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RFC 8587

NFS Version 4.0 Trunking Update

Pages: 22
Proposed STD
Updates:  7530

Top   ToC   RFC8587 - Page 1
Internet Engineering Task Force (IETF)                     C. Lever, Ed.
Request for Comments: 8587                                        Oracle
Updates: 7530                                                  D. Noveck
Category: Standards Track                                         NetApp
ISSN: 2070-1721                                                 May 2019


                    NFS Version 4.0 Trunking Update

Abstract

   In NFS version 4.0, the fs_locations attribute informs clients about
   alternate locations of file systems.  An NFS version 4.0 client can
   use this information to handle migration and replication of server
   file systems.  This document describes how an NFS version 4.0 client
   can also use this information to discover an NFS version 4.0 server's
   trunking capabilities.  This document updates RFC 7530.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8587.

Copyright Notice

   Copyright (c) 2019 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.
Top   ToC   RFC8587 - Page 2
Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Document Organization . . . . . . . . . . . . . . . . . . . .   6
   5.  Changes within Section 8 of RFC 7530  . . . . . . . . . . . .   7
     5.1.  Updated Section "Location Attributes"
           (Currently Section 8.1) . . . . . . . . . . . . . . . . .   8
     5.2.  Updates to "Uses of Location Information"
           (Currently Section 8.4) . . . . . . . . . . . . . . . . .   9
       5.2.1.  Updates to the Introductory Text of the Current
               Section 8.4 . . . . . . . . . . . . . . . . . . . . .   9
       5.2.2.  New Subsection Titled "Trunking Discovery and
               Detection" (Becomes Section 8.4.1)  . . . . . . . . .  11
       5.2.3.  New Subsection Titled "Location Attributes and
               Selection of Connection Type" (Becomes Section 8.4.2)  12
       5.2.4.  Updated Section "File System Replication" (Becomes
               Section 8.4.3 Retitled "File System Replication and
               Trunking" . . . . . . . . . . . . . . . . . . . . . .  12
       5.2.5.  Updated Section "File System Migration" (Becomes
               Section 8.4.4)  . . . . . . . . . . . . . . . . . . .  13
       5.2.6.  New Subsection Titled "Interaction of Trunking,
               Migration, and Replication" (Becomes Section 8.4.5) .  14
     5.3.  Updated Section "Location Entries and Server Identity"
           (Section 8.5) . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Updates to RFC 7530 outside Section 8 . . . . . . . . . . . .  16
   7.  Updates to the Security Considerations Section of RFC 7530  .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.  Updates to the References Section in RFC 7530 . . . . . . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     10.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Section Classification . . . . . . . . . . . . . . .  22
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22
Top   ToC   RFC8587 - Page 3
1.  Introduction

   The NFS version 4.0 specification [RFC7530] defines a migration
   feature that enables the transfer of a file system from one server to
   another without disruption of client activity.  There were a number
   of issues with the original definition of this feature, now resolved
   with the publication of [RFC7931].

   After a migration event, a client must determine whether state
   recovery is necessary.  To do this, it needs to determine whether 1)
   the source and destination server addresses represent the same server
   instance, 2) if the client has already established a lease on the
   destination server for other file systems, and 3) if the destination
   server instance has lock state for the migrated file system.

   As part of addressing this need, [RFC7931] introduces trunking into
   NFS version 4.0 along with a trunking detection mechanism.  A
   trunking detection mechanism enables a client to determine whether
   two distinct network addresses are connected to the same NFS version
   4.0 server instance.  Without this knowledge, a client unaware of a
   trunking relationship between paths it is using simultaneously is
   likely to become confused in ways described in [RFC7530].

   NFSv4.1 was defined with an integral means of trunking detection,
   which is described in [RFC5661].  NFSv4.0 initially did not have
   trunking detection; it was added by [RFC7931].  Nevertheless, the use
   of the concept of server-trunkability is the same in both protocol
   versions.

