extension to an
      extensible minor version, it is not required to be known in that
      minor version but is required to be known by the next minor
      version.  In the earlier minor version, it might not be defined in
      the XDR definition document, while in the later version it needs
      to be defined in the XDR definition document.  In either case, if
      it is defined, it might or might not be supported.

   o  When knowledge of protocol elements is optional in a given minor
      version, the responder's knowledge of such optional elements must
      obey the rule that if one such element is known, then all the
      protocol elements defined in the same minor version definition
      document must be known as well.

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   For many minor versions, all existing protocol elements are required
   to be known by both the client and the server, and so requesters do
   not have to test for the presence or absence of knowledge regarding
   protocol elements.  This is the case if there has been no extension
   for the minor version in question.  Extensions can be added to
   extensible minor versions as described in Section 6 and can be used
   to correct protocol flaws as described in Section 9.

   Requesters can ascertain the knowledge of the responder in two ways:

   o  By issuing a request using the protocol element and looking at the
      response.  Note that, even if the protocol element used is not
      supported by the responder, the requester can still determine if
      the element is known by the responder.

   o  By receiving a request from the responder, acting in the role of
      requester.  For example, a client may issue a request enabling the
      server to infer that it is aware of a corresponding callback.

   In making this determination, the requester can rely on two basic
   facts:

   o  If the responder is aware of a single protocol element within a
      feature package, it must be aware of all protocol elements within
      that feature package.

   o  If a protocol element is one defined by the minor version
      specified by a request (and not in an extension), or in a previous
      minor version, the responder must be aware of it.

4.4.2.  Establishing Interoperability

   When a client and a server interact, they need to able to take
   advantage of the compatibility provided by NFSv4's use of XDR
   extension.

   In this context, the client and server would arrive at a common
   variant, which the client uses to send requests that the server would
   then accept.  The server would use that variant to send callbacks
   that the client would then accept.  This state of affairs could arise
   in a number of ways:

   o  Client and server have been built using XDR variants that belong
      to the same minor version.

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   o  The client's minor version is lower than that of the server.  In
      this case the server, in accord with Section 8.2, accepts the
      client's minor version, and acts as if it has no knowledge of
      extensions made in subsequent minor versions.  It has knowledge of
      protocol elements within the current (i.e., effectively final)
      variant of the lower minor version.

   o  The client's minor version is higher than that of the server.  In
      this case the client, in accord with Section 8.2, uses a lower
      minor version that the server will accept.  In this case, the
      server has no knowledge of extensions made in subsequent minor
      versions.

   There are a number of cases to consider based on the characteristics
   of the minor version chosen.

   o  When the minor version consists of only a single variant (no
      extension or XDR corrections), the client and the server are using
      the same XDR description and have knowledge of the same protocol
      elements.

   o  When the minor version consists of multiple variants (i.e., there
      are one or more XDR extensions or XDR corrections), the client and
      the server are using compatible XDR descriptions.  The client is
      aware of some set of extensions while the server may be aware of a
      different set.  The client can use the approach described in
      Section 4.4.3 to determine which of the extensions it knows about
      are also known by the server.  Once this is done, the client and
      server will both be using a common variant.  The variants that the
      client and the server were built with will both either be
      identical to this variant or a valid extension of it.  Similarly,
      the variants that the client and the server actually use will be a
      subset of this variant, in that certain OPTIONAL features will not
      be used.

   In either case, the client must determine which of the OPTIONAL
   protocol elements within the common version are supported by the
   server, just as it does for OPTIONAL features introduced as part of a
   minor version.

   It is best if client implementations make the determination as to the
   support provided by the server before acting on user requests.  This
   includes the determination of the common protocol variant and the
   level of support for OPTIONAL protocol elements.

