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RFC 5661

 
 
 

Network File System (NFS) Version 4 Minor Version 1 Protocol

Part 3 of 20, p. 34 to 65
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2.7.  Minor Versioning

   To address the requirement of an NFS protocol that can evolve as the
   need arises, the NFSv4.1 protocol contains the rules and framework to
   allow for future minor changes or versioning.

   The base assumption with respect to minor versioning is that any
   future accepted minor version will be documented in one or more
   Standards Track RFCs.  Minor version 0 of the NFSv4 protocol is
   represented by [30], and minor version 1 is represented by this RFC.
   The COMPOUND and CB_COMPOUND procedures support the encoding of the
   minor version being requested by the client.

   The following items represent the basic rules for the development of
   minor versions.  Note that a future minor version may modify or add
   to the following rules as part of the minor version definition.

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   1.   Procedures are not added or deleted.

        To maintain the general RPC model, NFSv4 minor versions will not
        add to or delete procedures from the NFS program.

   2.   Minor versions may add operations to the COMPOUND and
        CB_COMPOUND procedures.

        The addition of operations to the COMPOUND and CB_COMPOUND
        procedures does not affect the RPC model.

        *  Minor versions may append attributes to the bitmap4 that
           represents sets of attributes and to the fattr4 that
           represents sets of attribute values.

           This allows for the expansion of the attribute model to allow
           for future growth or adaptation.

        *  Minor version X must append any new attributes after the last
           documented attribute.

           Since attribute results are specified as an opaque array of
           per-attribute, XDR-encoded results, the complexity of adding
           new attributes in the midst of the current definitions would
           be too burdensome.

   3.   Minor versions must not modify the structure of an existing
        operation's arguments or results.

        Again, the complexity of handling multiple structure definitions
        for a single operation is too burdensome.  New operations should
        be added instead of modifying existing structures for a minor
        version.

        This rule does not preclude the following adaptations in a minor
        version:

        *  adding bits to flag fields, such as new attributes to
           GETATTR's bitmap4 data type, and providing corresponding
           variants of opaque arrays, such as a notify4 used together
           with such bitmaps

        *  adding bits to existing attributes like ACLs that have flag
           words

        *  extending enumerated types (including NFS4ERR_*) with new
           values

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        *  adding cases to a switched union

   4.   Minor versions must not modify the structure of existing
        attributes.

   5.   Minor versions must not delete operations.

        This prevents the potential reuse of a particular operation
        "slot" in a future minor version.

   6.   Minor versions must not delete attributes.

   7.   Minor versions must not delete flag bits or enumeration values.

   8.   Minor versions may declare an operation MUST NOT be implemented.

        Specifying that an operation MUST NOT be implemented is
        equivalent to obsoleting an operation.  For the client, it means
        that the operation MUST NOT be sent to the server.  For the
        server, an NFS error can be returned as opposed to "dropping"
        the request as an XDR decode error.  This approach allows for
        the obsolescence of an operation while maintaining its structure
        so that a future minor version can reintroduce the operation.

        1.  Minor versions may declare that an attribute MUST NOT be
            implemented.

        2.  Minor versions may declare that a flag bit or enumeration
            value MUST NOT be implemented.

   9.   Minor versions may downgrade features from REQUIRED to
        RECOMMENDED, or RECOMMENDED to OPTIONAL.

   10.  Minor versions may upgrade features from OPTIONAL to
        RECOMMENDED, or RECOMMENDED to REQUIRED.

   11.  A client and server that support minor version X SHOULD support
        minor versions zero through X-1 as well.

   12.  Except for infrastructural changes, a minor version must not
        introduce REQUIRED new features.

        This rule allows for the introduction of new functionality and
        forces the use of implementation experience before designating a
        feature as REQUIRED.  On the other hand, some classes of
        features are infrastructural and have broad effects.  Allowing
        infrastructural features to be RECOMMENDED or OPTIONAL
        complicates implementation of the minor version.

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   13.  A client MUST NOT attempt to use a stateid, filehandle, or
        similar returned object from the COMPOUND procedure with minor
        version X for another COMPOUND procedure with minor version Y,
        where X != Y.

2.8.  Non-RPC-Based Security Services

   As described in Section 2.2.1.1.1.1, NFSv4.1 relies on RPC for
   identification, authentication, integrity, and privacy.  NFSv4.1
   itself provides or enables additional security services as described
   in the next several subsections.

2.8.1.  Authorization

   Authorization to access a file object via an NFSv4.1 operation is
   ultimately determined by the NFSv4.1 server.  A client can
   predetermine its access to a file object via the OPEN (Section 18.16)
   and the ACCESS (Section 18.1) operations.

   Principals with appropriate access rights can modify the
   authorization on a file object via the SETATTR (Section 18.30)
   operation.  Attributes that affect access rights include mode, owner,
   owner_group, acl, dacl, and sacl.  See Section 5.

2.8.2.  Auditing

   NFSv4.1 provides auditing on a per-file object basis, via the acl and
   sacl attributes as described in Section 6.  It is outside the scope
   of this specification to specify audit log formats or management
   policies.

2.8.3.  Intrusion Detection

   NFSv4.1 provides alarm control on a per-file object basis, via the
   acl and sacl attributes as described in Section 6.  Alarms may serve
   as the basis for intrusion detection.  It is outside the scope of
   this specification to specify heuristics for detecting intrusion via
   alarms.

2.9.  Transport Layers

2.9.1.  REQUIRED and RECOMMENDED Properties of Transports

   NFSv4.1 works over Remote Direct Memory Access (RDMA) and non-RDMA-
   based transports with the following attributes:

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   o  The transport supports reliable delivery of data, which NFSv4.1
      requires but neither NFSv4.1 nor RPC has facilities for ensuring
      [34].

   o  The transport delivers data in the order it was sent.  Ordered
      delivery simplifies detection of transmit errors, and simplifies
      the sending of arbitrary sized requests and responses via the
      record marking protocol [3].

   Where an NFSv4.1 implementation supports operation over the IP
   network protocol, any transport used between NFS and IP MUST be among
   the IETF-approved congestion control transport protocols.  At the
   time this document was written, the only two transports that had the
   above attributes were TCP and the Stream Control Transmission
   Protocol (SCTP).  To enhance the possibilities for interoperability,
   an NFSv4.1 implementation MUST support operation over the TCP
   transport protocol.

   Even if NFSv4.1 is used over a non-IP network protocol, it is
   RECOMMENDED that the transport support congestion control.

   It is permissible for a connectionless transport to be used under
   NFSv4.1; however, reliable and in-order delivery of data combined
   with congestion control by the connectionless transport is REQUIRED.
   As a consequence, UDP by itself MUST NOT be used as an NFSv4.1
   transport.  NFSv4.1 assumes that a client transport address and
   server transport address used to send data over a transport together
   constitute a connection, even if the underlying transport eschews the
   concept of a connection.

2.9.2.  Client and Server Transport Behavior

   If a connection-oriented transport (e.g., TCP) is used, the client
   and server SHOULD use long-lived connections for at least three
   reasons:

   1.  This will prevent the weakening of the transport's congestion
       control mechanisms via short-lived connections.

   2.  This will improve performance for the WAN environment by
       eliminating the need for connection setup handshakes.

   3.  The NFSv4.1 callback model differs from NFSv4.0, and requires the
       client and server to maintain a client-created backchannel (see
       Section 2.10.3.1) for the server to use.

   In order to reduce congestion, if a connection-oriented transport is
   used, and the request is not the NULL procedure:

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   o  A requester MUST NOT retry a request unless the connection the
      request was sent over was lost before the reply was received.

   o  A replier MUST NOT silently drop a request, even if the request is
      a retry.  (The silent drop behavior of RPCSEC_GSS [4] does not
      apply because this behavior happens at the RPCSEC_GSS layer, a
      lower layer in the request processing.)  Instead, the replier
      SHOULD return an appropriate error (see Section 2.10.6.1), or it
      MAY disconnect the connection.