   File system migration, replication, and referrals are distinct
   protocol features.  However, it is not appropriate to treat each of
   these features in isolation.  For example, recovery processing of
   client migration needs to deal with the possibility of multiple
   server addresses in a returned fs_locations attribute.  In addition,
   the content of the fs_locations attribute, which provides both
   trunking-related and replication information, may change over
   repeated retrievals, requiring an integrated description of how
   clients are to deal with such changes.  The issues discussed in the
   current document relate to the interpretation of the fs_locations
   attribute and to the proper client and server handling of changes in
   fs_locations attribute values.
Top   ToC   RFC8587 - Page 4
   Therefore, the goals of the current document are as follows:

   o  To provide NFS version 4.0 with a means of finding addresses that
      are trunkable with a given address, i.e., trunking discovery,
      compatible with the means of trunking detection introduced by
      [RFC7931].  For an explanation of trunking detection and
      discovery, see Section 3.

   o  To describe how NFS version 4.0 clients are to handle the presence
      of multiple network addresses associated with the same server when
      recovering from a replication and migration event.

   o  To describe how NFS version 4.0 clients are to handle changes in
      the contents of returned fs_locations attributes, including those
      that indicate changes in the responding NFS version 4.0 server's
      trunking configuration.

   The current document pursues these goals by presenting a set of
   updates to [RFC7530], as summarized in Sections 5 and 6.

2.  Requirements Language

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

3.  Terminology

   Most of the terms related to handling the fs_locations attribute are
   appropriately defined in Section 5.1.  However, there are a few
   terminological issues regarding the use of terms outside the context
   of text updating [RFC7530] that are explained in this section.  Note
   that the definitions of trunking-related terms in Section 5.1 apply
   throughout this document, including in explanatory sections that will
   not replace any text in [RFC7530].

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

   o  Each NFSv4 server is assumed to have a set of IP addresses to
      which NFSv4 requests may be sent by clients.  These are referred
      to as the server's "network addresses".  Access to a specific
      server network address might involve the use of multiple network
      ports, since the ports to be used for particular types of
      connections might be required to be different.
Top   ToC   RFC8587 - Page 5
   o  Clients may establish connections to NFSv4 servers via one of
      several connection types, supporting the NFSv4 protocol layered on
      top of an RPC stream transport, as described in [RFC5531], or on
      top of RPC-over-RDMA, as described in [RFC8166].  The combination
      of a server network address and a particular connection type is
      referred to as a "server endpoint".

   o  Each network address, when combined with a pathname providing the
      location of a file system root directory relative to the
      associated server root filehandle, defines a file system network
      access path.

   o  Two network addresses connected to the same server are said to be
      server-trunkable.  Unlike subsequent NFSv4 minor versions, NFSv4.0
      recognizes only a single type of trunking relationship between
      addresses.

   Discussion of the term "replica" is complicated for a number of
   reasons.  Even though the term is used in explaining the issues in
   [RFC7530] that need to be addressed in the current document, a full
   explanation of this term requires explanation of related terms
   connected to the fs_locations attribute, which is provided in
   Section 5.1 of the current document.

   The term is also used in previous documents about NFSv4.0 (i.e.,
   [RFC7530] and [RFC7931]) with a meaning different from that in the
   current document.  In these documents, each replica is identified by
   a single network access path.  However, in the current document, a
   set of network access paths that have server-trunkable network
   addresses and the same root-relative file system pathname is
   considered to be a single replica with multiple network access paths.
   Although [RFC7931] enables an NFSv4.0 client to determine whether two
   network addresses are server-trunkable, it never describes the
   addresses as connected to a single replica, in effect leaving the
   approach established in [RFC7530].

   Note that this document, except when explaining problems in
   [RFC7530], always uses the new definition, including in text intended
   to replace existing sections of [RFC7530].
Top   ToC   RFC8587 - Page 6
4.  Document Organization

   The sections of the current document are divided into four types
   based on how they relate to the eventual updating of the NFS version
   4.0 specification.  Once this update is published, NFS version 4.0
   will be specified by multiple documents that need to be read together
   until such time as a consolidated replacement specification is
   produced.

   o  The base specification [RFC7530]

   o  The migration-related update [RFC7931]

   o  This document [RFC8587]

   The section types are as follows.  See Appendix A for a
   classification of each section of the current document.

   o  An explanatory section does not contain any material that is meant
      to update the specification of NFS version 4.0.  Such sections may
      contain an explanation about why and how changes are to be made,
      but they do not include any text that is to update [RFC7530] or
      appear in an eventual consolidated document.

