Requirements for Parallel NFS (pNFS) Layout Types
RFC 8434

Internet Engineering Task Force (IETF)                         T. Haynes
Request for Comments: 8434                                   Hammerspace
Updates: 5661                                                August 2018
Category: Standards Track
ISSN: 2070-1721

           Requirements for Parallel NFS (pNFS) Layout Types

Abstract

   This document defines the requirements that individual Parallel NFS
   (pNFS) layout types need to meet in order to work within the pNFS
   framework as defined in RFC 5661.  In so doing, this document aims to
   clearly distinguish between requirements for pNFS as a whole and
   those specifically directed to the pNFS file layout.  The lack of a
   clear separation between the two sets of requirements has been
   troublesome for those specifying and evaluating new layout types.  In
   this regard, this document updates RFC 5661.

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
   https://www.rfc-editor.org/info/rfc8434.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................2
   2. Definitions .....................................................3
      2.1. Use of the Terms "Data Server" and "Storage Device" ........5
      2.2. Requirements Language ......................................6
   3. The Control Protocol ............................................6
      3.1. Control Protocol Requirements ..............................8
      3.2. Previously Undocumented Protocol Requirements ..............9
      3.3. Editorial Requirements ....................................10
   4. Specifications of Original Layout Types ........................11
      4.1. File Layout Type ..........................................11
      4.2. Block Layout Type .........................................12
      4.3. Object Layout Type ........................................13
   5. Summary ........................................................14
   6. Security Considerations ........................................15
   7. IANA Considerations ............................................15
   8. References .....................................................16
      8.1. Normative References ......................................16
      8.2. Informative References ....................................16
   Acknowledgments ...................................................17
   Author's Address ..................................................17

1.  Introduction

   The concept of "layout type" has a central role in the definition and
   implementation of Parallel NFS (pNFS) (see [RFC5661]).  Clients and
   servers implementing different layout types behave differently in
   many ways while conforming to the overall pNFS framework defined in
   [RFC5661] and this document.  Layout types may differ as to:

   o  The method used to do I/O operations directed to data storage
      devices.

   o  The requirements for communication between the metadata server
      (MDS) and the storage devices.

   o  The means used to ensure that I/O requests are only processed when
      the client holds an appropriate layout.

   o  The format and interpretation of nominally opaque data fields in
      pNFS-related NFSv4.x data structures.

   Each layout type will define the needed details for its usage in the
   specification for that layout type; layout type specifications are
   always Standards Track RFCs.  Except for the file layout type defined

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   in Section 13 of [RFC5661], existing layout types are defined in
   their own Standards Track documents, and it is anticipated that new
   layout types will be defined in similar documents.

   The file layout type was defined in the Network File System (NFS)
   version 4.1 protocol specification [RFC5661].  The block layout type
   was defined in [RFC5663], and the object layout type was defined in
   [RFC5664].  Subsequently, the Small Computer System Interface (SCSI)
   layout type was defined in [RFC8154].

   Some implementers have interpreted the text in Sections 12 ("Parallel
   NFS (pNFS)") and 13 ("NFSv4.1 as a Storage Protocol in pNFS: the File
   Layout Type") of [RFC5661] as applying only to the file layout type.
   Because Section 13 was not covered in a separate Standards Track
   document such as those for both the block and object layout types,
   there was some confusion as to the responsibilities of both the
   metadata server and the data servers (DSs) that were laid out in
   Section 12.

   As a consequence, authors of new specifications (see [RFC8435] and
   [Lustre]) may struggle to meet the requirements to be a pNFS layout
   type.  This document gathers the requirements from all of the
   original Standards Track documents regarding layout type and then
   specifies the requirements placed on all layout types independent of
   the particular type chosen.

2.  Definitions

   control communication requirement:  the specification for information
      on layouts, stateids, file metadata, and file data that must be
      communicated between the metadata server and the storage devices.
      There is a separate set of requirements for each layout type.

   control protocol:  the particular mechanism that an implementation of
      a layout type would use to meet the control communication
      requirement for that layout type.  This need not be a protocol as
      normally understood.  In some cases, the same protocol may be used
      as both a control protocol and storage protocol.

   storage protocol:  the protocol used by clients to do I/O operations
      to the storage device.  Each layout type specifies the set of
      storage protocols.

   loose coupling:  when the control protocol is a storage protocol.

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   tight coupling:  an arrangement in which the control protocol is one
      designed specifically for control communication.  It may be either
      a proprietary protocol adapted specifically to a particular
      metadata server or a protocol based on a Standards Track document.