   When sending a reply, the replier MUST send the reply to the same
   full network address (e.g., if using an IP-based transport, the
   source port of the requester is part of the full network address)
   from which the requester sent the request.  If using a connection-
   oriented transport, replies MUST be sent on the same connection from
   which the request was received.

   If a connection is dropped after the replier receives the request but
   before the replier sends the reply, the replier might have a pending
   reply.  If a connection is established with the same source and
   destination full network address as the dropped connection, then the
   replier MUST NOT send the reply until the requester retries the
   request.  The reason for this prohibition is that the requester MAY
   retry a request over a different connection (provided that connection
   is associated with the original request's session).

   When using RDMA transports, there are other reasons for not
   tolerating retries over the same connection:

   o  RDMA transports use "credits" to enforce flow control, where a
      credit is a right to a peer to transmit a message.  If one peer
      were to retransmit a request (or reply), it would consume an
      additional credit.  If the replier retransmitted a reply, it would
      certainly result in an RDMA connection loss, since the requester
      would typically only post a single receive buffer for each
      request.  If the requester retransmitted a request, the additional
      credit consumed on the server might lead to RDMA connection
      failure unless the client accounted for it and decreased its
      available credit, leading to wasted resources.

   o  RDMA credits present a new issue to the reply cache in NFSv4.1.
      The reply cache may be used when a connection within a session is
      lost, such as after the client reconnects.  Credit information is
      a dynamic property of the RDMA connection, and stale values must
      not be replayed from the cache.  This implies that the reply cache
      contents must not be blindly used when replies are sent from it,
      and credit information appropriate to the channel must be
      refreshed by the RPC layer.

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   In addition, as described in Section 2.10.6.2, while a session is
   active, the NFSv4.1 requester MUST NOT stop waiting for a reply.

2.9.3.  Ports

   Historically, NFSv3 servers have listened over TCP port 2049.  The
   registered port 2049 [35] for the NFS protocol should be the default
   configuration.  NFSv4.1 clients SHOULD NOT use the RPC binding
   protocols as described in [36].

2.10.  Session

   NFSv4.1 clients and servers MUST support and MUST use the session
   feature as described in this section.

2.10.1.  Motivation and Overview

   Previous versions and minor versions of NFS have suffered from the
   following:

   o  Lack of support for Exactly Once Semantics (EOS).  This includes
      lack of support for EOS through server failure and recovery.

   o  Limited callback support, including no support for sending
      callbacks through firewalls, and races between replies to normal
      requests and callbacks.

   o  Limited trunking over multiple network paths.

   o  Requiring machine credentials for fully secure operation.

   Through the introduction of a session, NFSv4.1 addresses the above
   shortfalls with practical solutions:

   o  EOS is enabled by a reply cache with a bounded size, making it
      feasible to keep the cache in persistent storage and enable EOS
      through server failure and recovery.  One reason that previous
      revisions of NFS did not support EOS was because some EOS
      approaches often limited parallelism.  As will be explained in
      Section 2.10.6, NFSv4.1 supports both EOS and unlimited
      parallelism.

   o  The NFSv4.1 client (defined in Section 1.6, Paragraph 2) creates
      transport connections and provides them to the server to use for
      sending callback requests, thus solving the firewall issue
      (Section 18.34).  Races between responses from client requests and

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      callbacks caused by the requests are detected via the session's
      sequencing properties that are a consequence of EOS
      (Section 2.10.6.3).

   o  The NFSv4.1 client can associate an arbitrary number of
      connections with the session, and thus provide trunking
      (Section 2.10.5).

   o  The NFSv4.1 client and server produces a session key independent
      of client and server machine credentials which can be used to
      compute a digest for protecting critical session management
      operations (Section 2.10.8.3).

   o  The NFSv4.1 client can also create secure RPCSEC_GSS contexts for
      use by the session's backchannel that do not require the server to
      authenticate to a client machine principal (Section 2.10.8.2).

   A session is a dynamically created, long-lived server object created
   by a client and used over time from one or more transport
   connections.  Its function is to maintain the server's state relative
   to the connection(s) belonging to a client instance.  This state is
   entirely independent of the connection itself, and indeed the state
   exists whether or not the connection exists.  A client may have one
   or more sessions associated with it so that client-associated state
   may be accessed using any of the sessions associated with that
   client's client ID, when connections are associated with those
   sessions.  When no connections are associated with any of a client
   ID's sessions for an extended time, such objects as locks, opens,
   delegations, layouts, etc. are subject to expiration.  The session
   serves as an object representing a means of access by a client to the
   associated client state on the server, independent of the physical
   means of access to that state.

   A single client may create multiple sessions.  A single session MUST
   NOT serve multiple clients.

2.10.2.  NFSv4 Integration

   Sessions are part of NFSv4.1 and not NFSv4.0.  Normally, a major
   infrastructure change such as sessions would require a new major
   version number to an Open Network Computing (ONC) RPC program like
   NFS.  However, because NFSv4 encapsulates its functionality in a
   single procedure, COMPOUND, and because COMPOUND can support an
   arbitrary number of operations, sessions have been added to NFSv4.1
   with little difficulty.  COMPOUND includes a minor version number
   field, and for NFSv4.1 this minor version is set to 1.  When the
   NFSv4 server processes a COMPOUND with the minor version set to 1, it
   expects a different set of operations than it does for NFSv4.0.

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   NFSv4.1 defines the SEQUENCE operation, which is required for every
   COMPOUND that operates over an established session, with the
   exception of some session administration operations, such as
   DESTROY_SESSION (Section 18.37).

2.10.2.1.  SEQUENCE and CB_SEQUENCE

   In NFSv4.1, when the SEQUENCE operation is present, it MUST be the
   first operation in the COMPOUND procedure.  The primary purpose of
   SEQUENCE is to carry the session identifier.  The session identifier
   associates all other operations in the COMPOUND procedure with a
   particular session.  SEQUENCE also contains required information for
   maintaining EOS (see Section 2.10.6).  Session-enabled NFSv4.1
   COMPOUND requests thus have the form:

       +-----+--------------+-----------+------------+-----------+----
       | tag | minorversion | numops    |SEQUENCE op | op + args | ...
       |     |   (== 1)     | (limited) |  + args    |           |
       +-----+--------------+-----------+------------+-----------+----

   and the replies have the form:

       +------------+-----+--------+-------------------------------+--//
       |last status | tag | numres |status + SEQUENCE op + results |  //
       +------------+-----+--------+-------------------------------+--//
               //-----------------------+----
               // status + op + results | ...
               //-----------------------+----

   A CB_COMPOUND procedure request and reply has a similar form to
   COMPOUND, but instead of a SEQUENCE operation, there is a CB_SEQUENCE
   operation.  CB_COMPOUND also has an additional field called
   "callback_ident", which is superfluous in NFSv4.1 and MUST be ignored
   by the client.  CB_SEQUENCE has the same information as SEQUENCE, and
   also includes other information needed to resolve callback races
   (Section 2.10.6.3).

2.10.2.2.  Client ID and Session Association

   Each client ID (Section 2.4) can have zero or more active sessions.
   A client ID and associated session are required to perform file
   access in NFSv4.1.  Each time a session is used (whether by a client
   sending a request to the server or the client replying to a callback
   request from the server), the state leased to its associated client
   ID is automatically renewed.

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   State (which can consist of share reservations, locks, delegations,
   and layouts (Section 1.7.4)) is tied to the client ID.  Client state
   is not tied to any individual session.  Successive state changing
   operations from a given state owner MAY go over different sessions,
   provided the session is associated with the same client ID.  A
   callback MAY arrive over a different session than that of the request
   that originally acquired the state pertaining to the callback.  For
   example, if session A is used to acquire a delegation, a request to
   recall the delegation MAY arrive over session B if both sessions are
   associated with the same client ID.  Sections 2.10.8.1 and 2.10.8.2
   discuss the security considerations around callbacks.

2.10.3.  Channels

   A channel is not a connection.  A channel represents the direction
   ONC RPC requests are sent.

