File: rfc3010.txt

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Network Working Group                                         S. Shepler
Request for Comments: 3010                                  B. Callaghan
Obsoletes: 1813, 1094                                        D. Robinson
Category: Standards Track                                     R. Thurlow
                                                   Sun Microsystems Inc.
                                                                C. Beame
                                                        Hummingbird Ltd.
                                                               M. Eisler
                                                           Zambeel, Inc.
                                                               D. Noveck
                                                 Network Appliance, Inc.
                                                           December 2000


                         NFS version 4 Protocol

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   NFS (Network File System) version 4 is a distributed file system
   protocol which owes heritage to NFS protocol versions 2 [RFC1094] and
   3 [RFC1813].  Unlike earlier versions, the NFS version 4 protocol
   supports traditional file access while integrating support for file
   locking and the mount protocol.  In addition, support for strong
   security (and its negotiation), compound operations, client caching,
   and internationalization have been added.  Of course, attention has
   been applied to making NFS version 4 operate well in an Internet
   environment.

Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.






Shepler, et al.             Standards Track                     [Page 1]

RFC 3010                 NFS version 4 Protocol            December 2000


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .   5
   1.1.  Overview of NFS Version 4 Features . . . . . . . . . . . .   6
   1.1.1.  RPC and Security . . . . . . . . . . . . . . . . . . . .   6
   1.1.2.  Procedure and Operation Structure  . . . . . . . . . . .   7
   1.1.3.  File System Model  . . . . . . . . . . . . . . . . . . .   8
   1.1.3.1.  Filehandle Types . . . . . . . . . . . . . . . . . . .   8
   1.1.3.2.  Attribute Types  . . . . . . . . . . . . . . . . . . .   8
   1.1.3.3.  File System Replication and Migration  . . . . . . . .   9
   1.1.4.  OPEN and CLOSE . . . . . . . . . . . . . . . . . . . . .   9
   1.1.5.  File locking . . . . . . . . . . . . . . . . . . . . . .   9
   1.1.6.  Client Caching and Delegation  . . . . . . . . . . . . .  10
   1.2.  General Definitions  . . . . . . . . . . . . . . . . . . .  11
   2.  Protocol Data Types  . . . . . . . . . . . . . . . . . . . .  12
   2.1.  Basic Data Types . . . . . . . . . . . . . . . . . . . . .  12
   2.2.  Structured Data Types  . . . . . . . . . . . . . . . . . .  14
   3.  RPC and Security Flavor  . . . . . . . . . . . . . . . . . .  18
   3.1.  Ports and Transports . . . . . . . . . . . . . . . . . . .  18
   3.2.  Security Flavors . . . . . . . . . . . . . . . . . . . . .  18
   3.2.1.  Security mechanisms for NFS version 4  . . . . . . . . .  19
   3.2.1.1.  Kerberos V5 as security triple . . . . . . . . . . . .  19
   3.2.1.2.  LIPKEY as a security triple  . . . . . . . . . . . . .  19
   3.2.1.3.  SPKM-3 as a security triple  . . . . . . . . . . . . .  20
   3.3.  Security Negotiation . . . . . . . . . . . . . . . . . . .  21
   3.3.1.  Security Error . . . . . . . . . . . . . . . . . . . . .  21
   3.3.2.  SECINFO  . . . . . . . . . . . . . . . . . . . . . . . .  21
   3.4.  Callback RPC Authentication  . . . . . . . . . . . . . . .  22
   4.  Filehandles  . . . . . . . . . . . . . . . . . . . . . . . .  23
   4.1.  Obtaining the First Filehandle . . . . . . . . . . . . . .  24
   4.1.1.  Root Filehandle  . . . . . . . . . . . . . . . . . . . .  24
   4.1.2.  Public Filehandle  . . . . . . . . . . . . . . . . . . .  24
   4.2.  Filehandle Types . . . . . . . . . . . . . . . . . . . . .  25
   4.2.1.  General Properties of a Filehandle . . . . . . . . . . .  25
   4.2.2.  Persistent Filehandle  . . . . . . . . . . . . . . . . .  26
   4.2.3.  Volatile Filehandle  . . . . . . . . . . . . . . . . . .  26
   4.2.4.  One Method of Constructing a Volatile Filehandle . . . .  28
   4.3.  Client Recovery from Filehandle Expiration . . . . . . . .  28
   5.  File Attributes  . . . . . . . . . . . . . . . . . . . . . .  29
   5.1.  Mandatory Attributes . . . . . . . . . . . . . . . . . . .  30
   5.2.  Recommended Attributes . . . . . . . . . . . . . . . . . .  30
   5.3.  Named Attributes . . . . . . . . . . . . . . . . . . . . .  31
   5.4.  Mandatory Attributes - Definitions . . . . . . . . . . . .  31
   5.5.  Recommended Attributes - Definitions . . . . . . . . . . .  33
   5.6.  Interpreting owner and owner_group . . . . . . . . . . . .  38
   5.7.  Character Case Attributes  . . . . . . . . . . . . . . . .  39
   5.8.  Quota Attributes . . . . . . . . . . . . . . . . . . . . .  39
   5.9.  Access Control Lists . . . . . . . . . . . . . . . . . . .  40



Shepler, et al.             Standards Track                     [Page 2]

RFC 3010                 NFS version 4 Protocol            December 2000


   5.9.1.  ACE type . . . . . . . . . . . . . . . . . . . . . . . .  41
   5.9.2.  ACE flag . . . . . . . . . . . . . . . . . . . . . . . .  41
   5.9.3.  ACE Access Mask  . . . . . . . . . . . . . . . . . . . .  43
   5.9.4.  ACE who  . . . . . . . . . . . . . . . . . . . . . . . .  44
   6.  File System Migration and Replication  . . . . . . . . . . .  44
   6.1.  Replication  . . . . . . . . . . . . . . . . . . . . . . .  45
   6.2.  Migration  . . . . . . . . . . . . . . . . . . . . . . . .  45
   6.3.  Interpretation of the fs_locations Attribute . . . . . . .  46
   6.4.  Filehandle Recovery for Migration or Replication . . . . .  47
   7.  NFS Server Name Space  . . . . . . . . . . . . . . . . . . .  47
   7.1.  Server Exports . . . . . . . . . . . . . . . . . . . . . .  47
   7.2.  Browsing Exports . . . . . . . . . . . . . . . . . . . . .  48
   7.3.  Server Pseudo File System  . . . . . . . . . . . . . . . .  48
   7.4.  Multiple Roots . . . . . . . . . . . . . . . . . . . . . .  49
   7.5.  Filehandle Volatility  . . . . . . . . . . . . . . . . . .  49
   7.6.  Exported Root  . . . . . . . . . . . . . . . . . . . . . .  49
   7.7.  Mount Point Crossing . . . . . . . . . . . . . . . . . . .  49
   7.8.  Security Policy and Name Space Presentation  . . . . . . .  50
   8.  File Locking and Share Reservations  . . . . . . . . . . . .  50
   8.1.  Locking  . . . . . . . . . . . . . . . . . . . . . . . . .  51
   8.1.1.  Client ID  . . . . . . . . . . . . . . . . . . . . . . .  51
   8.1.2.  Server Release of Clientid . . . . . . . . . . . . . . .  53
   8.1.3.  nfs_lockowner and stateid Definition . . . . . . . . . .  54
   8.1.4.  Use of the stateid . . . . . . . . . . . . . . . . . . .  55
   8.1.5.  Sequencing of Lock Requests  . . . . . . . . . . . . . .  56
   8.1.6.  Recovery from Replayed Requests  . . . . . . . . . . . .  56
   8.1.7.  Releasing nfs_lockowner State  . . . . . . . . . . . . .  57
   8.2.  Lock Ranges  . . . . . . . . . . . . . . . . . . . . . . .  57
   8.3.  Blocking Locks . . . . . . . . . . . . . . . . . . . . . .  58
   8.4.  Lease Renewal  . . . . . . . . . . . . . . . . . . . . . .  58
   8.5.  Crash Recovery . . . . . . . . . . . . . . . . . . . . . .  59
   8.5.1.  Client Failure and Recovery  . . . . . . . . . . . . . .  59
   8.5.2.  Server Failure and Recovery  . . . . . . . . . . . . . .  60
   8.5.3.  Network Partitions and Recovery  . . . . . . . . . . . .  62
   8.6.  Recovery from a Lock Request Timeout or Abort  . . . . . .  63
   8.7.  Server Revocation of Locks . . . . . . . . . . . . . . . .  63
   8.8.  Share Reservations . . . . . . . . . . . . . . . . . . . .  65
   8.9.  OPEN/CLOSE Operations  . . . . . . . . . . . . . . . . . .  65
   8.10.  Open Upgrade and Downgrade  . . . . . . . . . . . . . . .  66
   8.11.  Short and Long Leases . . . . . . . . . . . . . . . . . .  66
   8.12.  Clocks and Calculating Lease Expiration . . . . . . . . .  67
   8.13.  Migration, Replication and State  . . . . . . . . . . . .  67
   8.13.1.  Migration and State . . . . . . . . . . . . . . . . . .  67
   8.13.2.  Replication and State . . . . . . . . . . . . . . . . .  68
   8.13.3.  Notification of Migrated Lease  . . . . . . . . . . . .  69
   9.  Client-Side Caching  . . . . . . . . . . . . . . . . . . . .  69
   9.1.  Performance Challenges for Client-Side Caching . . . . . .  70
   9.2.  Delegation and Callbacks . . . . . . . . . . . . . . . . .  71



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   9.2.1.  Delegation Recovery  . . . . . . . . . . . . . . . . . .  72
   9.3.  Data Caching . . . . . . . . . . . . . . . . . . . . . . .  74
   9.3.1.  Data Caching and OPENs . . . . . . . . . . . . . . . . .  74
   9.3.2.  Data Caching and File Locking  . . . . . . . . . . . . .  75
   9.3.3.  Data Caching and Mandatory File Locking  . . . . . . . .  77
   9.3.4.  Data Caching and File Identity . . . . . . . . . . . . .  77
   9.4.  Open Delegation  . . . . . . . . . . . . . . . . . . . . .  78
   9.4.1.  Open Delegation and Data Caching . . . . . . . . . . . .  80
   9.4.2.  Open Delegation and File Locks . . . . . . . . . . . . .  82
   9.4.3.  Recall of Open Delegation  . . . . . . . . . . . . . . .  82
   9.4.4.  Delegation Revocation  . . . . . . . . . . . . . . . . .  84
   9.5.  Data Caching and Revocation  . . . . . . . . . . . . . . .  84
   9.5.1.  Revocation Recovery for Write Open Delegation  . . . . .  85
   9.6.  Attribute Caching  . . . . . . . . . . . . . . . . . . . .  85
   9.7.  Name Caching . . . . . . . . . . . . . . . . . . . . . . .  86
   9.8.  Directory Caching  . . . . . . . . . . . . . . . . . . . .  87
   10.  Minor Versioning  . . . . . . . . . . . . . . . . . . . . .  88
   11.  Internationalization  . . . . . . . . . . . . . . . . . . .  91
   11.1.  Universal Versus Local Character Sets . . . . . . . . . .  91
   11.2.  Overview of Universal Character Set Standards . . . . . .  92
   11.3.  Difficulties with UCS-4, UCS-2, Unicode . . . . . . . . .  93
   11.4.  UTF-8 and its solutions . . . . . . . . . . . . . . . . .  94
   11.5.  Normalization . . . . . . . . . . . . . . . . . . . . . .  94
   12.  Error Definitions . . . . . . . . . . . . . . . . . . . . .  95
   13.  NFS Version 4 Requests  . . . . . . . . . . . . . . . . . .  99
   13.1.  Compound Procedure  . . . . . . . . . . . . . . . . . . . 100
   13.2.  Evaluation of a Compound Request  . . . . . . . . . . . . 100
   13.3.  Synchronous Modifying Operations  . . . . . . . . . . . . 101
   13.4.  Operation Values  . . . . . . . . . . . . . . . . . . . . 102
   14.  NFS Version 4 Procedures  . . . . . . . . . . . . . . . . . 102
   14.1.  Procedure 0: NULL - No Operation  . . . . . . . . . . . . 102
   14.2.  Procedure 1: COMPOUND - Compound Operations . . . . . . . 102
   14.2.1.  Operation 3: ACCESS - Check Access Rights . . . . . . . 105
   14.2.2.  Operation 4: CLOSE - Close File . . . . . . . . . . . . 108
   14.2.3.  Operation 5: COMMIT - Commit Cached Data  . . . . . . . 109
   14.2.4.  Operation 6: CREATE - Create a Non-Regular File Object. 112
   14.2.5.  Operation 7: DELEGPURGE - Purge Delegations Awaiting
            Recovery  . . . . . . . . . . . . . . . . . . . . . . . 114
   14.2.6.  Operation 8: DELEGRETURN - Return Delegation  . . . . . 115
   14.2.7.  Operation 9: GETATTR - Get Attributes . . . . . . . . . 115
   14.2.8.  Operation 10: GETFH - Get Current Filehandle  . . . . . 117
   14.2.9.  Operation 11: LINK - Create Link to a File  . . . . . . 118
   14.2.10.  Operation 12: LOCK - Create Lock . . . . . . . . . . . 119
   14.2.11.  Operation 13: LOCKT - Test For Lock  . . . . . . . . . 121
   14.2.12.  Operation 14: LOCKU - Unlock File  . . . . . . . . . . 122
   14.2.13.  Operation 15: LOOKUP - Lookup Filename . . . . . . . . 123
   14.2.14.  Operation 16: LOOKUPP - Lookup Parent Directory  . . . 126




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   14.2.15.  Operation 17: NVERIFY - Verify Difference in
             Attributes . . . . . . . . . . . . . . . . . . . . . . 127
   14.2.16.  Operation 18: OPEN - Open a Regular File . . . . . . . 128
   14.2.17.  Operation 19: OPENATTR - Open Named Attribute
             Directory  . . . . . . . . . . . . . . . . . . . . . . 137
   14.2.18.  Operation 20: OPEN_CONFIRM - Confirm Open  . . . . . . 138
   14.2.19.  Operation 21: OPEN_DOWNGRADE - Reduce Open File Access 140
   14.2.20.  Operation 22: PUTFH - Set Current Filehandle . . . . . 141
   14.2.21.  Operation 23: PUTPUBFH - Set Public Filehandle . . . . 142
   14.2.22.  Operation 24: PUTROOTFH - Set Root Filehandle  . . . . 143
   14.2.23.  Operation 25: READ - Read from File  . . . . . . . . . 144
   14.2.24.  Operation 26: READDIR - Read Directory . . . . . . . . 146
   14.2.25.  Operation 27: READLINK - Read Symbolic Link  . . . . . 150
   14.2.26.  Operation 28: REMOVE - Remove Filesystem Object  . . . 151
   14.2.27.  Operation 29: RENAME - Rename Directory Entry  . . . . 153
   14.2.28.  Operation 30: RENEW - Renew a Lease  . . . . . . . . . 155
   14.2.29.  Operation 31: RESTOREFH - Restore Saved Filehandle . . 156
   14.2.30.  Operation 32: SAVEFH - Save Current Filehandle . . . . 157
   14.2.31.  Operation 33: SECINFO - Obtain Available Security  . . 158
   14.2.32.  Operation 34: SETATTR - Set Attributes . . . . . . . . 160
   14.2.33.  Operation 35: SETCLIENTID - Negotiate Clientid . . . . 162
   14.2.34.  Operation 36: SETCLIENTID_CONFIRM - Confirm Clientid . 163
   14.2.35.  Operation 37: VERIFY - Verify Same Attributes  . . . . 164
   14.2.36.  Operation 38: WRITE - Write to File  . . . . . . . . . 166
   15.  NFS Version 4 Callback Procedures . . . . . . . . . . . . . 170
   15.1.  Procedure 0: CB_NULL - No Operation . . . . . . . . . . . 170
   15.2.  Procedure 1: CB_COMPOUND - Compound Operations  . . . . . 171
   15.2.1.  Operation 3: CB_GETATTR - Get Attributes  . . . . . . . 172
   15.2.2.  Operation 4: CB_RECALL - Recall an Open Delegation  . . 173
   16.  Security Considerations . . . . . . . . . . . . . . . . . . 174
   17.  IANA Considerations . . . . . . . . . . . . . . . . . . . . 174
   17.1.  Named Attribute Definition  . . . . . . . . . . . . . . . 174
   18.  RPC definition file . . . . . . . . . . . . . . . . . . . . 175
   19.  Bibliography  . . . . . . . . . . . . . . . . . . . . . . . 206
   20.  Authors . . . . . . . . . . . . . . . . . . . . . . . . . . 210
   20.1.  Editor's Address  . . . . . . . . . . . . . . . . . . . . 210
   20.2.  Authors' Addresses  . . . . . . . . . . . . . . . . . . . 210
   20.3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . 211
   21.  Full Copyright Statement  . . . . . . . . . . . . . . . . . 212

1.  Introduction

   The NFS version 4 protocol is a further revision of the NFS protocol
   defined already by versions 2 [RFC1094] and 3 [RFC1813].  It retains
   the essential characteristics of previous versions: design for easy
   recovery, independent of transport protocols, operating systems and
   filesystems, simplicity, and good performance.  The NFS version 4
   revision has the following goals:



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   o  Improved access and good performance on the Internet.

      The protocol is designed to transit firewalls easily, perform well
      where latency is high and bandwidth is low, and scale to very
      large numbers of clients per server.

   o  Strong security with negotiation built into the protocol.

      The protocol builds on the work of the ONCRPC working group in
      supporting the RPCSEC_GSS protocol.  Additionally, the NFS version
      4 protocol provides a mechanism to allow clients and servers the
      ability to negotiate security and require clients and servers to
      support a minimal set of security schemes.

   o  Good cross-platform interoperability.

      The protocol features a file system model that provides a useful,
      common set of features that does not unduly favor one file system
      or operating system over another.

   o  Designed for protocol extensions.

      The protocol is designed to accept standard extensions that do not
      compromise backward compatibility.

1.1.  Overview of NFS Version 4 Features

   To provide a reasonable context for the reader, the major features of
   NFS version 4 protocol will be reviewed in brief.  This will be done
   to provide an appropriate context for both the reader who is familiar
   with the previous versions of the NFS protocol and the reader that is
   new to the NFS protocols.  For the reader new to the NFS protocols,
   there is still a fundamental knowledge that is expected.  The reader
   should be familiar with the XDR and RPC protocols as described in
   [RFC1831] and [RFC1832].  A basic knowledge of file systems and
   distributed file systems is expected as well.

1.1.1.  RPC and Security

   As with previous versions of NFS, the External Data Representation
   (XDR) and Remote Procedure Call (RPC) mechanisms used for the NFS
   version 4 protocol are those defined in [RFC1831] and [RFC1832].  To
   meet end to end security requirements, the RPCSEC_GSS framework
   [RFC2203] will be used to extend the basic RPC security.  With the
   use of RPCSEC_GSS, various mechanisms can be provided to offer
   authentication, integrity, and privacy to the NFS version 4 protocol.
   Kerberos V5 will be used as described in [RFC1964] to provide one
   security framework.  The LIPKEY GSS-API mechanism described in



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   [RFC2847] will be used to provide for the use of user password and
   server public key by the NFS version 4 protocol.  With the use of
   RPCSEC_GSS, other mechanisms may also be specified and used for NFS
   version 4 security.

   To enable in-band security negotiation, the NFS version 4 protocol
   has added a new operation which provides the client a method of
   querying the server about its policies regarding which security
   mechanisms must be used for access to the server's file system
   resources.  With this, the client can securely match the security
   mechanism that meets the policies specified at both the client and
   server.

1.1.2.  Procedure and Operation Structure

   A significant departure from the previous versions of the NFS
   protocol is the introduction of the COMPOUND procedure.  For the NFS
   version 4 protocol, there are two RPC procedures, NULL and COMPOUND.
   The COMPOUND procedure is defined in terms of operations and these
   operations correspond more closely to the traditional NFS procedures.
   With the use of the COMPOUND procedure, the client is able to build
   simple or complex requests.  These COMPOUND requests allow for a
   reduction in the number of RPCs needed for logical file system
   operations.  For example, without previous contact with a server a
   client will be able to read data from a file in one request by
   combining LOOKUP, OPEN, and READ operations in a single COMPOUND RPC.
   With previous versions of the NFS protocol, this type of single
   request was not possible.

   The model used for COMPOUND is very simple.  There is no logical OR
   or ANDing of operations.  The operations combined within a COMPOUND
   request are evaluated in order by the server.  Once an operation
   returns a failing result, the evaluation ends and the results of all
   evaluated operations are returned to the client.

   The NFS version 4 protocol continues to have the client refer to a
   file or directory at the server by a "filehandle".  The COMPOUND
   procedure has a method of passing a filehandle from one operation to
   another within the sequence of operations.  There is a concept of a
   "current filehandle" and "saved filehandle".  Most operations use the
   "current filehandle" as the file system object to operate upon.  The
   "saved filehandle" is used as temporary filehandle storage within a
   COMPOUND procedure as well as an additional operand for certain
   operations.







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1.1.3.  File System Model

   The general file system model used for the NFS version 4 protocol is
   the same as previous versions.  The server file system is
   hierarchical with the regular files contained within being treated as
   opaque byte streams.  In a slight departure, file and directory names
   are encoded with UTF-8 to deal with the basics of
   internationalization.

   The NFS version 4 protocol does not require a separate protocol to
   provide for the initial mapping between path name and filehandle.
   Instead of using the older MOUNT protocol for this mapping, the
   server provides a ROOT filehandle that represents the logical root or
   top of the file system tree provided by the server.  The server
   provides multiple file systems by gluing them together with pseudo
   file systems.  These pseudo file systems provide for potential gaps
   in the path names between real file systems.

1.1.3.1.  Filehandle Types

   In previous versions of the NFS protocol, the filehandle provided by
   the server was guaranteed to be valid or persistent for the lifetime
   of the file system object to which it referred.  For some server
   implementations, this persistence requirement has been difficult to
   meet.  For the NFS version 4 protocol, this requirement has been
   relaxed by introducing another type of filehandle, volatile.  With
   persistent and volatile filehandle types, the server implementation
   can match the abilities of the file system at the server along with
   the operating environment.  The client will have knowledge of the
   type of filehandle being provided by the server and can be prepared
   to deal with the semantics of each.

1.1.3.2.  Attribute Types

   The NFS version 4 protocol introduces three classes of file system or
   file attributes.  Like the additional filehandle type, the
   classification of file attributes has been done to ease server
   implementations along with extending the overall functionality of the
   NFS protocol.  This attribute model is structured to be extensible
   such that new attributes can be introduced in minor revisions of the
   protocol without requiring significant rework.

   The three classifications are: mandatory, recommended and named
   attributes.  This is a significant departure from the previous
   attribute model used in the NFS protocol.  Previously, the attributes
   for the file system and file objects were a fixed set of mainly Unix
   attributes.  If the server or client did not support a particular
   attribute, it would have to simulate the attribute the best it could.



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   Mandatory attributes are the minimal set of file or file system
   attributes that must be provided by the server and must be properly
   represented by the server.  Recommended attributes represent
   different file system types and operating environments.  The
   recommended attributes will allow for better interoperability and the
   inclusion of more operating environments.  The mandatory and
   recommended attribute sets are traditional file or file system
   attributes.  The third type of attribute is the named attribute.  A
   named attribute is an opaque byte stream that is associated with a
   directory or file and referred to by a string name.  Named attributes
   are meant to be used by client applications as a method to associate
   application specific data with a regular file or directory.

   One significant addition to the recommended set of file attributes is
   the Access Control List (ACL) attribute.  This attribute provides for
   directory and file access control beyond the model used in previous
   versions of the NFS protocol.  The ACL definition allows for
   specification of user and group level access control.

1.1.3.3.  File System Replication and Migration

   With the use of a special file attribute, the ability to migrate or
   replicate server file systems is enabled within the protocol.  The
   file system locations attribute provides a method for the client to
   probe the server about the location of a file system.  In the event
   of a migration of a file system, the client will receive an error
   when operating on the file system and it can then query as to the new
   file system location.  Similar steps are used for replication, the
   client is able to query the server for the multiple available
   locations of a particular file system.  From this information, the
   client can use its own policies to access the appropriate file system
   location.

1.1.4.  OPEN and CLOSE

   The NFS version 4 protocol introduces OPEN and CLOSE operations.  The
   OPEN operation provides a single point where file lookup, creation,
   and share semantics can be combined.  The CLOSE operation also
   provides for the release of state accumulated by OPEN.

