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NFSv4                                                     D. Noveck, Ed.
Internet-Draft                                                    NetApp
Intended status: Informational                                 P. Shivam
Expires: November 17, 2017                                      C. Lever
                                                                B. Baker
                                                                  ORACLE
                                                            May 16, 2017


 NFSv4 Migration and Trunking: Implementation and Specification Issues
                  draft-ietf-nfsv4-migration-issues-13

Abstract

   This document discusses a range of implementation and specification
   issues concerning features related to the use of location-related
   attributes in NFSv4.  These include migration, which transfers
   responsibility for a file system from one server to another, and
   trunking which deals with the discovery and control of the set of
   network addresses to use to access a file system.  The focus of the
   discussion, which relates to multiple minor versions, is on providing
   appropriate clarification and correction of existing specifications.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 17, 2017.

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



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Language  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     2.2.  Use of Normative Terms  . . . . . . . . . . . . . . . . .   4
     2.3.  Terminology Used in this Document . . . . . . . . . . . .   5
   3.  Multi-version Issues and Their Resolution . . . . . . . . . .   6
     3.1.  Multi-Version Issues  . . . . . . . . . . . . . . . . . .   6
     3.2.  Resolution of Multi-Version Issues  . . . . . . . . . . .   7
       3.2.1.  Providing Trunking Discovery  . . . . . . . . . . . .   7
       3.2.2.  Interaction of Trunking and Migration . . . . . . . .   9
   4.  NFSv4.0 Issues  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Core NFSv4.0 Migration Issues . . . . . . . . . . . . . .  11
     4.2.  Resolution of Core Migration Protocol Difficulties in
           NFSv4.0 . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.3.  Additional NFSv4.0 Issues . . . . . . . . . . . . . . . .  12
     4.4.  Resolution of Additional NFSv4.0 Issues . . . . . . . . .  12
   5.  Issues for NFSv4.1  . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Issues to Address for NFSv4.1 . . . . . . . . . . . . . .  14
       5.1.1.  Addressing State Merger in NFSv4.1  . . . . . . . . .  15
       5.1.2.  Addressing pNFS Relationship with Migration . . . . .  15
       5.1.3.  Addressing Server_owner Changes in NFSv4.1  . . . . .  16
       5.1.4.  Addressing Confirmation Status of Migrated
               Client IDs in NFSv4.1 . . . . . . . . . . . . . . . .  17
       5.1.5.  Addressing Changes in Trunking Configuration  . . . .  18
       5.1.6.  Addressing Session Migration in NFSv4.1 . . . . . . .  18
     5.2.  Possible Resolutions for NFSv4.1 Protocol Issues  . . . .  19
       5.2.1.  Client ID Confirmation Issues . . . . . . . . . . . .  19
       5.2.2.  Dealing with Multiple Location Entries  . . . . . . .  20
       5.2.3.  Migration and pNFS  . . . . . . . . . . . . . . . . .  22
     5.3.  Defining Server Responsibilities for NFSv4.1  . . . . . .  23
       5.3.1.  Server Responsibilities in Effecting Transparent
               State Migration . . . . . . . . . . . . . . . . . . .  24
       5.3.2.  Synchronizing Session Transfer  . . . . . . . . . . .  25
     5.4.  Defining Client Responsibilities for NFSv4.1  . . . . . .  27
       5.4.1.  Client Recovery from Migration Events . . . . . . . .  28
       5.4.2.  The Migration Discovery Process . . . . . . . . . . .  30
       5.4.3.  Overview of Client Response to NFS4ERR_MOVED  . . . .  31
       5.4.4.  Obtaining Access to Sessions and State after
               Migration . . . . . . . . . . . . . . . . . . . . . .  33



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       5.4.5.  Obtaining Access to Sessions and State after Network
               Address Transfer  . . . . . . . . . . . . . . . . . .  35
     5.5.  Resolution of NFSv4.1 Issues  . . . . . . . . . . . . . .  35
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  38
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  38
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  38
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  38
   Appendix B.  History of this Document . . . . . . . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1.  Introduction

   This is an informational document that discusses a number of related
   issues in multiple versions of NFSv4.

   Many of these relate to the migration feature of NFSv4, which
   provides for moving responsibility for a single filesystem from one
   server to another, without disruption to clients.  A number of
   problems in the specification of this feature in NFSv4.0 were
   resolved by the publication of [RFC7931], which added trunking
   detection to NFSV4.0.  However, NFSv4.0 remains without an
   appropriate discussion of trunking discovery, which has many
   important connections with migration.  As a result, NFSv4.0 requires
   clarification of how the client is to respond to changes in the
   trunking arrangements to use, both when migration occurs and when it
   does not.

   In addition, there are specification issues to be resolved with
   regard to the NFSv4.1 version of these features which are discussed
   in this document.

   All of the issues discussed relate to the handling and interpretation
   of the location-related attributes fs_locations and fs_locations_info
   and to the proper client and server handling of changes in the values
   of these attributes

   These issues are all related to the protocol features for effecting
   file system migration, or to trunking discovery but it is not
   possible to treat each of these features in isolation.  These
   features are inherently linked because migration needs to deal with
   the possibility of multiple server addresses in location attributes
   and because location attributes, which provide trunking-related
   information, may change, which might or might not involve migration.








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2.  Language

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.2.  Use of Normative Terms

   This document, which deals with existing issues/problems in
   standards-track documents, is in the informational category, and
   while the facts it reports may have normative implications, any such
   normative significance reflects the readers' preferences.  For
   example, we may report that the existing definition of migration for
   NFSv4.1 does not properly describe how migrating state is to be
   merged with existing state for the destination server.  While it is
   to be expected that client and server implementers will judge this to
   be a situation that it would be appropriate to resolve, the judgment
   as to how pressing this issue should be considered is a judgment for
   the reader, and eventually the nfsv4 working group to make.

   We do explore possible ways in which such issues can be dealt with,
   with minimal negative effects, given that the working group has
   decided to address these issues, but the choice of exactly how to
   address these is best given effect in one or more standards-track
   documents and/or errata.

   In the context of this informational document, these normative
   keywords will generally occur in the context of a quotation, most
   often direct but sometimes indirect.  The context will make it clear
   whether the quotation is from:

   o  The base definition of the NFSv4.0 protocol [RFC7530].

   o  The document updating the handling of migration in NFSv4.0
      [RFC7931].

   o  The current definition of the NFSv4.1 protocol [RFC5661].

   An additional possibility is that these terms may appear in a
   proposed or possible text to serve as a replacement for a current
   protocol specification.  Sometimes, a number of possible alternative
   texts may be listed and benefits and detriments of each examined in
   turn.






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2.3.  Terminology Used in this Document

   In this document the phrase "client ID" always refers to the 64-bit
   shorthand identifier assigned by the server (a clientid4) and never
   to the structure which the client uses to identify itself to the
   server (called an nfs_client_id4 or client_owner in NFSv4.0 and
   NFSv4.1 respectively).  The opaque identifier within those structures
   is referred to as a client id string".

   Regarding trunking of network addresses, we use the following
   terminology:

   o  Trunking detection refers to ways of deciding whether two specific
      network addresses are connected to the same NFSv4 server.  The
      means available to make this determination depends on the protocol
      version, and, in some cases, on the client implementation.

   o  Two network addresses connected to the same server are said to be
      server-trunkable.

   o  Two network addresses connected to the same server such that those
      addresses can be used to support a single common session are
      referred to as session-trunkable.  Note that two addresses may be
      server-trunkable without being session-trunkable.

   o  Trunking discovery is a process by which a client using one
      network address can obtain other addresses that are trunkable
      (either server-trunkable or session-trunkable) with it.

   Regarding terminology relating to attributes used in trunking
   discovery and other multi-server namespace features:

   o  Location attributes include the fs_locations and fs_locations_info
      attributes.

   o  Location entries are the individual file system locations in the
      location attributes.

   o  Location elements are derived from location entries.  If a
      location entry specifies an IP address there is only a single
      corresponding location element.  Location entries that contain a
      host name, are resolved using DNS, and may result in one or more
      location elements.  All location elements consist of a location
      address which is the IP address of an interface to a server and an
      fs name which is the location of the file system within the
      server's pseudo-fs.  The fs name is empty if the server has no
      pseudo-fs and only a single exported file system at the root
      filehandle.



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   o  Two location elements are said to be server-trunkable if they
      specify the same fs name and the location addresses are such that
      the location addresses are server-trunkable.

   o  Two location elements are said to be session-trunkable if they
      specify the same fs name and the location addresses are such that
      the location addresses are session-trunkable.

   Each set of server-trunkable location elements defines the available
   access paths to a particular file system.  When there are multiple
   such file systems, each of these, which contains the same data, is a
   replica of the others.  Logically, such replication is symmetric,
   since the fs currently in use and an alternate fs are replicas of
   each other.  Often, in other documents, the term "replica" is not
   applied to the fs currently in use, despite the fact that the
   replication relation is inherently symmetric.