   (file) data:  that part of the file system object that contains the
      data to be read or written.  It is the contents of the object
      rather than the attributes of the object.

   data server (DS):  a pNFS server that provides the file's data when
      the file system object is accessed over a file-based protocol.
      Note that this usage differs from that in [RFC5661], which applies
      the term in some cases even when other sorts of protocols are
      being used.  Depending on the layout, there might be one or more
      data servers over which the data is striped.  While the metadata
      server is strictly accessed over the NFSv4.1 protocol, the data
      server could be accessed via any file access protocol that meets
      the pNFS requirements.

      See Section 2.1 for a comparison of this term and "storage
      device".

   storage device:  the target to which clients may direct I/O requests
      when they hold an appropriate layout.  Note that each data server
      is a storage device but that some storage device are not data
      servers.  See Section 2.1 for further discussion.

   fencing:  the process by which the metadata server prevents the
      storage devices from processing I/O from a specific client to a
      specific file.

   layout:  the information a client uses to access file data on a
      storage device.  This information includes specification of the
      protocol (layout type) and the identity of the storage devices to
      be used.

      The bulk of the contents of the layout are defined in [RFC5661] as
      nominally opaque, but individual layout types are responsible for
      specifying the format of the layout data.

   layout iomode:  a grant of either read-only or read/write I/O to the
      client.

   layout stateid:  a 128-bit quantity returned by a server that
      uniquely defines the layout state provided by the server for a
      specific layout that describes a layout type and file (see

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      Section 12.5.2 of [RFC5661]).  Further, Section 12.5.3 of
      [RFC5661] describes differences in handling between layout
      stateids and other stateid types.

   layout type:  a specification of both the storage protocol used to
      access the data and the aggregation scheme used to lay out the
      file data on the underlying storage devices.

   recalling a layout:  a graceful recall, via a callback, of a specific
      layout by the metadata server to the client.  Graceful here means
      that the client would have the opportunity to flush any WRITEs,
      etc., before returning the layout to the metadata server.

   revoking a layout:  an invalidation of a specific layout by the
      metadata server.  Once revocation occurs, the metadata server will
      not accept as valid any reference to the revoked layout, and a
      storage device will not accept any client access based on the
      layout.

   (file) metadata:  the part of the file system object that contains
      various descriptive data relevant to the file object, as opposed
      to the file data itself.  This could include the time of last
      modification, access time, EOF position, etc.

   metadata server (MDS):  the pNFS server that provides metadata
      information for a file system object.  It is also responsible for
      generating, recalling, and revoking layouts for file system
      objects, for performing directory operations, and for performing
      I/O operations to regular files when the clients direct these to
      the metadata server itself.

   stateid:  a 128-bit quantity returned by a server that uniquely
      defines the set of locking-related state provided by the server.
      Stateids may designate state related to open files, byte-range
      locks, delegations, or layouts.

2.1.  Use of the Terms "Data Server" and "Storage Device"

   In [RFC5661], the terms "data server" and "storage device" are used
   somewhat inconsistently:

   o  In Section 12, where pNFS in general is discussed, the term
      "storage device" is used.

   o  In Section 13, where the file layout type is discussed, the term
      "data server" is used.

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   o  In other sections, the term "data server" is used, even in
      contexts where the storage access type is not NFSv4.1 or any other
      file access protocol.

   As this document deals with pNFS in general, it uses the more generic
   term "storage device" in preference to "data server".  The term "data
   server" is used only in contexts in which a file server is used as a
   storage device.  Note that every data server is a storage device, but
   storage devices that use protocols that are not file access protocols
   (such as NFS) are not data servers.

   Since a given storage device may support multiple layout types, a
   given device can potentially act as a data server for some set of
   storage protocols while simultaneously acting as a storage device for
   others.

2.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document differs from most Standards Track documents in that it
   specifies requirements for those defining future layout types rather
   than defining the requirements for implementations directly.  This
   document makes clear whether:

   (1)  any particular requirement applies to implementations.

   (2)  any particular requirement applies to those defining layout
        types.

   (3)  the requirement is a general requirement that implementations
        need to conform to, with the specific means left to layout type
        definitions type to specify.

3.  The Control Protocol

   A layout type has to meet the requirements that apply to the
   interaction between the metadata server and the storage device such
   that they present to the client a consistent view of stored data and
   locking state (Section 12.2.6 of [RFC5661]).  Particular
   implementations may satisfy these requirements in any manner they
   choose, and the mechanism chosen need not be described as a protocol.
   Specifications defining layout types need to clearly show how
   implementations can meet the requirements discussed below, especially

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   with respect to those that have security implications.  In addition,
   such specifications may find it necessary to impose requirements on
   implementations of the layout type to ensure appropriate
   interoperability.