1.1.5.  File locking

   With the NFS version 4 protocol, the support for byte range file
   locking is part of the NFS protocol.  The file locking support is
   structured so that an RPC callback mechanism is not required.  This
   is a departure from the previous versions of the NFS file locking
   protocol, Network Lock Manager (NLM).  The state associated with file
   locks is maintained at the server under a lease-based model.  The



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   server defines a single lease period for all state held by a NFS
   client.  If the client does not renew its lease within the defined
   period, all state associated with the client's lease may be released
   by the server.  The client may renew its lease with use of the RENEW
   operation or implicitly by use of other operations (primarily READ).

1.1.6.  Client Caching and Delegation

   The file, attribute, and directory caching for the NFS version 4
   protocol is similar to previous versions.  Attributes and directory
   information are cached for a duration determined by the client.  At
   the end of a predefined timeout, the client will query the server to
   see if the related file system object has been updated.

   For file data, the client checks its cache validity when the file is
   opened.  A query is sent to the server to determine if the file has
   been changed.  Based on this information, the client determines if
   the data cache for the file should kept or released.  Also, when the
   file is closed, any modified data is written to the server.

   If an application wants to serialize access to file data, file
   locking of the file data ranges in question should be used.

   The major addition to NFS version 4 in the area of caching is the
   ability of the server to delegate certain responsibilities to the
   client.  When the server grants a delegation for a file to a client,
   the client is guaranteed certain semantics with respect to the
   sharing of that file with other clients.  At OPEN, the server may
   provide the client either a read or write delegation for the file.
   If the client is granted a read delegation, it is assured that no
   other client has the ability to write to the file for the duration of
   the delegation.  If the client is granted a write delegation, the
   client is assured that no other client has read or write access to
   the file.

   Delegations can be recalled by the server.  If another client
   requests access to the file in such a way that the access conflicts
   with the granted delegation, the server is able to notify the initial
   client and recall the delegation.  This requires that a callback path
   exist between the server and client.  If this callback path does not
   exist, then delegations can not be granted.  The essence of a
   delegation is that it allows the client to locally service operations
   such as OPEN, CLOSE, LOCK, LOCKU, READ, WRITE without immediate
   interaction with the server.







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1.2.  General Definitions

   The following definitions are provided for the purpose of providing
   an appropriate context for the reader.

   Client    The "client" is the entity that accesses the NFS server's
             resources.  The client may be an application which contains
             the logic to access the NFS server directly.  The client
             may also be the traditional operating system client remote
             file system services for a set of applications.

             In the case of file locking the client is the entity that
             maintains a set of locks on behalf of one or more
             applications.  This client is responsible for crash or
             failure recovery for those locks it manages.

             Note that multiple clients may share the same transport and
             multiple clients may exist on the same network node.

   Clientid  A 64-bit quantity used as a unique, short-hand reference to
             a client supplied Verifier and ID.  The server is
             responsible for supplying the Clientid.

   Lease     An interval of time defined by the server for which the
             client is irrevocably granted a lock.  At the end of a
             lease period the lock may be revoked if the lease has not
             been extended.  The lock must be revoked if a conflicting
             lock has been granted after the lease interval.

             All leases granted by a server have the same fixed
             interval.  Note that the fixed interval was chosen to
             alleviate the expense a server would have in maintaining
             state about variable length leases across server failures.

   Lock      The term "lock" is used to refer to both record (byte-
             range) locks as well as file (share) locks unless
             specifically stated otherwise.

   Server    The "Server" is the entity responsible for coordinating
             client access to a set of file systems.

   Stable Storage
             NFS version 4 servers must be able to recover without data
             loss from multiple power failures (including cascading
             power failures, that is, several power failures in quick
             succession), operating system failures, and hardware
             failure of components other than the storage medium itself
             (for example, disk, nonvolatile RAM).



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             Some examples of stable storage that are allowable for an
             NFS server include:

             1. Media commit of data, that is, the modified data has
                been successfully written to the disk media, for
                example, the disk platter.

             2. An immediate reply disk drive with battery-backed on-
                drive intermediate storage or uninterruptible power
                system (UPS).

             3. Server commit of data with battery-backed intermediate
                storage and recovery software.

             4. Cache commit with uninterruptible power system (UPS) and
                recovery software.

   Stateid   A 64-bit quantity returned by a server that uniquely
             defines the locking state granted by the server for a
             specific lock owner for a specific file.

             Stateids composed of all bits 0 or all bits 1 have special
             meaning and are reserved values.

   Verifier  A 64-bit quantity generated by the client that the server
             can use to determine if the client has restarted and lost
             all previous lock state.

2.  Protocol Data Types

   The syntax and semantics to describe the data types of the NFS
   version 4 protocol are defined in the XDR [RFC1832] and RPC [RFC1831]
   documents.  The next sections build upon the XDR data types to define
   types and structures specific to this protocol.

2.1.  Basic Data Types

   Data Type     Definition
   _____________________________________________________________________
   int32_t       typedef int             int32_t;

   uint32_t      typedef unsigned int    uint32_t;

   int64_t       typedef hyper           int64_t;

   uint64_t      typedef unsigned hyper  uint64_t;





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RFC 3010                 NFS version 4 Protocol            December 2000


   attrlist4     typedef opaque        attrlist4<>;
                 Used for file/directory attributes

   bitmap4       typedef uint32_t        bitmap4<>;
                 Used in attribute array encoding.

   changeid4     typedef       uint64_t        changeid4;
                 Used in definition of change_info

   clientid4     typedef uint64_t        clientid4;
                 Shorthand reference to client identification

   component4    typedef utf8string      component4;
                 Represents path name components

   count4        typedef uint32_t        count4;
                 Various count parameters (READ, WRITE, COMMIT)

   length4       typedef uint64_t        length4;
                 Describes LOCK lengths

   linktext4     typedef utf8string      linktext4;
                 Symbolic link contents

   mode4         typedef uint32_t        mode4;
                 Mode attribute data type

   nfs_cookie4   typedef uint64_t        nfs_cookie4;
                 Opaque cookie value for READDIR

   nfs_fh4       typedef opaque          nfs_fh4<NFS4_FHSIZE>;
                 Filehandle definition; NFS4_FHSIZE is defined as 128

   nfs_ftype4    enum nfs_ftype4;
                 Various defined file types

   nfsstat4      enum nfsstat4;
                 Return value for operations

   offset4       typedef uint64_t        offset4;
                 Various offset designations (READ, WRITE, LOCK, COMMIT)

   pathname4     typedef component4      pathname4<>;
                 Represents path name for LOOKUP, OPEN and others

   qop4          typedef uint32_t        qop4;
                 Quality of protection designation in SECINFO




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RFC 3010                 NFS version 4 Protocol            December 2000


   sec_oid4      typedef opaque          sec_oid4<>;
                 Security Object Identifier
                 The sec_oid4 data type is not really opaque.
                 Instead contains an ASN.1 OBJECT IDENTIFIER as used
                 by GSS-API in the mech_type argument to
                 GSS_Init_sec_context.  See [RFC2078] for details.

   seqid4        typedef uint32_t        seqid4;
                 Sequence identifier used for file locking

   stateid4      typedef uint64_t        stateid4;
                 State identifier used for file locking and delegation

   utf8string    typedef opaque          utf8string<>;
                 UTF-8 encoding for strings

   verifier4     typedef opaque        verifier4[NFS4_VERIFIER_SIZE];
                 Verifier used for various operations (COMMIT, CREATE,
                 OPEN, READDIR, SETCLIENTID, WRITE)
                 NFS4_VERIFIER_SIZE is defined as 8

2.2.  Structured Data Types

   nfstime4
                  struct nfstime4 {
                          int64_t seconds;
                          uint32_t nseconds;
                  }

      The nfstime4 structure gives the number of seconds and nanoseconds
      since midnight or 0 hour January 1, 1970 Coordinated Universal
      Time (UTC).  Values greater than zero for the seconds field denote
      dates after the 0 hour January 1, 1970.  Values less than zero for
      the seconds field denote dates before the 0 hour January 1, 1970.
      In both cases, the nseconds field is to be added to the seconds
      field for the final time representation.  For example, if the time
      to be represented is one-half second before 0 hour January 1,
      1970, the seconds field would have a value of negative one (-1)
      and the nseconds fields would have a value of one-half second
      (500000000).  Values greater than 999,999,999 for nseconds are
      considered invalid.

      This data type is used to pass time and date information.  A
      server converts to and from its local representation of time when
      processing time values, preserving as much accuracy as possible.
      If the precision of timestamps stored for a file system object is





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RFC 3010                 NFS version 4 Protocol            December 2000


      less than defined, loss of precision can occur.  An adjunct time
      maintenance protocol is recommended to reduce client and server
      time skew.

   time_how4

                  enum time_how4 {
                          SET_TO_SERVER_TIME4 = 0,
                          SET_TO_CLIENT_TIME4 = 1
                  };


   settime4

                  union settime4 switch (time_how4 set_it) {
                   case SET_TO_CLIENT_TIME4:
                           nfstime4       time;
                   default:
                           void;
                  };

        The above definitions are used as the attribute definitions to
        set time values.  If set_it is SET_TO_SERVER_TIME4, then the
        server uses its local representation of time for the time value.


   specdata4

                  struct specdata4 {
                          uint32_t specdata1;
                          uint32_t specdata2;
                  };

        This data type represents additional information for the device
        file types NF4CHR and NF4BLK.


   fsid4

                  struct fsid4 {
                    uint64_t        major;
                    uint64_t        minor;
                  };

        This type is the file system identifier that is used as a
        mandatory attribute.





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RFC 3010                 NFS version 4 Protocol            December 2000


   fs_location4

                  struct fs_location4 {
                          utf8string    server<>;
                          pathname4     rootpath;
                  };


   fs_locations4

                  struct fs_locations4 {
                          pathname4     fs_root;
                          fs_location4  locations<>;
                  };

        The fs_location4 and fs_locations4 data types are used for the
        fs_locations recommended attribute which is used for migration
        and replication support.


   fattr4

                  struct fattr4 {
                          bitmap4       attrmask;
                          attrlist4     attr_vals;
                  };

        The fattr4 structure is used to represent file and directory
        attributes.

        The bitmap is a counted array of 32 bit integers used to contain
        bit values.  The position of the integer in the array that
        contains bit n can be computed from the expression (n / 32) and
        its bit within that integer is (n mod 32).

                                      0            1
                    +-----------+-----------+-----------+--
                    |  count    | 31  ..  0 | 63  .. 32 |
                    +-----------+-----------+-----------+--


   change_info4

                  struct change_info4 {
                          bool          atomic;
                          changeid4     before;
                          changeid4     after;
                  };



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RFC 3010                 NFS version 4 Protocol            December 2000


        This structure is used with the CREATE, LINK, REMOVE, RENAME
        operations to let the client the know value of the change
        attribute for the directory in which the target file system
        object resides.


   clientaddr4

                  struct clientaddr4 {
                          /* see struct rpcb in RFC 1833 */
                          string r_netid<>;    /* network id */
                          string r_addr<>;     /* universal address */
                  };

        The clientaddr4 structure is used as part of the SETCLIENT
        operation to either specify the address of the client that is
        using a clientid or as part of the call back registration.


   cb_client4

                  struct cb_client4 {
                          unsigned int  cb_program;
                          clientaddr4   cb_location;
                  };

        This structure is used by the client to inform the server of its
        call back address; includes the program number and client
        address.


   nfs_client_id4

                  struct nfs_client_id4 {
                          verifier4     verifier;
                          opaque        id<>;
                  };

        This structure is part of the arguments to the SETCLIENTID
        operation.


   nfs_lockowner4

                  struct nfs_lockowner4 {
                          clientid4     clientid;
                          opaque        owner<>;
                  };



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RFC 3010                 NFS version 4 Protocol            December 2000


        This structure is used to identify the owner of a OPEN share or
        file lock.

3.  RPC and Security Flavor

   The NFS version 4 protocol is a Remote Procedure Call (RPC)
   application that uses RPC version 2 and the corresponding eXternal
   Data Representation (XDR) as defined in [RFC1831] and [RFC1832].  The
   RPCSEC_GSS security flavor as defined in [RFC2203] MUST be used as
   the mechanism to deliver stronger security for the NFS version 4
   protocol.

3.1.  Ports and Transports

   Historically, NFS version 2 and version 3 servers have resided on
   port 2049.  The registered port 2049 [RFC1700] for the NFS protocol
   should be the default configuration.  Using the registered port for
   NFS services means the NFS client will not need to use the RPC
   binding protocols as described in [RFC1833]; this will allow NFS to
   transit firewalls.

   The transport used by the RPC service for the NFS version 4 protocol
   MUST provide congestion control comparable to that defined for TCP in
   [RFC2581].  If the operating environment implements TCP, the NFS
   version 4 protocol SHOULD be supported over TCP.  The NFS client and
   server may use other transports if they support congestion control as
   defined above and in those cases a mechanism may be provided to
   override TCP usage in favor of another transport.

   If TCP is used as the transport, the client and server SHOULD use
   persistent connections.  This will prevent the weakening of TCP's
   congestion control via short lived connections and will improve
   performance for the WAN environment by eliminating the need for SYN
   handshakes.

   Note that for various timers, the client and server should avoid
   inadvertent synchronization of those timers.  For further discussion
   of the general issue refer to [Floyd].

3.2.  Security Flavors

   Traditional RPC implementations have included AUTH_NONE, AUTH_SYS,
   AUTH_DH, and AUTH_KRB4 as security flavors.  With [RFC2203] an
   additional security flavor of RPCSEC_GSS has been introduced which
   uses the functionality of GSS-API [RFC2078].  This allows for the use
   of varying security mechanisms by the RPC layer without the
   additional implementation overhead of adding RPC security flavors.
   For NFS version 4, the RPCSEC_GSS security flavor MUST be used to



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RFC 3010                 NFS version 4 Protocol            December 2000


   enable the mandatory security mechanism.  Other flavors, such as,
   AUTH_NONE, AUTH_SYS, and AUTH_DH MAY be implemented as well.

3.2.1.  Security mechanisms for NFS version 4

   The use of RPCSEC_GSS requires selection of: mechanism, quality of
   protection, and service (authentication, integrity, privacy).  The
   remainder of this document will refer to these three parameters of
   the RPCSEC_GSS security as the security triple.

3.2.1.1.  Kerberos V5 as security triple

   The Kerberos V5 GSS-API mechanism as described in [RFC1964] MUST be
   implemented and provide the following security triples.

   column descriptions:

   1 == number of pseudo flavor
   2 == name of pseudo flavor
   3 == mechanism's OID
   4 == mechanism's algorithm(s)
   5 == RPCSEC_GSS service

1      2     3                    4              5
-----------------------------------------------------------------------
390003 krb5  1.2.840.113554.1.2.2 DES MAC MD5    rpc_gss_svc_none
390004 krb5i 1.2.840.113554.1.2.2 DES MAC MD5    rpc_gss_svc_integrity
390005 krb5p 1.2.840.113554.1.2.2 DES MAC MD5    rpc_gss_svc_privacy
                                  for integrity,
                                  and 56 bit DES
                                  for privacy.

   Note that the pseudo flavor is presented here as a mapping aid to the
   implementor.  Because this NFS protocol includes a method to
   negotiate security and it understands the GSS-API mechanism, the
   pseudo flavor is not needed.  The pseudo flavor is needed for NFS
   version 3 since the security negotiation is done via the MOUNT
   protocol.

   For a discussion of NFS' use of RPCSEC_GSS and Kerberos V5, please
   see [RFC2623].

3.2.1.2.  LIPKEY as a security triple

   The LIPKEY GSS-API mechanism as described in [RFC2847] MUST be
   implemented and provide the following security triples. The
   definition of the columns matches the previous subsection "Kerberos
   V5 as security triple"



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RFC 3010                 NFS version 4 Protocol            December 2000


1      2        3                    4              5
-----------------------------------------------------------------------
390006 lipkey   1.3.6.1.5.5.9        negotiated  rpc_gss_svc_none
390007 lipkey-i 1.3.6.1.5.5.9        negotiated  rpc_gss_svc_integrity
390008 lipkey-p 1.3.6.1.5.5.9        negotiated  rpc_gss_svc_privacy

   The mechanism algorithm is listed as "negotiated".  This is because
   LIPKEY is layered on SPKM-3 and in SPKM-3 [RFC2847] the
   confidentiality and integrity algorithms are negotiated.  Since
   SPKM-3 specifies HMAC-MD5 for integrity as MANDATORY, 128 bit
   cast5CBC for confidentiality for privacy as MANDATORY, and further
   specifies that HMAC-MD5 and cast5CBC MUST be listed first before
   weaker algorithms, specifying "negotiated" in column 4 does not
   impair interoperability.  In the event an SPKM-3 peer does not
   support the mandatory algorithms, the other peer is free to accept or
   reject the GSS-API context creation.

   Because SPKM-3 negotiates the algorithms, subsequent calls to
   LIPKEY's GSS_Wrap() and GSS_GetMIC() by RPCSEC_GSS will use a quality
   of protection value of 0 (zero).  See section 5.2 of [RFC2025] for an
   explanation.

   LIPKEY uses SPKM-3 to create a secure channel in which to pass a user
   name and password from the client to the user.  Once the user name
   and password have been accepted by the server, calls to the LIPKEY
   context are redirected to the SPKM-3 context.  See [RFC2847] for more
   details.

3.2.1.3.  SPKM-3 as a security triple

   The SPKM-3 GSS-API mechanism as described in [RFC2847] MUST be
   implemented and provide the following security triples. The
   definition of the columns matches the previous subsection "Kerberos
   V5 as security triple".

1      2        3                    4              5
-----------------------------------------------------------------------
390009 spkm3    1.3.6.1.5.5.1.3      negotiated  rpc_gss_svc_none
390010 spkm3i   1.3.6.1.5.5.1.3      negotiated  rpc_gss_svc_integrity
390011 spkm3p   1.3.6.1.5.5.1.3      negotiated  rpc_gss_svc_privacy

   For a discussion as to why the mechanism algorithm is listed as
   "negotiated", see the previous section "LIPKEY as a security triple."

   Because SPKM-3 negotiates the algorithms, subsequent calls to SPKM-
   3's GSS_Wrap() and GSS_GetMIC() by RPCSEC_GSS will use a quality of
   protection value of 0 (zero).  See section 5.2 of [RFC2025] for an
   explanation.



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RFC 3010                 NFS version 4 Protocol            December 2000


   Even though LIPKEY is layered over SPKM-3, SPKM-3 is specified as a
   mandatory set of triples to handle the situations where the initiator
   (the client) is anonymous or where the initiator has its own
   certificate.  If the initiator is anonymous, there will not be a user
   name and password to send to the target (the server).  If the
   initiator has its own certificate, then using passwords is
   superfluous.

3.3.  Security Negotiation

   With the NFS version 4 server potentially offering multiple security
   mechanisms, the client needs a method to determine or negotiate which
   mechanism is to be used for its communication with the server.  The
   NFS server may have multiple points within its file system name space
   that are available for use by NFS clients.  In turn the NFS server
   may be configured such that each of these entry points may have
   different or multiple security mechanisms in use.

   The security negotiation between client and server must be done with
   a secure channel to eliminate the possibility of a third party
   intercepting the negotiation sequence and forcing the client and
   server to choose a lower level of security than required or desired.

3.3.1.  Security Error

   Based on the assumption that each NFS version 4 client and server
   must support a minimum set of security (i.e. LIPKEY, SPKM-3, and
   Kerberos-V5 all under RPCSEC_GSS), the NFS client will start its
   communication with the server with one of the minimal security
   triples.  During communication with the server, the client may
   receive an NFS error of NFS4ERR_WRONGSEC.  This error allows the
   server to notify the client that the security triple currently being
   used is not appropriate for access to the server's file system
   resources.  The client is then responsible for determining what
   security triples are available at the server and choose one which is
   appropriate for the client.

3.3.2.  SECINFO

   The new SECINFO operation will allow the client to determine, on a
   per filehandle basis, what security triple is to be used for server
   access.  In general, the client will not have to use the SECINFO
   procedure except during initial communication with the server or when
   the client crosses policy boundaries at the server.  It is possible
   that the server's policies change during the client's interaction
   therefore forcing the client to negotiate a new security triple.





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RFC 3010                 NFS version 4 Protocol            December 2000


3.4.  Callback RPC Authentication

   The callback RPC (described later) must mutually authenticate the NFS
   server to the principal that acquired the clientid (also described
   later), using the same security flavor the original SETCLIENTID
   operation used. Because LIPKEY is layered over SPKM-3, it is
   permissible for the server to use SPKM-3 and not LIPKEY for the
   callback even if the client used LIPKEY for SETCLIENTID.

   For AUTH_NONE, there are no principals, so this is a non-issue.

   For AUTH_SYS, the server simply uses the AUTH_SYS credential that the
   user used when it set up the delegation.

   For AUTH_DH, one commonly used convention is that the server uses the
   credential corresponding to this AUTH_DH principal:

         unix.host@domain

   where host and domain are variables corresponding to the name of
   server host and directory services domain in which it lives such as a
   Network Information System domain or a DNS domain.

   Regardless of what security mechanism under RPCSEC_GSS is being used,
   the NFS server, MUST identify itself in GSS-API via a
   GSS_C_NT_HOSTBASED_SERVICE name type.  GSS_C_NT_HOSTBASED_SERVICE
   names are of the form:

         service@hostname

   For NFS, the "service" element is

         nfs

   Implementations of security mechanisms will convert nfs@hostname to
   various different forms. For Kerberos V5 and LIPKEY, the following
   form is RECOMMENDED:

         nfs/hostname

   For Kerberos V5, nfs/hostname would be a server principal in the
   Kerberos Key Distribution Center database.  For LIPKEY, this would be
   the username passed to the target (the NFS version 4 client that
   receives the callback).

   It should be noted that LIPKEY may not work for callbacks, since the
   LIPKEY client uses a user id/password.  If the NFS client receiving
   the callback can authenticate the NFS server's user name/password



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RFC 3010                 NFS version 4 Protocol            December 2000


   pair, and if the user that the NFS server is authenticating to has a
   public key certificate, then it works.

   In situations where NFS client uses LIPKEY and uses a per-host
   principal for the SETCLIENTID operation, instead of using LIPKEY for
   SETCLIENTID, it is RECOMMENDED that SPKM-3 with mutual authentication
   be used.  This effectively means that the client will use a
   certificate to authenticate and identify the initiator to the target
   on the NFS server.  Using SPKM-3 and not LIPKEY has the following
   advantages:

   o  When the server does a callback, it must authenticate to the
      principal used in the SETCLIENTID.  Even if LIPKEY is used,
      because LIPKEY is layered over SPKM-3, the NFS client will need to
      have a certificate that corresponds to the principal used in the
      SETCLIENTID operation.  From an administrative perspective, having
      a user name, password, and certificate for both the client and
      server is redundant.

   o  LIPKEY was intended to minimize additional infrastructure
      requirements beyond a certificate for the target, and the
      expectation is that existing password infrastructure can be
      leveraged for the initiator.  In some environments, a per-host
      password does not exist yet.  If certificates are used for any
      per-host principals, then additional password infrastructure is
      not needed.

   o  In cases when a host is both an NFS client and server, it can
      share the same per-host certificate.

4.  Filehandles

   The filehandle in the NFS protocol is a per server unique identifier
   for a file system object.  The contents of the filehandle are opaque
   to the client.  Therefore, the server is responsible for translating
   the filehandle to an internal representation of the file system
   object.  Since the filehandle is the client's reference to an object
   and the client may cache this reference, the server SHOULD not reuse
   a filehandle for another file system object.  If the server needs to
   reuse a filehandle value, the time elapsed before reuse SHOULD be
   large enough such that it is unlikely the client has a cached copy of
   the reused filehandle value.  Note that a client may cache a
   filehandle for a very long time.  For example, a client may cache NFS
   data to local storage as a method to expand its effective cache size
   and as a means to survive client restarts.  Therefore, the lifetime
   of a cached filehandle may be extended.





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RFC 3010                 NFS version 4 Protocol            December 2000


4.1.  Obtaining the First Filehandle

   The operations of the NFS protocol are defined in terms of one or
   more filehandles.  Therefore, the client needs a filehandle to
   initiate communication with the server.  With the NFS version 2
   protocol [RFC1094] and the NFS version 3 protocol [RFC1813], there
   exists an ancillary protocol to obtain this first filehandle.  The
   MOUNT protocol, RPC program number 100005, provides the mechanism of
   translating a string based file system path name to a filehandle
   which can then be used by the NFS protocols.