3.  Multi-version Issues and Their Resolution

3.1.  Multi-Version Issues

   The source of these issues rises from a lack of clarity regarding the
   meaning of and proper handling for location attributes that specify
   more than a single server address.  Such situations can arise as a
   result of multiple entries in the same attribute or because a single
   entry has a server name which, when processed by DNS, is mapped to
   multiple server addresses.

   Both [RFC7530] and [RFC5661] indicate that multiple addresses may be
   present and that these addresses may be different paths to the same
   server as well as different copies of the same data.  However, the
   following issues have, for both protocols, interfered with the
   recognition of the existing location attributes as a way of providing
   a trunking discovery function:

   o  There is no discussion of the use of these attributes when a file
      system is first accessed, giving the impression that they are only
      to be used as a way of overcoming access difficulties.

   o  The treatment of migration (and in the case of NFSv4.1 of file
      system transitions in general) is written as if only a single
      server address will be accessed.

   o  Although location attributes can contains the addresses of
      migration targets and of additional replicas as well, the issues
      that arise when both of these are specified are not clearly
      discussed.




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   In addition, there are factors that relates to specific protocol
   versions and documents:

   o  In NFSv4.0, as described solely by [RFC7530], trunking is treated
      as a problem to be avoided, making the whole matter moot.

   o  In NFSv4.0, as described by [RFC7530] together with [RFC7931], the
      situation is different.  There is a means of trunking detection
      suggested in [RFC7931] but it is a suggestion only valid when the
      client chooses to use the uniform client id string model.

   o  For NFSv4.1, as described by [RFC5661], there is a standard method
      of trunking detection, which can be relied upon.

   The issues that need to be resolved for both versions are:

   o  Provision of a trunking discovery facility to allow a client to
      find out about other addresses that may be used to access the
      current server.

   o  Better integration of migration with trunking changes, including
      situations in which the set of addresses to access the same server
      changes (without migration) and those in which there is a shift to
      a different server, but trunking of addresses on either the source
      or destination is involved

3.2.  Resolution of Multi-Version Issues

   Although the specifics of addressing these issues will be different
   for different versions, there are some common aspects discussed in
   the subsections below:

   o  The trunking discovery function, except for the additional
      location attribute fs_locations_info, is to be addressed in
      substantially the same way in both versions, as explained in
      Section 3.2.1.

   o  The interaction of trunking and migration is discussed in general
      terms in Section 3.2.2.  However, the specifics of the NFSv4.1
      client's response to NFS4ERR_MOVED are discussed in Sections
      5.4.3, 5.4.4, and 5.4.5.

3.2.1.  Providing Trunking Discovery

   A client can discover a set network addresses to use to access a file
   system using an NFSv4 server in a number of ways:





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   o  If the client is accessing a server using its name, that name can
      be mapped to a set of IP addresses using DNS and if multiple
      addresses are available, those addresses can generally be used
      together to access the server.

   o  A client connected to a server without knowledge of its name can
      obtain the value of a location attribute (e.g. fs_locations).
      Where an entry within that attribute specifies a server name, DNS
      can be used to obtain one or more network addresses corresponding
      to that name.  In cases in which one of those is the address being
      used, the others that corresponding to that name can also be used
      to access the server.

   o  A client can obtain the value of a location attribute (e.g.
      fs_locations) and use location entries that specify network
      addresses.  When there is a means of trunking detection available
      (see below), all of addresses that are determined to correspond to
      the same server can be used to access the server.

   Note that the last two of these are usable in situations in which
   NFS4ERR_MOVED was returned.  Note that this does not necessarily mean
   that migration has occurred since there may be a shift in the set of
   network addresses to be used without changing to a different server.
   See Section 3.2.2 for further discussion.

   Which of the above means of providing trunking information is
   appropriate to use in a given environment will depend on security
   considerations, the possible need for the server to direct different
   clients to different sets of addresses, and the availability of
   trunking detection facilities on the clients.

   With regard to security, the possibility that requests to determine
   the set of network addresses corresponding to a given server might be
   interfered with or have their responses corrupted needs to be taken
   account of.  As a result, when use of DNSSEC is not available, it
   might not be advisable to present server names in location attributes
   and present the network addresses directly, eliminating the need to
   use DNS to effect this translation.  Fetching of location attributes
   should be done with integrity protection.

   In some situations, the server will want to direct clients to use
   specific sets of network addresses, to effect load balancing, or to
   meet quality-of-service goals.  In such environments, presentation of
   network addresses directly in the location attribute can help give
   the server the necessary control over the paths to be used when
   accessing particular file systems.  When such techniques are used,
   servers typically present their own network addresses in the location




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   attribute while adding the names of other servers, such as those used
   to access replicas.

   Trunking detection allows the client to determine whether two network
   addresses can be used to access the same server.  The availability of
   trunking detection depends on the protocol version, and, in some
   case, on client implementation choices:

   o  For NFSv4.0, a means by which it can be determined if two network
      addresses correspond to the same server is suggested in [RFC7931].
      However is it is optional and only available to clients using the
      uniform client id string approach.

   o  For NFSv4.1, the client can compare the server_owner returned in
      the response to EXCHANGE_ID to determine if two network addresses
      correspond to the same server.

   As a result, direct presentation of network addresses in location
   entries may be problematic for NFSv4.0, since some clients might not
   have the trunking detection facilities that allow them to take
   advantage of this information.  For further discussion of issues
   related to NFSv4.0, see Section 4.4.

3.2.2.  Interaction of Trunking and Migration

   When the set of network addresses designated by a location attribute
   changes, NFS4ERR_MOVED may or may not result, and in some of the
   cases in which it is returned migration will occur, while in others
   there a shift in the network addresses used to access a particular
   file system with no migration.

   o  When the list of networks addresses is a superset of that
      previously in effect, there is no need for migration or any other
      sort of client adjustment.  Nevertheless, the client is free to
      use an additional address if it provides another path to the same
      server.  If, on the other hand, it does not do so, the client may
      treat it as it does a replica, to be used if the current server
      addresses become unavailable.

   o  When the list of networks addresses is a subset of that previously
      in effect, immediate action is not needed if the address was not
      being used.  The client should avoid using it in the future,
      whether the address is for a replica or a potential additional
      path to the server being used.

   o  When an address being removed is one of a number of paths to the
      current server, the client can cease to use it but it can continue
      to use it until NFS4ERR_MOVED is received.  This is not considered



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      a migration event, unless it is the last available path to the
      server that has become unusable.

   When migration does occur, multiple addresses may be in use on the
   server previous to migration and multiple addresses may be available
   for use on the destination server.

   With regard to the server in use, it may be that return of
   NFS4ERR_MOVED indicates that a particular network address is no
   longer to be used, without implying that migration of the file system
   to a different server is needed In light of this possibility, clients
   are best off not concluding that migration has occurred until
   concluding that all the network addresses known to be associated with
   the server are not usable.

   It should be noted that the need to defer this determination is not
   absolute.  If a client is not aware of all network addresses for any
   reason, if may conclude that migration has occurred when it has not
   and treat a switch to a different server address as if it were a
   migration event.  This is generally harmless since the use of the
   same server via a new address will appear as a successful Transparent
   State Migration.

   While significant harm will not arise from this misapprehension, it
   can give rise to disconcerting situations.  For example, if a lock
   has been revoked during the address shift, it will appear to the
   client as if the lock has been lost during migration, normally
   calling for it to be recoverable via an fs-specific grace period
   associated with the migration event.

   With regard to the destination server, it is desirable for the client
   to be aware of all the valid network addresses that can be used to
   access the destination server.  However, there is no need for this to
   be done immediately.  Implementations can process the additional
   location elements in parallel with normal use of the first valid
   location entry found to access the destination.

   Because a location attribute may include entries relating to the
   current server, the migration destination and possible replicas to
   use, scanning for available network addresses could potentially be a
   long process.  The following list of helpful practices, here
   presented as suggestions, could become RECOMMENDATIONs or
   REQUIREMENTs in future standards-track documents

   o  Servers are well advised to place location entries that represent
      addresses usable with the current server or a migration target
      before those associated with replicas.




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   o  A client can cease scanning for trunkable location entries once it
      encounters one whose fs_name differs from the current fs name.

   o  A client can cease scanning for trunkable location entries once it
      encounters a location element whose address in not server-
      trunkable with the one it is using.

4.  NFSv4.0 Issues

4.1.  Core NFSv4.0 Migration Issues

   Many of the problems seen with Transparent State Migration derived
   from the inability of NFSv4.0servers to determine whether two client
   IDs, issued on different servers, corresponded to the same client.
   This difficulty derived in turn from the common practice, recommended
   by [RFC7530], in which each client presented different client
   identification strings to different servers, rather than presenting
   the same identification string to all servers.