   In some cases, there may be no control protocol other than the
   storage protocol.  This is often described as using a "loosely
   coupled" model.  In such cases, the assumption is that the metadata
   server, storage devices, and client may be changed independently and
   that the implementation requirements in the layout type specification
   need to ensure this degree of interoperability.  This model is used
   in the block and object layout type specification.

   In other cases, it is assumed that there will be a purpose-built
   control protocol that may be different for different implementations
   of the metadata server and data server.  The assumption here is that
   the metadata server and data servers are designed and implemented as
   a unit and interoperability needs to be assured between clients and
   metadata-data server pairs, developed independently.  This is the
   model used for the file layout.

   Another possibility is for the definition of a control protocol to be
   specified in a Standards Track document.  There are two subcases to
   consider:

   o  A new layout type includes a definition of a particular control
      protocol whose use is obligatory for metadata servers and storage
      devices implementing the layout type.  In this case, the
      interoperability model is similar to the first case above, and the
      defining document should assure interoperability among metadata
      servers, storage devices, and clients developed independently.

   o  A control protocol is defined in a Standards Track document that
      meets the control protocol requirements for one of the existing
      layout types.  In this case, the new document's job is to assure
      interoperability between metadata servers and storage devices
      developed separately.  The existing definition document for the
      selected layout type retains the function of assuring
      interoperability between clients and a given collection of
      metadata servers and storage devices.  In this context,
      implementations that implement the new protocol are treated in the
      same way as those that use an internal control protocol or a
      functional equivalent.

   An example of this last case is the SCSI layout type [RFC8154], which
   extends the block layout type.  The block layout type had a
   requirement for fencing of clients but did not present a way for the

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   control protocol (in this case, the SCSI storage protocol) to fence
   the client.  The SCSI layout type remedies that in [RFC8154] and, in
   effect, has a tightly coupled model.

3.1.  Control Protocol Requirements

   The requirements of interactions between the metadata server and the
   storage devices are:

   (1)  The metadata server MUST be able to service the client's I/O
        requests if the client decides to make such requests to the
        metadata server instead of to the storage device.  The metadata
        server must be able to retrieve the data from the constituent
        storage devices and present it back to the client.  A corollary
        to this is that even though the metadata server has successfully
        given the client a layout, the client MAY still send I/O
        requests to the metadata server.

   (2)  The metadata server MUST be able to restrict access to a file on
        the storage devices when it revokes a layout.  The metadata
        server typically would revoke a layout whenever a client fails
        to respond to a recall or a client's lease is expired due to
        non-renewal.  It might also revoke the layout as a means of
        enforcing a change in locking state or access permissions that
        the storage device cannot directly enforce.

        Effective revocation may require client cooperation in using a
        particular stateid (file layout) or principal (e.g., flexible
        file layout) when performing I/O.

        In contrast, there is no requirement to restrict access to a
        file on the storage devices when a layout is recalled.  It is
        only after the metadata server determines that the client is not
        gracefully returning the layout and starts the revocation that
        this requirement is enforced.

   (3)  A pNFS implementation MUST NOT allow the violation of NFSv4.1's
        access controls: Access Control Lists (ACLs) and file open
        modes.  Section 12.9 of [RFC5661] specifically lays this burden
        on the combination of clients, storage devices, and the metadata
        server.  However, the specification of the individual layout
        type might create requirements as to how this is to be done.
        This may include a possible requirement for the metadata server
        to update the storage device so that it can enforce security.

        The file layout requires the storage device to enforce access
        whereas the flexible file layout requires both the storage
        device and the client to enforce security.

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   (4)  Interactions between locking and I/O operations MUST obey
        existing semantic restrictions.  In particular, if an I/O
        operation would be invalid when directed at the metadata server,
        it is not to be allowed when performed on the storage device.

        For the block and SCSI layouts, as the storage device is not
        able to reject the I/O operation, the client is responsible for
        enforcing this requirement.

   (5)  Any disagreement between the metadata server and the data server
        as to the value of attributes such as modify time, the change
        attribute, and the EOF position MUST be of limited duration with
        clear means of resolution of any discrepancies being provided.
        Note the following:

        (a)  Discrepancies need not be resolved unless any client has
             accessed the file in question via the metadata server,
             typically by performing a GETATTR.