   o  A replacement section contains text that is to replace and thus
      supersede text within [RFC7530] and then appear in an eventual
      consolidated document.  The titles of the replacement sections
      indicate what section of [RFC7530] is to be replaced.

   o  An additional section contains text that, although not replacing
      anything in [RFC7530], will be part of the specification of NFS
      version 4.0 and will be expected to be part of an eventual
      consolidated document.  The titles of the additional sections
      provide an indication of where the new section would appear when
      consolidated with [RFC7530].

   o  An editing section contains some text that replaces text within
      [RFC7530], although the entire section will not consist of such
      text and will include other text as well.  Such sections make
      relatively minor adjustments in the existing NFS version 4.0
      specification, which are expected to be reflected in an eventual
      consolidated document.  Generally, such replacement text appears
      as a quotation, possibly taking the form of an indented set of
      paragraphs.
Top   ToC   RFC8587 - Page 7
   Additional and replacement sections sometimes contain references to
   the "current document" by which RFC 8587 is meant.  When those
   sections are incorporated in a consolidated document, those
   references will need to be updated to refer to the appropriate
   sections in that new document.

5.  Changes within Section 8 of RFC 7530

   Most of the updates to [RFC7530] that provide support for trunking
   using the fs_locations attribute apply to Section 8 ("Multi-Server
   Namespace") of that document.  In the following list, the replacing
   section refers to its numbering in this document.

   o  Section 5.1 replaces Section 8.1 ("Location Attributes") of
      [RFC7530].  The text in the original section has been reorganized
      and extended to explicitly allow the use of fs_locations to
      provide trunking-related information that appropriately interacts
      with the migration, replication, and referral features of
      fs_locations.  Terminology used to describe the interactions is
      added.

   o  Section 5.2 updates Section 8.4 ("Uses of Location Information")
      of [RFC7530].  This section comprises the bulk of the updates.
      Each paragraph of Section 8.4 and its subsections have been
      reviewed to clarify the provision of trunking-related information
      using the fs_locations attribute.

      *  Section 5.2.1 replaces the introductory material within
         Section 8.4 of [RFC7530], i.e., the material within Section 8.4
         exclusive of subsections.

      *  Section 5.2.2 is to be added as a new subsection of Section 8.4
         before the updated Section 8.4.1 of [RFC7530].  In a
         consolidated document, it would appear as Section 8.4.1.

      *  Section 5.2.3 is to be added as a new subsection of Section 8.4
         before the updated Section 8.4.1 of [RFC7530].  In a
         consolidated document, it would appear as Section 8.4.2.

      *  Section 5.2.4 replaces Section 8.4.1 ("File System
         Replication") of [RFC7530].  In a consolidated document, it
         would appear as Section 8.4.3.

      *  Section 5.2.5 replaces Section 8.4.2 ("File System Migration")
         of [RFC7530].  In a consolidated document, it would appear as
         Section 8.4.4.
Top   ToC   RFC8587 - Page 8
      *  Section 5.2.6 is to be added as a new subsection of Section 8.4
         before Section 8.4.3 of [RFC7530].  In a consolidated document,
         it would appear as Section 8.4.5, while the existing
         Section 8.3 would appear as Section 8.4.6.

   o  Section 5.3 replaces Section 8.5 ("Location Entries and Server
      Identity") of [RFC7530].  The last paragraph of the existing
      section has been removed.

5.1.  Updated Section "Location Attributes" (Currently Section 8.1)

   The fs_locations attribute allows specification of file system
   locations where the data corresponding to a given file system may be
   accessed.  This attribute represents such file system instances as a
   server address target (as either a DNS hostname representing one or
   more network addresses or as a single literal network address)
   together with the path of that file system within the associated
   single-server namespace.  Individual fs_locations entries can express
   trunkable addresses, locations of file system replicas on other
   servers, migration targets, or pure referrals.

   We introduce the following terminology:

   o  "Trunking" is a situation in which multiple network addresses are
      connected to the same NFS server.  Network addresses connected to
      the same NFS server instance are said to be "server-trunkable".

   o  "Trunking detection" refers to ways of confirming that two
      distinct network addresses are connected to the same NFSv4 server
      instance.