   The MOUNT protocol has deficiencies in the area of security and use
   via firewalls.  This is one reason that the use of the public
   filehandle was introduced in [RFC2054] and [RFC2055].  With the use
   of the public filehandle in combination with the LOOKUP procedure in
   the NFS version 2 and 3 protocols, it has been demonstrated that the
   MOUNT protocol is unnecessary for viable interaction between NFS
   client and server.

   Therefore, the NFS version 4 protocol will not use an ancillary
   protocol for translation from string based path names to a
   filehandle.  Two special filehandles will be used as starting points
   for the NFS client.

4.1.1.  Root Filehandle

   The first of the special filehandles is the ROOT filehandle.  The
   ROOT filehandle is the "conceptual" root of the file system name
   space at the NFS server.  The client uses or starts with the ROOT
   filehandle by employing the PUTROOTFH operation.  The PUTROOTFH
   operation instructs the server to set the "current" filehandle to the
   ROOT of the server's file tree.  Once this PUTROOTFH operation is
   used, the client can then traverse the entirety of the server's file
   tree with the LOOKUP procedure.  A complete discussion of the server
   name space is in the section "NFS Server Name Space".

4.1.2.  Public Filehandle

   The second special filehandle is the PUBLIC filehandle.  Unlike the
   ROOT filehandle, the PUBLIC filehandle may be bound or represent an
   arbitrary file system object at the server.  The server is
   responsible for this binding.  It may be that the PUBLIC filehandle
   and the ROOT filehandle refer to the same file system object.
   However, it is up to the administrative software at the server and
   the policies of the server administrator to define the binding of the
   PUBLIC filehandle and server file system object.  The client may not
   make any assumptions about this binding.




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RFC 3010                 NFS version 4 Protocol            December 2000


4.2.  Filehandle Types

   In the NFS version 2 and 3 protocols, there was one type of
   filehandle with a single set of semantics.  The NFS version 4
   protocol introduces a new type of filehandle in an attempt to
   accommodate certain server environments.  The first type of
   filehandle is 'persistent'.  The semantics of a persistent filehandle
   are the same as the filehandles of the NFS version 2 and 3 protocols.
   The second or new type of filehandle is the "volatile" filehandle.

   The volatile filehandle type is being introduced to address server
   functionality or implementation issues which make correct
   implementation of a persistent filehandle infeasible.  Some server
   environments do not provide a file system level invariant that can be
   used to construct a persistent filehandle.  The underlying server
   file system may not provide the invariant or the server's file system
   programming interfaces may not provide access to the needed
   invariant.  Volatile filehandles may ease the implementation of
   server functionality such as hierarchical storage management or file
   system reorganization or migration.  However, the volatile filehandle
   increases the implementation burden for the client.  However this
   increased burden is deemed acceptable based on the overall gains
   achieved by the protocol.

   Since the client will need to handle persistent and volatile
   filehandle differently, a file attribute is defined which may be used
   by the client to determine the filehandle types being returned by the
   server.

4.2.1.  General Properties of a Filehandle

   The filehandle contains all the information the server needs to
   distinguish an individual file.  To the client, the filehandle is
   opaque. The client stores filehandles for use in a later request and
   can compare two filehandles from the same server for equality by
   doing a byte-by-byte comparison.  However, the client MUST NOT
   otherwise interpret the contents of filehandles.  If two filehandles
   from the same server are equal, they MUST refer to the same file.  If
   they are not equal, the client may use information provided by the
   server, in the form of file attributes, to determine whether they
   denote the same files or different files.  The client would do this
   as necessary for client side caching.  Servers SHOULD try to maintain
   a one-to-one correspondence between filehandles and files but this is
   not required.  Clients MUST use filehandle comparisons only to
   improve performance, not for correct behavior.  All clients need to
   be prepared for situations in which it cannot be determined whether
   two filehandles denote the same object and in such cases, avoid
   making invalid assumptions which might cause incorrect behavior.



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RFC 3010                 NFS version 4 Protocol            December 2000


   Further discussion of filehandle and attribute comparison in the
   context of data caching is presented in the section "Data Caching and
   File Identity".

   As an example, in the case that two different path names when
   traversed at the server terminate at the same file system object, the
   server SHOULD return the same filehandle for each path.  This can
   occur if a hard link is used to create two file names which refer to
   the same underlying file object and associated data.  For example, if
   paths /a/b/c and /a/d/c refer to the same file, the server SHOULD
   return the same filehandle for both path names traversals.

4.2.2.  Persistent Filehandle

   A persistent filehandle is defined as having a fixed value for the
   lifetime of the file system object to which it refers.  Once the
   server creates the filehandle for a file system object, the server
   MUST accept the same filehandle for the object for the lifetime of
   the object.  If the server restarts or reboots the NFS server must
   honor the same filehandle value as it did in the server's previous
   instantiation.  Similarly, if the file system is migrated, the new
   NFS server must honor the same file handle as the old NFS server.

   The persistent filehandle will be become stale or invalid when the
   file system object is removed.  When the server is presented with a
   persistent filehandle that refers to a deleted object, it MUST return
   an error of NFS4ERR_STALE.  A filehandle may become stale when the
   file system containing the object is no longer available.  The file
   system may become unavailable if it exists on removable media and the
   media is no longer available at the server or the file system in
   whole has been destroyed or the file system has simply been removed
   from the server's name space (i.e. unmounted in a Unix environment).

4.2.3.  Volatile Filehandle

   A volatile filehandle does not share the same longevity
   characteristics of a persistent filehandle.  The server may determine
   that a volatile filehandle is no longer valid at many different
   points in time.  If the server can definitively determine that a
   volatile filehandle refers to an object that has been removed, the
   server should return NFS4ERR_STALE to the client (as is the case for
   persistent filehandles).  In all other cases where the server
   determines that a volatile filehandle can no longer be used, it
   should return an error of NFS4ERR_FHEXPIRED.







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   The mandatory attribute "fh_expire_type" is used by the client to
   determine what type of filehandle the server is providing for a
   particular file system.  This attribute is a bitmask with the
   following values:

   FH4_PERSISTENT
         The value of FH4_PERSISTENT is used to indicate a persistent
         filehandle, which is valid until the object is removed from the
         file system.  The server will not return NFS4ERR_FHEXPIRED for
         this filehandle.  FH4_PERSISTENT is defined as a value in which
         none of the bits specified below are set.

   FH4_NOEXPIRE_WITH_OPEN
         The filehandle will not expire while client has the file open.
         If this bit is set, then the values FH4_VOLATILE_ANY or
         FH4_VOL_RENAME do not impact expiration while the file is open.
         Once the file is closed or if the FH4_NOEXPIRE_WITH_OPEN bit is
         false, the rest of the volatile related bits apply.

   FH4_VOLATILE_ANY
         The filehandle may expire at any time and will expire during
         system migration and rename.

   FH4_VOL_MIGRATION
         The filehandle will expire during file system migration.  May
         only be set if FH4_VOLATILE_ANY is not set.

   FH4_VOL_RENAME
         The filehandle may expire due to a rename.  This includes a
         rename by the requesting client or a rename by another client.
         May only be set if FH4_VOLATILE_ANY is not set.

   Servers which provide volatile filehandles should deny a RENAME or
   REMOVE that would affect an OPEN file or any of the components
   leading to the OPEN file.  In addition, the server should deny all
   RENAME or REMOVE requests during the grace or lease period upon
   server restart.

   The reader may be wondering why there are three FH4_VOL* bits and why
   FH4_VOLATILE_ANY is exclusive of FH4_VOL_MIGRATION and
   FH4_VOL_RENAME.  If the a filehandle is normally persistent but
   cannot persist across a file set migration, then the presence of the
   FH4_VOL_MIGRATION or FH4_VOL_RENAME tells the client that it can
   treat the file handle as persistent for purposes of maintaining a
   file name to file handle cache, except for the specific event
   described by the bit.  However, FH4_VOLATILE_ANY tells the client
   that it should not maintain such a cache for unopened files.  A
   server MUST not present FH4_VOLATILE_ANY with FH4_VOL_MIGRATION or



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   FH4_VOL_RENAME as this will lead to confusion.  FH4_VOLATILE_ANY
   implies that the file handle will expire upon migration or rename, in
   addition to other events.

4.2.4.  One Method of Constructing a Volatile Filehandle

   As mentioned, in some instances a filehandle is stale (no longer
   valid; perhaps because the file was removed from the server) or it is
   expired (the underlying file is valid but since the filehandle is
   volatile, it may have expired).  Thus the server needs to be able to
   return NFS4ERR_STALE in the former case and NFS4ERR_FHEXPIRED in the
   latter case. This can be done by careful construction of the volatile
   filehandle.  One possible implementation follows.

   A volatile filehandle, while opaque to the client could contain:

   [volatile bit = 1 | server boot time | slot | generation number]

   o  slot is an index in the server volatile filehandle table

   o  generation number is the generation number for the table
      entry/slot

   If the server boot time is less than the current server boot time,
   return NFS4ERR_FHEXPIRED.  If slot is out of range, return
   NFS4ERR_BADHANDLE.  If the generation number does not match, return
   NFS4ERR_FHEXPIRED.

   When the server reboots, the table is gone (it is volatile).

   If volatile bit is 0, then it is a persistent filehandle with a
   different structure following it.

4.3.  Client Recovery from Filehandle Expiration

   If possible, the client SHOULD recover from the receipt of an
   NFS4ERR_FHEXPIRED error.  The client must take on additional
   responsibility so that it may prepare itself to recover from the
   expiration of a volatile filehandle.  If the server returns
   persistent filehandles, the client does not need these additional
   steps.

   For volatile filehandles, most commonly the client will need to store
   the component names leading up to and including the file system
   object in question.  With these names, the client should be able to
   recover by finding a filehandle in the name space that is still
   available or by starting at the root of the server's file system name
   space.



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   If the expired filehandle refers to an object that has been removed
   from the file system, obviously the client will not be able to
   recover from the expired filehandle.

   It is also possible that the expired filehandle refers to a file that
   has been renamed.  If the file was renamed by another client, again
   it is possible that the original client will not be able to recover.
   However, in the case that the client itself is renaming the file and
   the file is open, it is possible that the client may be able to
   recover.  The client can determine the new path name based on the
   processing of the rename request.  The client can then regenerate the
   new filehandle based on the new path name.  The client could also use
   the compound operation mechanism to construct a set of operations
   like:

            RENAME A B
            LOOKUP B
            GETFH

5.  File Attributes

   To meet the requirements of extensibility and increased
   interoperability with non-Unix platforms, attributes must be handled
   in a flexible manner.  The NFS Version 3 fattr3 structure contains a
   fixed list of attributes that not all clients and servers are able to
   support or care about.  The fattr3 structure can not be extended as
   new needs arise and it provides no way to indicate non-support.  With
   the NFS Version 4 protocol, the client will be able to ask what
   attributes the server supports and will be able to request only those
   attributes in which it is interested.

   To this end, attributes will be divided into three groups: mandatory,
   recommended, and named.  Both mandatory and recommended attributes
   are supported in the NFS version 4 protocol by a specific and well-
   defined encoding and are identified by number.  They are requested by
   setting a bit in the bit vector sent in the GETATTR request; the
   server response includes a bit vector to list what attributes were
   returned in the response.  New mandatory or recommended attributes
   may be added to the NFS protocol between major revisions by
   publishing a standards-track RFC which allocates a new attribute
   number value and defines the encoding for the attribute.  See the
   section "Minor Versioning" for further discussion.

   Named attributes are accessed by the new OPENATTR operation, which
   accesses a hidden directory of attributes associated with a file
   system object.  OPENATTR takes a filehandle for the object and
   returns the filehandle for the attribute hierarchy.  The filehandle
   for the named attributes is a directory object accessible by LOOKUP



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   or READDIR and contains files whose names represent the named
   attributes and whose data bytes are the value of the attribute.  For
   example:

         LOOKUP     "foo"       ; look up file
         GETATTR    attrbits
         OPENATTR               ; access foo's named attributes
         LOOKUP     "x11icon"   ; look up specific attribute
         READ       0,4096      ; read stream of bytes

   Named attributes are intended for data needed by applications rather
   than by an NFS client implementation.  NFS implementors are strongly
   encouraged to define their new attributes as recommended attributes
   by bringing them to the IETF standards-track process.

   The set of attributes which are classified as mandatory is
   deliberately small since servers must do whatever it takes to support
   them.  The recommended attributes may be unsupported; though a server
   should support as many as it can.  Attributes are deemed mandatory if
   the data is both needed by a large number of clients and is not
   otherwise reasonably computable by the client when support is not
   provided on the server.

5.1.  Mandatory Attributes

   These MUST be supported by every NFS Version 4 client and server in
   order to ensure a minimum level of interoperability.  The server must
   store and return these attributes and the client must be able to
   function with an attribute set limited to these attributes.  With
   just the mandatory attributes some client functionality may be
   impaired or limited in some ways.  A client may ask for any of these
   attributes to be returned by setting a bit in the GETATTR request and
   the server must return their value.

5.2.  Recommended Attributes

   These attributes are understood well enough to warrant support in the
   NFS Version 4 protocol.  However, they may not be supported on all
   clients and servers.  A client may ask for any of these attributes to
   be returned by setting a bit in the GETATTR request but must handle
   the case where the server does not return them.  A client may ask for
   the set of attributes the server supports and should not request
   attributes the server does not support.  A server should be tolerant
   of requests for unsupported attributes and simply not return them
   rather than considering the request an error.  It is expected that
   servers will support all attributes they comfortably can and only
   fail to support attributes which are difficult to support in their
   operating environments.  A server should provide attributes whenever



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   they don't have to "tell lies" to the client.  For example, a file
   modification time should be either an accurate time or should not be
   supported by the server.  This will not always be comfortable to
   clients but it seems that the client has a better ability to
   fabricate or construct an attribute or do without the attribute.

5.3.  Named Attributes

   These attributes are not supported by direct encoding in the NFS
   Version 4 protocol but are accessed by string names rather than
   numbers and correspond to an uninterpreted stream of bytes which are
   stored with the file system object.  The name space for these
   attributes may be accessed by using the OPENATTR operation.  The
   OPENATTR operation returns a filehandle for a virtual "attribute
   directory" and further perusal of the name space may be done using
   READDIR and LOOKUP operations on this filehandle.  Named attributes
   may then be examined or changed by normal READ and WRITE and CREATE
   operations on the filehandles returned from READDIR and LOOKUP.
   Named attributes may have attributes.

   It is recommended that servers support arbitrary named attributes.  A
   client should not depend on the ability to store any named attributes
   in the server's file system.  If a server does support named
   attributes, a client which is also able to handle them should be able
   to copy a file's data and meta-data with complete transparency from
   one location to another; this would imply that names allowed for
   regular directory entries are valid for named attribute names as
   well.

   Names of attributes will not be controlled by this document or other
   IETF standards track documents.  See the section "IANA
   Considerations" for further discussion.

5.4.  Mandatory Attributes - Definitions

   Name              #    DataType     Access   Description
   ___________________________________________________________________
   supp_attr         0    bitmap       READ     The bit vector which
                                                would retrieve all
                                                mandatory and
                                                recommended attributes
                                                that are supported for
                                                this object.

   type              1    nfs4_ftype   READ     The type of the object
                                                (file, directory,
                                                symlink)




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   fh_expire_type    2    uint32       READ     Server uses this to
                                                specify filehandle
                                                expiration behavior to
                                                the client.  See the
                                                section "Filehandles"
                                                for additional
                                                description.

   change            3    uint64       READ     A value created by the
                                                server that the client
                                                can use to determine
                                                if file data,
                                                directory contents or
                                                attributes of the
                                                object have been
                                                modified.  The server
                                                may return the
                                                object's time_modify
                                                attribute for this
                                                attribute's value but
                                                only if the file
                                                system object can not
                                                be updated more
                                                frequently than the
                                                resolution of
                                                time_modify.

   size              4    uint64       R/W      The size of the object
                                                in bytes.

   link_support      5    boolean      READ     Does the object's file
                                                system supports hard
                                                links?

   symlink_support   6    boolean      READ     Does the object's file
                                                system supports
                                                symbolic links?

   named_attr        7    boolean      READ     Does this object have
                                                named attributes?

   fsid              8    fsid4        READ     Unique file system
                                                identifier for the
                                                file system holding
                                                this object.  fsid
                                                contains major and
                                                minor components each
                                                of which are uint64.



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   unique_handles    9    boolean      READ     Are two distinct
                                                filehandles guaranteed
                                                to refer to two
                                                different file system
                                                objects?

   lease_time        10   nfs_lease4   READ     Duration of leases at
                                                server in seconds.

   rdattr_error      11   enum         READ     Error returned from
                                                getattr during
                                                readdir.

5.5.  Recommended Attributes - Definitions

   Name               #    Data Type      Access   Description
   _____________________________________________________________________
   ACL                12   nfsace4<>      R/W      The access control
                                                   list for the object.

   aclsupport         13   uint32         READ     Indicates what types
                                                   of ACLs are supported
                                                   on the current file
                                                   system.

   archive            14   boolean        R/W      Whether or not this
                                                   file has been
                                                   archived since the
                                                   time of last
                                                   modification
                                                   (deprecated in favor
                                                   of time_backup).

   cansettime         15   boolean        READ     Is the server able to
                                                   change the times for
                                                   a file system object
                                                   as specified in a
                                                   SETATTR operation?

   case_insensitive   16   boolean        READ     Are filename
                                                   comparisons on this
                                                   file system case
                                                   insensitive?

   case_preserving    17   boolean        READ     Is filename case on
                                                   this file system
                                                   preserved?




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   chown_restricted   18   boolean        READ     If TRUE, the server
                                                   will reject any
                                                   request to change
                                                   either the owner or
                                                   the group associated
                                                   with a file if the
                                                   caller is not a
                                                   privileged user (for
                                                   example, "root" in
                                                   Unix operating
                                                   environments or in NT
                                                   the "Take Ownership"
                                                   privilege)

   filehandle         19   nfs4_fh        READ     The filehandle of
                                                   this object
                                                   (primarily for
                                                   readdir requests).

   fileid             20   uint64         READ     A number uniquely
                                                   identifying the file
                                                   within the file
                                                   system.

   files_avail        21   uint64         READ     File slots available
                                                   to this user on the
                                                   file system
                                                   containing this
                                                   object - this should
                                                   be the smallest
                                                   relevant limit.

   files_free         22   uint64         READ     Free file slots on
                                                   the file system
                                                   containing this
                                                   object - this should
                                                   be the smallest
                                                   relevant limit.

   files_total        23   uint64         READ     Total file slots on
                                                   the file system
                                                   containing this
                                                   object.








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   fs_locations       24   fs_locations   READ     Locations where this
                                                   file system may be
                                                   found.  If the server
                                                   returns NFS4ERR_MOVED
                                                   as an error, this
                                                   attribute must be
                                                   supported.

   hidden             25   boolean        R/W      Is file considered
                                                   hidden with respect
                                                   to the WIN32 API?

   homogeneous        26   boolean        READ     Whether or not this
                                                   object's file system
                                                   is homogeneous, i.e.
                                                   are per file system
                                                   attributes the same
                                                   for all file system's
                                                   objects.

   maxfilesize        27   uint64         READ     Maximum supported
                                                   file size for the
                                                   file system of this
                                                   object.

   maxlink            28   uint32         READ     Maximum number of
                                                   links for this
                                                   object.

   maxname            29   uint32         READ     Maximum filename size
                                                   supported for this
                                                   object.

   maxread            30   uint64         READ     Maximum read size
                                                   supported for this
                                                   object.

   maxwrite           31   uint64         READ     Maximum write size
                                                   supported for this
                                                   object.  This
                                                   attribute SHOULD be
                                                   supported if the file
                                                   is writable.  Lack of
                                                   this attribute can
                                                   lead to the client
                                                   either wasting





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                                                   bandwidth or not
                                                   receiving the best
                                                   performance.

   mimetype           32   utf8<>         R/W      MIME body
                                                   type/subtype of this
                                                   object.

   mode               33   mode4          R/W      Unix-style permission
                                                   bits for this object
                                                   (deprecated in favor
                                                   of ACLs)

   no_trunc           34   boolean        READ     If a name longer than
                                                   name_max is used,
                                                   will an error be
                                                   returned or will the
                                                   name be truncated?

   numlinks           35   uint32         READ     Number of hard links
                                                   to this object.

   owner              36   utf8<>         R/W      The string name of
                                                   the owner of this
                                                   object.

   owner_group        37   utf8<>         R/W      The string name of
                                                   the group ownership
                                                   of this object.

   quota_avail_hard   38   uint64         READ     For definition see
                                                   "Quota Attributes"
                                                   section below.

   quota_avail_soft   39   uint64         READ     For definition see
                                                   "Quota Attributes"
                                                   section below.

   quota_used         40   uint64         READ     For definition see
                                                   "Quota Attributes"
                                                   section below.

   rawdev             41   specdata4      READ     Raw device
                                                   identifier.  Unix
                                                   device major/minor
                                                   node information.





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   space_avail        42   uint64         READ     Disk space in bytes
                                                   available to this
                                                   user on the file
                                                   system containing
                                                   this object - this
                                                   should be the
                                                   smallest relevant
                                                   limit.

   space_free         43   uint64         READ     Free disk space in
                                                   bytes on the file
                                                   system containing
                                                   this object - this
                                                   should be the
                                                   smallest relevant
                                                   limit.

   space_total        44   uint64         READ     Total disk space in
                                                   bytes on the file
                                                   system containing
                                                   this object.

   space_used         45   uint64         READ     Number of file system
                                                   bytes allocated to
                                                   this object.

   system             46   boolean        R/W      Is this file a system
                                                   file with respect to
                                                   the WIN32 API?

   time_access        47   nfstime4       READ     The time of last
                                                   access to the object.

   time_access_set    48   settime4       WRITE    Set the time of last
                                                   access to the object.
                                                   SETATTR use only.

   time_backup        49   nfstime4       R/W      The time of last
                                                   backup of the object.

   time_create        50   nfstime4       R/W      The time of creation
                                                   of the object. This
                                                   attribute does not
                                                   have any relation to
                                                   the traditional Unix
                                                   file attribute
                                                   "ctime" or "change
                                                   time".



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   time_delta         51   nfstime4       READ     Smallest useful
                                                   server time
                                                   granularity.

   time_metadata      52   nfstime4       R/W      The time of last
                                                   meta-data
                                                   modification of the
                                                   object.

   time_modify        53   nfstime4       READ     The time of last
                                                   modification to the
                                                   object.

   time_modify_set    54   settime4       WRITE    Set the time of last
                                                   modification to the
                                                   object.  SETATTR use
                                                   only.

5.6.  Interpreting owner and owner_group

   The recommended attributes "owner" and "owner_group" are represented
   in terms of a UTF-8 string.  To avoid a representation that is tied
   to a particular underlying implementation at the client or server,
   the use of the UTF-8 string has been chosen.  Note that section 6.1
   of [RFC2624] provides additional rationale.  It is expected that the
   client and server will have their own local representation of owner
   and owner_group that is used for local storage or presentation to the
   end user.  Therefore, it is expected that when these attributes are
   transferred between the client and server that the local
   representation is translated to a syntax of the form
   "user@dns_domain".  This will allow for a client and server that do
   not use the same local representation the ability to translate to a
   common syntax that can be interpreted by both.

   The translation is not specified as part of the protocol.  This
   allows various solutions to be employed.  For example, a local
   translation table may be consulted that maps between a numeric id to
   the user@dns_domain syntax.  A name service may also be used to
   accomplish the translation.  The "dns_domain" portion of the owner
   string is meant to be a DNS domain name.  For example, user@ietf.org.

   In the case where there is no translation available to the client or
   server, the attribute value must be constructed without the "@".
   Therefore, the absence of the @ from the owner or owner_group
   attribute signifies that no translation was available and the
   receiver of the attribute should not place any special meaning with





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   the attribute value.  Even though the attribute value can not be
   translated, it may still be useful.  In the case of a client, the
   attribute string may be used for local display of ownership.

5.7.  Character Case Attributes

   With respect to the case_insensitive and case_preserving attributes,
   each UCS-4 character (which UTF-8 encodes) has a "long descriptive
   name" [RFC1345] which may or may not included the word "CAPITAL" or
   "SMALL".  The presence of SMALL or CAPITAL allows an NFS server to
   implement unambiguous and efficient table driven mappings for case
   insensitive comparisons, and non-case-preserving storage.  For
   general character handling and internationalization issues, see the
   section "Internationalization".

