RFC 3530: Network File System (NFS) version 4 Protocol
- S. Shepler,
- B. Callaghan,
- D. Robinson,
- R. Thurlow,
- C. Beame,
- M. Eisler,
- D. Noveck
Proposed Standard
This RFC is now obsolete, see
Network Working Group S. Shepler Request for Comments: 3530 B. Callaghan Obsoletes: 3010 D. Robinson Category: Standards Track R. Thurlow Sun Microsystems, Inc. C. Beame Hummingbird Ltd. M. Eisler D. Noveck Network Appliance, Inc. April 2003 Network File System (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 (2003). All Rights Reserved. Abstract The Network File System (NFS) version 4 is a distributed filesystem protocol which owes heritage to NFS protocol version 2, RFC 1094, and version 3, RFC 1813. 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. This document replaces RFC 3010 as the definition of the NFS version 4 protocol. 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 [RFC2119]. Shepler, et al. Standards Track [Page 1]
RFC 3530 NFS version 4 Protocol April 2003 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 8 1.1. Changes since RFC 3010 . . . . . . . . . . . . . . . 8 1.2. NFS version 4 Goals. . . . . . . . . . . . . . . . . 9 1.3. Inconsistencies of this Document with Section 18 . . 9 1.4. Overview of NFS version 4 Features . . . . . . . . . 10 1.4.1. RPC and Security . . . . . . . . . . . . . . 10 1.4.2. Procedure and Operation Structure. . . . . . 10 1.4.3. Filesystem Mode. . . . . . . . . . . . . . . 11 1.4.3.1. Filehandle Types . . . . . . . . . 11 1.4.3.2. Attribute Types. . . . . . . . . . 12 1.4.3.3. Filesystem Replication and Migration. . . . . . . . . . . . . 13 1.4.4. OPEN and CLOSE . . . . . . . . . . . . . . . 13 1.4.5. File locking . . . . . . . . . . . . . . . . 13 1.4.6. Client Caching and Delegation. . . . . . . . 13 1.5. General Definitions. . . . . . . . . . . . . . . . . 14 2. Protocol Data Types. . . . . . . . . . . . . . . . . . . . 16 2.1. Basic Data Types . . . . . . . . . . . . . . . . . . 16 2.2. Structured Data Types. . . . . . . . . . . . . . . . 18 3. RPC and Security Flavor. . . . . . . . . . . . . . . . . . 23 3.1. Ports and Transports . . . . . . . . . . . . . . . . 23 3.1.1. Client Retransmission Behavior . . . . . . . 24 3.2. Security Flavors . . . . . . . . . . . . . . . . . . 25 3.2.1. Security mechanisms for NFS version 4. . . . 25 3.2.1.1. Kerberos V5 as a security triple . 25 3.2.1.2. LIPKEY as a security triple. . . . 26 3.2.1.3. SPKM-3 as a security triple. . . . 27 3.3. Security Negotiation . . . . . . . . . . . . . . . . 27 3.3.1. SECINFO. . . . . . . . . . . . . . . . . . . 28 3.3.2. Security Error . . . . . . . . . . . . . . . 28 3.4. Callback RPC Authentication. . . . . . . . . . . . . 28 4. Filehandles . . . . . . . . . . . . . . . . . . . . . . . . 30 4.1. Obtaining the First Filehandle . . . . . . . . . . . 30 4.1.1. Root Filehandle. . . . . . . . . . . . . . . 31 4.1.2. Public Filehandle. . . . . . . . . . . . . . 31 4.2. Filehandle Types . . . . . . . . . . . . . . . . . . 31 4.2.1. General Properties of a Filehandle . . . . . 32 4.2.2. Persistent Filehandle. . . . . . . . . . . . 32 4.2.3. Volatile Filehandle. . . . . . . . . . . . . 33 4.2.4. One Method of Constructing a Volatile Filehandle. . . . . . . . . . . . . 34 4.3. Client Recovery from Filehandle Expiration . . . . . 35 5. File Attributes. . . . . . . . . . . . . . . . . . . . . . 35 5.1. Mandatory Attributes . . . . . . . . . . . . . . . . 37 5.2. Recommended Attributes . . . . . . . . . . . . . . . 37 5.3. Named Attributes . . . . . . . . . . . . . . . . . . 37 Shepler, et al. Standards Track [Page 2]