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4.4.3.  Determining Knowledge of Protocol Elements

   A requester may test the responder's knowledge of particular protocol
   elements as defined below, based on the type of protocol element.
   Note that in the case of attribute or flag bits, use of a request
   that refers to 2 or more bits of undetermined status ("known" versus
   "unknown") may return results that are not particularly helpful.  In
   such cases, when the response is NFS4ERR_INVAL, the requester can
   only conclude that at least one of the bits is unknown.

   o  When a GETATTR request is made specifying an attribute bit to be
      tested and that attribute is not a set-only attribute, if the
      GETATTR returns with the error NFS4ERR_INVAL, then it can be
      concluded that the responder has no knowledge of the attribute in
      question.  Other responses, including NFS4ERR_ATTRNOTSUPP,
      indicate that the responder is aware of the attribute in question.

   o  When a SETATTR request is made specifying the attribute bit to be
      tested and that attribute is not a get-only attribute, if the
      SETATTR returns with the error NFS4ERR_INVAL, then it can be
      concluded that the responder has no knowledge of the attribute in
      question.  Other responses, including NFS4ERR_ATTRNOTSUPP,
      indicate that the responder is aware of the attribute in question.

   o  When a request is made including an operation with a new flag bit,
      if the operation returns with the error NFS4ERR_INVAL, then it can
      generally be concluded that the responder has no knowledge of the
      flag bit in question, as long as the requester is careful to avoid
      other error situations in which the operation in question is
      defined as returning NFS4ERR_INVAL.  Other responses indicate that
      the responder is aware of the flag bit in question.

   o  When a request is made including the operation to be tested, if
      the responder returns an RPC XDR decode error, or a response
      indicating that the operation in question resulted in
      NFS4ERR_OP_ILLEGAL or NFS4ERR_BADXDR, then it can be concluded
      that the responder has no knowledge of the operation in question.
      Other responses, including NFS4ERR_NOTSUPP, indicate that the
      responder is aware of the operation in question.

   o  When a request is made including the switch arm to be tested, if
      the responder returns an RPC XDR decode error, or a response
      indicating that the operation in question resulted in
      NFS4ERR_BADXDR, then it can be concluded that the responder has no
      knowledge of the operation in question.  Other responses,
      including NFS4ERR_UNION_NOTSUPP, indicate that the responder is
      aware of the protocol element in question.

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   A determination of the knowledge or lack of knowledge of a particular
   protocol element is expected to remain valid as long as the clientid
   associated with the request remains valid.

   The above assumes, as should be the case, that the server will accept
   the minor version used by the client.  For more detail regarding this
   issue, see Section 8.2.

4.5.  XDR Overlay

   XDR additions may also be made by defining XDR structures that
   overlay nominally opaque fields that are defined to allow such
   incremental extensions.

   For example, each parallel NFS (pNFS) mapping type provides its own
   XDR definition for various pNFS-related fields defined in [RFC5661]
   as opaque arrays.

   Because such additions provide new interpretations of existing
   fields, they may be made outside of the extension framework as long
   as they obey the rules previously established when the nominally
   opaque protocol elements were added to the protocol.

5.  Other NFSv4 Protocol Changes

   There are a number of types of protocol changes that are outside the
   XDR extension framework discussed in Section 4.  These changes are
   also managed within the NFSv4 versioning framework and may be of a
   number of types, which are discussed in the sections below.

   Despite the previous emphasis on XDR changes, additions and changes
   to the NFSv4 protocols have not been limited to those that involve
   changes (in the form of extensions) to the protocol XDR.  Examples of
   other sorts of changes have been taken from NFSv4.1.

   All such changes that have been made in the past have been made as
   part of new minor version.  Future change of these sorts may not be
   done in an extension but can only be made in a new minor version.

5.1.  Field Interpretation and Use

   The XDR description of a protocol does not constitute a complete
   description of the protocol.  Therefore, versioning needs to consider
   the role of changes in the use of fields, even when there is no
   change to the underlying XDR.

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   Although any XDR element is potentially subject to a change in its
   interpretation and use, the likelihood of such change will vary with
   the XDR-specified type of the element, as discussed below:

   o  When XDR elements are defined as strings, rules regarding the
      appropriate string values are specified in protocol specification
      text with changes in such rules documented in minor version
      definition documents.  Some types of strings within NFS4 are used
      in server names (in location-related attributes), user and group
      names, and in the names of file objects within directories.  Rules
      regarding what strings are acceptable appear in [RFC7530] and
      [RFC5661] with the role of the XDR limited to hints regarding
      UTF-8 and capitalization issues via XDR typedefs.

   o  Fields that are XDR-defined as opaque elements and that are truly
      opaque, do not raise versioning issues, except as regards inter-
      version use, which is effectively foreclosed by the rules in
      Section 8.1.