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

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

   o  Location attributes include only the fs_locations GETATTR
      attribute.

   o  Location entries (fs_location4, defined in [RFC7530],
      Section 2.2.6) are the individual file system locations in the
      fs_locations attribute (defined in [RFC7530], Section 2.2.7).  A
      file system location entry designates a set of network addresses
      to which clients may establish connections.  The entry may
      designate multiple such addresses because the server hostname may
      map to multiple network addresses and because multiple connection
Top   ToC   RFC8587 - Page 9
      types may be used to communicate with each specified network
      address.  Such addresses provide multiple ways of connecting to a
      single server.

      Clients use the NFSv4.0 trunking detection mechanism [RFC7931] to
      confirm that such addresses are connected to the same server.  The
      client can ignore non-confirmed trunking relationships and treat
      the corresponding addresses as connected to different servers.

   o  File system location elements are derived from file system
      location entries.  If a file system location entry specifies a
      network address, there is only a single corresponding location
      element.  When a file system location entry contains a hostname,
      the client resolves the hostname, producing one file system
      location element for each of the resulting network addresses.
      Issues regarding the trustworthiness of hostname resolutions are
      further discussed in Section 7 of the current document.

   o  All file system location elements consist of a file system
      location address, which is the network 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.

   o  If the server has no pseudo-fs and only has a single exported file
      system at the root filehandle, the fs_name may be empty.

5.2.  Updates to "Uses of Location Information" (Currently Section 8.4)

   The subsections below provide replacement sections for existing
   sections within Section 8.4 of [RFC7530] or new subsections to be
   added to that section.

5.2.1.  Updates to the Introductory Text of the Current Section 8.4

   Together with the possibility of absent file systems, the
   fs_locations attribute bears file system locations and a number of
   important facilities that enable reliable, manageable, and scalable
   data access.

   When a file system is present on the queried server, this attribute
   can provide a set of alternate locations that clients may use to
   access the file system, when necessary.  Provision of such alternate
   file system locations is referred to as "replication" and is further
   described in Section 5.2.4 of the current document.

   When alternative file system locations are provided, they may
   represent distinct physical copies of the same file system data or
   separate NFS server instances that provide access to the same
Top   ToC   RFC8587 - Page 10
   physical file system.  Another possible use of the provision of
   multiple file system location entries is trunking, wherein the file
   system location entries do not, in fact, represent different servers
   but rather are distinct network paths to the same server.

   A client may use file system location elements simultaneously to
   provide higher-performance access to the target file system.  This
   can be done using trunking, although the use of multiple replicas
   simultaneously is possible.  To enable simultaneous access, the
   client utilizes trunking detection and/or discovery, further
   described in Section 5.2.2 of the current document, to determine a
   set of network paths that are server-trunkable with the path
   currently being used to access the file system.  Once this
   determination is made, requests may be routed across multiple paths
   using the existing state management mechanism.

   Multiple replicas may also be used simultaneously, typically when
   accessing read-only datasets.  In this case, each replica requires
   its own state management.  The client performs multiple file opens to
   read the same file content from multiple replicas.

   When a file system is present and subsequently becomes absent,
   clients can be given the opportunity to have continued access to
   their data at an alternative file system location.  Transfer of the
   file system contents to the new file system location is referred to
   as "migration".  The client's responsibilities in dealing with this
   transition depend on the specific nature of the new access path as
   well as how and whether data was, in fact, migrated.  See Sections
   5.2.5 and 5.2.6 of the current document for details.

   The fs_locations attribute can designate one or more remote file
   system locations in place of an absent file system.  This is known as
   a "referral".  A particularly important case is that of a "pure
   referral", in which the absent file system has never been present on
   the NFS server.  Such a referral is a means by which a file system
   located on one server can redirect clients to file systems located on
   other servers, thus enabling the creation of a multi-server
   namespace.

   Because client support for the fs_locations attribute is OPTIONAL, a
   server may (but is not required to) take action to hide migration and
   referral events from such clients by acting as a proxy, for example.
Top   ToC   RFC8587 - Page 11
5.2.2.  New Subsection Titled "Trunking Discovery and Detection"
        (Becomes Section 8.4.1)

   "Trunking" is a situation in which multiple distinct network
   addresses are associated with the same NFS server instance.  As a
   matter of convenience, we say that two network addresses connected to
   the same NFS server instance are server-trunkable.  Section 5.4 of
   [RFC7931] explains why NFSv4 clients need to be aware of the NFS
   server identity to manage lease and lock states effectively when
   multiple connections to the same server exist.