        (b)  A particular storage device might be striped, and as such,
             its local view of the EOF position does not match the
             global EOF position.

        (c)  Both clock skew and network delay can lead to the metadata
             server and the storage device having different values of
             the time attributes.  As long as those differences can be
             accounted for in what is presented to the client in a
             GETATTR, then no violation results.

        (d)  A LAYOUTCOMMIT requires that changes in attributes
             resulting from operations on the storage device need to be
             reflected in the metadata server by the completion of the
             operation.

   These requirements may be satisfied in different ways by different
   layout types.  As an example, while the file layout type uses the
   stateid to fence off the client, there is no requirement that other
   layout types use this stateid approach.

   Each new Standards Track document for a layout type MUST address how
   the client, metadata server, and storage devices are to interact to
   meet these requirements.

3.2.  Previously Undocumented Protocol Requirements

   While not explicitly stated as requirements in Section 12 of
   [RFC5661], the existing layout types do have more requirements that
   they need to enforce.

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   The client has these obligations when making I/O requests to the
   storage devices:

   (1)  Clients MUST NOT perform I/O to the storage device if they do
        not have layouts for the files in question.

   (2)  Clients MUST NOT perform I/O operations outside of the specified
        ranges in the layout segment.

   (3)  Clients MUST NOT perform I/O operations that would be
        inconsistent with the iomode specified in the layout segments it
        holds.

   Under the file layout type, the storage devices are able to reject
   any request made not conforming to these requirements.  This may not
   be possible for other known layout types, which puts the burden of
   enforcing such violations solely on the client.  For these layout
   types:

   (1)  The metadata server MAY use fencing operations to the storage
        devices to enforce layout revocation against the client.

   (2)  The metadata server MUST allow the clients to perform data I/O
        against it, even if it has already granted the client a layout.
        A layout type might discourage such I/O, but it cannot forbid
        it.

   (3)  The metadata server MUST be able to do storage allocation,
        whether that is to create, delete, extend, or truncate files.

   The means to address these requirements will vary with the layout
   type.  A control protocol will be used to effect these; the control
   protocol could be a purpose-built one, one identical to the storage
   protocol, or a new Standards Track control protocol.

3.3.  Editorial Requirements

   This section discusses how the protocol requirements discussed above
   need to be addressed in documents specifying a new layout type.
   Depending on the interoperability model for the layout type in
   question, this may involve the imposition of layout-type-specific
   requirements that ensure appropriate interoperability of pNFS
   components that are developed separately.

   The specification of the layout type needs to make clear how the
   client, metadata server, and storage device act together to meet the
   protocol requirements discussed previously.  If the document does not

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   impose implementation requirements sufficient to ensure that these
   semantic requirements are met, it is not appropriate for publication
   as an RFC from the IETF stream.

   Some examples include:

   o  If the metadata server does not have a means to invalidate a
      stateid issued to the storage device to keep a particular client
      from accessing a specific file, then the layout type specification
      has to document how the metadata server is going to fence the
      client from access to the file on that storage device.

   o  If the metadata server implements mandatory byte-range locking
      when accessed directly by the client, then the layout type
      specification must require that this also be done when data is
      read or written using the designated storage protocol.

4.  Specifications of Original Layout Types

   This section discusses how the original layout types interact with
   Section 12 of [RFC5661], which enumerates the requirements of pNFS
   layout type specifications.  It is not normative with regards to the
   file layout type presented in Section 13 of [RFC5661], the block
   layout type [RFC5663], and the object layout type [RFC5664].  These
   are discussed here only to illuminate the updates Section 3 of this
   document makes to Section 12 of [RFC5661].

4.1.  File Layout Type

   Because the storage protocol is a subset of NFSv4.1, the semantics of
   the file layout type comes closest to the semantics of NFSv4.1 in the
   absence of pNFS.  In particular, the stateid and principal used for
   I/O MUST have the same effect and be subject to the same validation
   on a data server as it would have if the I/O were being performed on
   the metadata server itself.  The same set of validations are applied
   whether or not pNFS is in effect.

   While for most implementations, the storage devices can do the
   following validations that are each presented as a "SHOULD" and not a
   "MUST" in [RFC5661]:

   (1)  client holds a valid layout,

   (2)  client I/O matches the layout iomode, and

   (3)  client does not go out of the byte ranges,

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   Actually, the first point is presented in [RFC5661] as both:

   "MUST":  in Section 13.6

      As described in Section 12.5.1, a client MUST NOT send an I/O to a
      data server for which it does not hold a valid layout; the data
      server MUST reject such an I/O.