      Note that sometimes a field will seem to be opaque but not
      actually be fully opaque when considered carefully.  For example,
      the "other" field of stateids is defined as an opaque array, while
      the specification text specially defines appropriate treatment
      when the "other" field within it is either all zeros or all ones.
      Given this context, creation or deletion of reserved values for
      "special" stateids will be a protocol change that versioning rules
      need to deal with.

   o  Some nominally opaque elements have external XDR definitions that
      overlay the nominally opaque arrays.  Such cases are discussed in
      Section 4.5.

5.2.  Behavioral Changes

   Changes in the behavior of NFSv4 operations are possible, even if
   there is no change in the underlying XDR or change to field
   interpretation and use.

   One class of behavioral change involves changes in the set of errors
   to be returned when various failure conditions occur.  When the set
   of valid requests remain the same, and the behavior for each of them
   remains the same, such changes can be implemented with only limited
   disruption to existing clients.

   Many more substantial behavioral changes have occurred in connection
   with the addition of the session concept in NFSv4.1.  Even though
   there was no change to the XDR for existing operations, many existing
   operations and COMPOUNDs consisting only of them became invalid.

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   Also, changes were made regarding the required server behavior as to
   the interaction of the MODE and Access Control List (ACL) attributes.

6.  Extending Existing Minor Versions

   Extensions to the most recently published NFSv4 minor version may be
   made by publishing the extension as a Proposed Standard, unless the
   minor version in question has been defined as non-extensible.  A
   document need not use the "Updates" header specifying the RFC that
   defines the minor version, which remains a valid description of the
   base variant of the minor version in question.

   In addition to following the rules for XDR extensions in Section 4.2,
   such extensions must also obey the rules listed below in order to
   allow interoperability to be established, as described in
   Section 4.4:

   o  Additions to the set of callback requests and extensions to the
      XDR for existing callback operations can only be made if the
      server can determine, based on the client's actions, that the
      client is aware of the changes.  This determination, for any
      particular client (as defined by its clientid), is made before
      sending those new or extended callbacks.

   o  XDR extensions that affect the structures of responses to existing
      operations can only be made if the server can determine, based on
      the client's actions, that it is aware of the existence of XDR
      changes, before sending responses containing those extensions.
      This determination can be based on the request being responded to,
      but that is not required.  Use of any protocol element defined in
      the extension can be the basis of the determination, provided that
      the requirements for determining client awareness are clearly
      stated.

   Corrections to protocol errors (see Section 9) may be accomplished by
   publishing an extension, including a compatible XDR change that
   follows the rules above.  Such documents will update the defining
   documents for the minor version to be corrected.

   In some cases, extensions will contain elements such as new
   operations or previously invalid switch cases.  Although it is
   possible to determine whether these OPTIONAL elements are supported
   using the rules described above, those defining an extension that
   contains such elements have the choice of defining a new attribute
   that indicates whether the feature is present and supported.  Since
   it is easy to determine whether a new attribute is supported using

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   the supported_attrs attribute, this can make it simple and convenient
   for clients to determine whether support is present, particularly
   when a feature involves support for multiple such elements.

7.  Minor Versions

7.1.  Creation of New Minor Versions

   It is important to note that this section, in describing situations
   that would require new minor versions to be created, does not thereby
   imply that situations will exist in the future.  Judgments regarding
   desirability of future changes will be made by the working group or
   its successors and any guidance that can be offered at this point is
   necessarily quite limited.

   Creation of a new minor version is an option that the working group
   retains.  The listing of situations below that would prompt such
   actions is not meant to be exhaustive.