5.8.  Quota Attributes

   For the attributes related to file system quotas, the following
   definitions apply:

   quota_avail_soft
         The value in bytes which represents the amount of additional
         disk space that can be allocated to this file or directory
         before the user may reasonably be warned.  It is understood
         that this space may be consumed by allocations to other files
         or directories though there is a rule as to which other files
         or directories.

   quota_avail_hard
         The value in bytes which represent the amount of additional
         disk space beyond the current allocation that can be allocated
         to this file or directory before further allocations will be
         refused.  It is understood that this space may be consumed by
         allocations to other files or directories.

   quota_used
         The value in bytes which represent the amount of disc space
         used by this file or directory and possibly a number of other
         similar files or directories, where the set of "similar" meets
         at least the criterion that allocating space to any file or
         directory in the set will reduce the "quota_avail_hard" of
         every other file or directory in the set.

         Note that there may be a number of distinct but overlapping
         sets of files or directories for which a quota_used value is
         maintained. E.g. "all files with a given owner", "all files
         with a given group owner". etc.




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         The server is at liberty to choose any of those sets but should
         do so in a repeatable way.  The rule may be configured per-
         filesystem or may be "choose the set with the smallest quota".

5.9.  Access Control Lists

   The NFS ACL attribute is an array of access control entries (ACE).
   There are various access control entry types.  The server is able to
   communicate which ACE types are supported by returning the
   appropriate value within the aclsupport attribute.  The types of ACEs
   are defined as follows:

   Type         Description
   _____________________________________________________
   ALLOW        Explicitly grants the access defined in
                acemask4 to the file or directory.

   DENY         Explicitly denies the access defined in
                acemask4 to the file or directory.

   AUDIT        LOG (system dependent) any access
                attempt to a file or directory which
                uses any of the access methods specified
                in acemask4.

   ALARM        Generate a system ALARM (system
                dependent) when any access attempt is
                made to a file or directory for the
                access methods specified in acemask4.

   The NFS ACE attribute is defined as follows:

   typedef uint32_t        acetype4;
   typedef uint32_t        aceflag4;
   typedef uint32_t        acemask4;

   struct nfsace4 {
           acetype4        type;
           aceflag4        flag;
           acemask4        access_mask;
           utf8string      who;
   };

   To determine if an ACCESS or OPEN request succeeds each nfsace4 entry
   is processed in order by the server.  Only ACEs which have a "who"
   that matches the requester are considered.  Each ACE is processed
   until all of the bits of the requester's access have been ALLOWED.
   Once a bit (see below) has been ALLOWED by an ACCESS_ALLOWED_ACE, it



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   is no longer considered in the processing of later ACEs. If an
   ACCESS_DENIED_ACE is encountered where the requester's mode still has
   unALLOWED bits in common with the "access_mask" of the ACE, the
   request is denied.

   The bitmask constants used to represent the above definitions within
   the aclsupport attribute are as follows:

   const ACL4_SUPPORT_ALLOW_ACL    = 0x00000001;
   const ACL4_SUPPORT_DENY_ACL     = 0x00000002;
   const ACL4_SUPPORT_AUDIT_ACL    = 0x00000004;
   const ACL4_SUPPORT_ALARM_ACL    = 0x00000008;

5.9.1.  ACE type

   The semantics of the "type" field follow the descriptions provided
   above.

   The bitmask constants used for the type field are as follows:

   const ACE4_ACCESS_ALLOWED_ACE_TYPE      = 0x00000000;
   const ACE4_ACCESS_DENIED_ACE_TYPE       = 0x00000001;
   const ACE4_SYSTEM_AUDIT_ACE_TYPE        = 0x00000002;
   const ACE4_SYSTEM_ALARM_ACE_TYPE        = 0x00000003;

5.9.2.  ACE flag

   The "flag" field contains values based on the following descriptions.

   ACE4_FILE_INHERIT_ACE

   Can be placed on a directory and indicates that this ACE should be
   added to each new non-directory file created.

   ACE4_DIRECTORY_INHERIT_ACE

   Can be placed on a directory and indicates that this ACE should be
   added to each new directory created.

   ACE4_INHERIT_ONLY_ACE

   Can be placed on a directory but does not apply to the directory,
   only to newly created files/directories as specified by the above two
   flags.

   ACE4_NO_PROPAGATE_INHERIT_ACE





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   Can be placed on a directory. Normally when a new directory is
   created and an ACE exists on the parent directory which is marked
   ACL4_DIRECTORY_INHERIT_ACE, two ACEs are placed on the new directory.
   One for the directory itself and one which is an inheritable ACE for
   newly created directories.  This flag tells the server to not place
   an ACE on the newly created directory which is inheritable by
   subdirectories of the created directory.

   ACE4_SUCCESSFUL_ACCESS_ACE_FLAG

   ACL4_FAILED_ACCESS_ACE_FLAG

   Both indicate for AUDIT and ALARM which state to log the event.  On
   every ACCESS or OPEN call which occurs on a file or directory which
   has an ACL that is of type ACE4_SYSTEM_AUDIT_ACE_TYPE or
   ACE4_SYSTEM_ALARM_ACE_TYPE, the attempted access is compared to the
   ace4mask of these ACLs. If the access is a subset of ace4mask and the
   identifier match, an AUDIT trail or an ALARM is generated.  By
   default this happens regardless of the success or failure of the
   ACCESS or OPEN call.

   The flag ACE4_SUCCESSFUL_ACCESS_ACE_FLAG only produces the AUDIT or
   ALARM if the ACCESS or OPEN call is successful. The
   ACE4_FAILED_ACCESS_ACE_FLAG causes the ALARM or AUDIT if the ACCESS
   or OPEN call fails.

   ACE4_IDENTIFIER_GROUP

   Indicates that the "who" refers to a GROUP as defined under Unix.

   The bitmask constants used for the flag field are as follows:

   const ACE4_FILE_INHERIT_ACE             = 0x00000001;
   const ACE4_DIRECTORY_INHERIT_ACE        = 0x00000002;
   const ACE4_NO_PROPAGATE_INHERIT_ACE     = 0x00000004;
   const ACE4_INHERIT_ONLY_ACE             = 0x00000008;
   const ACE4_SUCCESSFUL_ACCESS_ACE_FLAG   = 0x00000010;
   const ACE4_FAILED_ACCESS_ACE_FLAG       = 0x00000020;
   const ACE4_IDENTIFIER_GROUP             = 0x00000040;












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5.9.3.  ACE Access Mask

   The access_mask field contains values based on the following:

   Access                 Description
   _______________________________________________________________
   READ_DATA              Permission to read the data of the file
   LIST_DIRECTORY         Permission to list the contents of a
                          directory
   WRITE_DATA             Permission to modify the file's data
   ADD_FILE               Permission to add a new file to a
                          directory
   APPEND_DATA            Permission to append data to a file
   ADD_SUBDIRECTORY       Permission to create a subdirectory to a
                          directory
   READ_NAMED_ATTRS       Permission to read the named attributes
                          of a file
   WRITE_NAMED_ATTRS      Permission to write the named attributes
                          of a file
   EXECUTE                Permission to execute a file
   DELETE_CHILD           Permission to delete a file or directory
                          within a directory
   READ_ATTRIBUTES        The ability to read basic attributes
                          (non-acls) of a file
   WRITE_ATTRIBUTES       Permission to change basic attributes
                          (non-acls) of a file

   DELETE                 Permission to Delete the file
   READ_ACL               Permission to Read the ACL
   WRITE_ACL              Permission to Write the ACL
   WRITE_OWNER            Permission to change the owner
   SYNCHRONIZE            Permission to access file locally at the
                          server with synchronous reads and writes

   The bitmask constants used for the access mask field are as follows:

   const ACE4_READ_DATA            = 0x00000001;
   const ACE4_LIST_DIRECTORY       = 0x00000001;
   const ACE4_WRITE_DATA           = 0x00000002;
   const ACE4_ADD_FILE             = 0x00000002;
   const ACE4_APPEND_DATA          = 0x00000004;
   const ACE4_ADD_SUBDIRECTORY     = 0x00000004;
   const ACE4_READ_NAMED_ATTRS     = 0x00000008;
   const ACE4_WRITE_NAMED_ATTRS    = 0x00000010;
   const ACE4_EXECUTE              = 0x00000020;
   const ACE4_DELETE_CHILD         = 0x00000040;
   const ACE4_READ_ATTRIBUTES      = 0x00000080;
   const ACE4_WRITE_ATTRIBUTES     = 0x00000100;



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   const ACE4_DELETE               = 0x00010000;
   const ACE4_READ_ACL             = 0x00020000;
   const ACE4_WRITE_ACL            = 0x00040000;
   const ACE4_WRITE_OWNER          = 0x00080000;
   const ACE4_SYNCHRONIZE          = 0x00100000;

5.9.4.  ACE who

   There are several special identifiers ("who") which need to be
   understood universally. Some of these identifiers cannot be
   understood when an NFS client accesses the server, but have meaning
   when a local process accesses the file. The ability to display and
   modify these permissions is permitted over NFS.

   Who                    Description
   _______________________________________________________________
   "OWNER"                The owner of the file.
   "GROUP"                The group associated with the file.
   "EVERYONE"             The world.
   "INTERACTIVE"          Accessed from an interactive terminal.
   "NETWORK"              Accessed via the network.
   "DIALUP"               Accessed as a dialup user to the server.
   "BATCH"                Accessed from a batch job.
   "ANONYMOUS"            Accessed without any authentication.
   "AUTHENTICATED"        Any authenticated user (opposite of
                          ANONYMOUS)
   "SERVICE"              Access from a system service.

   To avoid conflict, these special identifiers are distinguish by an
   appended "@" and should appear in the form "xxxx@" (note: no domain
   name after the "@").  For example: ANONYMOUS@.

6.  File System Migration and Replication

   With the use of the recommended attribute "fs_locations", the NFS
   version 4 server has a method of providing file system migration or
   replication services.  For the purposes of migration and replication,
   a file system will be defined as all files that share a given fsid
   (both major and minor values are the same).

   The fs_locations attribute provides a list of file system locations.
   These locations are specified by providing the server name (either
   DNS domain or IP address) and the path name representing the root of
   the file system.  Depending on the type of service being provided,
   the list will provide a new location or a set of alternate locations
   for the file system.  The client will use this information to
   redirect its requests to the new server.




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6.1.  Replication

   It is expected that file system replication will be used in the case
   of read-only data.  Typically, the file system will be replicated on
   two or more servers.  The fs_locations attribute will provide the
   list of these locations to the client.  On first access of the file
   system, the client should obtain the value of the fs_locations
   attribute.  If, in the future, the client finds the server
   unresponsive, the client may attempt to use another server specified
   by fs_locations.

   If applicable, the client must take the appropriate steps to recover
   valid filehandles from the new server.  This is described in more
   detail in the following sections.

6.2.  Migration

   File system migration is used to move a file system from one server
   to another.  Migration is typically used for a file system that is
   writable and has a single copy.  The expected use of migration is for
   load balancing or general resource reallocation.  The protocol does
   not specify how the file system will be moved between servers.  This
   server-to-server transfer mechanism is left to the server
   implementor.  However, the method used to communicate the migration
   event between client and server is specified here.

   Once the servers participating in the migration have completed the
   move of the file system, the error NFS4ERR_MOVED will be returned for
   subsequent requests received by the original server.  The
   NFS4ERR_MOVED error is returned for all operations except GETATTR.
   Upon receiving the NFS4ERR_MOVED error, the client will obtain the
   value of the fs_locations attribute.  The client will then use the
   contents of the attribute to redirect its requests to the specified
   server.  To facilitate the use of GETATTR, operations such as PUTFH
   must also be accepted by the server for the migrated file system's
   filehandles.  Note that if the server returns NFS4ERR_MOVED, the
   server MUST support the fs_locations attribute.

   If the client requests more attributes than just fs_locations, the
   server may return fs_locations only.  This is to be expected since
   the server has migrated the file system and may not have a method of
   obtaining additional attribute data.

   The server implementor needs to be careful in developing a migration
   solution.  The server must consider all of the state information
   clients may have outstanding at the server.  This includes but is not
   limited to locking/share state, delegation state, and asynchronous




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   file writes which are represented by WRITE and COMMIT verifiers.  The
   server should strive to minimize the impact on its clients during and
   after the migration process.

6.3.  Interpretation of the fs_locations Attribute

   The fs_location attribute is structured in the following way:

   struct fs_location {
           utf8string      server<>;
           pathname4       rootpath;
   };

   struct fs_locations {
           pathname4       fs_root;
           fs_location     locations<>;
   };

   The fs_location struct is used to represent the location of a file
   system by providing a server name and the path to the root of the
   file system.  For a multi-homed server or a set of servers that use
   the same rootpath, an array of server names may be provided.  An
   entry in the server array is an UTF8 string and represents one of a
   traditional DNS host name, IPv4 address, or IPv6 address.  It is not
   a requirement that all servers that share the same rootpath be listed
   in one fs_location struct.  The array of server names is provided for
   convenience.  Servers that share the same rootpath may also be listed
   in separate fs_location entries in the fs_locations attribute.

   The fs_locations struct and attribute then contains an array of
   locations.  Since the name space of each server may be constructed
   differently, the "fs_root" field is provided.  The path represented
   by fs_root represents the location of the file system in the server's
   name space.  Therefore, the fs_root path is only associated with the
   server from which the fs_locations attribute was obtained.  The
   fs_root path is meant to aid the client in locating the file system
   at the various servers listed.

   As an example, there is a replicated file system located at two
   servers (servA and servB).  At servA the file system is located at
   path "/a/b/c".  At servB the file system is located at path "/x/y/z".
   In this example the client accesses the file system first at servA
   with a multi-component lookup path of "/a/b/c/d".  Since the client
   used a multi-component lookup to obtain the filehandle at "/a/b/c/d",
   it is unaware that the file system's root is located in servA's name
   space at "/a/b/c".  When the client switches to servB, it will need
   to determine that the directory it first referenced at servA is now
   represented by the path "/x/y/z/d" on servB.  To facilitate this, the



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   fs_locations attribute provided by servA would have a fs_root value
   of "/a/b/c" and two entries in fs_location.  One entry in fs_location
   will be for itself (servA) and the other will be for servB with a
   path of "/x/y/z".  With this information, the client is able to
   substitute "/x/y/z" for the "/a/b/c" at the beginning of its access
   path and construct "/x/y/z/d" to use for the new server.

6.4.  Filehandle Recovery for Migration or Replication

   Filehandles for file systems that are replicated or migrated
   generally have the same semantics as for file systems that are not
   replicated or migrated.  For example, if a file system has persistent
   filehandles and it is migrated to another server, the filehandle
   values for the file system will be valid at the new server.

   For volatile filehandles, the servers involved likely do not have a
   mechanism to transfer filehandle format and content between
   themselves.  Therefore, a server may have difficulty in determining
   if a volatile filehandle from an old server should return an error of
   NFS4ERR_FHEXPIRED.  Therefore, the client is informed, with the use
   of the fh_expire_type attribute, whether volatile filehandles will
   expire at the migration or replication event.  If the bit
   FH4_VOL_MIGRATION is set in the fh_expire_type attribute, the client
   must treat the volatile filehandle as if the server had returned the
   NFS4ERR_FHEXPIRED error.  At the migration or replication event in
   the presence of the FH4_VOL_MIGRATION bit, the client will not
   present the original or old volatile file handle to the new server.
   The client will start its communication with the new server by
   recovering its filehandles using the saved file names.

7.  NFS Server Name Space

7.1.  Server Exports

   On a UNIX server the name space describes all the files reachable by
   pathnames under the root directory or "/".  On a Windows NT server
   the name space constitutes all the files on disks named by mapped
   disk letters.  NFS server administrators rarely make the entire
   server's file system name space available to NFS clients.  More often
   portions of the name space are made available via an "export"
   feature.  In previous versions of the NFS protocol, the root
   filehandle for each export is obtained through the MOUNT protocol;
   the client sends a string that identifies the export of name space
   and the server returns the root filehandle for it.  The MOUNT
   protocol supports an EXPORTS procedure that will enumerate the
   server's exports.





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7.2.  Browsing Exports

   The NFS version 4 protocol provides a root filehandle that clients
   can use to obtain filehandles for these exports via a multi-component
   LOOKUP.  A common user experience is to use a graphical user
   interface (perhaps a file "Open" dialog window) to find a file via
   progressive browsing through a directory tree.  The client must be
   able to move from one export to another export via single-component,
   progressive LOOKUP operations.

   This style of browsing is not well supported by the NFS version 2 and
   3 protocols.  The client expects all LOOKUP operations to remain
   within a single server file system.  For example, the device
   attribute will not change.  This prevents a client from taking name
   space paths that span exports.

   An automounter on the client can obtain a snapshot of the server's
   name space using the EXPORTS procedure of the MOUNT protocol.  If it
   understands the server's pathname syntax, it can create an image of
   the server's name space on the client.  The parts of the name space
   that are not exported by the server are filled in with a "pseudo file
   system" that allows the user to browse from one mounted file system
   to another.  There is a drawback to this representation of the
   server's name space on the client: it is static.  If the server
   administrator adds a new export the client will be unaware of it.

7.3.  Server Pseudo File System

   NFS version 4 servers avoid this name space inconsistency by
   presenting all the exports within the framework of a single server
   name space.  An NFS version 4 client uses LOOKUP and READDIR
   operations to browse seamlessly from one export to another.  Portions
   of the server name space that are not exported are bridged via a
   "pseudo file system" that provides a view of exported directories
   only.  A pseudo file system has a unique fsid and behaves like a
   normal, read only file system.

   Based on the construction of the server's name space, it is possible
   that multiple pseudo file systems may exist.  For example,

   /a         pseudo file system
   /a/b       real file system
   /a/b/c     pseudo file system
   /a/b/c/d   real file system

   Each of the pseudo file systems are consider separate entities and
   therefore will have a unique fsid.




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7.4.  Multiple Roots

   The DOS and Windows operating environments are sometimes described as
   having "multiple roots".  File systems are commonly represented as
   disk letters.  MacOS represents file systems as top level names.  NFS
   version 4 servers for these platforms can construct a pseudo file
   system above these root names so that disk letters or volume names
   are simply directory names in the pseudo root.

7.5.  Filehandle Volatility

   The nature of the server's pseudo file system is that it is a logical
   representation of file system(s) available from the server.
   Therefore, the pseudo file system is most likely constructed
   dynamically when the server is first instantiated.  It is expected
   that the pseudo file system may not have an on disk counterpart from
   which persistent filehandles could be constructed.  Even though it is
   preferable that the server provide persistent filehandles for the
   pseudo file system, the NFS client should expect that pseudo file
   system filehandles are volatile.  This can be confirmed by checking
   the associated "fh_expire_type" attribute for those filehandles in
   question.  If the filehandles are volatile, the NFS client must be
   prepared to recover a filehandle value (e.g. with a multi-component
   LOOKUP) when receiving an error of NFS4ERR_FHEXPIRED.

7.6.  Exported Root

   If the server's root file system is exported, one might conclude that
   a pseudo-file system is not needed.  This would be wrong.  Assume the
   following file systems on a server:

            /       disk1  (exported)
            /a      disk2  (not exported)
            /a/b    disk3  (exported)

   Because disk2 is not exported, disk3 cannot be reached with simple
   LOOKUPs.  The server must bridge the gap with a pseudo-file system.

7.7.  Mount Point Crossing

   The server file system environment may be constructed in such a way
   that one file system contains a directory which is 'covered' or
   mounted upon by a second file system.  For example:

            /a/b            (file system 1)
            /a/b/c/d        (file system 2)





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   The pseudo file system for this server may be constructed to look
   like:

            /               (place holder/not exported)
            /a/b            (file system 1)
            /a/b/c/d        (file system 2)

   It is the server's responsibility to present the pseudo file system
   that is complete to the client.  If the client sends a lookup request
   for the path "/a/b/c/d", the server's response is the filehandle of
   the file system "/a/b/c/d".  In previous versions of the NFS
   protocol, the server would respond with the directory "/a/b/c/d"
   within the file system "/a/b".

   The NFS client will be able to determine if it crosses a server mount
   point by a change in the value of the "fsid" attribute.

7.8.  Security Policy and Name Space Presentation

   The application of the server's security policy needs to be carefully
   considered by the implementor.  One may choose to limit the
   viewability of portions of the pseudo file system based on the
   server's perception of the client's ability to authenticate itself
   properly.  However, with the support of multiple security mechanisms
   and the ability to negotiate the appropriate use of these mechanisms,
   the server is unable to properly determine if a client will be able
   to authenticate itself.  If, based on its policies, the server
   chooses to limit the contents of the pseudo file system, the server
   may effectively hide file systems from a client that may otherwise
   have legitimate access.

8.  File Locking and Share Reservations

   Integrating locking into the NFS protocol necessarily causes it to be
   state-full.  With the inclusion of "share" file locks the protocol
   becomes substantially more dependent on state than the traditional
   combination of NFS and NLM [XNFS].  There are three components to
   making this state manageable:

   o  Clear division between client and server

   o  Ability to reliably detect inconsistency in state between client
      and server

   o  Simple and robust recovery mechanisms






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   In this model, the server owns the state information.  The client
   communicates its view of this state to the server as needed.  The
   client is also able to detect inconsistent state before modifying a
   file.

   To support Win32 "share" locks it is necessary to atomically OPEN or
   CREATE files.  Having a separate share/unshare operation would not
   allow correct implementation of the Win32 OpenFile API.  In order to
   correctly implement share semantics, the previous NFS protocol
   mechanisms used when a file is opened or created (LOOKUP, CREATE,
   ACCESS) need to be replaced.  The NFS version 4 protocol has an OPEN
   operation that subsumes the functionality of LOOKUP, CREATE, and
   ACCESS.  However, because many operations require a filehandle, the
   traditional LOOKUP is preserved to map a file name to filehandle
   without establishing state on the server.  The policy of granting
   access or modifying files is managed by the server based on the
   client's state.  These mechanisms can implement policy ranging from
   advisory only locking to full mandatory locking.

8.1.  Locking

   It is assumed that manipulating a lock is rare when compared to READ
   and WRITE operations.  It is also assumed that crashes and network
   partitions are relatively rare.  Therefore it is important that the
   READ and WRITE operations have a lightweight mechanism to indicate if
   they possess a held lock.  A lock request contains the heavyweight
   information required to establish a lock and uniquely define the lock
   owner.

   The following sections describe the transition from the heavy weight
   information to the eventual stateid used for most client and server
   locking and lease interactions.

8.1.1.  Client ID

   For each LOCK request, the client must identify itself to the server.

   This is done in such a way as to allow for correct lock
   identification and crash recovery.  Client identification is
   accomplished with two values.

   o  A verifier that is used to detect client reboots.

   o  A variable length opaque array to uniquely define a client.

         For an operating system this may be a fully qualified host name
         or IP address.  For a user level NFS client it may additionally
         contain a process id or other unique sequence.



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   The data structure for the Client ID would then appear as:

            struct nfs_client_id {
                    opaque verifier[4];
                    opaque id<>;
            }

   It is possible through the mis-configuration of a client or the
   existence of a rogue client that two clients end up using the same
   nfs_client_id.  This situation is avoided by "negotiating" the
   nfs_client_id between client and server with the use of the
   SETCLIENTID and SETCLIENTID_CONFIRM operations.  The following
   describes the two scenarios of negotiation.

   1  Client has never connected to the server

      In this case the client generates an nfs_client_id and unless
      another client has the same nfs_client_id.id field, the server
      accepts the request. The server also records the principal (or
      principal to uid mapping) from the credential in the RPC request
      that contains the nfs_client_id negotiation request (SETCLIENTID
      operation).

      Two clients might still use the same nfs_client_id.id due to
      perhaps configuration error.  For example, a High Availability
      configuration where the nfs_client_id.id is derived from the
      ethernet controller address and both systems have the same
      address.  In this case, the result is a switched union that
      returns, in addition to NFS4ERR_CLID_INUSE, the network address
      (the rpcbind netid and universal address) of the client that is
      using the id.

   2  Client is re-connecting to the server after a client reboot

      In this case, the client still generates an nfs_client_id but the
      nfs_client_id.id field will be the same as the nfs_client_id.id
      generated prior to reboot.  If the server finds that the
      principal/uid is equal to the previously "registered"
      nfs_client_id.id, then locks associated with the old nfs_client_id
      are immediately released.  If the principal/uid is not equal, then
      this is a rogue client and the request is returned in error.  For
      more discussion of crash recovery semantics, see the section on
      "Crash Recovery".