RFC 3530 NFS version 4 Protocol April 2003 5.4. Classification of Attributes . . . . . . . . . . . . 38 5.5. Mandatory Attributes - Definitions . . . . . . . . . 39 5.6. Recommended Attributes - Definitions . . . . . . . . 41 5.7. Time Access. . . . . . . . . . . . . . . . . . . . . 46 5.8. Interpreting owner and owner_group . . . . . . . . . 47 5.9. Character Case Attributes. . . . . . . . . . . . . . 49 5.10. Quota Attributes . . . . . . . . . . . . . . . . . . 49 5.11. Access Control Lists . . . . . . . . . . . . . . . . 50 5.11.1. ACE type . . . . . . . . . . . . . . . . . 51 5.11.2. ACE Access Mask. . . . . . . . . . . . . . 52 5.11.3. ACE flag . . . . . . . . . . . . . . . . . 54 5.11.4. ACE who . . . . . . . . . . . . . . . . . 55 5.11.5. Mode Attribute . . . . . . . . . . . . . . 56 5.11.6. Mode and ACL Attribute . . . . . . . . . . 57 5.11.7. mounted_on_fileid. . . . . . . . . . . . . 57 6. Filesystem Migration and Replication . . . . . . . . . . . 58 6.1. Replication. . . . . . . . . . . . . . . . . . . . . 58 6.2. Migration. . . . . . . . . . . . . . . . . . . . . . 59 6.3. Interpretation of the fs_locations Attribute . . . . 60 6.4. Filehandle Recovery for Migration or Replication . . 61 7. NFS Server Name Space . . . . . . . . . . . . . . . . . . . 61 7.1. Server Exports . . . . . . . . . . . . . . . . . . . 61 7.2. Browsing Exports . . . . . . . . . . . . . . . . . . 62 7.3. Server Pseudo Filesystem . . . . . . . . . . . . . . 62 7.4. Multiple Roots . . . . . . . . . . . . . . . . . . . 63 7.5. Filehandle Volatility. . . . . . . . . . . . . . . . 63 7.6. Exported Root. . . . . . . . . . . . . . . . . . . . 63 7.7. Mount Point Crossing . . . . . . . . . . . . . . . . 63 7.8. Security Policy and Name Space Presentation. . . . . 64 8. File Locking and Share Reservations. . . . . . . . . . . . 65 8.1. Locking. . . . . . . . . . . . . . . . . . . . . . . 65 8.1.1. Client ID. . . . . . . . . . . . . . . . . 66 8.1.2. Server Release of Clientid . . . . . . . . 69 8.1.3. lock_owner and stateid Definition. . . . . 69 8.1.4. Use of the stateid and Locking . . . . . . 71 8.1.5. Sequencing of Lock Requests. . . . . . . . 73 8.1.6. Recovery from Replayed Requests. . . . . . 74 8.1.7. Releasing lock_owner State . . . . . . . . 74 8.1.8. Use of Open Confirmation . . . . . . . . . 75 8.2. Lock Ranges. . . . . . . . . . . . . . . . . . . . . 76 8.3. Upgrading and Downgrading Locks. . . . . . . . . . . 76 8.4. Blocking Locks . . . . . . . . . . . . . . . . . . . 77 8.5. Lease Renewal. . . . . . . . . . . . . . . . . . . . 77 8.6. Crash Recovery . . . . . . . . . . . . . . . . . . . 78 8.6.1. Client Failure and Recovery. . . . . . . . 79 8.6.2. Server Failure and Recovery. . . . . . . . 79 8.6.3. Network Partitions and Recovery. . . . . . 81 8.7. Recovery from a Lock Request Timeout or Abort . . . 85 Shepler, et al. Standards Track [Page 3]