   The following sorts of features are not allowed as extensions and
   would require creation of a new minor version:

   o  Features that incorporate any of the non-XDR-based changes
      discussed in Sections 5.1 and 5.2.

   o  Features whose XDR changes do not follow the rules in Section 6.

   o  Addition of REQUIRED new features.

   o  Changes to the status of existing features including converting
      features to be mandatory to not implement.

8.  Minor Version Interaction Rules

   This section addresses issues related to rules #11 and #13 in the
   minor versioning rules in [RFC5661].  With regard to the supersession
   of minor versioning rules, the treatment here overrides that in
   [RFC5661] when either of the potentially interacting minor versions
   has not yet been published as a Proposed Standard.

   Note that these rules are the only ones directed to minor version
   implementers, rather than to those specifying new minor versions.

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8.1.  Minor Version Identifier Transfer Issues

   Each relationship between a client instance and a server instance, as
   represented by a clientid, is to be devoted to a single minor
   version.  If a server detects that a COMPOUND with an inappropriate
   minor version is being used, it MUST reject the request.  In doing
   so, it may return either NFS4ERR_BAD_CLIENTID or
   NFS4RR_MINOR_VERS_MISMATCH.

   As a result of the above, the client has the assurance that the set
   of REQUIRED and OPTIONAL features will not change within the context
   of a single clientid.  Server implementations MUST ensure that the
   set of supported features and protocol elements does not change
   within such a context.

8.2.  Minor Version Compatibility

   The goal of the NFSv4 extension model is to enable compatibility
   including compatibility between clients and servers implementing
   different minor versions.

   Within a set of minor versions that define the same set of features
   as REQUIRED and mandatory to not implement, it is relatively easy for
   clients and servers to provide the needed compatibility by adhering
   to the following practices:

   o  Servers supporting a given minor version should support earlier
      minor versions within that set and return appropriate errors for
      use of protocol elements that were not a valid part of that
      earlier minor version.  For details, see below.

   o  Clients should deal with an NFS4ERR_MINOR_VERS_MISMATCH error by
      searching for a lower minor version number that the server will
      accept.

   Servers supporting a given minor version MUST, in returning errors
   for operations that were a valid part of the minor version, return
   the errors allowed for the current operation in the minor version
   actually being used.

   With regard to protocol elements not known in a given minor version,
   the appropriate error codes are given below.  Essentially, the
   server, although it has a more extensive XDR reflective of a newer
   minor version, must act as a server with a more limited XDR would.

   o  When an operation is used that is not known in the specified minor
      version, NFS4ERR_OP_ILLEGAL (as opposed to NFS4ERR_NOTSUPP) should
      be returned.

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   o  When an attribute is used that is not known in the specified minor
      version, NFS4ERR_INVAL (as opposed to NFS4ERR_ATTRNOTSUPP) should
      be returned.

   o  When a switch case is used that is not known in the specified
      minor version, NFS4ERR_BADXDR (as opposed to
      NFS4ERR_UNION_NOTSUPP) should be returned.  Even though the
      message may be XDR-decodable by the server's current XDR, it is
      not so according to the minor version being used.

   o  When a flag bit is used that is not known in the specified minor
      version, NFS4ERR_INVAL (as opposed to NFS4ERR_NOTSUPP or any other
      error defined as indicating non-support of a flag bit) should be
      returned.

9.  Correction of Existing Minor Versions and Features

   The possibility always exists that there will be a need to correct an
   existing feature in some way after the acceptance of that feature, or
   a minor version containing it, as a Proposed Standard.  While the
   working group can reduce the probability of such situations arising
   by waiting for running code before considering a feature as done, it
   cannot reduce the probability to zero.  As features are used more
   extensively and interact with other features, previously unseen flaws
   may be discovered and will need to be corrected.

   Such corrections are best done in a document obsoleting or updating
   the RFC defining the relevant feature or minor version.  In making
   such corrections, the working group will have to carefully consider
   how to assure interoperability with older clients and servers.

   Often, corrections can be done without changing the protocol XDR.  In
   many cases, a change in client and server behavior can be implemented
   without taking special provision with regard to interoperability with
   earlier implementations.  In those cases, and in cases in which a
   revision merely clarifies an earlier protocol definition document, a
   new document can be published that simply updates the earlier
   protocol definition document.