   This practice, later referred to as the "non-uniform" client id
   string approach, derived from concern that, since NFSv4.0 provided no
   means to determine whether two IP addresses correspond to the server,
   a single client connected to both might be confused by the fact that
   state changes made via one IP address might unexpectedly affect the
   state maintained with respect to the second IP address, thought of as
   a separate server

   To avoid this unexpected behavior, clients used the non-uniform
   client id string approach.  By doing so, a client connected to two
   different servers (or to two IP addresses connected to the same
   server) appeared to be two different servers.  Since the server is
   under the impression that two different clients are involved, state
   changes made on each distinct IP address cannot be reflected on
   another.

   However, by doing things in that way, state migrated from server to
   server cannot be referred to the actual client which generated it,
   leading to confusion.

   In addition to this core problem, the following issues with regard to
   Transparent State Migration needed to be addressed:

   o  Clarification regarding the ability to merge state from different
      leases even though their expiration times might not be precisely
      synchronized.

   o  Clarifying the treatment of client IDs since it is not always
      clear when clientid4 and when nfs_client_id4 was intended.



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   o  Clarifying the logic of returning NFS4ERR_LEASE_MOVED.

   o  Clarifying the handling NFS4ERR_CLID_INUSE.

4.2.  Resolution of Core Migration Protocol Difficulties in NFSv4.0

   The client string identification issue was addressed in [RFC7931] as
   follows:

   o  Defining both the uniform and non-uniform client id string
      approaches as valid choices but indicating that the latter posed
      difficulties for Transparent Stare Migration.

   o  Providing a way that clients using the uniform approach could use
      to determine whether two IP addresses are connected to the same
      server.

   o  Allowing clients using the uniform approach to avoid negative
      consequences due to otherwise unexpected behavior since behavior
      that is a consequence of known trunking relationships is not
      unexpected.

   o  As a result, servers migrating state can be aware of the fact that
      the same client is associated with two different items of state
      even when that state was originally created on two different
      servers.

   Since all of the other issues noted in Section 4.1 were also
   addressed by [RFC7931], publication of that document updating
   [RFC7530] addressed all issues with Transparent State Migration in
   NFSv4.0 known at that time.

4.3.  Additional NFSv4.0 Issues

   In light of the fact that a large set of migration-specific issues
   were addressed by the publication of [RFC7931], the remaining issues
   derive from those mentioned in Section 3.1.  These include:

   o  Introducing facilities for trunking discovery.

   o  Clarifying the relationship between migration and trunking.

4.4.  Resolution of Additional NFSv4.0 Issues

   One possible approach to addressing these issues would entail
   publication of an additional standards-track document updating
   [RFC7530].




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   Fortunately, it appears that all of the material to be updated
   appears in Section 8 of that document, whether it concerns the
   provision of trunking discovery or the interaction of trunking and
   migration.  It also appears that none of the material to be updated
   is in sections updated by [RFC7931].

   A review of the existing Section 8 of [RFC7530], shows the following
   sections as requiring significant attention:

   o  The existing Section 8.1 requires a considerable expansion to
      explain the various uses of the fs_locations and the possible
      interactions among them.

   o  The existing Section 8.4 may require substantial re-organization
      to reflect the facts that fs_locations has multiple functions and
      may be referenced on multiple occasions.

   o  The existing Section 8.5 follows the previous approach for NFSv4.0
      in assuming that trunking simply cannot and should not happen.
      For example, the last paragraph says:

         If a single location entry designates multiple server IP
         addresses, the client should choose a single one to use.  When
         two server addresses are designated by a single location entry
         and they correspond to different servers, this normally
         indicates some sort of misconfiguration, and so the client
         should avoid using such location entries when alternatives are
         available.  When they are not, clients should pick one of the
         IP addresses and use it, without using others that are not
         directed to the same server.

      As written, this seems to foreclose any use of trunking in
      connection with migration.  In retrospect, it appears that this
      section should have been revised as part of [RFC7931], but since
      that was not done then, the issue needs to be addressed now.

   Overall, it appears that, in addition to the revision of Section 8.1,
   Sections 8.4 and 8.5 need to be reorganized.  One likely approach is
   to divide the material into three main sections based on the
   circumstances in which the attribute is accessed:

      A section dealing with initial use and how it is trunking
      discovery and preparation for replication.

      A section dealing with subsequent attribute change and how it
      affects trunking discovery and preparation for replication.





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      A section dealing with how the attribute is used to deal with
      communication problems including receiving NFS4ERR_MOVED but also
      including other communication difficulties driving selection of a
      new replica.

5.  Issues for NFSv4.1

5.1.  Issues to Address for NFSv4.1

   Because NFSv4.1 embraces the uniform client-string approach, as
   advised by section 2.4 of [RFC5661], addressing migration issues is
   simpler, in that a shift in client id string models is not required.
   Instead, NFSv4 returns information in the EXCHANGE_ID response to
   enable trunking relationships to be determined by the client.

   Despite this simplification, there are substantial issues that need
   to be dealt with:

   o  The other necessary part of addressing migration issues, providing
      for the server's merger of leases that relate to the same client,
      is not currently addressed by [RFC5661] and changes need to be
      made to make it clear that state needs to be appropriately merged
      as part of migration, to avoid multiple client IDs between a
      client-server pair.

   o  The current discussion (in [RFC5661]), of the possibility of
      server_owner changes is incomplete and confusing.

   o  As with NFSV4.0, the interaction of trunking with migration and
      other aspects of multi-server namespace needs to be clarified.

   Addressing migration in NFSv41 will also require adaptation of the
   approaches used in [RFC7931] to the NFSv4.1 environment including:

   o  The use of EXCHANGE_ID needs to be accommodated including issues
      associated with the expected confirmation status of client IDs
      transferred by Transparent State Migration.

   o  The use of sessions needs to be addressed including discussion of
      the proper use of the status bits returned by the SEQUENCE
      operation.

   In addition, there are a number of new features within NFSv4.1 whose
   relationship with migration needs to be clarified.  Some examples:

   o  There needs to be some clarification of how migration, and
      particularly Transparent State Migration, should interact with
      pNFS layouts.



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   o  There are a number of issues related to the migration of sessions
      that need to be addressed.

   Discussion of how to resolve these issues will appear in the sections
   below.

5.1.1.  Addressing State Merger in NFSv4.1

   The existing treatment of state transfer in [RFC5661], has similar
   problems to that in [RFC7530] in that it assumes that the state for
   multiple filesystems formerly on different servers will not be merged
   so that it appears under a single common client ID.  We've already
   seen the reasons that this is a problem with regard to NFSv4.0.

   Although we don't have the problems stemming from the non-uniform
   client-string approach, there are a number of complexities in the
   existing treatment of state management in the section entitled "Lock
   State and File System Transitions" in [RFC5661] that make this non-
   trivial to address:

   o  Migration is currently treated together with other sorts of
      filesystem transitions including transitioning between replicas
      without any NFS4ERR_MOVED errors.

   o  There is separate handling and discussion of the cases of matching
      and non-matching server scopes.

   o  In the case of matching server scopes, the text calls for an
      unrealistic degree of transparency, suggesting that the source and
      destination servers need to cooperate in stateid and client ID
      assignment.

   o  In the case of non-matching server scopes, the text does not
      mention the possibility of the transparent migration of state at
      all, resulting in a functional regression from NFSV4.0

5.1.2.  Addressing pNFS Relationship with Migration

   This is made difficult because, within the pNFS framework, migration
   might mean any of several things:

   o  Transfer of the MDS, leaving DS's as they are.

      This would be minimally disruptive to those using layouts but
      would require the pNFS control protocol being used to support the
      DS being directed to a new MDS.

   o  Transfer of a DS, leaving everything else in place.



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      Such a transfer can be handled without using migration at all.
      The server can recall/revoke layouts, and issue new ones, as
      appropriate.

   o  Transfer of the filesystem to a new filesystem with both MDS and
      DS's moving.

      In such a transfer, an entirely different set of DS's will be at
      the target location.  There may even be no pNFS support on the
      destination filesystem at all.

   Migration needs to support both the first and last of these models.

5.1.3.  Addressing Server_owner Changes in NFSv4.1

   Section 2.10.5 of [RFC5661] states the following.

      The client should be prepared for the possibility that
      eir_server_owner values may be different on subsequent EXCHANGE_ID
      requests made to the same network address, as a result of various
      sorts of reconfiguration events.  When this happens and the
      changes result in the invalidation of previously valid forms of
      trunking, the client should cease to use those forms, either by
      dropping connections or by adding sessions.  For a discussion of
      lock reclaim as it relates to such reconfiguration events, see
      Section 8.4.2.1.