   Each session has one or two channels: the fore channel and the
   backchannel.  Because there are at most two channels per session, and
   because each channel has a distinct purpose, channels are not
   assigned identifiers.

   The fore channel is used for ordinary requests from the client to the
   server, and carries COMPOUND requests and responses.  A session
   always has a fore channel.

   The backchannel is used for callback requests from server to client,
   and carries CB_COMPOUND requests and responses.  Whether or not there
   is a backchannel is a decision made by the client; however, many
   features of NFSv4.1 require a backchannel.  NFSv4.1 servers MUST
   support backchannels.

   Each session has resources for each channel, including separate reply
   caches (see Section 2.10.6.1).  Note that even the backchannel
   requires a reply cache (or, at least, a slot table in order to detect
   retries) because some callback operations are nonidempotent.

2.10.3.1.  Association of Connections, Channels, and Sessions

   Each channel is associated with zero or more transport connections
   (whether of the same transport protocol or different transport
   protocols).  A connection can be associated with one channel or both
   channels of a session; the client and server negotiate whether a
   connection will carry traffic for one channel or both channels via
   the CREATE_SESSION (Section 18.36) and the BIND_CONN_TO_SESSION
   (Section 18.34) operations.  When a session is created via
   CREATE_SESSION, the connection that transported the CREATE_SESSION
   request is automatically associated with the fore channel, and

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   optionally the backchannel.  If the client specifies no state
   protection (Section 18.35) when the session is created, then when
   SEQUENCE is transmitted on a different connection, the connection is
   automatically associated with the fore channel of the session
   specified in the SEQUENCE operation.

   A connection's association with a session is not exclusive.  A
   connection associated with the channel(s) of one session may be
   simultaneously associated with the channel(s) of other sessions
   including sessions associated with other client IDs.

   It is permissible for connections of multiple transport types to be
   associated with the same channel.  For example, both TCP and RDMA
   connections can be associated with the fore channel.  In the event an
   RDMA and non-RDMA connection are associated with the same channel,
   the maximum number of slots SHOULD be at least one more than the
   total number of RDMA credits (Section 2.10.6.1).  This way, if all
   RDMA credits are used, the non-RDMA connection can have at least one
   outstanding request.  If a server supports multiple transport types,
   it MUST allow a client to associate connections from each transport
   to a channel.

   It is permissible for a connection of one type of transport to be
   associated with the fore channel, and a connection of a different
   type to be associated with the backchannel.

2.10.4.  Server Scope

   Servers each specify a server scope value in the form of an opaque
   string eir_server_scope returned as part of the results of an
   EXCHANGE_ID operation.  The purpose of the server scope is to allow a
   group of servers to indicate to clients that a set of servers sharing
   the same server scope value has arranged to use compatible values of
   otherwise opaque identifiers.  Thus, the identifiers generated by one
   server of that set may be presented to another of that same scope.

   The use of such compatible values does not imply that a value
   generated by one server will always be accepted by another.  In most
   cases, it will not.  However, a server will not accept a value
   generated by another inadvertently.  When it does accept it, it will
   be because it is recognized as valid and carrying the same meaning as
   on another server of the same scope.

   When servers are of the same server scope, this compatibility of
   values applies to the follow identifiers:

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   o  Filehandle values.  A filehandle value accepted by two servers of
      the same server scope denotes the same object.  A WRITE operation
      sent to one server is reflected immediately in a READ sent to the
      other, and locks obtained on one server conflict with those
      requested on the other.

   o  Session ID values.  A session ID value accepted by two servers of
      the same server scope denotes the same session.

   o  Client ID values.  A client ID value accepted as valid by two
      servers of the same server scope is associated with two clients
      with the same client owner and verifier.

   o  State ID values.  A state ID value is recognized as valid when the
      corresponding client ID is recognized as valid.  If the same
      stateid value is accepted as valid on two servers of the same
      scope and the client IDs on the two servers represent the same
      client owner and verifier, then the two stateid values designate
      the same set of locks and are for the same file.

   o  Server owner values.  When the server scope values are the same,
      server owner value may be validly compared.  In cases where the
      server scope values are different, server owner values are treated
      as different even if they contain all identical bytes.

   The coordination among servers required to provide such compatibility
   can be quite minimal, and limited to a simple partition of the ID
   space.  The recognition of common values requires additional
   implementation, but this can be tailored to the specific situations
   in which that recognition is desired.

   Clients will have occasion to compare the server scope values of
   multiple servers under a number of circumstances, each of which will
   be discussed under the appropriate functional section:

   o  When server owner values received in response to EXCHANGE_ID
      operations sent to multiple network addresses are compared for the
      purpose of determining the validity of various forms of trunking,
      as described in Section 2.10.5.

   o  When network or server reconfiguration causes the same network
      address to possibly be directed to different servers, with the
      necessity for the client to determine when lock reclaim should be
      attempted, as described in Section 8.4.2.1.

   o  When file system migration causes the transfer of responsibility
      for a file system between servers and the client needs to
      determine whether state has been transferred with the file system

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      (as described in Section 11.7.7) or whether the client needs to
      reclaim state on a similar basis as in the case of server restart,
      as described in Section 8.4.2.

   When two replies from EXCHANGE_ID, each from two different server
   network addresses, have the same server scope, there are a number of
   ways a client can validate that the common server scope is due to two
   servers cooperating in a group.

   o  If both EXCHANGE_ID requests were sent with RPCSEC_GSS
      authentication and the server principal is the same for both
      targets, the equality of server scope is validated.  It is
      RECOMMENDED that two servers intending to share the same server
      scope also share the same principal name.

   o  The client may accept the appearance of the second server in the
      fs_locations or fs_locations_info attribute for a relevant file
      system.  For example, if there is a migration event for a
      particular file system or there are locks to be reclaimed on a
      particular file system, the attributes for that particular file
      system may be used.  The client sends the GETATTR request to the
      first server for the fs_locations or fs_locations_info attribute
      with RPCSEC_GSS authentication.  It may need to do this in advance
      of the need to verify the common server scope.  If the client
      successfully authenticates the reply to GETATTR, and the GETATTR
      request and reply containing the fs_locations or fs_locations_info
      attribute refers to the second server, then the equality of server
      scope is supported.  A client may choose to limit the use of this
      form of support to information relevant to the specific file
      system involved (e.g. a file system being migrated).

2.10.5.  Trunking

   Trunking is the use of multiple connections between a client and
   server in order to increase the speed of data transfer.  NFSv4.1
   supports two types of trunking: session trunking and client ID
   trunking.

   NFSv4.1 servers MUST support both forms of trunking within the
   context of a single server network address and MUST support both
   forms within the context of the set of network addresses used to
   access a single server.  NFSv4.1 servers in a clustered configuration
   MAY allow network addresses for different servers to use client ID
   trunking.

   Clients may use either form of trunking as long as they do not, when
   trunking between different server network addresses, violate the
   servers' mandates as to the kinds of trunking to be allowed (see

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   below).  With regard to callback channels, the client MUST allow the
   server to choose among all callback channels valid for a given client
   ID and MUST support trunking when the connections supporting the
   backchannel allow session or client ID trunking to be used for
   callbacks.

   Session trunking is essentially the association of multiple
   connections, each with potentially different target and/or source
   network addresses, to the same session.  When the target network
   addresses (server addresses) of the two connections are the same, the
   server MUST support such session trunking.  When the target network
   addresses are different, the server MAY indicate such support using
   the data returned by the EXCHANGE_ID operation (see below).

   Client ID trunking is the association of multiple sessions to the
   same client ID.  Servers MUST support client ID trunking for two
   target network addresses whenever they allow session trunking for
   those same two network addresses.  In addition, a server MAY, by
   presenting the same major server owner ID (Section 2.5) and server
   scope (Section 2.10.4), allow an additional case of client ID
   trunking.  When two servers return the same major server owner and
   server scope, it means that the two servers are cooperating on
   locking state management, which is a prerequisite for client ID
   trunking.