      It is possible for a retransmission of request to be received by
      the server after the server has acted upon and responded to the
      original client request.  Therefore to mitigate effects of the
      retransmission of the SETCLIENTID operation, the client and server



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      use a confirmation step.  The server returns a confirmation
      verifier that the client then sends to the server in the
      SETCLIENTID_CONFIRM operation.  Once the server receives the
      confirmation from the client, the locking state for the client is
      released.

   In both cases, upon success, NFS4_OK is returned.  To help reduce the
   amount of data transferred on OPEN and LOCK, the server will also
   return a unique 64-bit clientid value that is a shorthand reference
   to the nfs_client_id values presented by the client.  From this point
   forward, the client will use the clientid to refer to itself.

   The clientid assigned by the server should be chosen so that it will
   not conflict with a clientid previously assigned by the server.  This
   applies across server restarts or reboots.  When a clientid is
   presented to a server and that clientid is not recognized, as would
   happen after a server reboot, the server will reject the request with
   the error NFS4ERR_STALE_CLIENTID.  When this happens, the client must
   obtain a new clientid by use of the SETCLIENTID operation and then
   proceed to any other necessary recovery for the server reboot case
   (See the section "Server Failure and Recovery").

   The client must also employ the SETCLIENTID operation when it
   receives a NFS4ERR_STALE_STATEID error using a stateid derived from
   its current clientid, since this also indicates a server reboot which
   has invalidated the existing clientid (see the next section
   "nfs_lockowner and stateid Definition" for details).

8.1.2.  Server Release of Clientid

   If the server determines that the client holds no associated state
   for its clientid, the server may choose to release the clientid.  The
   server may make this choice for an inactive client so that resources
   are not consumed by those intermittently active clients.  If the
   client contacts the server after this release, the server must ensure
   the client receives the appropriate error so that it will use the
   SETCLIENTID/SETCLIENTID_CONFIRM sequence to establish a new identity.
   It should be clear that the server must be very hesitant to release a
   clientid since the resulting work on the client to recover from such
   an event will be the same burden as if the server had failed and
   restarted.  Typically a server would not release a clientid unless
   there had been no activity from that client for many minutes.









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8.1.3.  nfs_lockowner and stateid Definition

   When requesting a lock, the client must present to the server the
   clientid and an identifier for the owner of the requested lock.
   These two fields are referred to as the nfs_lockowner and the
   definition of those fields are:

   o  A clientid returned by the server as part of the client's use of
      the SETCLIENTID operation.

   o  A variable length opaque array used to uniquely define the owner
      of a lock managed by the client.

         This may be a thread id, process id, or other unique value.

   When the server grants the lock, it responds with a unique 64-bit
   stateid.  The stateid is used as a shorthand reference to the
   nfs_lockowner, since the server will be maintaining the
   correspondence between them.

   The server is free to form the stateid in any manner that it chooses
   as long as it is able to recognize invalid and out-of-date stateids.
   This requirement includes those stateids generated by earlier
   instances of the server.  From this, the client can be properly
   notified of a server restart.  This notification will occur when the
   client presents a stateid to the server from a previous
   instantiation.

   The server must be able to distinguish the following situations and
   return the error as specified:

   o  The stateid was generated by an earlier server instance (i.e.
      before a server reboot).  The error NFS4ERR_STALE_STATEID should
      be returned.

   o  The stateid was generated by the current server instance but the
      stateid no longer designates the current locking state for the
      lockowner-file pair in question (i.e. one or more locking
      operations has occurred).  The error NFS4ERR_OLD_STATEID should be
      returned.

      This error condition will only occur when the client issues a
      locking request which changes a stateid while an I/O request that
      uses that stateid is outstanding.







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   o  The stateid was generated by the current server instance but the
      stateid does not designate a locking state for any active
      lockowner-file pair.  The error NFS4ERR_BAD_STATEID should be
      returned.

      This error condition will occur when there has been a logic error
      on the part of the client or server.  This should not happen.

   One mechanism that may be used to satisfy these requirements is for
   the server to divide stateids into three fields:

   o  A server verifier which uniquely designates a particular server
      instantiation.

   o  An index into a table of locking-state structures.

   o  A sequence value which is incremented for each stateid that is
      associated with the same index into the locking-state table.

   By matching the incoming stateid and its field values with the state
   held at the server, the server is able to easily determine if a
   stateid is valid for its current instantiation and state.  If the
   stateid is not valid, the appropriate error can be supplied to the
   client.

8.1.4.  Use of the stateid

   All READ and WRITE operations contain a stateid.  If the
   nfs_lockowner performs a READ or WRITE on a range of bytes within a
   locked range, the stateid (previously returned by the server) must be
   used to indicate that the appropriate lock (record or share) is held.
   If no state is established by the client, either record lock or share
   lock, a stateid of all bits 0 is used.  If no conflicting locks are
   held on the file, the server may service the READ or WRITE operation.
   If a conflict with an explicit lock occurs, an error is returned for
   the operation (NFS4ERR_LOCKED). This allows "mandatory locking" to be
   implemented.

   A stateid of all bits 1 (one) allows READ operations to bypass record
   locking checks at the server.  However, WRITE operations with stateid
   with bits all 1 (one) do not bypass record locking checks.  File
   locking checks are handled by the OPEN operation (see the section
   "OPEN/CLOSE Operations").

   An explicit lock may not be granted while a READ or WRITE operation
   with conflicting implicit locking is being performed.





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8.1.5.  Sequencing of Lock Requests

   Locking is different than most NFS operations as it requires "at-
   most-one" semantics that are not provided by ONCRPC.  ONCRPC over a
   reliable transport is not sufficient because a sequence of locking
   requests may span multiple TCP connections.  In the face of
   retransmission or reordering, lock or unlock requests must have a
   well defined and consistent behavior.  To accomplish this, each lock
   request contains a sequence number that is a consecutively increasing
   integer.  Different nfs_lockowners have different sequences.  The
   server maintains the last sequence number (L) received and the
   response that was returned.

   Note that for requests that contain a sequence number, for each
   nfs_lockowner, there should be no more than one outstanding request.

   If a request with a previous sequence number (r < L) is received, it
   is rejected with the return of error NFS4ERR_BAD_SEQID.  Given a
   properly-functioning client, the response to (r) must have been
   received before the last request (L) was sent.  If a duplicate of
   last request (r == L) is received, the stored response is returned.
   If a request beyond the next sequence (r == L + 2) is received, it is
   rejected with the return of error NFS4ERR_BAD_SEQID.  Sequence
   history is reinitialized whenever the client verifier changes.

   Since the sequence number is represented with an unsigned 32-bit
   integer, the arithmetic involved with the sequence number is mod
   2^32.

   It is critical the server maintain the last response sent to the
   client to provide a more reliable cache of duplicate non-idempotent
   requests than that of the traditional cache described in [Juszczak].
   The traditional duplicate request cache uses a least recently used
   algorithm for removing unneeded requests. However, the last lock
   request and response on a given nfs_lockowner must be cached as long
   as the lock state exists on the server.

8.1.6.  Recovery from Replayed Requests

   As described above, the sequence number is per nfs_lockowner.  As
   long as the server maintains the last sequence number received and
   follows the methods described above, there are no risks of a
   Byzantine router re-sending old requests.  The server need only
   maintain the nfs_lockowner, sequence number state as long as there
   are open files or closed files with locks outstanding.






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   LOCK, LOCKU, OPEN, OPEN_DOWNGRADE, and CLOSE each contain a sequence
   number and therefore the risk of the replay of these operations
   resulting in undesired effects is non-existent while the server
   maintains the nfs_lockowner state.

8.1.7.  Releasing nfs_lockowner State

   When a particular nfs_lockowner no longer holds open or file locking
   state at the server, the server may choose to release the sequence
   number state associated with the nfs_lockowner.  The server may make
   this choice based on lease expiration, for the reclamation of server
   memory, or other implementation specific details.  In any event, the
   server is able to do this safely only when the nfs_lockowner no
   longer is being utilized by the client.  The server may choose to
   hold the nfs_lockowner state in the event that retransmitted requests
   are received.  However, the period to hold this state is
   implementation specific.

   In the case that a LOCK, LOCKU, OPEN_DOWNGRADE, or CLOSE is
   retransmitted after the server has previously released the
   nfs_lockowner state, the server will find that the nfs_lockowner has
   no files open and an error will be returned to the client.  If the
   nfs_lockowner does have a file open, the stateid will not match and
   again an error is returned to the client.

   In the case that an OPEN is retransmitted and the nfs_lockowner is
   being used for the first time or the nfs_lockowner state has been
   previously released by the server, the use of the OPEN_CONFIRM
   operation will prevent incorrect behavior.  When the server observes
   the use of the nfs_lockowner for the first time, it will direct the
   client to perform the OPEN_CONFIRM for the corresponding OPEN.  This
   sequence establishes the use of an nfs_lockowner and associated
   sequence number.  See the section "OPEN_CONFIRM - Confirm Open" for
   further details.

8.2.  Lock Ranges

   The protocol allows a lock owner to request a lock with one byte
   range and then either upgrade or unlock a sub-range of the initial
   lock.  It is expected that this will be an uncommon type of request.
   In any case, servers or server file systems may not be able to
   support sub-range lock semantics.  In the event that a server
   receives a locking request that represents a sub-range of current
   locking state for the lock owner, the server is allowed to return the
   error NFS4ERR_LOCK_RANGE to signify that it does not support sub-
   range lock operations.  Therefore, the client should be prepared to
   receive this error and, if appropriate, report the error to the
   requesting application.



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   The client is discouraged from combining multiple independent locking
   ranges that happen to be adjacent into a single request since the
   server may not support sub-range requests and for reasons related to
   the recovery of file locking state in the event of server failure.
   As discussed in the section "Server Failure and Recovery" below, the
   server may employ certain optimizations during recovery that work
   effectively only when the client's behavior during lock recovery is
   similar to the client's locking behavior prior to server failure.

8.3.  Blocking Locks

   Some clients require the support of blocking locks.  The NFS version
   4 protocol must not rely on a callback mechanism and therefore is
   unable to notify a client when a previously denied lock has been
   granted.  Clients have no choice but to continually poll for the
   lock.  This presents a fairness problem.  Two new lock types are
   added, READW and WRITEW, and are used to indicate to the server that
   the client is requesting a blocking lock.  The server should maintain
   an ordered list of pending blocking locks.  When the conflicting lock
   is released, the server may wait the lease period for the first
   waiting client to re-request the lock.  After the lease period
   expires the next waiting client request is allowed the lock.  Clients
   are required to poll at an interval sufficiently small that it is
   likely to acquire the lock in a timely manner.  The server is not
   required to maintain a list of pending blocked locks as it is used to
   increase fairness and not correct operation.  Because of the
   unordered nature of crash recovery, storing of lock state to stable
   storage would be required to guarantee ordered granting of blocking
   locks.

   Servers may also note the lock types and delay returning denial of
   the request to allow extra time for a conflicting lock to be
   released, allowing a successful return.  In this way, clients can
   avoid the burden of needlessly frequent polling for blocking locks.
   The server should take care in the length of delay in the event the
   client retransmits the request.

8.4.  Lease Renewal

   The purpose of a lease is to allow a server to remove stale locks
   that are held by a client that has crashed or is otherwise
   unreachable.  It is not a mechanism for cache consistency and lease
   renewals may not be denied if the lease interval has not expired.

   The following events cause implicit renewal of all of the leases for
   a given client (i.e. all those sharing a given clientid).  Each of
   these is a positive indication that the client is still active and




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   that the associated state held at the server, for the client, is
   still valid.

   o  An OPEN with a valid clientid.

   o  Any operation made with a valid stateid (CLOSE, DELEGRETURN, LOCK,
      LOCKU, OPEN, OPEN_CONFIRM, READ, RENEW, SETATTR, WRITE).  This
      does not include the special stateids of all bits 0 or all bits 1.

         Note that if the client had restarted or rebooted, the client
         would not be making these requests without issuing the
         SETCLIENTID operation.  The use of the SETCLIENTID operation
         (possibly with the addition of the optional SETCLIENTID_CONFIRM
         operation) notifies the server to drop the locking state
         associated with the client.

         If the server has rebooted, the stateids (NFS4ERR_STALE_STATEID
         error) or the clientid (NFS4ERR_STALE_CLIENTID error) will not
         be valid hence preventing spurious renewals.

   This approach allows for low overhead lease renewal which scales
   well.  In the typical case no extra RPC calls are required for lease
   renewal and in the worst case one RPC is required every lease period
   (i.e. a RENEW operation).  The number of locks held by the client is
   not a factor since all state for the client is involved with the
   lease renewal action.

   Since all operations that create a new lease also renew existing
   leases, the server must maintain a common lease expiration time for
   all valid leases for a given client.  This lease time can then be
   easily updated upon implicit lease renewal actions.

8.5.  Crash Recovery

   The important requirement in crash recovery is that both the client
   and the server know when the other has failed.  Additionally, it is
   required that a client sees a consistent view of data across server
   restarts or reboots.  All READ and WRITE operations that may have
   been queued within the client or network buffers must wait until the
   client has successfully recovered the locks protecting the READ and
   WRITE operations.

8.5.1.  Client Failure and Recovery

   In the event that a client fails, the server may recover the client's
   locks when the associated leases have expired.  Conflicting locks
   from another client may only be granted after this lease expiration.
   If the client is able to restart or reinitialize within the lease



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   period the client may be forced to wait the remainder of the lease
   period before obtaining new locks.

   To minimize client delay upon restart, lock requests are associated
   with an instance of the client by a client supplied verifier.  This
   verifier is part of the initial SETCLIENTID call made by the client.
   The server returns a clientid as a result of the SETCLIENTID
   operation.  The client then confirms the use of the verifier with
   SETCLIENTID_CONFIRM.  The clientid in combination with an opaque
   owner field is then used by the client to identify the lock owner for
   OPEN.  This chain of associations is then used to identify all locks
   for a particular client.

   Since the verifier will be changed by the client upon each
   initialization, the server can compare a new verifier to the verifier
   associated with currently held locks and determine that they do not
   match.  This signifies the client's new instantiation and subsequent
   loss of locking state.  As a result, the server is free to release
   all locks held which are associated with the old clientid which was
   derived from the old verifier.

   For secure environments, a change in the verifier must only cause the
   release of locks associated with the authenticated requester.  This
   is required to prevent a rogue entity from freeing otherwise valid
   locks.

   Note that the verifier must have the same uniqueness properties of
   the verifier for the COMMIT operation.

8.5.2.  Server Failure and Recovery

   If the server loses locking state (usually as a result of a restart
   or reboot), it must allow clients time to discover this fact and re-
   establish the lost locking state.  The client must be able to re-
   establish the locking state without having the server deny valid
   requests because the server has granted conflicting access to another
   client.  Likewise, if there is the possibility that clients have not
   yet re-established their locking state for a file, the server must
   disallow READ and WRITE operations for that file.  The duration of
   this recovery period is equal to the duration of the lease period.

   A client can determine that server failure (and thus loss of locking
   state) has occurred, when it receives one of two errors.  The
   NFS4ERR_STALE_STATEID error indicates a stateid invalidated by a
   reboot or restart.  The NFS4ERR_STALE_CLIENTID error indicates a
   clientid invalidated by reboot or restart.  When either of these are
   received, the client must establish a new clientid (See the section
   "Client ID") and re-establish the locking state as discussed below.



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   The period of special handling of locking and READs and WRITEs, equal
   in duration to the lease period, is referred to as the "grace
   period".  During the grace period, clients recover locks and the
   associated state by reclaim-type locking requests (i.e. LOCK requests
   with reclaim set to true and OPEN operations with a claim type of
   CLAIM_PREVIOUS).  During the grace period, the server must reject
   READ and WRITE operations and non-reclaim locking requests (i.e.
   other LOCK and OPEN operations) with an error of NFS4ERR_GRACE.

   If the server can reliably determine that granting a non-reclaim
   request will not conflict with reclamation of locks by other clients,
   the NFS4ERR_GRACE error does not have to be returned and the non-
   reclaim client request can be serviced.  For the server to be able to
   service READ and WRITE operations during the grace period, it must
   again be able to guarantee that no possible conflict could arise
   between an impending reclaim locking request and the READ or WRITE
   operation.  If the server is unable to offer that guarantee, the
   NFS4ERR_GRACE error must be returned to the client.

   For a server to provide simple, valid handling during the grace
   period, the easiest method is to simply reject all non-reclaim
   locking requests and READ and WRITE operations by returning the
   NFS4ERR_GRACE error.  However, a server may keep information about
   granted locks in stable storage.  With this information, the server
   could determine if a regular lock or READ or WRITE operation can be
   safely processed.

   For example, if a count of locks on a given file is available in
   stable storage, the server can track reclaimed locks for the file and
   when all reclaims have been processed, non-reclaim locking requests
   may be processed.  This way the server can ensure that non-reclaim
   locking requests will not conflict with potential reclaim requests.
   With respect to I/O requests, if the server is able to determine that
   there are no outstanding reclaim requests for a file by information
   from stable storage or another similar mechanism, the processing of
   I/O requests could proceed normally for the file.

   To reiterate, for a server that allows non-reclaim lock and I/O
   requests to be processed during the grace period, it MUST determine
   that no lock subsequently reclaimed will be rejected and that no lock
   subsequently reclaimed would have prevented any I/O operation
   processed during the grace period.

   Clients should be prepared for the return of NFS4ERR_GRACE errors for
   non-reclaim lock and I/O requests.  In this case the client should
   employ a retry mechanism for the request.  A delay (on the order of
   several seconds) between retries should be used to avoid overwhelming
   the server.  Further discussion of the general is included in



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   [Floyd].  The client must account for the server that is able to
   perform I/O and non-reclaim locking requests within the grace period
   as well as those that can not do so.

   A reclaim-type locking request outside the server's grace period can
   only succeed if the server can guarantee that no conflicting lock or
   I/O request has been granted since reboot or restart.

8.5.3.  Network Partitions and Recovery

   If the duration of a network partition is greater than the lease
   period provided by the server, the server will have not received a
   lease renewal from the client.  If this occurs, the server may free
   all locks held for the client.  As a result, all stateids held by the
   client will become invalid or stale.  Once the client is able to
   reach the server after such a network partition, all I/O submitted by
   the client with the now invalid stateids will fail with the server
   returning the error NFS4ERR_EXPIRED.  Once this error is received,
   the client will suitably notify the application that held the lock.

   As a courtesy to the client or as an optimization, the server may
   continue to hold locks on behalf of a client for which recent
   communication has extended beyond the lease period.  If the server
   receives a lock or I/O request that conflicts with one of these
   courtesy locks, the server must free the courtesy lock and grant the
   new request.

   If the server continues to hold locks beyond the expiration of a
   client's lease, the server MUST employ a method of recording this
   fact in its stable storage.  Conflicting locks requests from another
   client may be serviced after the lease expiration.  There are various
   scenarios involving server failure after such an event that require
   the storage of these lease expirations or network partitions.  One
   scenario is as follows:

         A client holds a lock at the server and encounters a network
         partition and is unable to renew the associated lease.  A
         second client obtains a conflicting lock and then frees the
         lock.  After the unlock request by the second client, the
         server reboots or reinitializes.  Once the server recovers, the
         network partition heals and the original client attempts to
         reclaim the original lock.

   In this scenario and without any state information, the server will
   allow the reclaim and the client will be in an inconsistent state
   because the server or the client has no knowledge of the conflicting
   lock.




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   The server may choose to store this lease expiration or network
   partitioning state in a way that will only identify the client as a
   whole.  Note that this may potentially lead to lock reclaims being
   denied unnecessarily because of a mix of conflicting and non-
   conflicting locks.  The server may also choose to store information
   about each lock that has an expired lease with an associated
   conflicting lock.  The choice of the amount and type of state
   information that is stored is left to the implementor.  In any case,
   the server must have enough state information to enable correct
   recovery from multiple partitions and multiple server failures.

8.6.  Recovery from a Lock Request Timeout or Abort

   In the event a lock request times out, a client may decide to not
   retry the request.  The client may also abort the request when the
   process for which it was issued is terminated (e.g. in UNIX due to a
   signal.  It is possible though that the server received the request
   and acted upon it.  This would change the state on the server without
   the client being aware of the change.  It is paramount that the
   client re-synchronize state with server before it attempts any other
   operation that takes a seqid and/or a stateid with the same
   nfs_lockowner. This is straightforward to do without a special re-
   synchronize operation.

   Since the server maintains the last lock request and response
   received on the nfs_lockowner, for each nfs_lockowner, the client
   should cache the last lock request it sent such that the lock request
   did not receive a response.  From this, the next time the client does
   a lock operation for the nfs_lockowner, it can send the cached
   request, if there is one, and if the request was one that established
   state (e.g. a LOCK or OPEN operation) the client can follow up with a
   request to remove the state (e.g. a LOCKU or CLOSE operation).  With
   this approach, the sequencing and stateid information on the client
   and server for the given nfs_lockowner will re-synchronize and in
   turn the lock state will re-synchronize.

8.7.  Server Revocation of Locks

   At any point, the server can revoke locks held by a client and the
   client must be prepared for this event.  When the client detects that
   its locks have been or may have been revoked, the client is
   responsible for validating the state information between itself and
   the server.  Validating locking state for the client means that it
   must verify or reclaim state for each lock currently held.







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   The first instance of lock revocation is upon server reboot or re-
   initialization.  In this instance the client will receive an error
   (NFS4ERR_STALE_STATEID or NFS4ERR_STALE_CLIENTID) and the client will
   proceed with normal crash recovery as described in the previous
   section.

   The second lock revocation event is the inability to renew the lease
   period.  While this is considered a rare or unusual event, the client
   must be prepared to recover.  Both the server and client will be able
   to detect the failure to renew the lease and are capable of
   recovering without data corruption.  For the server, it tracks the
   last renewal event serviced for the client and knows when the lease
   will expire.  Similarly, the client must track operations which will
   renew the lease period.  Using the time that each such request was
   sent and the time that the corresponding reply was received, the
   client should bound the time that the corresponding renewal could
   have occurred on the server and thus determine if it is possible that
   a lease period expiration could have occurred.

   The third lock revocation event can occur as a result of
   administrative intervention within the lease period.  While this is
   considered a rare event, it is possible that the server's
   administrator has decided to release or revoke a particular lock held
   by the client.  As a result of revocation, the client will receive an
   error of NFS4ERR_EXPIRED and the error is received within the lease
   period for the lock.  In this instance the client may assume that
   only the nfs_lockowner's locks have been lost.  The client notifies
   the lock holder appropriately.  The client may not assume the lease
   period has been renewed as a result of failed operation.

   When the client determines the lease period may have expired, the
   client must mark all locks held for the associated lease as
   "unvalidated".  This means the client has been unable to re-establish
   or confirm the appropriate lock state with the server.  As described
   in the previous section on crash recovery, there are scenarios in
   which the server may grant conflicting locks after the lease period
   has expired for a client.  When it is possible that the lease period
   has expired, the client must validate each lock currently held to
   ensure that a conflicting lock has not been granted. The client may
   accomplish this task by issuing an I/O request, either a pending I/O
   or a zero-length read, specifying the stateid associated with the
   lock in question. If the response to the request is success, the
   client has validated all of the locks governed by that stateid and
   re-established the appropriate state between itself and the server.
   If the I/O request is not successful, then one or more of the locks
   associated with the stateid was revoked by the server and the client
   must notify the owner.




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8.8.  Share Reservations

   A share reservation is a mechanism to control access to a file.  It
   is a separate and independent mechanism from record locking.  When a
   client opens a file, it issues an OPEN operation to the server
   specifying the type of access required (READ, WRITE, or BOTH) and the
   type of access to deny others (deny NONE, READ, WRITE, or BOTH).  If
   the OPEN fails the client will fail the application's open request.

   Pseudo-code definition of the semantics:

               if ((request.access & file_state.deny)) ||
                     (request.deny & file_state.access))
                             return (NFS4ERR_DENIED)

   The constants used for the OPEN and OPEN_DOWNGRADE operations for the
   access and deny fields are as follows:

   const OPEN4_SHARE_ACCESS_READ   = 0x00000001;
   const OPEN4_SHARE_ACCESS_WRITE  = 0x00000002;
   const OPEN4_SHARE_ACCESS_BOTH   = 0x00000003;

   const OPEN4_SHARE_DENY_NONE     = 0x00000000;
   const OPEN4_SHARE_DENY_READ     = 0x00000001;
   const OPEN4_SHARE_DENY_WRITE    = 0x00000002;
   const OPEN4_SHARE_DENY_BOTH     = 0x00000003;

8.9.  OPEN/CLOSE Operations

   To provide correct share semantics, a client MUST use the OPEN
   operation to obtain the initial filehandle and indicate the desired
   access and what if any access to deny.  Even if the client intends to
   use a stateid of all 0's or all 1's, it must still obtain the
   filehandle for the regular file with the OPEN operation so the
   appropriate share semantics can be applied.  For clients that do not
   have a deny mode built into their open programming interfaces, deny
   equal to NONE should be used.