   "SHOULD":  in Section 13.8

      The iomode need not be checked by the data servers when clients
      perform I/O.  However, the data servers SHOULD still validate that
      the client holds a valid layout and return an error if the client
      does not.

   It should be noted that it is just these layout-specific checks that
   are optional, not the normal file access semantics.  The storage
   devices MUST make all of the required access checks on each READ or
   WRITE I/O as determined by the NFSv4.1 protocol.  If the metadata
   server would deny a READ or WRITE operation on a file due to its ACL,
   mode attribute, open access mode, open deny mode, mandatory byte-
   range locking state, or any other attributes and state, the storage
   device MUST also deny the READ or WRITE operation.  Also, while the
   NFSv4.1 protocol does not mandate export access checks based on the
   client's IP address, if the metadata server implements such a policy,
   then that counts as such state as outlined above.

   The data filehandle provided by the PUTFH operation to the data
   server provides sufficient context to enable the data server to
   ensure that the client has a valid layout for the I/O being performed
   for the subsequent READ or WRITE operation in the compound.

   Finally, the data server can check the stateid presented in the READ
   or WRITE operation to see if that stateid has been rejected by the
   metadata server; if so, the data server will cause the I/O to be
   fenced.  Whilst it might just be the open owner or lock owner on that
   client being fenced, the client should take the NFS4ERR_BAD_STATEID
   error code to mean it has been fenced from the file and contact the
   metadata server.

4.2.  Block Layout Type

   With the block layout type, the storage devices are generally not
   able to enforce file-based security.  Typically, storage area network
   (SAN) disk arrays and SAN protocols provide coarse-grained access
   control mechanisms (e.g., Logical Unit Number (LUN) mapping and/or
   masking), with a target granularity of disks rather than individual
   blocks and a source granularity of individual hosts rather than of

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   users or owners.  Access to block storage is logically at a lower
   layer of the I/O stack than NFSv4.  Since NFSv4 security is not
   directly applicable to protocols that access such storage directly,
   Section 2.1 of [RFC5663] specifies that:

      in environments where pNFS clients cannot be trusted to enforce
      such policies, pNFS block layout types SHOULD NOT be used.

   Due to these granularity issues, the security burden has been shifted
   from the storage devices to the client.  Those deploying
   implementations of this layout type need to be sure that the client
   implementation can be trusted.  This is not a new sort of requirement
   in the context of SAN protocols.  In such environments, the client is
   expected to provide block-based protection.

   This shift of the burden also extends to locks and layouts.  The
   storage devices are not able to enforce any of these, and the burden
   is pushed to the client to make the appropriate checks before sending
   I/O to the storage devices.  For example, the server may use a layout
   iomode only allowing reading to enforce a mandatory read-only lock.
   In such cases, the client has to support that use by not sending
   WRITEs to the storage devices.  The fundamental issue here is that
   the storage device is treated by this layout type in the same fashion
   as a local disk device.  Once the client has access to the storage
   device, it is able to perform both READ and WRITE I/O to the entire
   storage device.  The byte ranges in the layout, any locks, the layout
   iomode, etc., can only be enforced by the client.  Therefore, the
   client is required to provide that enforcement.

   In the context of fencing off of the client upon revocation of a
   layout, these limitations come into play again, i.e., the granularity
   of the fencing can only be at the level of the host and logical unit.
   Thus, if one of a client's layouts is revoked by the server, it will
   effectively revoke all of the client's layouts for files located on
   the storage units comprising the logical volume.  This may extend to
   the client's layouts for files in other file systems.  Clients need
   to be prepared for such revocations and reacquire layouts as needed.

4.3.  Object Layout Type

   With the object layout type, security checks occur during the
   allocation of the layout.  The client will typically ask for layouts
   covering all of the file and may do so for either READ or READ/WRITE.
   This enables it to do subsequent I/O operations without the need to
   obtain layouts for specific byte ranges.  At that time, the metadata
   server should verify permissions against the layout iomode, the file
   mode bits or ACLs, etc.  As the client may be acting for multiple

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   local users, it MUST authenticate and authorize the user by issuing
   respective OPEN and ACCESS calls to the metadata server, similar to
   having NFSv4 data delegations.