   While this paragraph is literally true in that such reconfiguration
   events can happen and clients have to deal with them, it is confusing
   in that it can be read as suggesting that clients have to deal with
   them without disruption, which in general is impossible.

   A clearer alternative would be:

      It is always possible that, as a result of various sorts of
      reconfiguration events, eir_server_scope and eir_server_owner
      values may be different on subsequent EXCHANGE_ID requests made to
      the same network address.

      In most cases such reconfiguration events will be disruptive and
      indicate that an IP address formerly connected to one server is
      now connected to an entirely different one.

      Some guidelines on client handling of such situations follow:

      o  When eir_server_scope changes, the client has no assurance that
         any id's it obtained previously (e.g. file handles) can be
         validly used on the new server, and, even if the new server



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         accepts them, there is no assurance that this is not due to
         accident.  Thus it is best to treat all such state as lost/
         stale although a client may assume that the probability of
         inadvertent acceptance is low and treat this situation as
         within the next case.

      o  When eir_server_scope remains the same and
         eir_server_owner.so_major_id changes, the client can use
         filehandles it has and attempt reclaims.  It may find that
         these are now stale but if NFS4ERR_STALE is not received, he
         can proceed to reclaim his opens.

      o  When eir_server_scope and eir_server_owner.so_major_id remain
         the same, the client has to use the now-current values of
         eir_server-owner.so_minor_id in deciding on appropriate forms
         of trunking.

5.1.4.  Addressing Confirmation Status of Migrated Client IDs in NFSv4.1

   When a client ID is transferred between systems as a part of
   migration, it has never been clear whether it should be considered
   confirmed or unconfirmed on the target server.  In the case in which
   an associated session is transferred together with the client ID, it
   is clear that the transferred client ID needs to be considered
   confirmed, as the existence of an associated session is incompatible
   with an unconfirmed client ID.

   The case in which a client ID is transferred without an associated
   session is less clear-cut, particularly since the treatment of
   EXCHANGE_ID in [RFC5661] assumes that CREATE_SESSION is the only
   means by which a client id may be confirmed.  While this assumption
   is valid in the absence of Transparent State Migration,
   implementation of migration means that if this assumption is
   maintained, it is not clear how migrated client ID s can be a
   accommodated.  If this assumption were maintained, we would have to
   choose between the following two alternatives, regarding whether the
   client ID to be reported as confirmed when EXCHANGE_ID is used to
   register an already-known client_owner with the server.

   o  Report the client ID unconfirmed, because of the lack of an
      associated session.  This makes it simpler for the client to
      determine whether there is an associated session transferred at
      the same time.  However, it is inconsistent with the fact there
      are stateids which have been transferred with the client ID.

   o  Report the client ID as confirmed, because it was confirmed on the
      source server and the transfer is not considered to have affected
      that.  Given the current description of EXCHANGE_ID in [RFC5661],



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      some modification in the treatment of client id confirmation is
      called for.  In particular, provision would have to be made to
      enable the client id slot sequence id to be used by the client to
      be determined.

   Although the first approach makes it simpler for the client to
   determine whether there is an associated session transferred at the
   same time, it makes it more difficult to determine whether
   Transparent State Migration has occurred.  Section 5.1.6.

   In any case, adjustments will be required to deal with the fact that
   [RFC5661] currently assumes that a client id can only be confirmed by
   issuing a CREATE_SESSION.  In order to properly deal with the status
   of migrated client ids, we have to distinguish among:

   o  The confirmation status as reported by EXCHANGE_ID.

   o  Whether the client id is considered confirmed as that term is used
      in the many other cases in which the confirmation status of a
      client ID affects how requests are handled.

   o  How the client is to determine the initial sequence id to be used
      when doing operations such as CREATE_SESSION.

   In [RFC5661] as it currently stands all of these are tied together
   and it is not obvious how migrated client IDs could be accommodated
   in this structure, and what changes are necessary to make this
   possible.  For more discussion of this issue, see Section 5.2.1.

5.1.5.  Addressing Changes in Trunking Configuration

   When the client us capable of finding out a set of network addresses
   to use in accessing a server, it is always possible for that set to
   change.

   This sometimes requires that a network address previously used to
   access a server becomes invalid for that purpose.  This requires a
   way of notifying the client and a way for the client to adapt to this
   change by using a new set of network addresses to access the server.
   his will involve recovery much like hat for migration although the
   same server and file system is used throughout.

5.1.6.  Addressing Session Migration in NFSv4.1

   Some issues that need to be addressed regard the migration of
   sessions, in addition to client IDs and stateids





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   o  It needs to be made clearer how the client can deal with the
      possibility that sessions might or might not be transferred as
      part of Transparent State Migration.

   o  Rules need to be clarified regarding possible transfer of sessions
      when either the source session is being used to access other file
      systems on source server or there is already a session connecting
      the client to the destination server.

   o  There needs to be more detail regarding how the protocol avoids
      situations in which the same session is subject to concurrent
      changes on two different servers at the same time.

5.2.  Possible Resolutions for NFSv4.1 Protocol Issues

   The subsections below explore some ways of dealing with clarifying
   the protocol to address issues discussed in Section 5.1

5.2.1.  Client ID Confirmation Issues

   As mentioned previously [RFC5661], makes no provision for client IDs
   that are confirmed other than through the use of CREATE_SESSION.  For
   example Section 18.35 of [RFC5661] states:

      The client uses the EXCHANGE_ID operation to register a particular
      client owner with the server.  The client ID returned from this
      operation will be necessary for requests that create state on the
      server and will serve as a parent object to sessions created by
      the client.  In order to confirm the client ID it must first be
      used, along with the returned eir_sequenceid, as arguments to
      CREATE_SESSION.  If the flag EXCHGID4_FLAG_CONFIRMED_R is set in
      the result, eir_flags, then eir_sequenceid MUST be ignored, as it
      has no relevancy.

   In deciding how to address the status of migrated client IDs in the
   case of Transparent State Migration, we should avoid giving undue
   weight to the last sentence of the above simply because it is stated
   in the form of a normative requirement.  We should instead focus on
   the reasons such terms (i.e. those defined by [RFC2119]) are to be
   used, to state interoperability constraints.  In this case, the
   "MUST" applies to a conclusion based on the premise that a
   CREATE_SESSION must have been done to assure that the client ID is
   reliably known to the server.

   In that light, let us consider a possible replacement, that treats
   confirmation by means of CREATE_SESSION as one of a number of
   possible means and avoids some the undesirable consequences of




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   adherence to the current approach, originally conceived without
   taking state migration into account.

      The client uses the EXCHANGE_ID operation to register a particular
      client_owner with the server.  However, when the client_owner has
      been already been registered by other means (e.g.  Transparent
      State Migration), the client may still use EXCHANGE_ID to obtain
      the client ID assigned previously.

      The client ID returned from this operation will be associated with
      the connection on which the EXHANGE_ID is received and will serve
      as a parent object for sessions created by the client on this
      connection or to which the connection is bound.  As a result of
      using those sessions to make requests involving the creation of
      state, that state will become associated with the client ID
      returned.

      In situations in which the registration of the client_owner has
      not occurred previously, the client ID must first be used, along
      with the returned eir_sequenceid, in creating an associated
      session using CREATE_SESSION.

      If the flag EXCHGID4_FLAG_CONFIRMED_R is set in the result,
      eir_flags, then it is an indication that the registration of the
      client_owner has already occurred and that a further
      CREATE_SESSION is not needed to confirm it.  Of course, subsequent
      CREATE_SESSION operations may be needed for other reasons.

      The value eir_seqenceid is used to establish an initial sequence
      value associate with the client ID returned.  In cases in which a
      CREATE_SESSION has already been done, there is no need for this
      value, since sequencing of such request has already been
      established and the client has no need for this value and will
      ignore it

5.2.2.  Dealing with Multiple Location Entries

   The possibility that more than one server address may be present in
   location attributes requires further clarification.  This is
   particularly the case, given the potential role of trunking for
   NFSv4.1, whose connection to migration needs to be clarified.

   The description of the location attributes in [RFC5661], while it
   indicates that multiple address entries in these attributes may be
   used to indicate alternate paths to the file system, does so mainly
   in the context of replication and does so without mentioning
   trunking.  The discussion of migration does not discuss the




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   possibility of multiple location entries or trunking, which we will
   explore here.

   We will cover cases in which multiple addresses appear directly in
   the attributes as well as those in which the multiple addresses
   result because a single location entry is expanded into multiple
   location elements using addresses provided by DNS.

   When the set of valid location elements by which a file system may be
   accessed changes, migration need not be involved.  Some cases to
   consider:

   o  When the set of location elements expands, migration is not
      involved.  In the case in which the additional elements are not
      trunkable with ones previously being used, the new elements serve
      as additional access locations, available in case of the failure
      of server addresses being used.  When additional elements are
      trunkable with those currently being used the client may use the
      additional addresses just as they might have if they had been
      available when use of the file system began.

