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rfc:rfc8178

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.

Noveck Standards Track [Page 1] RFC 8178 NFSv4 Extension July 2017

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

Noveck Standards Track [Page 2] RFC 8178 NFSv4 Extension July 2017

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.

Noveck Standards Track [Page 3] RFC 8178 NFSv4 Extension July 2017

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.

Noveck Standards Track [Page 4] RFC 8178 NFSv4 Extension July 2017

 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.

Noveck Standards Track [Page 5] RFC 8178 NFSv4 Extension July 2017

 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.

Noveck Standards Track [Page 6] RFC 8178 NFSv4 Extension July 2017

 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.

Noveck Standards Track [Page 7] RFC 8178 NFSv4 Extension July 2017

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

Noveck Standards Track [Page 8] RFC 8178 NFSv4 Extension July 2017

    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.

Noveck Standards Track [Page 9] RFC 8178 NFSv4 Extension July 2017

 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.

Noveck Standards Track [Page 10] RFC 8178 NFSv4 Extension July 2017

 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.

Noveck Standards Track [Page 11] RFC 8178 NFSv4 Extension July 2017

 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.

Noveck Standards Track [Page 14] RFC 8178 NFSv4 Extension July 2017

 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.

Noveck Standards Track [Page 15] RFC 8178 NFSv4 Extension July 2017

 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

Noveck Standards Track [Page 17] RFC 8178 NFSv4 Extension July 2017

 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.

Noveck Standards Track [Page 19] RFC 8178 NFSv4 Extension July 2017

 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.

Noveck Standards Track [Page 21] RFC 8178 NFSv4 Extension July 2017

 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

Noveck Standards Track [Page 22] RFC 8178 NFSv4 Extension July 2017

 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

Noveck Standards Track [Page 23] RFC 8178 NFSv4 Extension July 2017

    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.

Noveck Standards Track [Page 24] RFC 8178 NFSv4 Extension July 2017

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.

Noveck Standards Track [Page 25] RFC 8178 NFSv4 Extension July 2017

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

Noveck Standards Track [Page 26]

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