   "Trunking detection" refers to a way for an NFSv4 client to confirm
   that two independently acquired network addresses are connected to
   the same NFSv4 server.  Section 5.8 of [RFC7931] describes an
   OPTIONAL means by which it can be determined whether two network
   addresses correspond to the same NFSv4.0 server instance.  Without
   trunking detection, an NFSv4.0 client has no other way to confirm
   that two network addresses are server-trunkable.

   In the particular context of NFS version 4.0, trunking detection
   requires that the client support the uniform client ID string (UCS)
   approach, described in Section 5.6 of [RFC7931].  Any NFSv4.0 client
   that supports migration or trunking detection needs to present a
   uniform client ID string to all NFSv4.0 servers.  If it does not do
   so, it will be unable to perform trunking detection.

   "Trunking discovery" is the process by which an NFSv4 client, using a
   hostname or one of an NFSv4 server's network addresses, can obtain
   other candidate network addresses that are trunkable with the NFSv4
   server's network address, i.e., a set of addresses that might be
   connected to the same NFSv4 server instance.  An NFSv4.0 client can
   discover server-trunkable network addresses in a number of ways:

   o  An NFS server's hostname is provided either at mount time or in a
      returned file system location entry.  A DNS query of this hostname
      can return more than one network address.  The returned network
      addresses are candidates for trunking.

   o  Location entries returned in an fs_locations attribute can specify
      network addresses.  These network addresses are candidates for
      trunking.

   When there is a means of trunking detection available, an NFSv4.0
   client can confirm that a set of network addresses corresponds to the
   same NFSv4.0 server instance; thus, any of them can be used to access
   that server.
Top   ToC   RFC8587 - Page 12
5.2.3.  New Subsection Titled "Location Attributes and Selection of
        Connection Type" (Becomes Section 8.4.2)

   NFS version 4.0 may be implemented using a number of different types
   of connections:

      Stream connections may be used to provide RPC service, as
      described in [RFC5531].

      RDMA-capable connections may be used to provide RPC service, as
      described in [RFC8166].

   Because of the need to support multiple connection types, clients
   face the issue of determining the proper connection type to use when
   establishing a connection to a server network address.  The
   fs_locations attribute provides no information to support selection
   of the connection type.  As a result, clients supporting multiple
   connection types need to attempt to establish a connection on various
   connection types, allowing it to determine, via a trial-and-error
   approach, which connection types are supported.

   If a client strongly prefers one connection type, it can perform
   these attempts serially in order of declining preference.  Once there
   is a successful attempt, the established connection can be used.
   Note that with this approach, network partitions can result in a
   sequence of long waits for a successful connection.

   To avoid waiting when there is at least one viable network path
   available, simultaneous attempts to establish multiple connection
   types are possible.  Once a viable connection is established, the
   client discards less-preferred connections.

5.2.4.  Updated Section "File System Replication" (Becomes Section 8.4.3
        Retitled "File System Replication and Trunking"

   On first access to a file system, the client should obtain the value
   of the set of alternative file system locations by interrogating the
   fs_locations attribute.  Trunking discovery and/or detection can then
   be applied to the file system location entries to separate the
   candidate server-trunkable addresses from the replica addresses that
   provide alternative locations of the file system.  Server-trunkable
   addresses may be used simultaneously to provide higher performance
   through the exploitation of multiple paths between the client and
   target file system.

   In the event that server failure, communication problems, or other
   difficulties make continued access to the current file system
   impossible or otherwise impractical, the client can use the
Top   ToC   RFC8587 - Page 13
   alternative file system locations as a way to maintain continued
   access to the file system.  See Section 5.2.6 of the current document
   for more detail.

5.2.5.  Updated Section "File System Migration" (Becomes Section 8.4.4)

   When a file system is present and becomes absent, clients can be
   given the opportunity to have continued access to their data at an
   alternative file system location specified by the fs_locations
   attribute.  Typically, a client will be accessing the file system in
   question, get an NFS4ERR_MOVED error, and then use the fs_locations
   attribute to determine the new location of the data.  See
   Section 5.2.6 of the current document for more detail.