      There is no current mechanism by which the client can be notified
      of a change in the set of available location for an fs.  Given the
      client has at least one IP address available to access the
      filesystem in question, periodic polling is an adequate mechanism
      for the client to find additional server addresses to use to
      access the file system.

   o  When the set of location elements contracts but none of the
      elements no longer usable were in fact being used by the client,
      then no migration is involved and no change in network addresses
      is needed.  Only if the client were to start using one of the
      unavailable elements would the client be notified (via
      NFS4ERR_MOVED) of the need to not use those elements and to use
      others provided by a location attribute.

   When a specific server address being used becomes unavailable to
   service a particular file system, NFS4ERR_MOVED will be returned, and
   the client will respond based on the available locations.  Whether
   continuity of locking state will be available depends on a number of
   factors:

   o  If there are still elements in use trunkable with the element that
      has become unavailable, there will still be a continuity of
      locking state, even though Transparent State Migration per se has
      not occurred.  If the in-use addresses are session-trunkable with
      the address becoming unavailable, only one connection is lost and
      all existing sessions will remain available.  If, on the other



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      hand, the in-use addresses are only clientid-trunkable with the
      address becoming unavailable, a session can be lost.  However,
      that session can be made available on those other nodes, just as
      they it would have been if Transparent State Migration were in
      effect, even though no migration has occurred.

   o  Otherwise, if there are available addresses trunkable with the one
      that has become unavailable, the client has access to existing
      locking state once it establishes a connection with the new
      addresses, using a new or existing session depending on the type
      of trunking in effect.  This is also similar to the case in which
      Transparent State Migration has occurred, even though there is no
      migration, with the state remaining on the existing server.

      Note that this case, as well as the previous one, can be expected
      in the case in which the server seeks to direct traffic with
      regard to particular file systems to choose addresses, in the
      interest of load balancing, to adjust to hardware availability
      constraints, or for other reasons.

   o  In other cases, migration has occurred and the client can
      determine whether Transparent State Migration occurred and whether
      any locking state was lost during the transfer.

   Whether migration has occurred or not, the client can use the
   procedure described in Section 5.4.3 to recover access to existing
   locking state and, in some cases, sessions.

   One should note the following differences between migration with
   Transparent State Migration and the similar cases in which there is a
   continuity of locking state with no change in the server.

   o  When locks are lost (as indicated when using them or via the
      SEQ4_STATUS flags) and migration has not been done, they are not
      to be reclaimed, except when SEQ4_STATUS_RESTART_RECLAIM_NEEDED is
      set.  Instead such losses are treated as lock revocations and
      acknowledged using FREE_STATEID.

   o  When migration has not been done, there is no need for a
      RECLAIM_COMPLETE (with rca_one_fs set to true).

5.2.3.  Migration and pNFS

   When pNFS is involved, the protocol is capable of supporting:

   o  Migration of the MDS, leaving DS's in place.





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   o  Migration of the file system as a whole, including the MDS and
      associated DS's.

   o  Replacement of one DS by another.

   o  Migration of a pNFS file system to one in which pNFS is not used.

   o  Migration of a file system not using pNFS to one in which layouts
      are available.

   Migration of the MDS function is directly supported by Transparent
   State Migration.  Layout state will normally be transparently
   transferred, just as other state is.  As a result, Transparent State
   Migration provides a framework in which, given appropriate inter-MDS
   data transfer, one MDS can be substituted for another.

   Migration of the file system function as a whole can be accomplished
   by recalling all layouts as part of the initial phase of the
   migration process.  As a result, IO will be done through the MDS
   during the migration process, and new layouts can be granted once the
   client is interacting with the new MDS.  An MDS can also effect this
   sort of transition by revoking all layouts as part of Transparent
   State Migration, as long as the client is notified about the loss of
   state.

   In order to allow migration to a file system on which pNFS is not
   supported, clients need to be prepared for a situation in which
   layouts are not available or supported on the destination file system
   and so direct IO requests to the destination server, rather than
   depending on layouts being available.

   Replacement of one DS by another is not addressed by migration as
   such but can be effected by an MDS recalling layouts for the DS to be
   replaced and issuing new ones to be served by the successor DS.

   Migration may transfer a file system from a server which does not
   support pNFS to one which does.  In order to properly adapt to this
   situation, clients which support pNFS, but function adequately in its
   absence, should check for pNFS support when a file system is migrated
   and be prepared to use pNFS when support is available.

5.3.  Defining Server Responsibilities for NFSv4.1

   The subsections below discuss the responsibilities of source and
   destination servers in effecting the necessary transfer of
   information to support Transparent State Migration.





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5.3.1.  Server Responsibilities in Effecting Transparent State Migration

   The basic responsibility of the source server in effecting
   Transparent State Migration is to make available to the destination
   server a description of each piece of locking state associated with
   the file system being migrated.  In addition to client id string and
   verifier, the source server needs to provide, for each stateid:

   o  The stateid including the current sequence value.

   o  The associated client ID.

   o  The handle of the associated file.

   o  The type of the lock, such as open, byte-range lock, delegation,
      layout.

   o  For locks such as opens and byte-range locks, there will be
      information about the owner(s) of the lock.

   o  For recallable/revocable lock types, the current recall status
      needs to be included.

   o  For each lock type there will by type-specific information, such
      as share and deny modes for opens and type and byte ranges for
      byte-range locks and layouts.

   A further server responsibility concerns locks that are revoked or
   otherwise lost during the process of file system migration.  Because
   locks that appear to be lost during the process of migration will be
   reclaimed by the client, the servers have to take steps to ensure
   that locks revoked soon before or soon after migration are not
   inadvertently allowed to be reclaimed in situations in which the
   continuity of lock possession cannot be assured.

   o  For locks lost on the source but whose loss has not yet been
      acknowledged by the client (by using FREE_STATEID), the
      destination must be aware of this loss so that it can deny a
      request to reclaim them.

   o  For locks lost on the destination after the state transfer but
      before the client's RECLAIM_COMPLTE is done, the destination
      server should note these and not allow them to be reclaimed.

   An additional responsibility of the cooperating servers concerns
   situations in which a stateid cannot be transferred transparently
   because it conflicts with an existing stateid held by the client and




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   associated with a different file system.  In this case there are two
   valid choices:

   o  Treat the transfer, as in NFSv4.0, as one without Transparent
      State Migration.  In this case, conflicting locks cannot be
      granted until the client does a RECLAIM_COMPLETE, after reclaiming
      the locks it had, with the exception of reclaims denied because
      they were attempts to reclaim locks that had been lost.

   o  Implement Transparent State Migration, except for the lock with
      the conflicting stateid.  In this case, the client will be aware
      of a lost lock (through the SEQ4_STATUS flags) and be allowed to
      reclaim it.

5.3.2.  Synchronizing Session Transfer

   When transferring state between the source and destination, the
   issues discussed in Section 7.2 of [RFC7931] must still be attended
   to.  In this case, the use of NFS4ERR_DELAY is still necessary in
   NFSv4.1, as it was in NFSv4.0, to prevent locking state changing
   while it is being transferred.

   There are a number of important differences in the NFS4.1 context:

   o  The absence of RELEASE_LOCKOWNER means that the one case in which
      an operation could not be deferred by use of NFS4ERR_DELAY no
      longer exists.

   o  Sequencing of operations is no longer done using owner-based
      operation sequences numbers.  Instead, sequencing is session-
      based

   As a result, when sessions are not transferred, the techniques
   discussed in [RFC7931] are adequate and will not be further
   discussed.

   When sessions are transferred, there are a number of issues that pose
   challenges since,

   o  A single session may be used to access multiple file systems, not
      all of which are being transferred.

   o  Requests made on a session may, even if rejected, affect the state
      of the session by advancing the sequence number associated with
      the slot used.

   As a result, when the filesystem state might otherwise be considered
   unmodifiable, the client might have any number of in-flight requests,



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   each of which is capable of changing session state, which may be of a
   number of types:

   1.  Those requests that were processed on the migrating file system,
       before migration began.

   2.  Those requests which got the error NFS4ERR_DELAY because the file
       system being accessed was in the process of being migrated.

   3.  Those requests which got the error NFS4ERR_MOVED because the file
       system being accessed had been migrated.

   4.  Those requests that accessed the migrating file system, in order
       to obtain location or status information.

   5.  Those requests that did not reference the migrating file system.

   It should be noted that the history of any particular slot is likely
   to include a number of these request classes.  In the case in which a
   session which is migrated is used by filesystems other than the one
   migrated, requests of class 5 may be common and be the last request
   processed, for many slots.