   The OPEN operation with the CREATE flag, also subsumes the CREATE
   operation for regular files as used in previous versions of the NFS
   protocol.  This allows a create with a share to be done atomically.

   The CLOSE operation removes all share locks held by the nfs_lockowner
   on that file.  If record locks are held, the client SHOULD release
   all locks before issuing a CLOSE.  The server MAY free all
   outstanding locks on CLOSE but some servers may not support the CLOSE
   of a file that still has record locks held.  The server MUST return
   failure if any locks would exist after the CLOSE.



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   The LOOKUP operation will return a filehandle without establishing
   any lock state on the server.  Without a valid stateid, the server
   will assume the client has the least access.  For example, a file
   opened with deny READ/WRITE cannot be accessed using a filehandle
   obtained through LOOKUP because it would not have a valid stateid
   (i.e. using a stateid of all bits 0 or all bits 1).

8.10.  Open Upgrade and Downgrade

   When an OPEN is done for a file and the lockowner for which the open
   is being done already has the file open, the result is to upgrade the
   open file status maintained on the server to include the access and
   deny bits specified by the new OPEN as well as those for the existing
   OPEN.  The result is that there is one open file, as far as the
   protocol is concerned, and it includes the union of the access and
   deny bits for all of the OPEN requests completed.  Only a single
   CLOSE will be done to reset the effects of both OPEN's.  Note that
   the client, when issuing the OPEN, may not know that the same file is
   in fact being opened.  The above only applies if both OPEN's result
   in the OPEN'ed object being designated by the same filehandle.

   When the server chooses to export multiple filehandles corresponding
   to the same file object and returns different filehandles on two
   different OPEN's of the same file object, the server MUST NOT "OR"
   together the access and deny bits and coalesce the two open files.
   Instead the server must maintain separate OPEN's with separate
   stateid's and will require separate CLOSE's to free them.

   When multiple open files on the client are merged into a single open
   file object on the server, the close of one of the open files (on the
   client) may necessitate change of the access and deny status of the
   open file on the server.  This is because the union of the access and
   deny bits for the remaining open's may be smaller (i.e. a proper
   subset) than previously.  The OPEN_DOWNGRADE operation is used to
   make the necessary change and the client should use it to update the
   server so that share reservation requests by other clients are
   handled properly.

8.11.  Short and Long Leases

   When determining the time period for the server lease, the usual
   lease tradeoffs apply.  Short leases are good for fast server
   recovery at a cost of increased RENEW or READ (with zero length)
   requests.  Longer leases are certainly kinder and gentler to large
   internet servers trying to handle very large numbers of clients.  The
   number of RENEW requests drop in proportion to the lease time.  The
   disadvantages of long leases are slower recovery after server failure
   (server must wait for leases to expire and grace period before



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   granting new lock requests) and increased file contention (if client
   fails to transmit an unlock request then server must wait for lease
   expiration before granting new locks).

   Long leases are usable if the server is able to store lease state in
   non-volatile memory.  Upon recovery, the server can reconstruct the
   lease state from its non-volatile memory and continue operation with
   its clients and therefore long leases are not an issue.

8.12.  Clocks and Calculating Lease Expiration

   To avoid the need for synchronized clocks, lease times are granted by
   the server as a time delta.  However, there is a requirement that the
   client and server clocks do not drift excessively over the duration
   of the lock.  There is also the issue of propagation delay across the
   network which could easily be several hundred milliseconds as well as
   the possibility that requests will be lost and need to be
   retransmitted.

   To take propagation delay into account, the client should subtract it
   from lease times (e.g. if the client estimates the one-way
   propagation delay as 200 msec, then it can assume that the lease is
   already 200 msec old when it gets it).  In addition, it will take
   another 200 msec to get a response back to the server.  So the client
   must send a lock renewal or write data back to the server 400 msec
   before the lease would expire.

8.13.  Migration, Replication and State

   When responsibility for handling a given file system is transferred
   to a new server (migration) or the client chooses to use an alternate
   server (e.g. in response to server unresponsiveness) in the context
   of file system replication, the appropriate handling of state shared
   between the client and server (i.e. locks, leases, stateid's, and
   clientid's) is as described below.  The handling differs between
   migration and replication.  For related discussion of file server
   state and recover of such see the sections under "File Locking and
   Share Reservations"

8.13.1.  Migration and State

   In the case of migration, the servers involved in the migration of a
   file system SHOULD transfer all server state from the original to the
   new server.  This must be done in a way that is transparent to the
   client.  This state transfer will ease the client's transition when a
   file system migration occurs.  If the servers are successful in
   transferring all state, the client will continue to use stateid's
   assigned by the original server.  Therefore the new server must



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   recognize these stateid's as valid.  This holds true for the clientid
   as well.  Since responsibility for an entire file system is
   transferred with a migration event, there is no possibility that
   conflicts will arise on the new server as a result of the transfer of
   locks.

   As part of the transfer of information between servers, leases would
   be transferred as well.  The leases being transferred to the new
   server will typically have a different expiration time from those for
   the same client, previously on the new server.  To maintain the
   property that all leases on a given server for a given client expire
   at the same time, the server should advance the expiration time to
   the later of the leases being transferred or the leases already
   present.  This allows the client to maintain lease renewal of both
   classes without special effort.

   The servers may choose not to transfer the state information upon
   migration.  However, this choice is discouraged.  In this case, when
   the client presents state information from the original server, the
   client must be prepared to receive either NFS4ERR_STALE_CLIENTID or
   NFS4ERR_STALE_STATEID from the new server.  The client should then
   recover its state information as it normally would in response to a
   server failure.  The new server must take care to allow for the
   recovery of state information as it would in the event of server
   restart.

8.13.2.  Replication and State

   Since client switch-over in the case of replication is not under
   server control, the handling of state is different.  In this case,
   leases, stateid's and clientid's do not have validity across a
   transition from one server to another.  The client must re-establish
   its locks on the new server.  This can be compared to the re-
   establishment of locks by means of reclaim-type requests after a
   server reboot.  The difference is that the server has no provision to
   distinguish requests reclaiming locks from those obtaining new locks
   or to defer the latter.  Thus, a client re-establishing a lock on the
   new server (by means of a LOCK or OPEN request), may have the
   requests denied due to a conflicting lock.  Since replication is
   intended for read-only use of filesystems, such denial of locks
   should not pose large difficulties in practice.  When an attempt to
   re-establish a lock on a new server is denied, the client should
   treat the situation as if his original lock had been revoked.








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8.13.3.  Notification of Migrated Lease

   In the case of lease renewal, the client may not be submitting
   requests for a file system that has been migrated to another server.
   This can occur because of the implicit lease renewal mechanism.  The
   client renews leases for all file systems when submitting a request
   to any one file system at the server.

   In order for the client to schedule renewal of leases that may have
   been relocated to the new server, the client must find out about
   lease relocation before those leases expire.  To accomplish this, all
   operations which implicitly renew leases for a client (i.e. OPEN,
   CLOSE, READ, WRITE, RENEW, LOCK, LOCKT, LOCKU), will return the error
   NFS4ERR_LEASE_MOVED if responsibility for any of the leases to be
   renewed has been transferred to a new server.  This condition will
   continue until the client receives an NFS4ERR_MOVED error and the
   server receives the subsequent GETATTR(fs_locations) for an access to
   each file system for which a lease has been moved to a new server.

   When a client receives an NFS4ERR_LEASE_MOVED error, it should
   perform some operation, such as a RENEW, on each file system
   associated with the server in question.  When the client receives an
   NFS4ERR_MOVED error, the client can follow the normal process to
   obtain the new server information (through the fs_locations
   attribute) and perform renewal of those leases on the new server.  If
   the server has not had state transferred to it transparently, it will
   receive either NFS4ERR_STALE_CLIENTID or NFS4ERR_STALE_STATEID from
   the new server, as described above, and can then recover state
   information as it does in the event of server failure.

9.  Client-Side Caching

   Client-side caching of data, of file attributes, and of file names is
   essential to providing good performance with the NFS protocol.
   Providing distributed cache coherence is a difficult problem and
   previous versions of the NFS protocol have not attempted it.
   Instead, several NFS client implementation techniques have been used
   to reduce the problems that a lack of coherence poses for users.
   These techniques have not been clearly defined by earlier protocol
   specifications and it is often unclear what is valid or invalid
   client behavior.

   The NFS version 4 protocol uses many techniques similar to those that
   have been used in previous protocol versions.  The NFS version 4
   protocol does not provide distributed cache coherence.  However, it
   defines a more limited set of caching guarantees to allow locks and
   share reservations to be used without destructive interference from
   client side caching.



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   In addition, the NFS version 4 protocol introduces a delegation
   mechanism which allows many decisions normally made by the server to
   be made locally by clients.  This mechanism provides efficient
   support of the common cases where sharing is infrequent or where
   sharing is read-only.

9.1.  Performance Challenges for Client-Side Caching

   Caching techniques used in previous versions of the NFS protocol have
   been successful in providing good performance.  However, several
   scalability challenges can arise when those techniques are used with
   very large numbers of clients.  This is particularly true when
   clients are geographically distributed which classically increases
   the latency for cache revalidation requests.

   The previous versions of the NFS protocol repeat their file data
   cache validation requests at the time the file is opened.  This
   behavior can have serious performance drawbacks.  A common case is
   one in which a file is only accessed by a single client.  Therefore,
   sharing is infrequent.

   In this case, repeated reference to the server to find that no
   conflicts exist is expensive.  A better option with regards to
   performance is to allow a client that repeatedly opens a file to do
   so without reference to the server.  This is done until potentially
   conflicting operations from another client actually occur.

   A similar situation arises in connection with file locking.  Sending
   file lock and unlock requests to the server as well as the read and
   write requests necessary to make data caching consistent with the
   locking semantics (see the section "Data Caching and File Locking")
   can severely limit performance.  When locking is used to provide
   protection against infrequent conflicts, a large penalty is incurred.
   This penalty may discourage the use of file locking by applications.

   The NFS version 4 protocol provides more aggressive caching
   strategies with the following design goals:

   o  Compatibility with a large range of server semantics.

   o  Provide the same caching benefits as previous versions of the NFS
      protocol when unable to provide the more aggressive model.

   o  Requirements for aggressive caching are organized so that a large
      portion of the benefit can be obtained even when not all of the
      requirements can be met.





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   The appropriate requirements for the server are discussed in later
   sections in which specific forms of caching are covered. (see the
   section "Open Delegation").

9.2.  Delegation and Callbacks

   Recallable delegation of server responsibilities for a file to a
   client improves performance by avoiding repeated requests to the
   server in the absence of inter-client conflict.  With the use of a
   "callback" RPC from server to client, a server recalls delegated
   responsibilities when another client engages in sharing of a
   delegated file.

   A delegation is passed from the server to the client, specifying the
   object of the delegation and the type of delegation.  There are
   different types of delegations but each type contains a stateid to be
   used to represent the delegation when performing operations that
   depend on the delegation.  This stateid is similar to those
   associated with locks and share reservations but differs in that the
   stateid for a delegation is associated with a clientid and may be
   used on behalf of all the nfs_lockowners for the given client.  A
   delegation is made to the client as a whole and not to any specific
   process or thread of control within it.

   Because callback RPCs may not work in all environments (due to
   firewalls, for example), correct protocol operation does not depend
   on them.  Preliminary testing of callback functionality by means of a
   CB_NULL procedure determines whether callbacks can be supported.  The
   CB_NULL procedure checks the continuity of the callback path.  A
   server makes a preliminary assessment of callback availability to a
   given client and avoids delegating responsibilities until it has
   determined that callbacks are supported.  Because the granting of a
   delegation is always conditional upon the absence of conflicting
   access, clients must not assume that a delegation will be granted and
   they must always be prepared for OPENs to be processed without any
   delegations being granted.

   Once granted, a delegation behaves in most ways like a lock.  There
   is an associated lease that is subject to renewal together with all
   of the other leases held by that client.

   Unlike locks, an operation by a second client to a delegated file
   will cause the server to recall a delegation through a callback.

   On recall, the client holding the delegation must flush modified
   state (such as modified data) to the server and return the
   delegation.  The conflicting request will not receive a response
   until the recall is complete.  The recall is considered complete when



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   the client returns the delegation or the server times out on the
   recall and revokes the delegation as a result of the timeout.
   Following the resolution of the recall, the server has the
   information necessary to grant or deny the second client's request.

   At the time the client receives a delegation recall, it may have
   substantial state that needs to be flushed to the server.  Therefore,
   the server should allow sufficient time for the delegation to be
   returned since it may involve numerous RPCs to the server.  If the
   server is able to determine that the client is diligently flushing
   state to the server as a result of the recall, the server may extend
   the usual time allowed for a recall.  However, the time allowed for
   recall completion should not be unbounded.

   An example of this is when responsibility to mediate opens on a given
   file is delegated to a client (see the section "Open Delegation").
   The server will not know what opens are in effect on the client.
   Without this knowledge the server will be unable to determine if the
   access and deny state for the file allows any particular open until
   the delegation for the file has been returned.

   A client failure or a network partition can result in failure to
   respond to a recall callback. In this case, the server will revoke
   the delegation which in turn will render useless any modified state
   still on the client.

9.2.1.  Delegation Recovery

   There are three situations that delegation recovery must deal with:

   o  Client reboot or restart

   o  Server reboot or restart

   o  Network partition (full or callback-only)

   In the event the client reboots or restarts, the failure to renew
   leases will result in the revocation of record locks and share
   reservations.  Delegations, however, may be treated a bit
   differently.

   There will be situations in which delegations will need to be
   reestablished after a client reboots or restarts.  The reason for
   this is the client may have file data stored locally and this data
   was associated with the previously held delegations.  The client will
   need to reestablish the appropriate file state on the server.





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   To allow for this type of client recovery, the server may extend the
   period for delegation recovery beyond the typical lease expiration
   period.  This implies that requests from other clients that conflict
   with these delegations will need to wait.  Because the normal recall
   process may require significant time for the client to flush changed
   state to the server, other clients need be prepared for delays that
   occur because of a conflicting delegation.  This longer interval
   would increase the window for clients to reboot and consult stable
   storage so that the delegations can be reclaimed.  For open
   delegations, such delegations are reclaimed using OPEN with a claim
   type of CLAIM_DELEGATE_PREV.  (see the sections on "Data Caching and
   Revocation" and "Operation 18: OPEN" for discussion of open
   delegation and the details of OPEN respectively).

   When the server reboots or restarts, delegations are reclaimed (using
   the OPEN operation with CLAIM_DELEGATE_PREV) in a similar fashion to
   record locks and share reservations.  However, there is a slight
   semantic difference.  In the normal case if the server decides that a
   delegation should not be granted, it performs the requested action
   (e.g. OPEN) without granting any delegation.  For reclaim, the server
   grants the delegation but a special designation is applied so that
   the client treats the delegation as having been granted but recalled
   by the server.  Because of this, the client has the duty to write all
   modified state to the server and then return the delegation.  This
   process of handling delegation reclaim reconciles three principles of
   the NFS Version 4 protocol:

   o  Upon reclaim, a client reporting resources assigned to it by an
      earlier server instance must be granted those resources.

   o  The server has unquestionable authority to determine whether
      delegations are to be granted and, once granted, whether they are
      to be continued.

   o  The use of callbacks is not to be depended upon until the client
      has proven its ability to receive them.

   When a network partition occurs, delegations are subject to freeing
   by the server when the lease renewal period expires.  This is similar
   to the behavior for locks and share reservations.  For delegations,
   however, the server may extend the period in which conflicting
   requests are held off.  Eventually the occurrence of a conflicting
   request from another client will cause revocation of the delegation.
   A loss of the callback path (e.g. by later network configuration
   change) will have the same effect.  A recall request will fail and
   revocation of the delegation will result.





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   A client normally finds out about revocation of a delegation when it
   uses a stateid associated with a delegation and receives the error
   NFS4ERR_EXPIRED.  It also may find out about delegation revocation
   after a client reboot when it attempts to reclaim a delegation and
   receives that same error.  Note that in the case of a revoked write
   open delegation, there are issues because data may have been modified
   by the client whose delegation is revoked and separately by other
   clients.  See the section "Revocation Recovery for Write Open
   Delegation" for a discussion of such issues.  Note also that when
   delegations are revoked, information about the revoked delegation
   will be written by the server to stable storage (as described in the
   section "Crash Recovery").  This is done to deal with the case in
   which a server reboots after revoking a delegation but before the
   client holding the revoked delegation is notified about the
   revocation.

9.3.  Data Caching

   When applications share access to a set of files, they need to be
   implemented so as to take account of the possibility of conflicting
   access by another application.  This is true whether the applications
   in question execute on different clients or reside on the same
   client.

   Share reservations and record locks are the facilities the NFS
   version 4 protocol provides to allow applications to coordinate
   access by providing mutual exclusion facilities.  The NFS version 4
   protocol's data caching must be implemented such that it does not
   invalidate the assumptions that those using these facilities depend
   upon.

9.3.1.  Data Caching and OPENs

   In order to avoid invalidating the sharing assumptions that
   applications rely on, NFS version 4 clients should not provide cached
   data to applications or modify it on behalf of an application when it
   would not be valid to obtain or modify that same data via a READ or
   WRITE operation.

   Furthermore, in the absence of open delegation (see the section "Open
   Delegation") two additional rules apply.  Note that these rules are
   obeyed in practice by many NFS version 2 and version 3 clients.

   o  First, cached data present on a client must be revalidated after
      doing an OPEN.  This is to ensure that the data for the OPENed
      file is still correctly reflected in the client's cache.  This
      validation must be done at least when the client's OPEN operation
      includes DENY=WRITE or BOTH thus terminating a period in which



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      other clients may have had the opportunity to open the file with
      WRITE access.  Clients may choose to do the revalidation more
      often (i.e. at OPENs specifying DENY=NONE) to parallel the NFS
      version 3 protocol's practice for the benefit of users assuming
      this degree of cache revalidation.

   o  Second, modified data must be flushed to the server before closing
      a file OPENed for write.  This is complementary to the first rule.
      If the data is not flushed at CLOSE, the revalidation done after
      client OPENs as file is unable to achieve its purpose.  The other
      aspect to flushing the data before close is that the data must be
      committed to stable storage, at the server, before the CLOSE
      operation is requested by the client.  In the case of a server
      reboot or restart and a CLOSEd file, it may not be possible to
      retransmit the data to be written to the file.  Hence, this
      requirement.

9.3.2.  Data Caching and File Locking

   For those applications that choose to use file locking instead of
   share reservations to exclude inconsistent file access, there is an
   analogous set of constraints that apply to client side data caching.
   These rules are effective only if the file locking is used in a way
   that matches in an equivalent way the actual READ and WRITE
   operations executed.  This is as opposed to file locking that is
   based on pure convention.  For example, it is possible to manipulate
   a two-megabyte file by dividing the file into two one-megabyte
   regions and protecting access to the two regions by file locks on
   bytes zero and one.  A lock for write on byte zero of the file would
   represent the right to do READ and WRITE operations on the first
   region.  A lock for write on byte one of the file would represent the
   right to do READ and WRITE operations on the second region.  As long
   as all applications manipulating the file obey this convention, they
   will work on a local file system.  However, they may not work with
   the NFS version 4 protocol unless clients refrain from data caching.

   The rules for data caching in the file locking environment are:

   o  First, when a client obtains a file lock for a particular region,
      the data cache corresponding to that region (if any cache data
      exists) must be revalidated.  If the change attribute indicates
      that the file may have been updated since the cached data was
      obtained, the client must flush or invalidate the cached data for
      the newly locked region.  A client might choose to invalidate all
      of non-modified cached data that it has for the file but the only
      requirement for correct operation is to invalidate all of the data
      in the newly locked region.




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   o  Second, before releasing a write lock for a region, all modified
      data for that region must be flushed to the server.  The modified
      data must also be written to stable storage.

   Note that flushing data to the server and the invalidation of cached
   data must reflect the actual byte ranges locked or unlocked.
   Rounding these up or down to reflect client cache block boundaries
   will cause problems if not carefully done.  For example, writing a
   modified block when only half of that block is within an area being
   unlocked may cause invalid modification to the region outside the
   unlocked area.  This, in turn, may be part of a region locked by
   another client.  Clients can avoid this situation by synchronously
   performing portions of write operations that overlap that portion
   (initial or final) that is not a full block.  Similarly, invalidating
   a locked area which is not an integral number of full buffer blocks
   would require the client to read one or two partial blocks from the
   server if the revalidation procedure shows that the data which the
   client possesses may not be valid.

   The data that is written to the server as a pre-requisite to the
   unlocking of a region must be written, at the server, to stable
   storage.  The client may accomplish this either with synchronous
   writes or by following asynchronous writes with a COMMIT operation.
   This is required because retransmission of the modified data after a
   server reboot might conflict with a lock held by another client.

   A client implementation may choose to accommodate applications which
   use record locking in non-standard ways (e.g. using a record lock as
   a global semaphore) by flushing to the server more data upon an LOCKU
   than is covered by the locked range.  This may include modified data
   within files other than the one for which the unlocks are being done.
   In such cases, the client must not interfere with applications whose
   READs and WRITEs are being done only within the bounds of record
   locks which the application holds.  For example, an application locks
   a single byte of a file and proceeds to write that single byte.  A
   client that chose to handle a LOCKU by flushing all modified data to
   the server could validly write that single byte in response to an
   unrelated unlock.  However, it would not be valid to write the entire
   block in which that single written byte was located since it includes
   an area that is not locked and might be locked by another client.
   Client implementations can avoid this problem by dividing files with
   modified data into those for which all modifications are done to
   areas covered by an appropriate record lock and those for which there
   are modifications not covered by a record lock.  Any writes done for
   the former class of files must not include areas not locked and thus
   not modified on the client.





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9.3.3.  Data Caching and Mandatory File Locking

   Client side data caching needs to respect mandatory file locking when
   it is in effect.  The presence of mandatory file locking for a given
   file is indicated in the result flags for an OPEN.  When mandatory
   locking is in effect for a file, the client must check for an
   appropriate file lock for data being read or written.  If a lock
   exists for the range being read or written, the client may satisfy
   the request using the client's validated cache.  If an appropriate
   file lock is not held for the range of the read or write, the read or
   write request must not be satisfied by the client's cache and the
   request must be sent to the server for processing.  When a read or
   write request partially overlaps a locked region, the request should
   be subdivided into multiple pieces with each region (locked or not)
   treated appropriately.

9.3.4.  Data Caching and File Identity

   When clients cache data, the file data needs to organized according
   to the file system object to which the data belongs.  For NFS version
   3 clients, the typical practice has been to assume for the purpose of
   caching that distinct filehandles represent distinct file system
   objects.  The client then has the choice to organize and maintain the
   data cache on this basis.

   In the NFS version 4 protocol, there is now the possibility to have
   significant deviations from a "one filehandle per object" model
   because a filehandle may be constructed on the basis of the object's
   pathname.  Therefore, clients need a reliable method to determine if
   two filehandles designate the same file system object.  If clients
   were simply to assume that all distinct filehandles denote distinct
   objects and proceed to do data caching on this basis, caching
   inconsistencies would arise between the distinct client side objects
   which mapped to the same server side object.

   By providing a method to differentiate filehandles, the NFS version 4
   protocol alleviates a potential functional regression in comparison
   with the NFS version 3 protocol.  Without this method, caching
   inconsistencies within the same client could occur and this has not
   been present in previous versions of the NFS protocol.  Note that it
   is possible to have such inconsistencies with applications executing
   on multiple clients but that is not the issue being addressed here.

   For the purposes of data caching, the following steps allow an NFS
   version 4 client to determine whether two distinct filehandles denote
   the same server side object:





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   o  If GETATTR directed to two filehandles have different values of
      the fsid attribute, then the filehandles represent distinct
      objects.

   o  If GETATTR for any file with an fsid that matches the fsid of the
      two filehandles in question returns a unique_handles attribute
      with a value of TRUE, then the two objects are distinct.

   o  If GETATTR directed to the two filehandles does not return the
      fileid attribute for one or both of the handles, then the it
      cannot be determined whether the two objects are the same.
      Therefore, operations which depend on that knowledge (e.g.  client
      side data caching) cannot be done reliably.

   o  If GETATTR directed to the two filehandles returns different
      values for the fileid attribute, then they are distinct objects.

   o  Otherwise they are the same object.