   Distinguishing when the client is allowed to use session and client
   ID trunking requires understanding how the results of the EXCHANGE_ID
   (Section 18.35) operation identify a server.  Suppose a client sends
   EXCHANGE_IDs over two different connections, each with a possibly
   different target network address, but each EXCHANGE_ID operation has
   the same value in the eia_clientowner field.  If the same NFSv4.1
   server is listening over each connection, then each EXCHANGE_ID
   result MUST return the same values of eir_clientid,
   eir_server_owner.so_major_id, and eir_server_scope.  The client can
   then treat each connection as referring to the same server (subject
   to verification; see Section 2.10.5.1 later in this section), and it
   can use each connection to trunk requests and replies.  The client's
   choice is whether session trunking or client ID trunking applies.

   Session Trunking.  If the eia_clientowner argument is the same in two
      different EXCHANGE_ID requests, and the eir_clientid,
      eir_server_owner.so_major_id, eir_server_owner.so_minor_id, and
      eir_server_scope results match in both EXCHANGE_ID results, then
      the client is permitted to perform session trunking.  If the
      client has no session mapping to the tuple of eir_clientid,
      eir_server_owner.so_major_id, eir_server_scope, and
      eir_server_owner.so_minor_id, then it creates the session via a
      CREATE_SESSION operation over one of the connections, which

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      associates the connection to the session.  If there is a session
      for the tuple, the client can send BIND_CONN_TO_SESSION to
      associate the connection to the session.

      Of course, if the client does not desire to use session trunking,
      it is not required to do so.  It can invoke CREATE_SESSION on the
      connection.  This will result in client ID trunking as described
      below.  It can also decide to drop the connection if it does not
      choose to use trunking.

   Client ID Trunking.  If the eia_clientowner argument is the same in
      two different EXCHANGE_ID requests, and the eir_clientid,
      eir_server_owner.so_major_id, and eir_server_scope results match
      in both EXCHANGE_ID results, then the client is permitted to
      perform client ID trunking (regardless of whether the
      eir_server_owner.so_minor_id results match).  The client can
      associate each connection with different sessions, where each
      session is associated with the same server.

      The client completes the act of client ID trunking by invoking
      CREATE_SESSION on each connection, using the same client ID that
      was returned in eir_clientid.  These invocations create two
      sessions and also associate each connection with its respective
      session.  The client is free to decline to use client ID trunking
      by simply dropping the connection at this point.

      When doing client ID trunking, locking state is shared across
      sessions associated with that same client ID.  This requires the
      server to coordinate state across sessions.

   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.

2.10.5.1.  Verifying Claims of Matching Server Identity

   When two servers over two connections claim matching or partially
   matching eir_server_owner, eir_server_scope, and eir_clientid values,
   the client does not have to trust the servers' claims.  The client
   may verify these claims before trunking traffic in the following
   ways:

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   o  For session trunking, clients SHOULD reliably verify if
      connections between different network paths are in fact associated
      with the same NFSv4.1 server and usable on the same session, and
      servers MUST allow clients to perform reliable verification.  When
      a client ID is created, the client SHOULD specify that
      BIND_CONN_TO_SESSION is to be verified according to the SP4_SSV or
      SP4_MACH_CRED (Section 18.35) state protection options.  For
      SP4_SSV, reliable verification depends on a shared secret (the
      SSV) that is established via the SET_SSV (Section 18.47)
      operation.

      When a new connection is associated with the session (via the
      BIND_CONN_TO_SESSION operation, see Section 18.34), if the client
      specified SP4_SSV state protection for the BIND_CONN_TO_SESSION
      operation, the client MUST send the BIND_CONN_TO_SESSION with
      RPCSEC_GSS protection, using integrity or privacy, and an
      RPCSEC_GSS handle created with the GSS SSV mechanism
      (Section 2.10.9).

      If the client mistakenly tries to associate a connection to a
      session of a wrong server, the server will either reject the
      attempt because it is not aware of the session identifier of the
      BIND_CONN_TO_SESSION arguments, or it will reject the attempt
      because the RPCSEC_GSS authentication fails.  Even if the server
      mistakenly or maliciously accepts the connection association
      attempt, the RPCSEC_GSS verifier it computes in the response will
      not be verified by the client, so the client will know it cannot
      use the connection for trunking the specified session.

      If the client specified SP4_MACH_CRED state protection, the
      BIND_CONN_TO_SESSION operation will use RPCSEC_GSS integrity or
      privacy, using the same credential that was used when the client
      ID was created.  Mutual authentication via RPCSEC_GSS assures the
      client that the connection is associated with the correct session
      of the correct server.


   o  For client ID trunking, the client has at least two options for
      verifying that the same client ID obtained from two different
      EXCHANGE_ID operations came from the same server.  The first
      option is to use RPCSEC_GSS authentication when sending each
      EXCHANGE_ID operation.  Each time an EXCHANGE_ID is sent with
      RPCSEC_GSS authentication, the client notes the principal name of
      the GSS target.  If the EXCHANGE_ID results indicate that client
      ID trunking is possible, and the GSS targets' principal names are
      the same, the servers are the same and client ID trunking is
      allowed.

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      The second option for verification is to use SP4_SSV protection.
      When the client sends EXCHANGE_ID, it specifies SP4_SSV
      protection.  The first EXCHANGE_ID the client sends always has to
      be confirmed by a CREATE_SESSION call.  The client then sends
      SET_SSV.  Later, the client sends EXCHANGE_ID to a second
      destination network address different from the one the first
      EXCHANGE_ID was sent to.  The client checks that each EXCHANGE_ID
      reply has the same eir_clientid, eir_server_owner.so_major_id, and
      eir_server_scope.  If so, the client verifies the claim by sending
      a CREATE_SESSION operation to the second destination address,
      protected with RPCSEC_GSS integrity using an RPCSEC_GSS handle
      returned by the second EXCHANGE_ID.  If the server accepts the
      CREATE_SESSION request, and if the client verifies the RPCSEC_GSS
      verifier and integrity codes, then the client has proof the second
      server knows the SSV, and thus the two servers are cooperating for
      the purposes of specifying server scope and client ID trunking.

2.10.6.  Exactly Once Semantics

   Via the session, NFSv4.1 offers exactly once semantics (EOS) for
   requests sent over a channel.  EOS is supported on both the fore
   channel and backchannel.

   Each COMPOUND or CB_COMPOUND request that is sent with a leading
   SEQUENCE or CB_SEQUENCE operation MUST be executed by the receiver
   exactly once.  This requirement holds regardless of whether the
   request is sent with reply caching specified (see
   Section 2.10.6.1.3).  The requirement holds even if the requester is
   sending the request over a session created between a pNFS data client
   and pNFS data server.  To understand the rationale for this
   requirement, divide the requests into three classifications:

   o  Non-idempotent requests.

   o  Idempotent modifying requests.

   o  Idempotent non-modifying requests.

   An example of a non-idempotent request is RENAME.  Obviously, if a
   replier executes the same RENAME request twice, and the first
   execution succeeds, the re-execution will fail.  If the replier
   returns the result from the re-execution, this result is incorrect.
   Therefore, EOS is required for non-idempotent requests.

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   An example of an idempotent modifying request is a COMPOUND request
   containing a WRITE operation.  Repeated execution of the same WRITE
   has the same effect as execution of that WRITE a single time.
   Nevertheless, enforcing EOS for WRITEs and other idempotent modifying
   requests is necessary to avoid data corruption.

   Suppose a client sends WRITE A to a noncompliant server that does not
   enforce EOS, and receives no response, perhaps due to a network
   partition.  The client reconnects to the server and re-sends WRITE A.
   Now, the server has outstanding two instances of A.  The server can
   be in a situation in which it executes and replies to the retry of A,
   while the first A is still waiting in the server's internal I/O
   system for some resource.  Upon receiving the reply to the second
   attempt of WRITE A, the client believes its WRITE is done so it is
   free to send WRITE B, which overlaps the byte-range of A.  When the
   original A is dispatched from the server's I/O system and executed
   (thus the second time A will have been written), then what has been
   written by B can be overwritten and thus corrupted.