   Since session state can change even after the locking state has been
   fixed as part of the migration process, the session state known to
   the client could be different from that on the destination server,
   which necessarily reflects the session state on the source server, at
   an earlier time.  In deciding how to deal with this situation, it is
   helpful to distinguish between two sorts of behavioral consequences
   of the choice of initial sequence ID values.

   o  The error NFS4ERR_SEQ_MISORDERED is returned when the sequence ID
      in a request is neither equal to the last one seen for the current
      slot nor the next greater one.

      In view of the difficulty of arriving at a mutually acceptable
      value for the correct last sequence value at the point of
      migration, it may be necessary for the server to show some degree
      of forbearance, when the sequence ID is one that would be
      considered unacceptable if session migration were not involved.

   o  Returning the cached reply for a previously executed request when
      the sequence ID in the request matches the last value recorded for
      the slot.

      In the cases in which an error is returned and there is no
      possibility of any non-idempotent operation having been executed,
      it may not be necessary to adhere to this as strictly as might be



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      proper if session migration were not involved.  For example, the
      fact that the error NFS4ERR_DELAY was returned may not assist the
      client in any material way, while the fact that NFS4ERR_MOVED was
      returned by the source server may not be relevant when the request
      was reissued, directed to the destination server.

   One part of adapting to these sorts of issues would restrict
   enforcement of normal slot sequence enforcement semantics until the
   client itself, by issuing a request using a particular slot on the
   destination server, established the new starting sequence for that
   slot on the migrated session.

   An important issue is that the specification needs to take note of
   all potential COMPOUNDs, even if they might be unlikely in practice.
   For example, a COMPOUND is allowed to access multiple file systems
   and might perform non-idempotent operations in some of them before
   accessing a file system being migrated.  Also, a COMPOUND may return
   considerable data in the response, before being rejected with
   NFS4ERR_DELAY or NFS4ERR_MOVED, and may in addition be marked as
   sa_cachethis.

   Some possibilities that need to be considered to address the issues:

   o  Do not enforce any sequencing semantics for a particular slot
      until the client has established the starting sequence for that
      slot on the destination server.

   o  For each slot, do not return a cached reply returning
      NFS4ERR_DELAY or NFS4ERR_MOVED until the client has established
      the starting sequence for that slot on the destination server.

   o  Until the client has established the starting sequence for a
      particular slot on the destination server, do not report
      NFS4ERR_SEQ_MISORDERED or return a cached reply returning
      NFS4ERR_DELAY or NFS4ERR_MOVED, where the reply consists solely of
      a series of operations where the response is NFS4_OK until the
      final error.

5.4.  Defining Client Responsibilities for NFSv4.1

   The subsections below discuss the responsibilities of the client in
   dealing with transition to a new server (migration) and to use of new
   network addresses in accessing existing servers.








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5.4.1.  Client Recovery from Migration Events

   When a file system is migrated, there a number of migration-related
   status indications with which clients need to deal:

   o  If an attempt is made to use or return a filehandle within a file
      system that has been migrated away from the server on which it was
      previously available, the error NFS4ERR_MOVED is returned.

      This condition continues on subsequent attempts to access the file
      system in question.  The only way the client can avoid the error
      is to cease accessing the filesystem in question at its old server
      location and access it instead on the server to which it has been
      migrated.

   o  Whenever a SEQUENCE operation is sent by a client to a server
      which generated state held on that client which is associated with
      a file system that has been migrated away from the server on which
      it was previously available, the status bit
      SEQ4_STATUS_LEASE_MOVED is set in the response.

      This condition continues until the client acknowledges the
      notification by fetching a location attribute for the migrated
      file system.  When there are multiple migrated file systems, a
      location attribute for each such migrated file system needs to be
      fetched, in order to clear the condition.  Even after the
      condition is cleared, the client needs to respond by using the
      location information to access the destination server to ensure
      that leases are not needlessly expired.

   Unlike the case of NFSv4.0 in which the corresponding conditions are
   both errors, in NFSv4.1 the client can, and often will, receive both
   indications on the same request.  As a result, implementations need
   to address the question of how to co-ordinate the necessary recovery
   actions when both indications arrive simultaneously.  It should be
   noted that when the server decides whether SEQ4_STATUS_LEASE_MOVED is
   to be set, it has no way of knowing which file system will be
   referenced or whether NFS4ERR_MOVED will be returned.

   While it is true that, when only a single migrated file system is
   involved, a single set of actions will clear both indications, the
   possibility of multiple migrated file systems calls for an approach
   in which there are separate recovery actions for each indication.  In
   general, the response to neither indication can be subsumed within
   the other since:

   o  If the client were to respond only to the MOVED indication, there
      would be no effective client response to a situation in which a



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      file system was not being actively accessed at the time migration
      occurred.  As a result, leases on the destination server might be
      needlessly expired.

   o  If the client were to respond only to the LEASE_MOVED indication,
      recovery for migrated file systems in active use could be deferred
      in order to accomplish recovery for others not being actively
      accessed.  The consequences of this choice can pose particular
      problems when there are a large number of file systems supported
      by a particular server, or when it happens that some servers,
      after receiving migrated file systems have periods of
      unavailability, such as occur as a result of server reboot.  This
      can result in recovery for actively accessed migrated file systems
      being unnecessarily delayed for long periods of time.

   Similar considerations apply to other arrangements in which one of
   the indications, while not ignored per se, is subsumed within a
   single recovery process focused on recovery for the other indication.

   Although clients are free to decide on their own approaches to
   recovery, we will explore below an approach with the following
   characteristics:

   o  All instances of the MOVED indication, whether they involve
      migration or not, should be dealt with promptly, either by doing
      the necessary recovery directly, providing that it be done
      asynchronously, or ensuring that it is already under way.

   o  All instances of the LEASE_MOVED indication should be dealt with
      asynchronously, in a migration discovery thread whose job is to
      clear that indication by fetching the appropriate location
      attribute.  Because this thread will only be fetching a location
      attribute and the fs_status attribute for the file systems
      referenced by the client, it cannot receive MOVED indications.
      Some useful guidance regarding possible implementation of a
      migration discovery thread can be found in Section 5.4.2.

   o  When a migration discovery thread happens upon a migrated file
      system (i.e. not present and not a referral), the thread is likely
      to have cleared one (out of an unknown number) of file systems
      whose migration needs to be responded to.  The discovery thread
      needs to schedule the appropriate migration recovery (as described
      in Section 5.4.3).  This is necessary to ensure that migrated file
      systems will be referenced on the destination server in order to
      avoid unnecessary lease expiration.

      For many of the migrated file systems discovered in this way, the
      client has not received any MOVED indication.  In such cases,



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      lease recovery needs to be scheduled but it should not interfere
      with continuation of the migration discovery function.

   o  When a migration discovery thread receives a LEASE_MOVED
      indication, it takes no special action but continues its normal
      operation.  On the other hand, if a LEASE_MOVED indication is not
      received, it indicates that the thread has completed its work
      successfully.

5.4.2.  The Migration Discovery Process

   As noted above, LEASE_MOVED indications are best dealt with in a
   migration discovery thread.  Because of this structure,

   o  No action needs to be taken for such indications received by the
      migration discovery threads, since continuation of that thread's
      work will address the issue.

   o  For such indications received in other contexts, the generally
      appropriate response is to initiate or otherwise provide for the
      execution of a migration discovery thread for file systems
      associated with the server IP address returning the indication.

   o  In all cases in which the appropriate migration discovery thread
      is running, nothing further needs to be done to respond to
      LEASE_MOVED indications.

   This leaves a potential difficulty in situations in which the
   migration discovery thread is near to completion but is still
   operating.  One should not ignore a LEASE_MOVED indication if the
   discovery thread is not able to respond to migrated file system
   without additional aid.  A further difficulty in addressing such
   situation is that a LEASE_MOVED indication may reflect the server's
   state at the time the SEQUENCE operation was processed, which may be
   different from that in effect at the time the response is received.

   A useful approach to this issue involves the use of separate
   externally-visible discovery thread states representing non-
   operation, normal operation, and completion/verification of migration
   discovery processing.

   Within that framework, discovery thread processing would proceed as
   follows.

   o  While in the normal-operation state, the thread would fetch, for
      successive file systems known to the client on the server being
      worked on, a location attribute plus the fs_status attribute.




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   o  If the fs_status attribute indicates that the file system is a
      migrated one (i.e. fss_absent is true and fss_type !=
      STATUS4_REFERRAL) and thus that it is likely that the fetch of the
      location attribute has cleared one the file systems contributing
      to the LEASE_MOVED indication.

   o  In cases in which that happened, the thread cannot know whether
      the LEASE_MOVED indication has been cleared and so it enters the
      completion/verification state and proceeds to issue a COMPOUND to
      see if the LEASE_MOVED indication has been cleared.

   o  When the discovery thread is in the completion/verification state,
      if others get a LEASE_MOVED indication they note this fact and it
      is used when the request completes, as described below.