   Upon successful authorization, the client receives within the layout
   a set of object capabilities allowing it I/O access to the specified
   objects corresponding to the requested iomode.  These capabilities
   are used to enforce access control and locking semantics at the
   storage devices.  Whenever one of the following occurs on the
   metadata server, then the metadata server MUST change the capability
   version attribute on all objects comprising the file in order to
   invalidate any outstanding capabilities before committing to one of
   these changes:

   o  the permissions on the object change,

   o  a conflicting mandatory byte-range lock is granted, or

   o  a layout is revoked and reassigned to another client.

   When the metadata server wishes to fence off a client to a particular
   object, then it can use the above approach to invalidate the
   capability attribute on the given object.  The client can be informed
   via the storage device that the capability has been rejected and is
   allowed to fetch a refreshed set of capabilities, i.e., reacquire the
   layout.

5.  Summary

   In the three original layout types, the burden of enforcing the
   security of NFSv4.1 can fall to either the storage devices (files),
   the client (blocks), or the metadata server (objects).  Such choices
   are conditioned by the native capabilities of the storage devices --
   if a control protocol can be implemented, then the burden can be
   shifted primarily to the storage devices.

   In the context of this document, we treat the control protocol as a
   set of requirements.  As new layout types are published, the defining
   documents MUST address:

   (1)  The fencing of clients after a layout is revoked.

   (2)  The security implications of the native capabilities of the
        storage devices with respect to the requirements of the NFSv4.1
        security model.

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   In addition, these defining documents need to make clear how other
   semantic requirements of NFSv4.1 (e.g., locking) are met in the
   context of the proposed layout type.

6.  Security Considerations

   This section does not deal directly with security considerations for
   existing or new layout types.  Instead, it provides a general
   framework for understating security-related issues within the pNFS
   framework.  Specific security considerations will be addressed in the
   Security Considerations sections of documents specifying layout
   types.  For example, in Section 3 of [RFC5663], the lack of finer-
   than-physical disk access control necessitates that the client is
   delegated the responsibility to enforce the access provided to them
   in the layout extent that they were granted by the metadata server.

   The layout type specification must ensure that only data access
   consistent with the NFSV4.1 security model is allowed.  It may do
   this directly, by providing that appropriate checks be performed at
   the time each access is performed.  It may do it indirectly by
   allowing the client or the storage device to be responsible for
   making the appropriate checks.  In the latter case, I/O access rights
   are reflected in layouts, and the layout type must provide a way to
   prevent inappropriate access due to permissions changes between the
   time a layout is granted and the time the access is performed.

   The metadata server MUST be able to fence off a client's access to
   the data file on a storage device.  When it revokes the layout, the
   client's access MUST be terminated at the storage devices.  The
   client has a subsequent opportunity to reacquire the layout and
   perform the security check in the context of the newly current access
   permissions.

7.  IANA Considerations

   This document has no IANA actions.

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8.  References

8.1.  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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc5661>.

   [RFC5663]  Black, D., Fridella, S., and J. Glasgow, "Parallel NFS
              (pNFS) Block/Volume Layout", RFC 5663,
              DOI 10.17487/RFC5663, January 2010,
              <https://www.rfc-editor.org/info/rfc5663>.

   [RFC5664]  Halevy, B., Welch, B., and J. Zelenka, "Object-Based
              Parallel NFS (pNFS) Operations", RFC 5664,
              DOI 10.17487/RFC5664, January 2010,
              <https://www.rfc-editor.org/info/rfc5664>.

   [RFC8154]  Hellwig, C., "Parallel NFS (pNFS) Small Computer System
              Interface (SCSI) Layout", RFC 8154, DOI 10.17487/RFC8154,
              May 2017, <https://www.rfc-editor.org/info/rfc8154>.

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

8.2.  Informative References

   [Lustre]   Faibish, S., Cote, D., and P. Tao, "Parallel NFS (pNFS)
              Lustre Layout Operations", Work in Progress,
              draft-faibish-nfsv4-pnfs-lustre-layout-07, May 2014.

   [RFC8435]  Halevy, B. and T. Haynes, "Parallel NFS (pNFS) Flexible
              File Layout", RFC 8435, DOI 10.17487/RFC8435, August 2018,
              <https://www.rfc-editor.org/info/rfc8435>.

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Acknowledgments

   Dave Noveck provided an early review that sharpened the clarity of
   the definitions.  He also provided a more comprehensive review of the
   document.

   Both Chuck Lever and Christoph Helwig provided insightful comments
   during the working group last call.

Author's Address

   Thomas Haynes
   Hammerspace
   4300 El Camino Real Ste 105
   Los Altos, CA  94022
   United States of America

   Email: loghyr@gmail.com

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