   In other cases, it is best if client or server behavior needs to
   change in a way that raises interoperability concerns.  In such
   cases, incompatible changes in server or client behavior should not
   be mandated in order to avoid XDR changes.

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9.1.  XDR Changes to Implement Protocol Corrections

   When XDR changes are necessary as part of correcting a flaw, these
   should be done in a manner similar to that used when implementing new
   minor versions or features within them.  In particular:

   o  Existing XDR structures may not be modified or deleted.

   o  XDR extensions may be used to correct existing protocol facilities
      in a manner similar to those used to add additional optional
      features.  Such corrections may be done in a minor version for
      which optional features may no longer be added, if the working
      group decides that it is an appropriate way to compatibly effect a
      correction.

   o  When a correction is made to an OPTIONAL feature, the result is
      similar to a situation in which there are two independent OPTIONAL
      features.  A server may choose to implement either or both.  See
      Section 9.2 for a detailed discussion of interoperability issues.

   o  When a correction is made to a REQUIRED feature, the situation
      becomes one in which the old version of the feature remains
      REQUIRED while the corrected version, while OPTIONAL, is intended
      to be adopted to provide correct operation.  Although use of the
      corrected version is ultimately better and may be recommended, it
      should not be described as "RECOMMENDED" since the choice of
      versions to support will depend on the needs of clients, which may
      be slow to adopt the updated version.  The nature of such
      corrections is such that it may result in situations in which
      different variants of the same minor version may not both support
      the corrected version.  See Section 9.3 for details.

   o  In all of the cases above, it is appropriate that the old version
      of the feature be considered obsolescent, with the expectation
      that the working group might, in a later minor version, change the
      status of the uncorrected version.  See Section 9.4 for more
      detail.

9.2.  XDR Corrections to OPTIONAL Features

   By defining the corrected and uncorrected version as independent
   OPTIONAL features, the protocol with the XDR modification can
   accommodate clients and servers that support either the corrected or
   the uncorrected version of the protocol, and also clients and servers
   aware of and capable of supporting both alternatives.

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   Based on the type of client:

   o  A client that uses only the earlier version of the feature (i.e.,
      an older unfixed client) can determine whether the server it is
      connecting to supports the older version of feature.  It is
      capable of interoperating with older servers that support only the
      unfixed protocol as well as ones that support both versions.

   o  A client that supports only the corrected version of the feature
      (i.e., a new or updated client) can determine whether the server
      it is connecting to supports the newer version of the feature.  It
      is capable of interoperating with newer servers that support only
      the updated feature as well as ones that support both versions.

   o  A client that supports both the older and newer version of the
      feature can determine which version of the particular feature is
      supported by the server it is working with.

   Based on the type of server:

   o  A server that supports only the earlier version of the feature
      (i.e., an older unfixed server) can only successfully interoperate
      with clients implementing the older version.  However, clients
      that do not implement the older version of the feature can easily
      determine that the feature cannot be used on that server.

   o  A server that supports only the newer version of the feature
      (i.e., a new or updated server) can only successfully interoperate
      with newer clients.  However, older clients can easily determine
      that the feature cannot be used on that server.  In the case of
      OPTIONAL features, clients can be expected to deal with non-
      support of that particular feature.

   o  A server that supports both the older and newer versions of the
      feature can interoperate with all client variants.

   By using extensions in this manner, the protocol creates a clear path
   that preserves the functioning of existing clients and servers and
   allows client and server implementers to adopt the new version of the
   feature at a reasonable pace.

9.3.  XDR Corrections to REQUIRED Features

   Interoperability issues in this case are similar to those for the
   OPTIONAL case described above (in Section 9.2).  However, because the
   use of the uncorrected version is REQUIRED, servers have to support
   this until there is a minor version change.  Nevertheless, there is
   the opportunity for clients and servers to implement the corrected

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   version, while maintaining necessary interoperability with earlier
   implementations.