   An example of an idempotent non-modifying request is a COMPOUND
   containing SEQUENCE, PUTFH, READLINK, and nothing else.  The re-
   execution of such a request will not cause data corruption or produce
   an incorrect result.  Nonetheless, to keep the implementation simple,
   the replier MUST enforce EOS for all requests, whether or not
   idempotent and non-modifying.

   Note that true and complete EOS is not possible unless the server
   persists the reply cache in stable storage, and unless the server is
   somehow implemented to never require a restart (indeed, if such a
   server exists, the distinction between a reply cache kept in stable
   storage versus one that is not is one without meaning).  See
   Section 2.10.6.5 for a discussion of persistence in the reply cache.
   Regardless, even if the server does not persist the reply cache, EOS
   improves robustness and correctness over previous versions of NFS
   because the legacy duplicate request/reply caches were based on the
   ONC RPC transaction identifier (XID).  Section 2.10.6.1 explains the
   shortcomings of the XID as a basis for a reply cache and describes
   how NFSv4.1 sessions improve upon the XID.

2.10.6.1.  Slot Identifiers and Reply Cache

   The RPC layer provides a transaction ID (XID), which, while required
   to be unique, is not convenient for tracking requests for two
   reasons.  First, the XID is only meaningful to the requester; it
   cannot be interpreted by the replier except to test for equality with
   previously sent requests.  When consulting an RPC-based duplicate
   request cache, the opaqueness of the XID requires a computationally
   expensive look up (often via a hash that includes XID and source

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   address).  NFSv4.1 requests use a non-opaque slot ID, which is an
   index into a slot table, which is far more efficient.  Second,
   because RPC requests can be executed by the replier in any order,
   there is no bound on the number of requests that may be outstanding
   at any time.  To achieve perfect EOS, using ONC RPC would require
   storing all replies in the reply cache.  XIDs are 32 bits; storing
   over four billion (2^32) replies in the reply cache is not practical.
   In practice, previous versions of NFS have chosen to store a fixed
   number of replies in the cache, and to use a least recently used
   (LRU) approach to replacing cache entries with new entries when the
   cache is full.  In NFSv4.1, the number of outstanding requests is
   bounded by the size of the slot table, and a sequence ID per slot is
   used to tell the replier when it is safe to delete a cached reply.

   In the NFSv4.1 reply cache, when the requester sends a new request,
   it selects a slot ID in the range 0..N, where N is the replier's
   current maximum slot ID granted to the requester on the session over
   which the request is to be sent.  The value of N starts out as equal
   to ca_maxrequests - 1 (Section 18.36), but can be adjusted by the
   response to SEQUENCE or CB_SEQUENCE as described later in this
   section.  The slot ID must be unused by any of the requests that the
   requester has already active on the session.  "Unused" here means the
   requester has no outstanding request for that slot ID.

   A slot contains a sequence ID and the cached reply corresponding to
   the request sent with that sequence ID.  The sequence ID is a 32-bit
   unsigned value, and is therefore in the range 0..0xFFFFFFFF (2^32 -
   1).  The first time a slot is used, the requester MUST specify a
   sequence ID of one (Section 18.36).  Each time a slot is reused, the
   request MUST specify a sequence ID that is one greater than that of
   the previous request on the slot.  If the previous sequence ID was
   0xFFFFFFFF, then the next request for the slot MUST have the sequence
   ID set to zero (i.e., (2^32 - 1) + 1 mod 2^32).

   The sequence ID accompanies the slot ID in each request.  It is for
   the critical check at the replier: it used to efficiently determine
   whether a request using a certain slot ID is a retransmit or a new,
   never-before-seen request.  It is not feasible for the requester to
   assert that it is retransmitting to implement this, because for any
   given request the requester cannot know whether the replier has seen
   it unless the replier actually replies.  Of course, if the requester
   has seen the reply, the requester would not retransmit.

   The replier compares each received request's sequence ID with the
   last one previously received for that slot ID, to see if the new
   request is:

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   o  A new request, in which the sequence ID is one greater than that
      previously seen in the slot (accounting for sequence wraparound).
      The replier proceeds to execute the new request, and the replier
      MUST increase the slot's sequence ID by one.

   o  A retransmitted request, in which the sequence ID is equal to that
      currently recorded in the slot.  If the original request has
      executed to completion, the replier returns the cached reply.  See
      Section 2.10.6.2 for direction on how the replier deals with
      retries of requests that are still in progress.

   o  A misordered retry, in which the sequence ID is less than
      (accounting for sequence wraparound) that previously seen in the
      slot.  The replier MUST return NFS4ERR_SEQ_MISORDERED (as the
      result from SEQUENCE or CB_SEQUENCE).

   o  A misordered new request, in which the sequence ID is two or more
      than (accounting for sequence wraparound) that previously seen in
      the slot.  Note that because the sequence ID MUST wrap around to
      zero once it reaches 0xFFFFFFFF, a misordered new request and a
      misordered retry cannot be distinguished.  Thus, the replier MUST
      return NFS4ERR_SEQ_MISORDERED (as the result from SEQUENCE or
      CB_SEQUENCE).

   Unlike the XID, the slot ID is always within a specific range; this
   has two implications.  The first implication is that for a given
   session, the replier need only cache the results of a limited number
   of COMPOUND requests.  The second implication derives from the first,
   which is that unlike XID-indexed reply caches (also known as
   duplicate request caches - DRCs), the slot ID-based reply cache
   cannot be overflowed.  Through use of the sequence ID to identify
   retransmitted requests, the replier does not need to actually cache
   the request itself, reducing the storage requirements of the reply
   cache further.  These facilities make it practical to maintain all
   the required entries for an effective reply cache.

   The slot ID, sequence ID, and session ID therefore take over the
   traditional role of the XID and source network address in the
   replier's reply cache implementation.  This approach is considerably
   more portable and completely robust -- it is not subject to the
   reassignment of ports as clients reconnect over IP networks.  In
   addition, the RPC XID is not used in the reply cache, enhancing
   robustness of the cache in the face of any rapid reuse of XIDs by the
   requester.  While the replier does not care about the XID for the
   purposes of reply cache management (but the replier MUST return the
   same XID that was in the request), nonetheless there are
   considerations for the XID in NFSv4.1 that are the same as all other

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   previous versions of NFS.  The RPC XID remains in each message and
   needs to be formulated in NFSv4.1 requests as in any other ONC RPC
   request.  The reasons include:

   o  The RPC layer retains its existing semantics and implementation.

   o  The requester and replier must be able to interoperate at the RPC
      layer, prior to the NFSv4.1 decoding of the SEQUENCE or
      CB_SEQUENCE operation.

   o  If an operation is being used that does not start with SEQUENCE or
      CB_SEQUENCE (e.g., BIND_CONN_TO_SESSION), then the RPC XID is
      needed for correct operation to match the reply to the request.

   o  The SEQUENCE or CB_SEQUENCE operation may generate an error.  If
      so, the embedded slot ID, sequence ID, and session ID (if present)
      in the request will not be in the reply, and the requester has
      only the XID to match the reply to the request.

   Given that well-formulated XIDs continue to be required, this begs
   the question: why do SEQUENCE and CB_SEQUENCE replies have a session
   ID, slot ID, and sequence ID?  Having the session ID in the reply
   means that the requester does not have to use the XID to look up the
   session ID, which would be necessary if the connection were
   associated with multiple sessions.  Having the slot ID and sequence
   ID in the reply means that the requester does not have to use the XID
   to look up the slot ID and sequence ID.  Furthermore, since the XID
   is only 32 bits, it is too small to guarantee the re-association of a
   reply with its request [37]; having session ID, slot ID, and sequence
   ID in the reply allows the client to validate that the reply in fact
   belongs to the matched request.

   The SEQUENCE (and CB_SEQUENCE) operation also carries a
   "highest_slotid" value, which carries additional requester slot usage
   information.  The requester MUST always indicate the slot ID
   representing the outstanding request with the highest-numbered slot
   value.  The requester should in all cases provide the most
   conservative value possible, although it can be increased somewhat
   above the actual instantaneous usage to maintain some minimum or
   optimal level.  This provides a way for the requester to yield unused
   request slots back to the replier, which in turn can use the
   information to reallocate resources.