9.4.  Open Delegation

   When a file is being OPENed, the server may delegate further handling
   of opens and closes for that file to the opening client.  Any such
   delegation is recallable, since the circumstances that allowed for
   the delegation are subject to change.  In particular, the server may
   receive a conflicting OPEN from another client, the server must
   recall the delegation before deciding whether the OPEN from the other
   client may be granted.  Making a delegation is up to the server and
   clients should not assume that any particular OPEN either will or
   will not result in an open delegation.  The following is a typical
   set of conditions that servers might use in deciding whether OPEN
   should be delegated:

   o  The client must be able to respond to the server's callback
      requests.  The server will use the CB_NULL procedure for a test of
      callback ability.

   o  The client must have responded properly to previous recalls.

   o  There must be no current open conflicting with the requested
      delegation.

   o  There should be no current delegation that conflicts with the
      delegation being requested.

   o  The probability of future conflicting open requests should be low
      based on the recent history of the file.




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   o  The existence of any server-specific semantics of OPEN/CLOSE that
      would make the required handling incompatible with the prescribed
      handling that the delegated client would apply (see below).

   There are two types of open delegations, read and write.  A read open
   delegation allows a client to handle, on its own, requests to open a
   file for reading that do not deny read access to others.  Multiple
   read open delegations may be outstanding simultaneously and do not
   conflict.  A write open delegation allows the client to handle, on
   its own, all opens.  Only one write open delegation may exist for a
   given file at a given time and it is inconsistent with any read open
   delegations.

   When a client has a read open delegation, it may not make any changes
   to the contents or attributes of the file but it is assured that no
   other client may do so.  When a client has a write open delegation,
   it may modify the file data since no other client will be accessing
   the file's data.  The client holding a write delegation may only
   affect file attributes which are intimately connected with the file
   data:  object_size, time_modify, change.

   When a client has an open delegation, it does not send OPENs or
   CLOSEs to the server but updates the appropriate status internally.
   For a read open delegation, opens that cannot be handled locally
   (opens for write or that deny read access) must be sent to the
   server.

   When an open delegation is made, the response to the OPEN contains an
   open delegation structure which specifies the following:

   o  the type of delegation (read or write)

   o  space limitation information to control flushing of data on close
      (write open delegation only, see the section "Open Delegation and
      Data Caching")

   o  an nfsace4 specifying read and write permissions

   o  a stateid to represent the delegation for READ and WRITE

   The stateid is separate and distinct from the stateid for the OPEN
   proper.  The standard stateid, unlike the delegation stateid, is
   associated with a particular nfs_lockowner and will continue to be
   valid after the delegation is recalled and the file remains open.







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   When a request internal to the client is made to open a file and open
   delegation is in effect, it will be accepted or rejected solely on
   the basis of the following conditions.  Any requirement for other
   checks to be made by the delegate should result in open delegation
   being denied so that the checks can be made by the server itself.

   o  The access and deny bits for the request and the file as described
      in the section "Share Reservations".

   o  The read and write permissions as determined below.

   The nfsace4 passed with delegation can be used to avoid frequent
   ACCESS calls.  The permission check should be as follows:

   o  If the nfsace4 indicates that the open may be done, then it should
      be granted without reference to the server.

   o  If the nfsace4 indicates that the open may not be done, then an
      ACCESS request must be sent to the server to obtain the definitive
      answer.

   The server may return an nfsace4 that is more restrictive than the
   actual ACL of the file.  This includes an nfsace4 that specifies
   denial of all access.  Note that some common practices such as
   mapping the traditional user "root" to the user "nobody" may make it
   incorrect to return the actual ACL of the file in the delegation
   response.

   The use of delegation together with various other forms of caching
   creates the possibility that no server authentication will ever be
   performed for a given user since all of the user's requests might be
   satisfied locally.  Where the client is depending on the server for
   authentication, the client should be sure authentication occurs for
   each user by use of the ACCESS operation.  This should be the case
   even if an ACCESS operation would not be required otherwise.  As
   mentioned before, the server may enforce frequent authentication by
   returning an nfsace4 denying all access with every open delegation.

9.4.1.  Open Delegation and Data Caching

   OPEN delegation allows much of the message overhead associated with
   the opening and closing files to be eliminated.  An open when an open
   delegation is in effect does not require that a validation message be
   sent to the server.  The continued endurance of the "read open
   delegation" provides a guarantee that no OPEN for write and thus no
   write has occurred.  Similarly, when closing a file opened for write
   and if write open delegation is in effect, the data written does not
   have to be flushed to the server until the open delegation is



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   recalled.  The continued endurance of the open delegation provides a
   guarantee that no open and thus no read or write has been done by
   another client.

   For the purposes of open delegation, READs and WRITEs done without an
   OPEN are treated as the functional equivalents of a corresponding
   type of OPEN.  This refers to the READs and WRITEs that use the
   special stateids consisting of all zero bits or all one bits.
   Therefore, READs or WRITEs with a special stateid done by another
   client will force the server to recall a write open delegation.  A
   WRITE with a special stateid done by another client will force a
   recall of read open delegations.

   With delegations, a client is able to avoid writing data to the
   server when the CLOSE of a file is serviced.  The CLOSE operation is
   the usual point at which the client is notified of a lack of stable
   storage for the modified file data generated by the application.  At
   the CLOSE, file data is written to the server and through normal
   accounting the server is able to determine if the available file
   system space for the data has been exceeded (i.e. server returns
   NFS4ERR_NOSPC or NFS4ERR_DQUOT).  This accounting includes quotas.
   The introduction of delegations requires that a alternative method be
   in place for the same type of communication to occur between client
   and server.

   In the delegation response, the server provides either the limit of
   the size of the file or the number of modified blocks and associated
   block size.  The server must ensure that the client will be able to
   flush data to the server of a size equal to that provided in the
   original delegation.  The server must make this assurance for all
   outstanding delegations.  Therefore, the server must be careful in
   its management of available space for new or modified data taking
   into account available file system space and any applicable quotas.
   The server can recall delegations as a result of managing the
   available file system space.  The client should abide by the server's
   state space limits for delegations.  If the client exceeds the stated
   limits for the delegation, the server's behavior is undefined.

   Based on server conditions, quotas or available file system space,
   the server may grant write open delegations with very restrictive
   space limitations.  The limitations may be defined in a way that will
   always force modified data to be flushed to the server on close.

   With respect to authentication, flushing modified data to the server
   after a CLOSE has occurred may be problematic.  For example, the user
   of the application may have logged off of the client and unexpired
   authentication credentials may not be present.  In this case, the
   client may need to take special care to ensure that local unexpired



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   credentials will in fact be available.  This may be accomplished by
   tracking the expiration time of credentials and flushing data well in
   advance of their expiration or by making private copies of
   credentials to assure their availability when needed.

9.4.2.  Open Delegation and File Locks

   When a client holds a write open delegation, lock operations are
   performed locally.  This includes those required for mandatory file
   locking.  This can be done since the delegation implies that there
   can be no conflicting locks.  Similarly, all of the revalidations
   that would normally be associated with obtaining locks and the
   flushing of data associated with the releasing of locks need not be
   done.

9.4.3.  Recall of Open Delegation

   The following events necessitate recall of an open delegation:

   o  Potentially conflicting OPEN request (or READ/WRITE done with
      "special" stateid)

   o  SETATTR issued by another client

   o  REMOVE request for the file

   o  RENAME request for the file as either source or target of the
      RENAME

   Whether a RENAME of a directory in the path leading to the file
   results in recall of an open delegation depends on the semantics of
   the server file system.  If that file system denies such RENAMEs when
   a file is open, the recall must be performed to determine whether the
   file in question is, in fact, open.

   In addition to the situations above, the server may choose to recall
   open delegations at any time if resource constraints make it
   advisable to do so.  Clients should always be prepared for the
   possibility of recall.

   The server needs to employ special handling for a GETATTR where the
   target is a file that has a write open delegation in effect.  In this
   case, the client holding the delegation needs to be interrogated.
   The server will use a CB_GETATTR callback, if the GETATTR attribute
   bits include any of the attributes that a write open delegate may
   modify (object_size, time_modify, change).





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   When a client receives a recall for an open delegation, it needs to
   update state on the server before returning the delegation.  These
   same updates must be done whenever a client chooses to return a
   delegation voluntarily.  The following items of state need to be
   dealt with:

   o  If the file associated with the delegation is no longer open and
      no previous CLOSE operation has been sent to the server, a CLOSE
      operation must be sent to the server.

   o  If a file has other open references at the client, then OPEN
      operations must be sent to the server.  The appropriate stateids
      will be provided by the server for subsequent use by the client
      since the delegation stateid will not longer be valid.  These OPEN
      requests are done with the claim type of CLAIM_DELEGATE_CUR.  This
      will allow the presentation of the delegation stateid so that the
      client can establish the appropriate rights to perform the OPEN.
      (see the section "Operation 18: OPEN" for details.)

   o  If there are granted file locks, the corresponding LOCK operations
      need to be performed.  This applies to the write open delegation
      case only.

   o  For a write open delegation, if at the time of recall the file is
      not open for write, all modified data for the file must be flushed
      to the server.  If the delegation had not existed, the client
      would have done this data flush before the CLOSE operation.

   o  For a write open delegation when a file is still open at the time
      of recall, any modified data for the file needs to be flushed to
      the server.

   o  With the write open delegation in place, it is possible that the
      file was truncated during the duration of the delegation.  For
      example, the truncation could have occurred as a result of an OPEN
      UNCHECKED with a object_size attribute value of zero.  Therefore,
      if a truncation of the file has occurred and this operation has
      not been propagated to the server, the truncation must occur
      before any modified data is written to the server.

   In the case of write open delegation, file locking imposes some
   additional requirements.  The flushing of any modified data in any
   region for which a write lock was released while the write open
   delegation was in effect is what is required to precisely maintain
   the associated invariant.  However, because the write open delegation
   implies no other locking by other clients, a simpler implementation





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   is to flush all modified data for the file (as described just above)
   if any write lock has been released while the write open delegation
   was in effect.

9.4.4.  Delegation Revocation

   At the point a delegation is revoked, if there are associated opens
   on the client, the applications holding these opens need to be
   notified.  This notification usually occurs by returning errors for
   READ/WRITE operations or when a close is attempted for the open file.

   If no opens exist for the file at the point the delegation is
   revoked, then notification of the revocation is unnecessary.
   However, if there is modified data present at the client for the
   file, the user of the application should be notified.  Unfortunately,
   it may not be possible to notify the user since active applications
   may not be present at the client.  See the section "Revocation
   Recovery for Write Open Delegation" for additional details.

9.5.  Data Caching and Revocation

   When locks and delegations are revoked, the assumptions upon which
   successful caching depend are no longer guaranteed.  The owner of the
   locks or share reservations which have been revoked needs to be
   notified.  This notification includes applications with a file open
   that has a corresponding delegation which has been revoked.  Cached
   data associated with the revocation must be removed from the client.
   In the case of modified data existing in the client's cache, that
   data must be removed from the client without it being written to the
   server.  As mentioned, the assumptions made by the client are no
   longer valid at the point when a lock or delegation has been revoked.
   For example, another client may have been granted a conflicting lock
   after the revocation of the lock at the first client.  Therefore, the
   data within the lock range may have been modified by the other
   client.  Obviously, the first client is unable to guarantee to the
   application what has occurred to the file in the case of revocation.

   Notification to a lock owner will in many cases consist of simply
   returning an error on the next and all subsequent READs/WRITEs to the
   open file or on the close.  Where the methods available to a client
   make such notification impossible because errors for certain
   operations may not be returned, more drastic action such as signals
   or process termination may be appropriate.  The justification for
   this is that an invariant for which an application depends on may be
   violated.  Depending on how errors are typically treated for the
   client operating environment, further levels of notification
   including logging, console messages, and GUI pop-ups may be
   appropriate.



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9.5.1.  Revocation Recovery for Write Open Delegation

   Revocation recovery for a write open delegation poses the special
   issue of modified data in the client cache while the file is not
   open.  In this situation, any client which does not flush modified
   data to the server on each close must ensure that the user receives
   appropriate notification of the failure as a result of the
   revocation.  Since such situations may require human action to
   correct problems, notification schemes in which the appropriate user
   or administrator is notified may be necessary.  Logging and console
   messages are typical examples.

   If there is modified data on the client, it must not be flushed
   normally to the server.  A client may attempt to provide a copy of
   the file data as modified during the delegation under a different
   name in the file system name space to ease recovery.  Unless the
   client can determine that the file has not modified by any other
   client, this technique must be limited to situations in which a
   client has a complete cached copy of the file in question.  Use of
   such a technique may be limited to files under a certain size or may
   only be used when sufficient disk space is guaranteed to be available
   within the target file system and when the client has sufficient
   buffering resources to keep the cached copy available until it is
   properly stored to the target file system.

9.6.  Attribute Caching

   The attributes discussed in this section do not include named
   attributes.  Individual named attributes are analogous to files and
   caching of the data for these needs to be handled just as data
   caching is for ordinary files.  Similarly, LOOKUP results from an
   OPENATTR directory are to be cached on the same basis as any other
   pathnames and similarly for directory contents.

   Clients may cache file attributes obtained from the server and use
   them to avoid subsequent GETATTR requests.  Such caching is write
   through in that modification to file attributes is always done by
   means of requests to the server and should not be done locally and
   cached.  The exception to this are modifications to attributes that
   are intimately connected with data caching.  Therefore, extending a
   file by writing data to the local data cache is reflected immediately
   in the object_size as seen on the client without this change being
   immediately reflected on the server.  Normally such changes are not
   propagated directly to the server but when the modified data is
   flushed to the server, analogous attribute changes are made on the
   server.  When open delegation is in effect, the modified attributes
   may be returned to the server in the response to a CB_RECALL call.




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   The result of local caching of attributes is that the attribute
   caches maintained on individual clients will not be coherent. Changes
   made in one order on the server may be seen in a different order on
   one client and in a third order on a different client.

   The typical file system application programming interfaces do not
   provide means to atomically modify or interrogate attributes for
   multiple files at the same time.  The following rules provide an
   environment where the potential incoherences mentioned above can be
   reasonably managed.  These rules are derived from the practice of
   previous NFS protocols.

   o  All attributes for a given file (per-fsid attributes excepted) are
      cached as a unit at the client so that no non-serializability can
      arise within the context of a single file.

   o  An upper time boundary is maintained on how long a client cache
      entry can be kept without being refreshed from the server.

   o  When operations are performed that change attributes at the
      server, the updated attribute set is requested as part of the
      containing RPC.  This includes directory operations that update
      attributes indirectly.  This is accomplished by following the
      modifying operation with a GETATTR operation and then using the
      results of the GETATTR to update the client's cached attributes.

   Note that if the full set of attributes to be cached is requested by
   READDIR, the results can be cached by the client on the same basis as
   attributes obtained via GETATTR.

   A client may validate its cached version of attributes for a file by
   fetching only the change attribute and assuming that if the change
   attribute has the same value as it did when the attributes were
   cached, then no attributes have changed.  The possible exception is
   the attribute time_access.

9.7.  Name Caching

   The results of LOOKUP and READDIR operations may be cached to avoid
   the cost of subsequent LOOKUP operations.  Just as in the case of
   attribute caching, inconsistencies may arise among the various client
   caches.  To mitigate the effects of these inconsistencies and given
   the context of typical file system APIs, the following rules should
   be followed:

   o  The results of unsuccessful LOOKUPs should not be cached, unless
      they are specifically reverified at the point of use.




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   o  An upper time boundary is maintained on how long a client name
      cache entry can be kept without verifying that the entry has not
      been made invalid by a directory change operation performed by
      another client.

   When a client is not making changes to a directory for which there
   exist name cache entries, the client needs to periodically fetch
   attributes for that directory to ensure that it is not being
   modified.  After determining that no modification has occurred, the
   expiration time for the associated name cache entries may be updated
   to be the current time plus the name cache staleness bound.

   When a client is making changes to a given directory, it needs to
   determine whether there have been changes made to the directory by
   other clients.  It does this by using the change attribute as
   reported before and after the directory operation in the associated
   change_info4 value returned for the operation.  The server is able to
   communicate to the client whether the change_info4 data is provided
   atomically with respect to the directory operation.  If the change
   values are provided atomically, the client is then able to compare
   the pre-operation change value with the change value in the client's
   name cache.  If the comparison indicates that the directory was
   updated by another client, the name cache associated with the
   modified directory is purged from the client.  If the comparison
   indicates no modification, the name cache can be updated on the
   client to reflect the directory operation and the associated timeout
   extended.  The post-operation change value needs to be saved as the
   basis for future change_info4 comparisons.

   As demonstrated by the scenario above, name caching requires that the
   client revalidate name cache data by inspecting the change attribute
   of a directory at the point when the name cache item was cached.
   This requires that the server update the change attribute for
   directories when the contents of the corresponding directory is
   modified.  For a client to use the change_info4 information
   appropriately and correctly, the server must report the pre and post
   operation change attribute values atomically.  When the server is
   unable to report the before and after values atomically with respect
   to the directory operation, the server must indicate that fact in the
   change_info4 return value.  When the information is not atomically
   reported, the client should not assume that other clients have not
   changed the directory.

9.8.  Directory Caching

   The results of READDIR operations may be used to avoid subsequent
   READDIR operations.  Just as in the cases of attribute and name
   caching, inconsistencies may arise among the various client caches.



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   To mitigate the effects of these inconsistencies, and given the
   context of typical file system APIs, the following rules should be
   followed:

   o  Cached READDIR information for a directory which is not obtained
      in a single READDIR operation must always be a consistent snapshot
      of directory contents.  This is determined by using a GETATTR
      before the first READDIR and after the last of READDIR that
      contributes to the cache.

   o  An upper time boundary is maintained to indicate the length of
      time a directory cache entry is considered valid before the client
      must revalidate the cached information.

   The revalidation technique parallels that discussed in the case of
   name caching.  When the client is not changing the directory in
   question, checking the change attribute of the directory with GETATTR
   is adequate.  The lifetime of the cache entry can be extended at
   these checkpoints.  When a client is modifying the directory, the
   client needs to use the change_info4 data to determine whether there
   are other clients modifying the directory.  If it is determined that
   no other client modifications are occurring, the client may update
   its directory cache to reflect its own changes.

   As demonstrated previously, directory caching requires that the
   client revalidate directory cache data by inspecting the change
   attribute of a directory at the point when the directory was cached.
   This requires that the server update the change attribute for
   directories when the contents of the corresponding directory is
   modified.  For a client to use the change_info4 information
   appropriately and correctly, the server must report the pre and post
   operation change attribute values atomically.  When the server is
   unable to report the before and after values atomically with respect
   to the directory operation, the server must indicate that fact in the
   change_info4 return value.  When the information is not atomically
   reported, the client should not assume that other clients have not
   changed the directory.

10.  Minor Versioning

   To address the requirement of an NFS protocol that can evolve as the
   need arises, the NFS version 4 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 must follow the IETF process and be
   documented in a standards track RFC.  Therefore, each minor version
   number will correspond to an RFC.  Minor version zero of the NFS



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   version 4 protocol is represented by this RFC.  The COMPOUND
   procedure will 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 decide to
   modify or add to the following rules as part of the minor version
   definition.

   1    Procedures are not added or deleted

        To maintain the general RPC model, NFS version 4 minor versions
        will not add 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.

   2.1  Minor versions may append attributes to GETATTR4args, bitmap4,
        and GETATTR4res.

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

   2.2  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 will 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.

        o  adding bits to flag fields such as new attributes to
           GETATTR's bitmap4 data type




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

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

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

   5    Minor versions may not delete operations.

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

   6    Minor versions may not delete attributes.

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

   8    Minor versions may declare an operation as mandatory to NOT
        implement.

        Specifying an operation as "mandatory to not implement" is
        equivalent to obsoleting an operation.  For the client, it means
        that the operation should 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.

   8.1  Minor versions may declare attributes mandatory to NOT
        implement.

   8.2  Minor versions may declare flag bits or enumeration values as
        mandatory to NOT implement.

   9    Minor versions may downgrade features from mandatory to
        recommended, or recommended to optional.

   10   Minor versions may upgrade features from optional to recommended
        or recommended to mandatory.

   11   A client and server that support minor version X must support
        minor versions 0 (zero) through X-1 as well.

   12   No new features may be introduced as mandatory in a minor
        version.





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        This rule allows for the introduction of new functionality and
        forces the use of implementation experience before designating a
        feature as mandatory.

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

11.  Internationalization

   The primary issue in which NFS needs to deal with
   internationalization, or I18n, is with respect to file names and
   other strings as used within the protocol.  The choice of string
   representation must allow reasonable name/string access to clients
   which use various languages.  The UTF-8 encoding of the UCS as
   defined by [ISO10646] allows for this type of access and follows the
   policy described in "IETF Policy on Character Sets and Languages",
   [RFC2277].  This choice is explained further in the following.

11.1.  Universal Versus Local Character Sets

   [RFC1345] describes a table of 16 bit characters for many different
   languages (the bit encodings match Unicode, though of course RFC1345
   is somewhat out of date with respect to current Unicode assignments).
   Each character from each language has a unique 16 bit value in the 16
   bit character set.  Thus this table can be thought of as a universal
   character set.  [RFC1345] then talks about groupings of subsets of
   the entire 16 bit character set into "Charset Tables".  For example
   one might take all the Greek characters from the 16 bit table (which
   are consecutively allocated), and normalize their offsets to a table
   that fits in 7 bits.  Thus it is determined that "lower case alpha"
   is in the same position as "upper case a" in the US-ASCII table, and
   "upper case alpha" is in the same position as "lower case a" in the
   US-ASCII table.

   These normalized subset character sets can be thought of as "local
   character sets", suitable for an operating system locale.

   Local character sets are not suitable for the NFS protocol.  Consider
   someone who creates a file with a name in a Swedish character set.
   If someone else later goes to access the file with their locale set
   to the Swedish language, then there are no problems.  But if someone
   in say the US-ASCII locale goes to access the file, the file name
   will look very different, because the Swedish characters in the 7 bit
   table will now be represented in US-ASCII characters on the display.
   It would be preferable to give the US-ASCII user a way to display the




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   file name using Swedish glyphs. In order to do that, the NFS protocol
   would have to include the locale with the file name on each operation
   to create a file.

   But then what of the situation when there is a path name on the
   server like:

         /component-1/component-2/component-3

   Each component could have been created with a different locale.  If
   one issues CREATE with multi-component path name, and if some of the
   leading components already exist, what is to be done with the
   existing components?  Is the current locale attribute replaced with
   the user's current one?  These types of situations quickly become too
   complex when there is an alternate solution.

   If the NFS version 4 protocol used a universal 16 bit or 32 bit
   character set (or an encoding of a 16 bit or 32 bit character set
   into octets), then the server and client need not care if the locale
   of the user accessing the file is different than the locale of the
   user who created the file.  The unique 16 bit or 32 bit encoding of
   the character allows for determination of what language the character
   is from and also how to display that character on the client.  The
   server need not know what locales are used.

11.2.  Overview of Universal Character Set Standards

   The previous section makes a case for using a universal character
   set.  This section makes the case for using UTF-8 as the specific
   universal character set for the NFS version 4 protocol.

   [RFC2279] discusses UTF-* (UTF-8 and other UTF-XXX encodings),
   Unicode, and UCS-*.  There are two standards bodies managing
   universal code sets:

   o  ISO/IEC which has the standard 10646-1

   o  Unicode which has the Unicode standard

   Both standards bodies have pledged to track each other's assignments
   of character codes.

   The following is a brief analysis of the various standards.

   UCS       Universal Character Set.  This is ISO/IEC 10646-1: "a
             multi-octet character set called the Universal Character
             Set (UCS), which encompasses most of the world's writing
             systems."



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   UCS-2     a two octet per character encoding that addresses the first
             2^16 characters of UCS. Currently there are no UCS
             characters beyond that range.

   UCS-4     a four octet per character encoding that permits the
             encoding of up to 2^31 characters.

   UTF       UTF is an abbreviation of the term "UCS transformation
             format" and is used in the naming of various standards for
             encoding of UCS characters as described below.