   When the request used in the completion/verification state completes:

   o  If a LEASE_MOVED indication is returned, the discovery thread
      resumes its normal work.

   o  Otherwise, if there is any record that other requests saw a
      LEASE_MOVED indication, that record is cleared and the
      verification request retried.  The discovery thread remains in
      completion/verification state.

   o  If there has been no LEASE_MOVED indication, the work of the
      discovery thread is considered completed and it enters the non-
      operating state.

5.4.3.  Overview of Client Response to NFS4ERR_MOVED

   This section outlines a way in which a client that receives
   NFS4ERR_MOVED can respond by using a new server or network address if
   one is available.  As part of that process, it will determine:

   o  Whether the NFS4ERR_MOVED indicates migration has occurred, or
      whether it indicates another sort of file system transition as
      discussed in Section 5.2.2.

   o  In the case of migration, whether Transparent State Migration has
      occurred.

   o  Whether any state has been lost during the process of Transparent
      State Migration.

   o  Whether sessions have been transferred as part of Transparent
      State Migration.




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   During the first phase of this process, the client proceeds to
   examine location entries to find the initial network address it will
   use to continue access to the file system or its replacement.  For
   each location entry that the client examines, the process consists of
   five steps:

   1.  Performing an EXCHANGE_ID is directed at the location address.
       This operation is used to register the client-owner with the
       server, to obtain a client ID to be use subsequently to
       communicate with it, to obtain tat client ID's confirmation
       status and, to determine server_owner and scope for the purpose
       of determining if the entry is trunkable with that previously
       being used to access the file system (i.e. that it represents
       another path to the same file system and can share locking state
       with it).

   2.  Making an initial determination of whether migration has
       occurred.  The initial determination will be based on whether the
       EXCHANGE_ID results indicate that the current location element is
       server-trunkable with that used to access the file system when
       access was terminated by receiving NFS4ERR_MOVED.  If it is, then
       migration has not occurred and the transition is dealt with, at
       least initially, as one involving continued access to the same
       file system on the same server through a new network address.

   3.  Obtaining access to existing session state or creating new
       sessions.  How this is done depends on the initial determination
       of whether migration has occurred and can be done as described in
       Section 5.4.4 in the case of migration or as described in
       Section 5.4.5 in the case of a network address transfer without
       migration.

   4.  Verification of the trunking relationship assumed in step 2 as
       discussed in Section 2.10.5.1 of [RFC5661].  Although this step
       will generally confirm the initial determination, it is possible
       for verification to fail with the result that an initial
       determination that a network address shift (without migration)
       has occurred may be invalidated and migration determined to have
       occurred.  There is no need to redo step 3 above, since it will
       be possible to continue use of the session established already.

   5.  Obtaining access to existing locking state and/or reobtaining it.
       How this is done depends on the final determination of whether
       migration has occurred and can be done as described in
       Section 5.4.4 in the case of migration or as described in
       Section 5.4.5 in the case of a network address transfer without
       migration.




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   Once the initial address has been determined, clients are free to
   apply an abbreviated process to find additional addresses trunkable
   with it (clients may seek session-trunkable or server trunkable
   addresses depending on whether they support clientid trunking).
   During this later phase of the process, further location entries are
   examined using the abbreviated procedure specified below:

   1.  Before the EXCHANGE_ID, the fs_name field is examined and if it
       does not match that currently being used, the entry is ignored.
       otherwise, one proceeds as specified by step 1 above,.

   2.  In the case that the network address is session-trunkable with
       one used previously a BIND_CONN_TO_SESSION is used to access that
       session using new network address.  Otherwise, or if the bind
       operation fails, a CREATE_SESSION is done.

   3.  The verification procedure referred to in step 4 above is used.
       However, if it fails, the entry is ignored and the next available
       entry is used.

5.4.4.  Obtaining Access to Sessions and State after Migration

   In the event that migration has occurred, the determination of
   whether Transparent State Migration has occurred is driven by the
   client ID returned by the EXCHANGE_ID and the reported confirmation
   status.

   o  If the client ID is an unconfirmed client ID not previously known
      to the client, then Transparent State Migration has not occurred.

   o  If the client ID is a confirmed client ID previously known to the
      client, then any transferred state would have been merged with an
      existing client ID representing the client to the destination
      server.  In this state merger case, Transparent State Migration
      might or might not have occurred and a determination as to whether
      it has occurred is deferred until sessions are established and we
      are ready to begin state recovery.

   o  If the client ID is a confirmed client ID not previously known to
      the client, then the client can conclude that the client ID was
      transferred as part of Transparent State Migration.  In this
      transferred client ID case, Transparent State Migration has
      occurred although some state may have been lost.

   Once the client ID has been obtained, it is necessary to obtain
   access to sessions to continue communication with the new server.  In
   any of the cases in which Transparent State Migration has occurred,
   it is possible that a session was transferred as well.  To deal with



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   that possibility, clients can, after doing the EXCHANGE_ID, issue a
   BIND_CONN_TO_SESSION to connect the transferred session to a
   connection to the new server.  If that fails, it is an indication
   that the session was not transferred and that a new session needs to
   be created to take its place.

   In some situations, it is possible for a BIND_CONN_TO_SESSION to
   succeed without session migration having occurred.  If state merger
   has taken place then the associated client ID may have already had a
   set of existing sessions, with it being possible that the sessionid
   of a given session is the same as one that might have been migrated.
   In that event, a BIND_CONN_TO_SESSION might succeed, even though
   there could have been no migration of the session with that
   sessionid.

   Once the client has determined the initial migration status, and
   determined that there was a shift to a new server, it needs to re-
   establish its lock state, if possible.  To enable this to happen
   without loss of the guarantees normally provided by locking, the
   destination server needs to implement a per-fs grace period in all
   cases in which lock state was lost, including those in which
   Transparent State Migration was not implemented.

   Clients need to be deal with the following cases:

   o  In the state merger case, it is possible that the server has not
      attempted Transparent State Migration, in which case state may
      have been lost without it being reflected in the SEQ4_STATUS bits.
      To determine whether this has happened, the client can use
      TEST_STATEID to check whether the stateids created on the source
      server are still accessible on the destination server.  Once a
      single stateid is found to have been successfully transferred, the
      client can conclude that Transparent State Migration was begun and
      any failure to transport all of the stateids will be reflected in
      the SEQ4_STATUS bits.  Otherwise.  Transparent State Migration has
      not occurred.

   o  In a case in which Transparent State Migration has not occurred,
      the client can use the per-fs grace period provided by the
      destination server to reclaim locks that were held on the source
      server.

   o  In a case in which Transparent State Migration has occurred, and
      no lock state was lost (as shown by SEQ4_STATUS flags), no lock
      reclaim is necessary.

   o  In a case in which Transparent State Migration has occurred, and
      some lock state was lost (as shown by SEQ4_STATUS flags), existing



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      stateids need to be checked for validity using TEST_STATEID, and
      reclaim used to re-establish any that were not transferred.

   For all of the cases above, RECLAIM_COMPLETE with an rca_one_fs value
   of true should be done before normal use of the file system including
   obtaining new locks for the file system.  This applies even if no
   locks were lost and needed to be reclaimed.

5.4.5.  Obtaining Access to Sessions and State after Network Address
        Transfer

   The case in which there is a transfer to a new network address
   without migration is similar to that described in Section 5.4.4 in
   that there is a need to obtain access to needed sessions and locking
   state.  However, the details are simpler and will vary depending on
   the type of trunking between the address receiving NFS4ERR_MOVED and
   that to which the transfer is to be made

   To make a session available for use, a BIND_CONN_TO_SESSION should be
   used to obtain access to the session previously in use.  Only if this
   fails, should a CREATE_SESSION be done.  While this procedure mirrors
   that in Section 5.4.4, there is an important difference in that
   preservation of the session is not purely optional but depends on the
   type of trunking.

   Access to appropriate locking state should need no actions beyond
   access to the session.  However. the SEQ4_STATUS bits should be
   checked for lost locking state, including the need to reclaim locks
   after a server reboot.

5.5.  Resolution of NFSv4.1 Issues

   One possibility is that addressing all of the NFSv4.1 issues would
   entail publication of a standards-track document updating [RFC5661].

   Such a document would have three major elements:

   o  A considerable expansion of the existing Section 11.4 explaining
      the various uses of the location attribute and the possible
      interactions among these various uses.  This, like the
      corresponding replacement section for NFSv4.0 would be based on
      our Section 3.2 above.  Information regarding the specifics of
      trunking discovery might appear in this section, in a new sub-
      section.  As part of this revision, the existing Section 11.4.2
      would need to be revised to explain all the possible results of
      NFS4ERR_MOVED including migration and a possible transparent
      transition in which the network address changes but the server
      does not.