   The replier responds with both a new target highest_slotid and an
   enforced highest_slotid, described as follows:

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   o  The target highest_slotid is an indication to the requester of the
      highest_slotid the replier wishes the requester to be using.  This
      permits the replier to withdraw (or add) resources from a
      requester that has been found to not be using them, in order to
      more fairly share resources among a varying level of demand from
      other requesters.  The requester must always comply with the
      replier's value updates, since they indicate newly established
      hard limits on the requester's access to session resources.
      However, because of request pipelining, the requester may have
      active requests in flight reflecting prior values; therefore, the
      replier must not immediately require the requester to comply.

   o  The enforced highest_slotid indicates the highest slot ID the
      requester is permitted to use on a subsequent SEQUENCE or
      CB_SEQUENCE operation.  The replier's enforced highest_slotid
      SHOULD be no less than the highest_slotid the requester indicated
      in the SEQUENCE or CB_SEQUENCE arguments.

      A requester can be intransigent with respect to lowering its
      highest_slotid argument to a Sequence operation, i.e. the
      requester continues to ignore the target highest_slotid in the
      response to a Sequence operation, and continues to set its
      highest_slotid argument to be higher than the target
      highest_slotid.  This can be considered particularly egregious
      behavior when the replier knows there are no outstanding requests
      with slot IDs higher than its target highest_slotid.  When faced
      with such intransigence, the replier is free to take more forceful
      action, and MAY reply with a new enforced highest_slotid that is
      less than its previous enforced highest_slotid.  Thereafter, if
      the requester continues to send requests with a highest_slotid
      that is greater than the replier's new enforced highest_slotid,
      the server MAY return NFS4ERR_BAD_HIGH_SLOT, unless the slot ID in
      the request is greater than the new enforced highest_slotid and
      the request is a retry.

      The replier SHOULD retain the slots it wants to retire until the
      requester sends a request with a highest_slotid less than or equal
      to the replier's new enforced highest_slotid.

      The requester can also be intransigent with respect to sending
      non-retry requests that have a slot ID that exceeds the replier's
      highest_slotid.  Once the replier has forcibly lowered the
      enforced highest_slotid, the requester is only allowed to send
      retries on slots that exceed the replier's highest_slotid.  If a
      request is received with a slot ID that is higher than the new
      enforced highest_slotid, and the sequence ID is one higher than
      what is in the slot's reply cache, then the server can both retire
      the slot and return NFS4ERR_BADSLOT (however, the server MUST NOT

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      do one and not the other).  The reason it is safe to retire the
      slot is because by using the next sequence ID, the requester is
      indicating it has received the previous reply for the slot.

   o  The requester SHOULD use the lowest available slot when sending a
      new request.  This way, the replier may be able to retire slot
      entries faster.  However, where the replier is actively adjusting
      its granted highest_slotid, it will not be able to use only the
      receipt of the slot ID and highest_slotid in the request.  Neither
      the slot ID nor the highest_slotid used in a request may reflect
      the replier's current idea of the requester's session limit,
      because the request may have been sent from the requester before
      the update was received.  Therefore, in the downward adjustment
      case, the replier may have to retain a number of reply cache
      entries at least as large as the old value of maximum requests
      outstanding, until it can infer that the requester has seen a
      reply containing the new granted highest_slotid.  The replier can
      infer that the requester has seen such a reply when it receives a
      new request with the same slot ID as the request replied to and
      the next higher sequence ID.

2.10.6.1.1.  Caching of SEQUENCE and CB_SEQUENCE Replies

   When a SEQUENCE or CB_SEQUENCE operation is successfully executed,
   its reply MUST always be cached.  Specifically, session ID, sequence
   ID, and slot ID MUST be cached in the reply cache.  The reply from
   SEQUENCE also includes the highest slot ID, target highest slot ID,
   and status flags.  Instead of caching these values, the server MAY
   re-compute the values from the current state of the fore channel,
   session, and/or client ID as appropriate.  Similarly, the reply from
   CB_SEQUENCE includes a highest slot ID and target highest slot ID.
   The client MAY re-compute the values from the current state of the
   session as appropriate.

   Regardless of whether or not a replier is re-computing highest slot
   ID, target slot ID, and status on replies to retries, the requester
   MUST NOT assume that the values are being re-computed whenever it
   receives a reply after a retry is sent, since it has no way of
   knowing whether the reply it has received was sent by the replier in
   response to the retry or is a delayed response to the original
   request.  Therefore, it may be the case that highest slot ID, target
   slot ID, or status bits may reflect the state of affairs when the
   request was first executed.  Although acting based on such delayed
   information is valid, it may cause the receiver of the reply to do
   unneeded work.  Requesters MAY choose to send additional requests to
   get the current state of affairs or use the state of affairs reported
   by subsequent requests, in preference to acting immediately on data
   that might be out of date.

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2.10.6.1.2.  Errors from SEQUENCE and CB_SEQUENCE

   Any time SEQUENCE or CB_SEQUENCE returns an error, the sequence ID of
   the slot MUST NOT change.  The replier MUST NOT modify the reply
   cache entry for the slot whenever an error is returned from SEQUENCE
   or CB_SEQUENCE.

2.10.6.1.3.  Optional Reply Caching

   On a per-request basis, the requester can choose to direct the
   replier to cache the reply to all operations after the first
   operation (SEQUENCE or CB_SEQUENCE) via the sa_cachethis or
   csa_cachethis fields of the arguments to SEQUENCE or CB_SEQUENCE.
   The reason it would not direct the replier to cache the entire reply
   is that the request is composed of all idempotent operations [34].
   Caching the reply may offer little benefit.  If the reply is too
   large (see Section 2.10.6.4), it may not be cacheable anyway.  Even
   if the reply to idempotent request is small enough to cache,
   unnecessarily caching the reply slows down the server and increases
   RPC latency.

   Whether or not the requester requests the reply to be cached has no
   effect on the slot processing.  If the results of SEQUENCE or
   CB_SEQUENCE are NFS4_OK, then the slot's sequence ID MUST be
   incremented by one.  If a requester does not direct the replier to
   cache the reply, the replier MUST do one of following:

   o  The replier can cache the entire original reply.  Even though
      sa_cachethis or csa_cachethis is FALSE, the replier is always free
      to cache.  It may choose this approach in order to simplify
      implementation.

   o  The replier enters into its reply cache a reply consisting of the
      original results to the SEQUENCE or CB_SEQUENCE operation, and
      with the next operation in COMPOUND or CB_COMPOUND having the
      error NFS4ERR_RETRY_UNCACHED_REP.  Thus, if the requester later
      retries the request, it will get NFS4ERR_RETRY_UNCACHED_REP.  If a
      replier receives a retried Sequence operation where the reply to
      the COMPOUND or CB_COMPOUND was not cached, then the replier,

      *  MAY return NFS4ERR_RETRY_UNCACHED_REP in reply to a Sequence
         operation if the Sequence operation is not the first operation
         (granted, a requester that does so is in violation of the
         NFSv4.1 protocol).

      *  MUST NOT return NFS4ERR_RETRY_UNCACHED_REP in reply to a
         Sequence operation if the Sequence operation is the first
         operation.

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   o  If the second operation is an illegal operation, or an operation
      that was legal in a previous minor version of NFSv4 and MUST NOT
      be supported in the current minor version (e.g., SETCLIENTID), the
      replier MUST NOT ever return NFS4ERR_RETRY_UNCACHED_REP.  Instead
      the replier MUST return NFS4ERR_OP_ILLEGAL or NFS4ERR_BADXDR or
      NFS4ERR_NOTSUPP as appropriate.

   o  If the second operation can result in another error status, the
      replier MAY return a status other than NFS4ERR_RETRY_UNCACHED_REP,
      provided the operation is not executed in such a way that the
      state of the replier is changed.  Examples of such an error status
      include: NFS4ERR_NOTSUPP returned for an operation that is legal
      but not REQUIRED in the current minor versions, and thus not
      supported by the replier; NFS4ERR_SEQUENCE_POS; and
      NFS4ERR_REQ_TOO_BIG.