RFC 3530 NFS version 4 Protocol April 2003 8.8. Server Revocation of Locks. . . . . . . . . . . . . 85 8.9. Share Reservations. . . . . . . . . . . . . . . . . 86 8.10. OPEN/CLOSE Operations . . . . . . . . . . . . . . . 87 8.10.1. Close and Retention of State Information. . . . . . . . . . . . . . . . 88 8.11. Open Upgrade and Downgrade. . . . . . . . . . . . . 88 8.12. Short and Long Leases . . . . . . . . . . . . . . . 89 8.13. Clocks, Propagation Delay, and Calculating Lease Expiration. . . . . . . . . . . . . . . . . . . . . 89 8.14. Migration, Replication and State. . . . . . . . . . 90 8.14.1. Migration and State. . . . . . . . . . . . 90 8.14.2. Replication and State. . . . . . . . . . . 91 8.14.3. Notification of Migrated Lease . . . . . . 92 8.14.4. Migration and the Lease_time Attribute . . 92 9. Client-Side Caching . . . . . . . . . . . . . . . . . . . . 93 9.1. Performance Challenges for Client-Side Caching. . . 93 9.2. Delegation and Callbacks. . . . . . . . . . . . . . 94 9.2.1. Delegation Recovery . . . . . . . . . . . . 96 9.3. Data Caching. . . . . . . . . . . . . . . . . . . . 98 9.3.1. Data Caching and OPENs . . . . . . . . . . 98 9.3.2. Data Caching and File Locking. . . . . . . 99 9.3.3. Data Caching and Mandatory File Locking. . 101 9.3.4. Data Caching and File Identity . . . . . . 101 9.4. Open Delegation . . . . . . . . . . . . . . . . . . 102 9.4.1. Open Delegation and Data Caching . . . . . 104 9.4.2. Open Delegation and File Locks . . . . . . 106 9.4.3. Handling of CB_GETATTR . . . . . . . . . . 106 9.4.4. Recall of Open Delegation. . . . . . . . . 109 9.4.5. Clients that Fail to Honor Delegation Recalls . . . . . . . . . . . . 111 9.4.6. Delegation Revocation. . . . . . . . . . . 112 9.5. Data Caching and Revocation . . . . . . . . . . . . 112 9.5.1. Revocation Recovery for Write Open Delegation . . . . . . . . . . . . . . . . 113 9.6. Attribute Caching . . . . . . . . . . . . . . . . . 113 9.7. Data and Metadata Caching and Memory Mapped Files . 115 9.8. Name Caching . . . . . . . . . . . . . . . . . . . 118 9.9. Directory Caching . . . . . . . . . . . . . . . . . 119 10. Minor Versioning . . . . . . . . . . . . . . . . . . . . . 120 11. Internationalization . . . . . . . . . . . . . . . . . . . 122 11.1. Stringprep profile for the utf8str_cs type. . . . . 123 11.1.1. Intended applicability of the nfs4_cs_prep profile . . . . . . . . . . . 123 11.1.2. Character repertoire of nfs4_cs_prep . . . 124 11.1.3. Mapping used by nfs4_cs_prep . . . . . . . 124 11.1.4. Normalization used by nfs4_cs_prep . . . . 124 11.1.5. Prohibited output for nfs4_cs_prep . . . . 125 11.1.6. Bidirectional output for nfs4_cs_prep. . . 125 Shepler, et al. Standards Track [Page 4]
RFC 3530 NFS version 4 Protocol April 2003 11.2. Stringprep profile for the utf8str_cis type . . . . 125 11.2.1. Intended applicability of the nfs4_cis_prep profile. . . . . . . . . . . 125 11.2.2. Character repertoire of nfs4_cis_prep . . 125 11.2.3. Mapping used by nfs4_cis_prep . . . . . . 125 11.2.4. Normalization used by nfs4_cis_prep . . . 125 11.2.5. Prohibited output for nfs4_cis_prep . . . 126 11.2.6. Bidirectional output for nfs4_cis_prep . . 126 11.3. Stringprep profile for the utf8str_mixed type . . . 126 11.3.1. Intended applicability of the nfs4_mixed_prep profile. . . . . . . . . . 126 11.3.2. Character repertoire of nfs4_mixed_prep . 126 11.3.3. Mapping used by nfs4_cis_prep . . . . . . 126 11.3.4. Normalization used by nfs4_mixed_prep . . 127 11.3.5. Prohibited output for nfs4_mixed_prep . . 127 11.3.6. Bidirectional output for nfs4_mixed_prep . 127 11.4. UTF-8 Related Errors. . . . . . . . . . . . . . . . 127 12. Error Definitions . . . . . . . . . . . . . . . . . . . . 128 13. NFS version 4 Requests . . . . . . . . . . . . . . . . . . 134 13.1. Compound Procedure. . . . . . . . . . . . . . . . . 134 13.2. Evaluation of a Compound Request. . . . . . . . . . 