   The following types of servers can exist:

   o  Servers only aware of and supporting the uncorrected version, such
      as servers developed before the issue requiring correction was
      known.

   o  Servers aware of both versions while only supporting the
      uncorrected version.

   o  Servers aware of and supporting both versions.

   With the exception of clients that do not use the feature in
   question, the following sorts of clients may exist:

   o  Clients only aware of and prepared to use the uncorrected version,
      such as those developed before the issue requiring correction was
      known.

      Clients developed before the correction was defined would be of
      this type.  They would be capable of interoperating with all of
      the types of servers listed above, but could not use the corrected
      version.

   o  Clients aware of both versions while only prepared to use the
      uncorrected version.

      Some clients developed or modified after the correction was
      defined would be of this type, until they were modified to support
      the corrected version.  They would also be capable of
      interoperating with all of the types of servers listed above, but
      could not use the corrected version.

   o  Clients aware of and prepared to use either version.

      Such clients would be capable of interoperating with all of the
      types of servers listed above, and could use the corrected version
      with servers that supported it.

   o  Clients aware of both versions while only prepared to use the
      newer, corrected version.

      Such clients would only be capable of interoperating with servers
      that supported the corrected version.  With other types of
      servers, they could determine the absence of appropriate support
      at an early stage and treat the minor version in question as

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      unsupported by the server.  Such clients are only likely to be
      deployed when the majority of servers support the corrected
      version.

9.4.  Addressing XDR Corrections in Later Minor Versions

   As described in Sections 9.2 and 9.3, a corrected XDR can be
   incorporated in an existing minor version and be used, while an
   existing uncorrected version is still supported.  Nevertheless, the
   uncorrected version will remain part of the protocol until its status
   is changed in a later minor version.

   One possible change that could be made in a later minor version is to
   define the uncorrected version as mandatory to not implement.
   Because of the difficulty of determining that no clients depend on
   support for the uncorrected version, it is unlikely that this step
   would be appropriate for a considerable time.

   In the case of a correction to a REQUIRED feature, there are a number
   of less disruptive changes that could be made earlier:

   o  Changing the uncorrected version from REQUIRED to OPTIONAL while
      REQUIRING that servers support at least one of the two versions.

      This would allow new server implementations to avoid support for
      the uncorrected version.

   o  Changing the corrected version from OPTIONAL to REQUIRED, making
      both versions REQUIRED.

      This would allow new clients to depend on support for the
      corrected version being present.

   o  Changing the uncorrected version from REQUIRED to OPTIONAL while
      changing the corrected version from OPTIONAL to REQUIRED.

      This would complete the shift to the corrected version once
      clients are prepared to use the corrected version.

   In making such changes, interoperability issues would need to be
   carefully considered.

10.  Security Considerations

   Since no substantive protocol changes are proposed here, no security
   considerations apply.

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11.  IANA Considerations

   The current document does not require any IANA actions.

12.  References

12.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5661]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
              "Network File System (NFS) Version 4 Minor Version 1
              Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
              <http://www.rfc-editor.org/info/rfc5661>.

   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <http://www.rfc-editor.org/info/rfc7530>.

   [RFC7862]  Haynes, T., "Network File System (NFS) Version 4 Minor
              Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
              November 2016, <http://www.rfc-editor.org/info/rfc7862>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.

12.2.  Informative References

   [RFC3530]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
              Beame, C., Eisler, M., and D. Noveck, "Network File System
              (NFS) version 4 Protocol", RFC 3530, DOI 10.17487/RFC3530,
              April 2003, <http://www.rfc-editor.org/info/rfc3530>.

Acknowledgements

   The author wishes to thank Tom Haynes of Primary Data for his role in
   getting the effort to revise NFSV4 version management started and for
   his work in coauthoring the first draft version of this document.

   The author also wishes to thank Chuck Lever and Mike Kupfer of
   Oracle, and Bruce Fields of Red Hat for their helpful reviews of this
   and other versioning-related documents.

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Author's Address

   David Noveck
   NetApp
   1601 Trapelo Road
   Waltham, MA  02451
   United States of America

   Phone: +1 781 572 8038
   Email: davenoveck@gmail.com