   The discussion above assumes that the retried request matches the
   original one.  Section 2.10.6.1.3.1 discusses what the replier might
   do, and MUST do when original and retried requests do not match.
   Since the replier may only cache a small amount of the information
   that would be required to determine whether this is a case of a false
   retry, the replier may send to the client any of the following
   responses:

   o  The cached reply to the original request (if the replier has
      cached it in its entirety and the users of the original request
      and retry match).

   o  A reply that consists only of the Sequence operation with the
      error NFS4ERR_FALSE_RETRY.

   o  A reply consisting of the response to Sequence with the status
      NFS4_OK, together with the second operation as it appeared in the
      retried request with an error of NFS4ERR_RETRY_UNCACHED_REP or
      other error as described above.

   o  A reply that consists of the response to Sequence with the status
      NFS4_OK, together with the second operation as it appeared in the
      original request with an error of NFS4ERR_RETRY_UNCACHED_REP or
      other error as described above.

2.10.6.1.3.1.  False Retry

   If a requester sent a Sequence operation with a slot ID and sequence
   ID that are in the reply cache but the replier detected that the
   retried request is not the same as the original request, including a
   retry that has different operations or different arguments in the
   operations from the original and a retry that uses a different

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   principal in the RPC request's credential field that translates to a
   different user, then this is a false retry.  When the replier detects
   a false retry, it is permitted (but not always obligated) to return
   NFS4ERR_FALSE_RETRY in response to the Sequence operation when it
   detects a false retry.

   Translations of particularly privileged user values to other users
   due to the lack of appropriately secure credentials, as configured on
   the replier, should be applied before determining whether the users
   are the same or different.  If the replier determines the users are
   different between the original request and a retry, then the replier
   MUST return NFS4ERR_FALSE_RETRY.

   If an operation of the retry is an illegal operation, or an operation
   that was legal in a previous minor version of NFSv4 and MUST NOT be
   supported in the current minor version (e.g., SETCLIENTID), the
   replier MAY return NFS4ERR_FALSE_RETRY (and MUST do so if the users
   of the original request and retry differ).  Otherwise, the replier
   MAY return NFS4ERR_OP_ILLEGAL or NFS4ERR_BADXDR or NFS4ERR_NOTSUPP as
   appropriate.  Note that the handling is in contrast for how the
   replier deals with retries requests with no cached reply.  The
   difference is due to NFS4ERR_FALSE_RETRY being a valid error for only
   Sequence operations, whereas NFS4ERR_RETRY_UNCACHED_REP is a valid
   error for all operations except illegal operations and operations
   that MUST NOT be supported in the current minor version of NFSv4.

2.10.6.2.  Retry and Replay of Reply

   A requester MUST NOT retry a request, unless the connection it used
   to send the request disconnects.  The requester can then reconnect
   and re-send the request, or it can re-send the request over a
   different connection that is associated with the same session.

   If the requester is a server wanting to re-send a callback operation
   over the backchannel of a session, the requester of course cannot
   reconnect because only the client can associate connections with the
   backchannel.  The server can re-send the request over another
   connection that is bound to the same session's backchannel.  If there
   is no such connection, the server MUST indicate that the session has
   no backchannel by setting the SEQ4_STATUS_CB_PATH_DOWN_SESSION flag
   bit in the response to the next SEQUENCE operation from the client.
   The client MUST then associate a connection with the session (or
   destroy the session).

   Note that it is not fatal for a requester to retry without a
   disconnect between the request and retry.  However, the retry does
   consume resources, especially with RDMA, where each request, retry or
   not, consumes a credit.  Retries for no reason, especially retries

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   sent shortly after the previous attempt, are a poor use of network
   bandwidth and defeat the purpose of a transport's inherent congestion
   control system.

   A requester MUST wait for a reply to a request before using the slot
   for another request.  If it does not wait for a reply, then the
   requester does not know what sequence ID to use for the slot on its
   next request.  For example, suppose a requester sends a request with
   sequence ID 1, and does not wait for the response.  The next time it
   uses the slot, it sends the new request with sequence ID 2.  If the
   replier has not seen the request with sequence ID 1, then the replier
   is not expecting sequence ID 2, and rejects the requester's new
   request with NFS4ERR_SEQ_MISORDERED (as the result from SEQUENCE or
   CB_SEQUENCE).

   RDMA fabrics do not guarantee that the memory handles (Steering Tags)
   within each RPC/RDMA "chunk" [8] are valid on a scope outside that of
   a single connection.  Therefore, handles used by the direct
   operations become invalid after connection loss.  The server must
   ensure that any RDMA operations that must be replayed from the reply
   cache use the newly provided handle(s) from the most recent request.

   A retry might be sent while the original request is still in progress
   on the replier.  The replier SHOULD deal with the issue by returning
   NFS4ERR_DELAY as the reply to SEQUENCE or CB_SEQUENCE operation, but
   implementations MAY return NFS4ERR_MISORDERED.  Since errors from
   SEQUENCE and CB_SEQUENCE are never recorded in the reply cache, this
   approach allows the results of the execution of the original request
   to be properly recorded in the reply cache (assuming that the
   requester specified the reply to be cached).

2.10.6.3.  Resolving Server Callback Races

   It is possible for server callbacks to arrive at the client before
   the reply from related fore channel operations.  For example, a
   client may have been granted a delegation to a file it has opened,
   but the reply to the OPEN (informing the client of the granting of
   the delegation) may be delayed in the network.  If a conflicting
   operation arrives at the server, it will recall the delegation using
   the backchannel, which may be on a different transport connection,
   perhaps even a different network, or even a different session
   associated with the same client ID.

   The presence of a session between the client and server alleviates
   this issue.  When a session is in place, each client request is
   uniquely identified by its { session ID, slot ID, sequence ID }
   triple.  By the rules under which slot entries (reply cache entries)
   are retired, the server has knowledge whether the client has "seen"

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   each of the server's replies.  The server can therefore provide
   sufficient information to the client to allow it to disambiguate
   between an erroneous or conflicting callback race condition.

   For each client operation that might result in some sort of server
   callback, the server SHOULD "remember" the { session ID, slot ID,
   sequence ID } triple of the client request until the slot ID
   retirement rules allow the server to determine that the client has,
   in fact, seen the server's reply.  Until the time the { session ID,
   slot ID, sequence ID } request triple can be retired, any recalls of
   the associated object MUST carry an array of these referring
   identifiers (in the CB_SEQUENCE operation's arguments), for the
   benefit of the client.  After this time, it is not necessary for the
   server to provide this information in related callbacks, since it is
   certain that a race condition can no longer occur.

   The CB_SEQUENCE operation that begins each server callback carries a
   list of "referring" { session ID, slot ID, sequence ID } triples.  If
   the client finds the request corresponding to the referring session
   ID, slot ID, and sequence ID to be currently outstanding (i.e., the
   server's reply has not been seen by the client), it can determine
   that the callback has raced the reply, and act accordingly.  If the
   client does not find the request corresponding to the referring
   triple to be outstanding (including the case of a session ID
   referring to a destroyed session), then there is no race with respect
   to this triple.  The server SHOULD limit the referring triples to
   requests that refer to just those that apply to the objects referred
   to in the CB_COMPOUND procedure.

   The client must not simply wait forever for the expected server reply
   to arrive before responding to the CB_COMPOUND that won the race,
   because it is possible that it will be delayed indefinitely.  The
   client should assume the likely case that the reply will arrive
   within the average round-trip time for COMPOUND requests to the
   server, and wait that period of time.  If that period of time
   expires, it can respond to the CB_COMPOUND with NFS4ERR_DELAY.  There
   are other scenarios under which callbacks may race replies.  Among
   them are pNFS layout recalls as described in Section 12.5.5.2.