135 13.3. Synchronous Modifying Operations. . . . . . . . . . 136 13.4. Operation Values. . . . . . . . . . . . . . . . . . 136 14. NFS version 4 Procedures . . . . . . . . . . . . . . . . . 136 14.1. Procedure 0: NULL - No Operation. . . . . . . . . . 136 14.2. Procedure 1: COMPOUND - Compound Operations . . . . 137 14.2.1. Operation 3: ACCESS - Check Access Rights. . . . . . . . . . . . . . . . . . 140 14.2.2. Operation 4: CLOSE - Close File . . . . . 142 14.2.3. Operation 5: COMMIT - Commit Cached Data . . . . . . . . . . . . . . . 144 14.2.4. Operation 6: CREATE - Create a Non-Regular File Object . . . . . . . . . 147 14.2.5. Operation 7: DELEGPURGE - Purge Delegations Awaiting Recovery . . . 150 14.2.6. Operation 8: DELEGRETURN - Return Delegation. . . . . . . . . . . . . . . . 151 14.2.7. Operation 9: GETATTR - Get Attributes . . 152 14.2.8. Operation 10: GETFH - Get Current Filehandle. . . . . . . . . . . . . . . . 153 14.2.9. Operation 11: LINK - Create Link to a File. . . . . . . . . . . . . . . . . . . 154 14.2.10. Operation 12: LOCK - Create Lock . . . . 156 14.2.11. Operation 13: LOCKT - Test For Lock . . . 160 14.2.12. Operation 14: LOCKU - Unlock File . . . . 162 14.2.13. Operation 15: LOOKUP - Lookup Filename. . 163 14.2.14. Operation 16: LOOKUPP - Lookup Parent Directory. . . . . . . . . . . . . 165 Shepler, et al. Standards Track [Page 5]
RFC 3530 NFS version 4 Protocol April 2003 14.2.15. Operation 17: NVERIFY - Verify Difference in Attributes . . . . . . . . 166 14.2.16. Operation 18: OPEN - Open a Regular File. . . . . . . . . . . . . . . . . . . 168 14.2.17. Operation 19: OPENATTR - Open Named Attribute Directory . . . . . . . . . . . 178 14.2.18. Operation 20: OPEN_CONFIRM - Confirm Open . . . . . . . . . . . . . . 180 14.2.19. Operation 21: OPEN_DOWNGRADE - Reduce Open File Access . . . . . . . . . 182 14.2.20. Operation 22: PUTFH - Set Current Filehandle. . . . . . . . . . . . 184 14.2.21. Operation 23: PUTPUBFH - Set Public Filehandle . . . . . . . . . . 185 14.2.22. Operation 24: PUTROOTFH - Set Root Filehandle . . . . . . . . . . . 186 14.2.23. Operation 25: READ - Read from File . . . 187 14.2.24. Operation 26: READDIR - Read Directory. . . . . . . . . . . . . . 190 14.2.25. Operation 27: READLINK - Read Symbolic Link. . . . . . . . . . . . 193 14.2.26. Operation 28: REMOVE - Remove Filesystem Object. . . . . . . . . 195 14.2.27. Operation 29: RENAME - Rename Directory Entry. . . . . . . . . . 197 14.2.28. Operation 30: RENEW - Renew a Lease . . . 200 14.2.29. Operation 31: RESTOREFH - Restore Saved Filehandle. . . . . . . . . 201 14.2.30. Operation 32: SAVEFH - Save Current Filehandle. . . . . . . . . . . . 202 14.2.31. Operation 33: SECINFO - Obtain Available Security. . . . . . . . . . . . 203 14.2.32. Operation 34: SETATTR - Set Attributes. . 206 14.2.33. Operation 35: SETCLIENTID - Negotiate Clientid. . . . . . . . . . . . 209 14.2.34. Operation 36: SETCLIENTID_CONFIRM - Confirm Clientid. . . . . . . . . . . . . 213 14.2.35. Operation 37: VERIFY - Verify Same Attributes. . . . . . . . . . 217 14.2.36. Operation 38: WRITE - Write to File . . . 218 14.2.37. Operation 39: RELEASE_LOCKOWNER - Release Lockowner State . . . . . . . . . 223 14.2.38. Operation 10044: ILLEGAL - Illegal operation . . . . . . . . . . . . 224 15. NFS version 4 Callback Procedures . . . . . . . . . . . . 225 15.1. Procedure 0: CB_NULL - No Operation . . . . . . . . 225 15.2. Procedure 1: CB_COMPOUND - Compound Operations. . . . . . . . . . . . . . . . . . . . . 226 Shepler, et al. Standards Track [Page 6]