   Such migration can help provide load balancing or general resource
   reallocation.  The protocol does not specify how the file system will
   be moved between servers.  It is anticipated that a number of
   different server-to-server transfer mechanisms might be used, with
   the choice left to the server implementer.  The NFSv4 protocol
   specifies the method used to communicate the migration event between
   the client and server.

   When the client receives indication of a migration event via an
   NFS4ERR_MOVED error, data propagation to the destination server must
   have already occurred.  Once the client proceeds to access the
   alternate file system location, it must see the same data.  Where
   file systems are writable, a change made on the original file system
   must be visible on all migration targets.  Where a file system is not
   writable but represents a read-only copy (possibly periodically
   updated) of a writable file system, similar requirements apply to the
   propagation of updates.  Any change visible in the original file
   system must already be effected on all migration targets to avoid any
   possibility that a client, in effecting a transition to the migration
   target, will see any reversion in the file system state.
Top   ToC   RFC8587 - Page 14
5.2.6.  New Subsection Titled "Interaction of Trunking, Migration, and
        Replication" (Becomes Section 8.4.5)

   When the set of network addresses on a server changes in a way that
   would affect a file system location attribute, there are several
   possible outcomes for clients currently accessing that file system.
   NFS4ERR_MOVED is returned only when the server cannot satisfy a
   request from the client, whether because the file system has been
   migrated to a different server or is only accessible at a different
   trunked address on the same server, or for some other reason.  In
   cases 1 and 2 below, NFS4ERR_MOVED is not returned.

   1.  When the list of network 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 in the replacement list if that address
       provides another path to the same server.  Alternatively, the
       client may treat that address as it does a replica -- to be used
       if the current server addresses become unavailable.

   2.  When the list of network addresses is a subset of that previously
       in effect, immediate action is not needed if an address missing
       in the replacement list is not currently in use by the client.
       The client should avoid using that address to access that file
       system in the future, whether the address is for a replica or an
       additional path to the server being used.

   3.  When an address being removed is one of a number of paths to the
       current server, the client may continue to use it until
       NFS4ERR_MOVED is received.  This is not considered a migration
       event unless the last available path to the server has become
       unusable.

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

   With regard to the server in use, a return of NFS4ERR_MOVED may
   indicate 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.  Clients should not conclude that migration has
   occurred until confirming that all network addresses known to be
   associated with that server are not usable.
Top   ToC   RFC8587 - Page 15
   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, it 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 harmless since the use of the same server
   via a new address will appear as a successful instance of transparent
   state migration.

   Although significant harm cannot 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.  When such a
   lock is lost, it is the responsibility of the destination server to
   provide for its recovery via the use of an fs-specific grace period.

   With regard to the destination server, it is desirable for the client
   to be aware of all 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 file
   system location elements in parallel with normal use of the first
   valid file system location entry found to access the destination.

   Because a file system location attribute may include entries relating
   to the current server, the migration destination, and possible
   replicas to use, scanning for available network addresses that might
   be trunkable with addresses the client has already seen could
   potentially be a long process.  To keep this process as short as
   possible, servers that provide information about trunkable network
   paths are REQUIRED to place file system location entries that
   represent addresses usable with the current server or a migration
   target before those associated with replicas.

   This ordering allows a client to cease scanning for trunkable file
   system location entries once it encounters a file system location
   element whose fs_name differs from the current fs_name or whose
   address is not server-trunkable with the address it is currently
   using.  Although the possibility exists that a client might
   prematurely cease scanning for trunkable addresses when receiving a
   location attribute from an older server that does not follow the
   ordering constraint above, the harm is expected to be limited since
   such servers would not be expected to present information about
   trunkable server access paths.
Top   ToC   RFC8587 - Page 16
5.3.  Updated Section "Location Entries and Server Identity"
      (Section 8.5)

   As mentioned above, a single file system location entry may have a
   server address target in the form of a DNS hostname that resolves to
   multiple network addresses; it is also possible for multiple file
   system location entries to have their own server address targets that
   reference the same server.