2.10.6.4.  COMPOUND and CB_COMPOUND Construction Issues

   Very large requests and replies may pose both buffer management
   issues (especially with RDMA) and reply cache issues.  When the
   session is created (Section 18.36), for each channel (fore and back),
   the client and server negotiate the maximum-sized request they will
   send or process (ca_maxrequestsize), the maximum-sized reply they
   will return or process (ca_maxresponsesize), and the maximum-sized
   reply they will store in the reply cache (ca_maxresponsesize_cached).

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   If a request exceeds ca_maxrequestsize, the reply will have the
   status NFS4ERR_REQ_TOO_BIG.  A replier MAY return NFS4ERR_REQ_TOO_BIG
   as the status for the first operation (SEQUENCE or CB_SEQUENCE) in
   the request (which means that no operations in the request executed
   and that the state of the slot in the reply cache is unchanged), or
   it MAY opt to return it on a subsequent operation in the same
   COMPOUND or CB_COMPOUND request (which means that at least one
   operation did execute and that the state of the slot in the reply
   cache does change).  The replier SHOULD set NFS4ERR_REQ_TOO_BIG on
   the operation that exceeds ca_maxrequestsize.

   If a reply exceeds ca_maxresponsesize, the reply will have the status
   NFS4ERR_REP_TOO_BIG.  A replier MAY return NFS4ERR_REP_TOO_BIG as the
   status for the first operation (SEQUENCE or CB_SEQUENCE) in the
   request, or it MAY opt to return it on a subsequent operation (in the
   same COMPOUND or CB_COMPOUND reply).  A replier MAY return
   NFS4ERR_REP_TOO_BIG in the reply to SEQUENCE or CB_SEQUENCE, even if
   the response would still exceed ca_maxresponsesize.

   If sa_cachethis or csa_cachethis is TRUE, then the replier MUST cache
   a reply except if an error is returned by the SEQUENCE or CB_SEQUENCE
   operation (see Section 2.10.6.1.2).  If the reply exceeds
   ca_maxresponsesize_cached (and sa_cachethis or csa_cachethis is
   TRUE), then the server MUST return NFS4ERR_REP_TOO_BIG_TO_CACHE.
   Even if NFS4ERR_REP_TOO_BIG_TO_CACHE (or any other error for that
   matter) is returned on an operation other than the first operation
   (SEQUENCE or CB_SEQUENCE), then the reply MUST be cached if
   sa_cachethis or csa_cachethis is TRUE.  For example, if a COMPOUND
   has eleven operations, including SEQUENCE, the fifth operation is a
   RENAME, and the tenth operation is a READ for one million bytes, the
   server may return NFS4ERR_REP_TOO_BIG_TO_CACHE on the tenth
   operation.  Since the server executed several operations, especially
   the non-idempotent RENAME, the client's request to cache the reply
   needs to be honored in order for the correct operation of exactly
   once semantics.  If the client retries the request, the server will
   have cached a reply that contains results for ten of the eleven
   requested operations, with the tenth operation having a status of
   NFS4ERR_REP_TOO_BIG_TO_CACHE.

   A client needs to take care that when sending operations that change
   the current filehandle (except for PUTFH, PUTPUBFH, PUTROOTFH, and
   RESTOREFH), it not exceed the maximum reply buffer before the GETFH
   operation.  Otherwise, the client will have to retry the operation
   that changed the current filehandle, in order to obtain the desired
   filehandle.  For the OPEN operation (see Section 18.16), retry is not
   always available as an option.  The following guidelines for the
   handling of filehandle-changing operations are advised:

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   o  Within the same COMPOUND procedure, a client SHOULD send GETFH
      immediately after a current filehandle-changing operation.  A
      client MUST send GETFH after a current filehandle-changing
      operation that is also non-idempotent (e.g., the OPEN operation),
      unless the operation is RESTOREFH.  RESTOREFH is an exception,
      because even though it is non-idempotent, the filehandle RESTOREFH
      produced originated from an operation that is either idempotent
      (e.g., PUTFH, LOOKUP), or non-idempotent (e.g., OPEN, CREATE).  If
      the origin is non-idempotent, then because the client MUST send
      GETFH after the origin operation, the client can recover if
      RESTOREFH returns an error.

   o  A server MAY return NFS4ERR_REP_TOO_BIG or
      NFS4ERR_REP_TOO_BIG_TO_CACHE (if sa_cachethis is TRUE) on a
      filehandle-changing operation if the reply would be too large on
      the next operation.

   o  A server SHOULD return NFS4ERR_REP_TOO_BIG or
      NFS4ERR_REP_TOO_BIG_TO_CACHE (if sa_cachethis is TRUE) on a
      filehandle-changing, non-idempotent operation if the reply would
      be too large on the next operation, especially if the operation is
      OPEN.

   o  A server MAY return NFS4ERR_UNSAFE_COMPOUND to a non-idempotent
      current filehandle-changing operation, if it looks at the next
      operation (in the same COMPOUND procedure) and finds it is not
      GETFH.  The server SHOULD do this if it is unable to determine in
      advance whether the total response size would exceed
      ca_maxresponsesize_cached or ca_maxresponsesize.

2.10.6.5.  Persistence

   Since the reply cache is bounded, it is practical for the reply cache
   to persist across server restarts.  The replier MUST persist the
   following information if it agreed to persist the session (when the
   session was created; see Section 18.36):

   o  The session ID.

   o  The slot table including the sequence ID and cached reply for each
      slot.

   The above are sufficient for a replier to provide EOS semantics for
   any requests that were sent and executed before the server restarted.
   If the replier is a client, then there is no need for it to persist
   any more information, unless the client will be persisting all other
   state across client restart, in which case, the server will never see
   any NFSv4.1-level protocol manifestation of a client restart.  If the

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   replier is a server, with just the slot table and session ID
   persisting, any requests the client retries after the server restart
   will return the results that are cached in the reply cache, and any
   new requests (i.e., the sequence ID is one greater than the slot's
   sequence ID) MUST be rejected with NFS4ERR_DEADSESSION (returned by
   SEQUENCE).  Such a session is considered dead.  A server MAY re-
   animate a session after a server restart so that the session will
   accept new requests as well as retries.  To re-animate a session, the
   server needs to persist additional information through server
   restart:

   o  The client ID.  This is a prerequisite to let the client create
      more sessions associated with the same client ID as the re-
      animated session.

   o  The client ID's sequence ID that is used for creating sessions
      (see Sections 18.35 and 18.36).  This is a prerequisite to let the
      client create more sessions.

   o  The principal that created the client ID.  This allows the server
      to authenticate the client when it sends EXCHANGE_ID.

   o  The SSV, if SP4_SSV state protection was specified when the client
      ID was created (see Section 18.35).  This lets the client create
      new sessions, and associate connections with the new and existing
      sessions.

   o  The properties of the client ID as defined in Section 18.35.

   A persistent reply cache places certain demands on the server.  The
   execution of the sequence of operations (starting with SEQUENCE) and
   placement of its results in the persistent cache MUST be atomic.  If
   a client retries a sequence of operations that was previously
   executed on the server, the only acceptable outcomes are either the
   original cached reply or an indication that the client ID or session
   has been lost (indicating a catastrophic loss of the reply cache or a
   session that has been deleted because the client failed to use the
   session for an extended period of time).

   A server could fail and restart in the middle of a COMPOUND procedure
   that contains one or more non-idempotent or idempotent-but-modifying
   operations.  This creates an even higher challenge for atomic
   execution and placement of results in the reply cache.  One way to
   view the problem is as a single transaction consisting of each
   operation in the COMPOUND followed by storing the result in
   persistent storage, then finally a transaction commit.  If there is a
   failure before the transaction is committed, then the server rolls

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   back the transaction.  If the server itself fails, then when it
   restarts, its recovery logic could roll back the transaction before
   starting the NFSv4.1 server.

   While the description of the implementation for atomic execution of
   the request and caching of the reply is beyond the scope of this
   document, an example implementation for NFSv2 [38] is described in
   [39].



(page 65 continued on part 4)

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