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   o  A revision of the existing section 18.35 (dealing with
      EXCHANGE_ID) addressing the issues discussed in Section 5.2.1.

   o  A major replacement of the existing Section 11.7, entitled
      "Effecting File System Transitions", as discussed below.

   In addition, there is a set of smaller changes necessary

   o  Update the existing Section 2.10.5 to clarify the proper response
      to server_owner changes, as described in our Section 5.1.3.

   o  Replacement of the existing Section 15.1.2.4 to reflect the fact
      that NFS4ERR_MOVED can occur when a file system is now accessible
      at a different network address.  A possible replacement text might
      be:

         The file system that contains the current filehandle object is
         not accessible using the network address which has been used.
         It may have been relocated, migrated to another server, be
         accessible using another network address on the current server,
         or it may have never been present.  The client may obtain the
         new file system location by obtaining the "fs_locations" or
         "fs_locations_info" attribute for the current filehandle.  For
         further discussion, refer to Section 11.4.2

   The replacement for the existing section 11.7 would maintain most
   sections essentially as they are, only making minor changes to
   include server-trunking in the discussion.  However, in some cases
   involving more significant changes to existing sub-sections, and
   potential new sub-sections are listed below:

   o  The existing Section 11.7.1 needs to be modified to refer
      explicitly to the previous discussion of trunking discovery.

      In addition, the term "multi-home single-server namespace", used
      nowhere else in [RFC5661], poses difficulties.  From the
      description given it appears that the case being referred to in
      one in which two network addresses return server_owners with the
      same major_id and different minor_id values, making the network
      addresses server-trunkable without being session trunkable.

      A better approach would be to refer to "server-trunking" as used
      elsewhere in this document and use the replacement for the
      existing Section 18.35 to identify clientid trunking as the means
      to adapt to network addresses which are server-trunkable without
      being session-trunkable and session trunking as the means to adapt
      to network addresses which are session-trunkable.




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   o  The existing Section 11.7.2 needs to be better connected to
      trunking discovery.  By calling these "transparent" transitions,
      it obscures the fact that some (or all) of the "transitions" it is
      discussing are not in fact transitions between servers or file
      systems but merely changes the set of communication paths in use.

   o  The existing Section 11.7.2.1, needs to address more clearly the
      case of server-trunkable addresses which are not session-
      trunkable.  As it is, it mentions the related concept of
      clustering, but only deals explicitly with the case in which two
      distinct servers share access to one or more file systems and does
      not mention the case in which the network addresses can be used to
      access a shared stateid space without being session-trunkable.

   o  The existing Section 11.7.2.2, while correct, needs to be part of
      a general re-organization since the characteristics it lists as
      necessary for a transparent transition will be of use in other
      contexts, particularly as they apply to Transparent State
      Migration as well.  It make sense to move these to a new sub-
      section within the equivalent of the Existing Section 11.7.

   o  The existing Section 11.7.7, needs the a major rework to deal with
      its basic assumption, that existing state can only be made
      available on the destination server if the source and destination
      co-operate in state management and maintain a common client id
      space.  It is not clear how this can be done, other than for
      servers working together so as to provide clientid trunking, a
      case that is probably considered as a "transparent transition".
      The section needs to modified to allow something along the lines
      of NFSv4.0-style Transparent State Migration with the details
      provided by a later section (see below).

      A related issues concerns the sentence, "In the case of migration,
      the servers involved in the migration of a file system SHOULD
      transfer all server state from the original to the new server.  It
      is unclear why this is a "SHOULD" as the rest of the paragraph
      essentially tells the client that it needs to be prepared for the
      server not to do this.  The equivalent is a "should" in [RFC7931],
      and there is no reason to add to confusion by making a "SHOULD" in
      NFSv4.1.  also, there is no mention of the need to provide a fs-
      specific grace period in the cases in which Transparent State
      Migration is not made available.

   o  Adding a new section (at level of the existing Section 11.7.7)
      about state transfer during migration.  Although the phrase
      "Transparent State Migration" is well established in the context
      of NFSv4.0, the word "transparent" could cause confusion given the




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      existing use of the phrase "transparent transitions".  A possible
      title for the new section is "State Transfer during Migration"

   The new section would present the NFSv4.1-equivalent of Transparent
   State Migration as described in [RFC7931].  This would address the
   issues presented in Section 5.1 along the lines suggested in Sections
   5.2, 5.3, and 5.4.

6.  Security Considerations

   With regard to NFSv4.0, the Security Considerations section of
   [RFC7530] as modified by that of [RFC7931] takes proper care of
   migration-related issues.  No change is needed.

   With regard to NFSv4.1, the Security Considerations section of
   [RFC5661] takes proper care of migration-related issues.  No change
   is needed.

7.  IANA Considerations

   This document does not require actions by IANA.

8.  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>.

   [RFC7931]  Noveck, D., Ed., Shivam, P., Lever, C., and B. Baker,
              "NFSv4.0 Migration: Specification Update", RFC 7931,
              DOI 10.17487/RFC7931, July 2016,
              <http://www.rfc-editor.org/info/rfc7931>.

Appendix A.  Acknowledgements

   The editor and authors of this document gratefully acknowledge the
   contributions of Trond Myklebust of Primary Data, Robert Thurlow of
   Oracle, and Andy Adamson of NetApp.  We also thank Tom Haynes of



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   Primary Data and Spencer Shepler of Microsoft for their guidance and
   suggestions.

   Special thanks go to members of the Oracle Solaris NFS team,
   especially Rick Mesta and James Wahlig, for their work implementing
   an NFSv4.0 migration prototype and identifying many of the issues
   documented here.  Also, the work of Xuan Qi of Oracle with NFSv4.1
   client and server prototypes was helpful.

Appendix B.  History of this Document

   The contents of successive versions of this document have changed
   because new issues have been discovered, because there here have
   changes in our understanding of how these features should interact,
   and because some of the issues have been adequately addressed with
   regard to certain protocol versions.

   As a result, it may be helpful to understand the history of these
   issues, which is complicated because multiple NFSv4 protocols have
   been involved.

   This history can be summarized as follows

   o  Initially, the focus was on the difficulties seen in NFSv4.0
      implementations of Transparent State Migration, and on identifying
      possible corrections to [RFC7530] that might address these issues.

      At this point, treatment of NFSv4.1 was minimal.

   o  As examination of the issues continued, it became clear that the
      use of the non-uniform client string model was a critical element
      of the problem and further work proceeded on that basis.

      During the period, treatment of NFSv4.1 was expanded but the fact
      that NFSv4.1 had existing facilities for trunking detection was
      taken as an indication that the problems would not be difficult to
      address..

   o  As work proceeded on a standards-track document addressing the
      NFSv4.0 issues, material that proposed changes to address the
      issues became less relevant, since the effective vehicle for
      addressing these issues became the standards-track document.

      During this period, and subsequently, treatment of NFSv4.1
      remained essentially unchanged.

   o  With the publication of [RFC7931], material regarding fixes for
      the NSV4.0 became vestigial but the material was retained for a



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      while together with a shift from proposing those changes to
      reporting that they had been made.

   o  Later, in response to experiences testing existing NFSv4.1
      implementations of migration, the focus of the document shifted
      decisively to NFSv4.1.  As part of the analysis of migration
      within NFSv4.1, it was realized that issues related to the
      appearance of multiple addresses were fundamental to clearly
      describing how migration would work and that changes in the set of
      such addresses might or might not involve migration.

      At this point, discussion of NFSV4.0 issues was further limited.
      The issues seen were noted but the discussion of the resolution
      was limited to explaining that they had been addressed by the
      publication of [RFC7931].

   o  Finally, based on the results of work to provide NFSv4 with
      trunking discovery facilities, a decision was made that this work
      was most appropriately dealt with together with migration, for
      reasons noted previously.

      Since the trunking discovery facilities apply to all NFSv4 minor
      versions, work was needed to define those for NFSv4.0as well,
      together with the necessary interactions with migration.

Authors' Addresses

   David Noveck (editor)
   NetApp
   26 Locust Avenue
   Lexington, MA  02421
   US

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


   Piyush Shivam
   Oracle Corporation
   5300 Riata Park Ct.
   Austin, TX  78727
   US

   Phone: +1 512 401 1019
   Email: piyush.shivam@oracle.com






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   Charles Lever
   Oracle Corporation
   1015 Granger Avenue
   Ann Arbor, MI  48104
   US

   Phone: +1 248 614 5091
   Email: chuck.lever@oracle.com


   Bill Baker
   Oracle Corporation
   5300 Riata Park Ct.
   Austin, TX  78727
   US

   Phone: +1 512 401 1081
   Email: bill.baker@oracle.com

































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