   When server-trunkable addresses for a server exist, the client may
   assume that for each file system in the namespace of a given server
   network address, file systems at corresponding namespace locations
   exist for each of the other server-trunkable network addresses.  It
   may do this even in the absence of explicit listing in fs_locations.
   Such corresponding file system locations can be used as alternative
   locations, just as those explicitly specified via the fs_locations
   attribute.

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

6.  Updates to RFC 7530 outside Section 8

   Since the existing description of NFS4ERR_MOVED in Section 13.1.2.4
   of [RFC7530] does not take proper account of trunking, it needs to be
   modified by replacing the first two sentences of the description with
   the following material:

      The file system that contains the current filehandle object cannot
      be accessed using the current network address.  It may be
      accessible using other network addresses connected to the same
      server, it may have been relocated to another server, or it may
      never have been present.

7.  Updates to the Security Considerations Section of RFC 7530

   The Security Considerations section of [RFC7530] needs the additions
   below to properly address some aspects of trunking discovery,
   referral, migration, and replication.
Top   ToC   RFC8587 - Page 17
      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 into account.

      o  When DNS is used to convert NFS server hostnames to network
         addresses and DNSSEC [RFC4033] is not available, the validity
         of the network addresses returned cannot be relied upon.
         However, when the client uses RPCSEC_GSS [RFC7861] to access
         NFS servers, it is possible for mutual authentication to detect
         invalid server addresses.  Other forms of transport layer
         security (e.g., [RFC8446]) can also offer strong authentication
         of NFS servers.

      o  Fetching file system location information SHOULD be performed
         using RPCSEC_GSS with integrity protection, as previously
         explained in the Security Considerations section of [RFC7530].
         Making a request of this sort without using strong integrity
         protection permits corruption during the transit of returned
         file system location information.  The client implementer needs
         to recognize that using such information to access an NFS
         server without use of RPCSEC_GSS (e.g., by using AUTH_SYS as
         defined in [RFC5531]) can result in the client interacting with
         an unverified network address that is posing as an NFSv4
         server.

      o  Despite the fact that [RFC7530] REQUIRES "implementations" to
         provide "support" for the use of RPCSEC_GSS, it cannot be
         assumed that use of RPCSEC_GSS is always possible between any
         particular client-server pair.

      o  Returning only network addresses to a client that has no
         trusted DNS resolution service can hamper its ability to use
         RPCSEC_GSS.

      Therefore, an NFSv4 server SHOULD present file system location
      entries that correspond to file systems on other servers using
      only hostnames.  This enables the client to interrogate the
      fs_locations on the destination server to obtain trunking
      information (as well as replica information) using RPCSEC_GSS with
      integrity, validating the hostname provided while ensuring that
      the response has not been corrupted.

      When RPCSEC_GSS is not available on an NFS server, returned file
      system location information is subject to corruption during
      transit and cannot be relied upon.  In the case of a client being
      directed to another server after NFS4ERR_MOVED, this could vitiate
      the authentication provided by the use of RPCSEC_GSS on the
      destination.  Even when RPCSEC_GSS authentication is available on
Top   ToC   RFC8587 - Page 18
      the destination, this server might validly represent itself as the
      server to which the client was erroneously directed.  Without a
      way to decide whether the server is a valid one, the client can
      only determine, using RPCSEC_GSS, that the server corresponds to
      the hostname provided, with no basis for trusting that server.
      The client should not use such unverified file system location
      entries as a basis for migration, even though RPCSEC_GSS might be
      available on the destination server.

      When a file system location attribute is fetched upon connecting
      with an NFSv4 server, it SHOULD, as stated above, be done using
      RPCSEC_GSS with integrity protection.

      When file system location information cannot be protected in
      transit, the client can subject it to additional filtering to
      prevent the client from being inappropriately directed.  For
      example, if a range of network addresses can be determined that
      ensure that the servers and clients using AUTH_SYS are subject to
      appropriate constraints (such as physical network isolation and
      the use of administrative controls within the operating systems),
      then network addresses in this range can be used, with others
      discarded or restricted in their use of AUTH_SYS.

      When neither integrity protection nor filtering is possible, it is
      best for the client to ignore trunking and replica information or
      simply not fetch the file system location information for these
      purposes.