   UTF-1     Only historical interest; it has been removed from 10646-1

   UTF-7     Encodes the entire "repertoire" of UCS "characters using
             only octets with the higher order bit clear".  [RFC2152]
             describes UTF-7. UTF-7 accomplishes this by reserving one
             of the 7bit US-ASCII characters as a "shift" character to
             indicate non-US-ASCII characters.

   UTF-8     Unlike UTF-7, uses all 8 bits of the octets. US-ASCII
             characters are encoded as before unchanged. Any octet with
             the high bit cleared can only mean a US-ASCII character.
             The high bit set means that a UCS character is being
             encoded.

   UTF-16    Encodes UCS-4 characters into UCS-2 characters using a
             reserved range in UCS-2.


   Unicode   Unicode and UCS-2 are the same; [RFC2279] states:

             Up to the present time, changes in Unicode and amendments
             to ISO/IEC 10646 have tracked each other, so that the
             character repertoires and code point assignments have
             remained in sync.  The relevant standardization committees
             have committed to maintain this very useful synchronism.

11.3.  Difficulties with UCS-4, UCS-2, Unicode

   Adapting existing applications, and file systems to multi-octet
   schemes like UCS and Unicode can be difficult.  A significant amount
   of code has been written to process streams of bytes. Also there are
   many existing stored objects described with 7 bit or 8 bit
   characters. Doubling or quadrupling the bandwidth and storage
   requirements seems like an expensive way to accomplish I18N.






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   UCS-2 and Unicode are "only" 16 bits long.  That might seem to be
   enough but, according to [Unicode1], 49,194 Unicode characters are
   already assigned.  According to [Unicode2] there are still more
   languages that need to be added.

11.4.  UTF-8 and its solutions

   UTF-8 solves problems for NFS that exist with the use of UCS and
   Unicode.  UTF-8 will encode 16 bit and 32 bit characters in a way
   that will be compact for most users. The encoding table from UCS-4 to
   UTF-8, as copied from [RFC2279]:

      UCS-4 range (hex.)           UTF-8 octet sequence (binary)
    0000 0000-0000 007F   0xxxxxxx
    0000 0080-0000 07FF   110xxxxx 10xxxxxx
    0000 0800-0000 FFFF   1110xxxx 10xxxxxx 10xxxxxx
    0001 0000-001F FFFF   11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
    0020 0000-03FF FFFF   111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
    0400 0000-7FFF FFFF   1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
                          10xxxxxx

   See [RFC2279] for precise encoding and decoding rules. Note because
   of UTF-16, the algorithm from Unicode/UCS-2 to UTF-8 needs to account
   for the reserved range between D800 and DFFF.

   Note that the 16 bit UCS or Unicode characters require no more than 3
   octets to encode into UTF-8

   Interestingly, UTF-8 has room to handle characters larger than 31
   bits, because the leading octet of form:

         1111111x

   is not defined. If needed, ISO could either use that octet to
   indicate a sequence of an encoded 8 octet character, or perhaps use
   11111110 to permit the next octet to indicate an even more expandable
   character set.

   So using UTF-8 to represent character encodings means never having to
   run out of room.

11.5.  Normalization

   The client and server operating environments may differ in their
   policies and operational methods with respect to character
   normalization (See [Unicode1] for a discussion of normalization
   forms).  This difference may also exist between applications on the
   same client.  This adds to the difficulty of providing a single



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   normalization policy for the protocol that allows for maximal
   interoperability.  This issue is similar to the character case issues
   where the server may or may not support case insensitive file name
   matching and may or may not preserve the character case when storing
   file names.  The protocol does not mandate a particular behavior but
   allows for the various permutations.

   The NFS version 4 protocol does not mandate the use of a particular
   normalization form at this time.  A later revision of this
   specification may specify a particular normalization form.
   Therefore, the server and client can expect that they may receive
   unnormalized characters within protocol requests and responses.  If
   the operating environment requires normalization, then the
   implementation must normalize the various UTF-8 encoded strings
   within the protocol before presenting the information to an
   application (at the client) or local file system (at the server).

12.  Error Definitions

   NFS error numbers are assigned to failed operations within a compound
   request.  A compound request contains a number of NFS operations that
   have their results encoded in sequence in a compound reply.  The
   results of successful operations will consist of an NFS4_OK status
   followed by the encoded results of the operation.  If an NFS
   operation fails, an error status will be entered in the reply and the
   compound request will be terminated.

   A description of each defined error follows:

   NFS4_OK               Indicates the operation completed successfully.

   NFS4ERR_ACCES         Permission denied. The caller does not have the
                         correct permission to perform the requested
                         operation. Contrast this with NFS4ERR_PERM,
                         which restricts itself to owner or privileged
                         user permission failures.

   NFS4ERR_BADHANDLE     Illegal NFS file handle. The file handle failed
                         internal consistency checks.

   NFS4ERR_BADTYPE       An attempt was made to create an object of a
                         type not supported by the server.

   NFS4ERR_BAD_COOKIE    READDIR cookie is stale.

   NFS4ERR_BAD_SEQID     The sequence number in a locking request is
                         neither the next expected number or the last
                         number processed.



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   NFS4ERR_BAD_STATEID   A stateid generated by the current server
                         instance, but which does not designate any
                         locking state (either current or superseded)
                         for a current lockowner-file pair, was used.

   NFS4ERR_CLID_INUSE    The SETCLIENTID procedure has found that a
                         client id is already in use by another client.

   NFS4ERR_DELAY         The server initiated the request, but was not
                         able to complete it in a timely fashion. The
                         client should wait and then try the request
                         with a new RPC transaction ID.  For example,
                         this error should be returned from a server
                         that supports hierarchical storage and receives
                         a request to process a file that has been
                         migrated. In this case, the server should start
                         the immigration process and respond to client
                         with this error.  This error may also occur
                         when a necessary delegation recall makes
                         processing a request in a timely fashion
                         impossible.

   NFS4ERR_DENIED        An attempt to lock a file is denied.  Since
                         this may be a temporary condition, the client
                         is encouraged to retry the lock request until
                         the lock is accepted.

   NFS4ERR_DQUOT         Resource (quota) hard limit exceeded. The
                         user's resource limit on the server has been
                         exceeded.

   NFS4ERR_EXIST         File exists. The file specified already exists.

   NFS4ERR_EXPIRED       A lease has expired that is being used in the
                         current procedure.

   NFS4ERR_FBIG          File too large. The operation would have caused
                         a file to grow beyond the server's limit.

   NFS4ERR_FHEXPIRED     The file handle provided is volatile and has
                         expired at the server.

   NFS4ERR_GRACE         The server is in its recovery or grace period
                         which should match the lease period of the
                         server.






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   NFS4ERR_INVAL         Invalid argument or unsupported argument for an
                         operation. Two examples are attempting a
                         READLINK on an object other than a symbolic
                         link or attempting to SETATTR a time field on a
                         server that does not support this operation.

   NFS4ERR_IO            I/O error. A hard error (for example, a disk
                         error) occurred while processing the requested
                         operation.

   NFS4ERR_ISDIR         Is a directory. The caller specified a
                         directory in a non-directory operation.

   NFS4ERR_LEASE_MOVED   A lease being renewed is associated with a file
                         system that has been migrated to a new server.

   NFS4ERR_LOCKED        A read or write operation was attempted on a
                         locked file.

   NFS4ERR_LOCK_RANGE    A lock request is operating on a sub-range of a
                         current lock for the lock owner and the server
                         does not support this type of request.

   NFS4ERR_MINOR_VERS_MISMATCH
                         The server has received a request that
                         specifies an unsupported minor version.  The
                         server must return a COMPOUND4res with a zero
                         length operations result array.

   NFS4ERR_MLINK         Too many hard links.

   NFS4ERR_MOVED         The filesystem which contains the current
                         filehandle object has been relocated or
                         migrated to another server.  The client may
                         obtain the new filesystem location by obtaining
                         the "fs_locations" attribute for the current
                         filehandle.  For further discussion, refer to
                         the section "Filesystem Migration or
                         Relocation".

   NFS4ERR_NAMETOOLONG   The filename in an operation was too long.

   NFS4ERR_NODEV         No such device.

   NFS4ERR_NOENT         No such file or directory. The file or
                         directory name specified does not exist.





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   NFS4ERR_NOFILEHANDLE  The logical current file handle value has not
                         been set properly.  This may be a result of a
                         malformed COMPOUND operation (i.e. no PUTFH or
                         PUTROOTFH before an operation that requires the
                         current file handle be set).

   NFS4ERR_NOSPC         No space left on device. The operation would
                         have caused the server's file system to exceed
                         its limit.

   NFS4ERR_NOTDIR        Not a directory. The caller specified a non-
                         directory in a directory operation.

   NFS4ERR_NOTEMPTY      An attempt was made to remove a directory that
                         was not empty.

   NFS4ERR_NOTSUPP       Operation is not supported.

   NFS4ERR_NOT_SAME      This error is returned by the VERIFY operation
                         to signify that the attributes compared were
                         not the same as provided in the client's
                         request.

   NFS4ERR_NXIO          I/O error. No such device or address.

   NFS4ERR_OLD_STATEID   A stateid which designates the locking state
                         for a lockowner-file at an earlier time was
                         used.

   NFS4ERR_PERM          Not owner. The operation was not allowed
                         because the caller is either not a privileged
                         user (root) or not the owner of the target of
                         the operation.

   NFS4ERR_READDIR_NOSPC The encoded response to a READDIR request
                         exceeds the size limit set by the initial
                         request.

   NFS4ERR_RESOURCE      For the processing of the COMPOUND procedure,
                         the server may exhaust available resources and
                         can not continue processing procedures within
                         the COMPOUND operation.  This error will be
                         returned from the server in those instances of
                         resource exhaustion related to the processing
                         of the COMPOUND procedure.

   NFS4ERR_ROFS          Read-only file system. A modifying operation
                         was attempted on a read-only file system.



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   NFS4ERR_SAME          This error is returned by the NVERIFY operation
                         to signify that the attributes compared were
                         the same as provided in the client's request.

   NFS4ERR_SERVERFAULT   An error occurred on the server which does not
                         map to any of the legal NFS version 4 protocol
                         error values.  The client should translate this
                         into an appropriate error.  UNIX clients may
                         choose to translate this to EIO.

   NFS4ERR_SHARE_DENIED  An attempt to OPEN a file with a share
                         reservation has failed because of a share
                         conflict.

   NFS4ERR_STALE         Invalid file handle. The file handle given in
                         the arguments was invalid. The file referred to
                         by that file handle no longer exists or access
                         to it has been revoked.

   NFS4ERR_STALE_CLIENTID A clientid not recognized by the server was
                         used in a locking or SETCLIENTID_CONFIRM
                         request.

   NFS4ERR_STALE_STATEID A stateid generated by an earlier server
                         instance was used.

   NFS4ERR_SYMLINK       The current file handle provided for a LOOKUP
                         is not a directory but a symbolic link.  Also
                         used if the final component of the OPEN path is
                         a symbolic link.

                         NFS4ERR_TOOSMALL      Buffer or request is too
                         small.

   NFS4ERR_WRONGSEC      The security mechanism being used by the client
                         for the procedure does not match the server's
                         security policy.  The client should change the
                         security mechanism being used and retry the
                         operation.

   NFS4ERR_XDEV          Attempt to do a cross-device hard link.

13.  NFS Version 4 Requests

   For the NFS version 4 RPC program, there are two traditional RPC
   procedures: NULL and COMPOUND.  All other functionality is defined as
   a set of operations and these operations are defined in normal
   XDR/RPC syntax and semantics.  However, these operations are



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   encapsulated within the COMPOUND procedure.  This requires that the
   client combine one or more of the NFS version 4 operations into a
   single request.

   The NFS4_CALLBACK program is used to provide server to client
   signaling and is constructed in a similar fashion as the NFS version
   4 program.  The procedures CB_NULL and CB_COMPOUND are defined in the
   same way as NULL and COMPOUND are within the NFS program.  The
   CB_COMPOUND request also encapsulates the remaining operations of the
   NFS4_CALLBACK program.  There is no predefined RPC program number for
   the NFS4_CALLBACK program.  It is up to the client to specify a
   program number in the "transient" program range.  The program and
   port number of the NFS4_CALLBACK program are provided by the client
   as part of the SETCLIENTID operation and therefore is fixed for the
   life of the client instantiation.

13.1.  Compound Procedure

   The COMPOUND procedure provides the opportunity for better
   performance within high latency networks.  The client can avoid
   cumulative latency of multiple RPCs by combining multiple dependent
   operations into a single COMPOUND procedure.  A compound operation
   may provide for protocol simplification by allowing the client to
   combine basic procedures into a single request that is customized for
   the client's environment.

   The CB_COMPOUND procedure precisely parallels the features of
   COMPOUND as described above.

   The basics of the COMPOUND procedures construction is:

                  +-----------+-----------+-----------+--
                  | op + args | op + args | op + args |
                  +-----------+-----------+-----------+--

   and the reply looks like this:

      +------------+-----------------------+-----------------------+--
      |last status | status + op + results | status + op + results |
      +------------+-----------------------+-----------------------+--

13.2.  Evaluation of a Compound Request

   The server will process the COMPOUND procedure by evaluating each of
   the operations within the COMPOUND procedure in order.  Each
   component operation consists of a 32 bit operation code, followed by
   the argument of length determined by the type of operation. The
   results of each operation are encoded in sequence into a reply



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   buffer.  The results of each operation are preceded by the opcode and
   a status code (normally zero).  If an operation results in a non-zero
   status code, the status will be encoded and evaluation of the
   compound sequence will halt and the reply will be returned.  Note
   that evaluation stops even in the event of "non error" conditions
   such as NFS4ERR_SAME.

   There are no atomicity requirements for the operations contained
   within the COMPOUND procedure.  The operations being evaluated as
   part of a COMPOUND request may be evaluated simultaneously with other
   COMPOUND requests that the server receives.

   It is the client's responsibility for recovering from any partially
   completed COMPOUND procedure.  Partially completed COMPOUND
   procedures may occur at any point due to errors such as
   NFS4ERR_RESOURCE and NFS4ERR_LONG_DELAY.  This may occur even given
   an otherwise valid operation string.  Further, a server reboot which
   occurs in the middle of processing a COMPOUND procedure may leave the
   client with the difficult task of determining how far COMPOUND
   processing has proceeded.  Therefore, the client should avoid overly
   complex COMPOUND procedures in the event of the failure of an
   operation within the procedure.

   Each operation assumes a "current" and "saved" filehandle that is
   available as part of the execution context of the compound request.
   Operations may set, change, or return the current filehandle.  The
   "saved" filehandle is used for temporary storage of a filehandle
   value and as operands for the RENAME and LINK operations.

13.3.  Synchronous Modifying Operations

   NFS version 4 operations that modify the file system are synchronous.
   When an operation is successfully completed at the server, the client
   can depend that any data associated with the request is now on stable
   storage (the one exception is in the case of the file data in a WRITE
   operation with the UNSTABLE option specified).

   This implies that any previous operations within the same compound
   request are also reflected in stable storage.  This behavior enables
   the client's ability to recover from a partially executed compound
   request which may resulted from the failure of the server.  For
   example, if a compound request contains operations A and B and the
   server is unable to send a response to the client, depending on the
   progress the server made in servicing the request the result of both
   operations may be reflected in stable storage or just operation A may
   be reflected.  The server must not have just the results of operation
   B in stable storage.




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13.4.  Operation Values

   The operations encoded in the COMPOUND procedure are identified by
   operation values.  To avoid overlap with the RPC procedure numbers,
   operations 0 (zero) and 1 are not defined.  Operation 2 is not
   defined but reserved for future use with minor versioning.

14.  NFS Version 4 Procedures

14.1.  Procedure 0: NULL - No Operation

   SYNOPSIS

      <null>

   ARGUMENT

      void;


   RESULT

      void;

   DESCRIPTION

      Standard NULL procedure.  Void argument, void response.  This
      procedure has no functionality associated with it.  Because of
      this it is sometimes used to measure the overhead of processing a
      service request.  Therefore, the server should ensure that no
      unnecessary work is done in servicing this procedure.

   ERRORS

      None.

14.2.  Procedure 1: COMPOUND - Compound Operations

   SYNOPSIS

      compoundargs -> compoundres

   ARGUMENT

      union nfs_argop4 switch (nfs_opnum4 argop) {
              case <OPCODE>: <argument>;
              ...
      };



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      struct COMPOUND4args {
              utf8string      tag;
              uint32_t        minorversion;
              nfs_argop4      argarray<>;
      };

   RESULT

         union nfs_resop4 switch (nfs_opnum4 resop){
                 case <OPCODE>: <result>;
                 ...
         };

         struct COMPOUND4res {
                 nfsstat4        status;
                 utf8string      tag;
                 nfs_resop4      resarray<>;
         };

   DESCRIPTION

      The COMPOUND procedure is used to combine one or more of the NFS
      operations into a single RPC request.  The main NFS RPC program
      has two main procedures: NULL and COMPOUND.  All other operations
      use the COMPOUND procedure as a wrapper.

      The COMPOUND procedure is used to combine individual operations
      into a single RPC request.  The server interprets each of the
      operations in turn.  If an operation is executed by the server and
      the status of that operation is NFS4_OK, then the next operation
      in the COMPOUND procedure is executed.  The server continues this
      process until there are no more operations to be executed or one
      of the operations has a status value other than NFS4_OK.

      In the processing of the COMPOUND procedure, the server may find
      that it does not have the available resources to execute any or
      all of the operations within the COMPOUND sequence.  In this case,
      the error NFS4ERR_RESOURCE will be returned for the particular
      operation within the COMPOUND procedure where the resource
      exhaustion occurred.  This assumes that all previous operations
      within the COMPOUND sequence have been evaluated successfully.
      The results for all of the evaluated operations must be returned
      to the client.

      The COMPOUND arguments contain a "minorversion" field.  The
      initial and default value for this field is 0 (zero).  This field
      will be used by future minor versions such that the client can
      communicate to the server what minor version is being requested.



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      If the server receives a COMPOUND procedure with a minorversion
      field value that it does not support, the server MUST return an
      error of NFS4ERR_MINOR_VERS_MISMATCH and a zero length resultdata
      array.

      Contained within the COMPOUND results is a "status" field.  If the
      results array length is non-zero, this status must be equivalent
      to the status of the last operation that was executed within the
      COMPOUND procedure.  Therefore, if an operation incurred an error
      then the "status" value will be the same error value as is being
      returned for the operation that failed.

      Note that operations, 0 (zero) and 1 (one) are not defined for the
      COMPOUND procedure.  If the server receives an operation array
      with either of these included, an error of NFS4ERR_NOTSUPP must be
      returned.  Operation 2 is not defined but reserved for future
      definition and use with minor versioning.  If the server receives
      a operation array that contains operation 2 and the minorversion
      field has a value of 0 (zero), an error of NFS4ERR_NOTSUPP is
      returned.  If an operation array contains an operation 2 and the
      minorversion field is non-zero and the server does not support the
      minor version, the server returns an error of
      NFS4ERR_MINOR_VERS_MISMATCH.  Therefore, the
      NFS4ERR_MINOR_VERS_MISMATCH error takes precedence over all other
      errors.

   IMPLEMENTATION

      Note that the definition of the "tag" in both the request and
      response are left to the implementor.  It may be used to summarize
      the content of the compound request for the benefit of packet
      sniffers and engineers debugging implementations.

      Since an error of any type may occur after only a portion of the
      operations have been evaluated, the client must be prepared to
      recover from any failure.  If the source of an NFS4ERR_RESOURCE
      error was a complex or lengthy set of operations, it is likely
      that if the number of operations were reduced the server would be
      able to evaluate them successfully.  Therefore, the client is
      responsible for dealing with this type of complexity in recovery.

   ERRORS

      All errors defined in the protocol







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14.2.1.  Operation 3: ACCESS - Check Access Rights

   SYNOPSIS

         (cfh), accessreq -> supported, accessrights

   ARGUMENT

         const ACCESS4_READ      = 0x00000001;
         const ACCESS4_LOOKUP    = 0x00000002;
         const ACCESS4_MODIFY    = 0x00000004;
         const ACCESS4_EXTEND    = 0x00000008;
         const ACCESS4_DELETE    = 0x00000010;
         const ACCESS4_EXECUTE   = 0x00000020;

         struct ACCESS4args {
                 /* CURRENT_FH: object */
                 uint32_t        access;
         };

   RESULT

         struct ACCESS4resok {
                 uint32_t        supported;
                 uint32_t        access;
         };

         union ACCESS4res switch (nfsstat4 status) {
          case NFS4_OK:
                  ACCESS4resok   resok4;
          default:
                  void;
         };

   DESCRIPTION

      ACCESS determines the access rights that a user, as identified by
      the credentials in the RPC request, has with respect to the file
      system object specified by the current filehandle.  The client
      encodes the set of access rights that are to be checked in the bit
      mask "access".  The server checks the permissions encoded in the
      bit mask.  If a status of NFS4_OK is returned, two bit masks are
      included in the response.  The first, "supported", represents the
      access rights for which the server can verify reliably.  The
      second, "access", represents the access rights available to the
      user for the filehandle provided.  On success, the current
      filehandle retains its value.




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      Note that the supported field will contain only as many values as
      was originally sent in the arguments.  For example, if the client
      sends an ACCESS operation with only the ACCESS4_READ value set and
      the server supports this value, the server will return only
      ACCESS4_READ even if it could have reliably checked other values.

      The results of this operation are necessarily advisory in nature.
      A return status of NFS4_OK and the appropriate bit set in the bit
      mask does not imply that such access will be allowed to the file
      system object in the future. This is because access rights can be
      revoked by the server at any time.

      The following access permissions may be requested:

   ACCESS4_READ    Read data from file or read a directory.

   ACCESS4_LOOKUP  Look up a name in a directory (no meaning for non-
                   directory objects).

   ACCESS4_MODIFY  Rewrite existing file data or modify existing
                   directory entries.

   ACCESS4_EXTEND  Write new data or add directory entries.

   ACCESS4_DELETE  Delete an existing directory entry (no meaning for
                   non-directory objects).

   ACCESS4_EXECUTE Execute file (no meaning for a directory).

   On success, the current filehandle retains its value.

   IMPLEMENTATION

      For the NFS version 4 protocol, the use of the ACCESS procedure
      when opening a regular file is deprecated in favor of using OPEN.

      In general, it is not sufficient for the client to attempt to
      deduce access permissions by inspecting the uid, gid, and mode
      fields in the file attributes or by attempting to interpret the
      contents of the ACL attribute.  This is because the server may
      perform uid or gid mapping or enforce additional access control
      restrictions.  It is also possible that the server may not be in
      the same ID space as the client.  In these cases (and perhaps
      others), the client can not reliably perform an access check with
      only current file attributes.






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      In the NFS version 2 protocol, the only reliable way to determine
      whether an operation was allowed was to try it and see if it
      succeeded or failed.  Using the ACCESS procedure in the NFS
      version 4 protocol, the client can ask the server to indicate
      whether or not one or more classes of operations are permitted.
      The ACCESS operation is provided to allow clients to check before
      doing a series of operations which will result in an access
      failure.  The OPEN operation provides a point where the server can
      verify access to the file object and method to return that
      information to the client.  The ACCESS operation is still useful
      for directory operations or for use in the case the UNIX API
      "access" is used on the client.

      The information returned by the server in response to an ACCESS
      call is not permanent.  It was correct at the exact time that the
      server performed the checks, but not necessarily afterwards.  The
      server can revoke access permission at any time.

      The client should use the effective credentials of the user to
      build the authentication information in the ACCESS request used to
      determine access rights.  It is the effective user and group
      credentials that are used in subsequent read and write operations.

      Many implementations do not directly support the ACCESS4_DELETE
      permission.  Operating systems like UNIX will ignore the
      ACCESS4_DELETE bit if set on an access request on a non-directory
      object.  In these systems, delete permission on a file is
      determined by the access permissions on the directory in which the
      file resides, instead of being determined by the permissions of
      the file itself.  Therefore, the mask returned enumerating which
      access rights can be determined will have the ACCESS4_DELETE value
      set to 0.  This indicates to the client that the server was unable
      to check that particular access right.  The ACCESS4_DELETE bit in
      the access mask returned will then be ignored by the client.

   ERRORS

         NFS4ERR_ACCES
         NFS4ERR_BADHANDLE
         NFS4ERR_DELAY
         NFS4ERR_FHEXPIRED
         NFS4ERR_IO
         NFS4ERR_MOVED
         NFS4ERR_NOFILEHANDLE
         NFS4ERR_RESOURCE
         NFS4ERR_SERVERFAULT
         NFS4ERR_STALE
         NFS4ERR_WRONGSEC