      To summarize considerations regarding the use of RPCSEC_GSS in
      fetching file system location information, consider the following
      recommendations for requests to interrogate location information,
      with interrogation approaches on the referring and destination
      servers arrived at separately:

      o  The use of RPCSEC_GSS with integrity protection is RECOMMENDED
         in all cases, since the absence of integrity protection exposes
         the client to the possibility of the results being modified in
         transit.

      o  The use of requests issued without RPCSEC_GSS (e.g., using
         AUTH_SYS), while undesirable, might be unavoidable in some
         cases.  Where the use of returned file system location
         information cannot be avoided, it should be subject to
         filtering to eliminate untrusted network addresses.  The
         specifics will vary depending on the degree of network
         isolation and whether the request is to the referring or
         destination servers.
Top   ToC   RFC8587 - Page 19
      Privacy considerations relating to uniform client strings (UCS)
      versus non-uniform client strings (non-UCS), discussed in
      Section 5.6 of [RFC7931], are also applicable to their usage for
      trunking detection in NFS version 4.0.

8.  IANA Considerations

   This document has no IANA actions.

9.  Updates to the References Section in RFC 7530

   The following references should be added to the Normative References
   section of [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,
              <https://www.rfc-editor.org/info/rfc7931>.

   [RFC8166]  Lever, C., Ed., Simpson, W., and T. Talpey, "Remote
              Direct Memory Access Transport for Remote Procedure
              Call Version 1", RFC 8166, DOI 10.17487/RFC8166,
              June 2017, <https://www.rfc-editor.org/info/rfc8166>.

   The following references should be added to the Informative
   References section of [RFC7530]:

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D.,
              and S. Rose, "DNS Security Introduction and
              Requirements", RFC 4033, DOI 10.17487/RFC4033,
              March 2005, <https://www.rfc-editor.org/info/rfc4033>.

   [RFC7861]  Adamson, A. and N. Williams, "Remote Procedure Call
              (RPC) Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
              November 2016, <https://www.rfc-editor.org/info/rfc7861>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.
Top   ToC   RFC8587 - Page 20
10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5531]  Thurlow, R., "RPC: Remote Procedure Call Protocol
              Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
              May 2009, <https://www.rfc-editor.org/info/rfc5531>.

   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <https://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,
              <https://www.rfc-editor.org/info/rfc7931>.

   [RFC8166]  Lever, C., Ed., Simpson, W., and T. Talpey, "Remote Direct
              Memory Access Transport for Remote Procedure Call Version
              1", RFC 8166, DOI 10.17487/RFC8166, June 2017,
              <https://www.rfc-editor.org/info/rfc8166>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.
Top   ToC   RFC8587 - Page 21
10.2.  Informative References

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

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

   [RFC7861]  Adamson, A. and N. Williams, "Remote Procedure Call (RPC)
              Security Version 3", RFC 7861, DOI 10.17487/RFC7861,
              November 2016, <https://www.rfc-editor.org/info/rfc7861>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.
Top   ToC   RFC8587 - Page 22
Appendix A.  Section Classification

   All sections of the current document are considered explanatory with
   the following exceptions.

   o  Sections 5.1, 5.2.4, 5.2.5, and 5.3 are replacement sections.

   o  Sections 5.2.2, 5.2.3, and 5.2.6 are additional sections.

   o  Sections 5.2.1, 6, 7, and Section 9 are editing sections.

Acknowledgments

   The authors wish to thank Andy Adamson, who wrote the original
   version of this document.  All the innovation in this document is the
   result of Andy's work, while mistakes are best ascribed to the
   current authors.

   The editor wishes to thank Greg Marsden for his support of this work
   and Robert Thurlow for his review and suggestions.

   Special thanks go to Transport Area Director Spencer Dawkins, NFSV4
   Working Group Chairs Spencer Shepler and Brian Pawlowski, and NFSV4
   Working Group Secretary Thomas Haynes for their ongoing support.  We
   are also grateful for the thorough review of this document by
   Benjamin Kaduk and Ben Campbell.

Authors' Addresses

   Charles Lever (editor)
   Oracle Corporation
   United States of America

   Email: chuck.lever@oracle.com


   David Noveck
   NetApp
   United States of America

   Email: davenoveck@gmail.com