RFC 3530 NFS version 4 Protocol April 2003 15.2.1. Operation 3: CB_GETATTR - Get Attributes . . . . . . . . . . . . . . . . 228 15.2.2. Operation 4: CB_RECALL - Recall an Open Delegation. . . . . . . . . 229 15.2.3. Operation 10044: CB_ILLEGAL - Illegal Callback Operation . . . . . . . . 230 16. Security Considerations . . . . . . . . . . . . . . . . . 231 17. IANA Considerations . . . . . . . . . . . . . . . . . . . 232 17.1. Named Attribute Definition. . . . . . . . . . . . . 232 17.2. ONC RPC Network Identifiers (netids). . . . . . . . 232 18. RPC definition file . . . . . . . . . . . . . . . . . . . 234 19. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 268 20. Normative References . . . . . . . . . . . . . . . . . . . 268 21. Informative References . . . . . . . . . . . . . . . . . . 270 22. Authors' Information . . . . . . . . . . . . . . . . . . . 273 22.1. Editor's Address. . . . . . . . . . . . . . . . . . 273 22.2. Authors' Addresses. . . . . . . . . . . . . . . . . 274 23. Full Copyright Statement . . . . . . . . . . . . . . . . . 275 Shepler, et al. Standards Track [Page 7]
RFC 3530 NFS version 4 Protocol April 2003 1. Introduction 1.1. Changes since RFC 3010 This definition of the NFS version 4 protocol replaces or obsoletes the definition present in [RFC3010]. While portions of the two documents have remained the same, there have been substantive changes in others. The changes made between [RFC3010] and this document represent implementation experience and further review of the protocol. While some modifications were made for ease of implementation or clarification, most updates represent errors or situations where the [RFC3010] definition were untenable. The following list is not all inclusive of all changes but presents some of the most notable changes or additions made: o The state model has added an open_owner4 identifier. This was done to accommodate Posix based clients and the model they use for file locking. For Posix clients, an open_owner4 would correspond to a file descriptor potentially shared amongst a set of processes and the lock_owner4 identifier would correspond to a process that is locking a file. o Clarifications and error conditions were added for the handling of the owner and group attributes. Since these attributes are string based (as opposed to the numeric uid/gid of previous versions of NFS), translations may not be available and hence the changes made. o Clarifications for the ACL and mode attributes to address evaluation and partial support. o For identifiers that are defined as XDR opaque, limits were set on their size. o Added the mounted_on_filed attribute to allow Posix clients to correctly construct local mounts. o Modified the SETCLIENTID/SETCLIENTID_CONFIRM operations to deal correctly with confirmation details along with adding the ability to specify new client callback information. Also added clarification of the callback information itself. o Added a new operation LOCKOWNER_RELEASE to enable notifying the server that a lock_owner4 will no longer be used by the client. o RENEW operation changes to identify the client correctly and allow for additional error returns. Shepler, et al. Standards Track [Page 8]
RFC 3530 NFS version 4 Protocol April 2003 o Verify error return possibilities for all operations. o Remove use of the pathname4 data type from LOOKUP and OPEN in favor of having the client construct a sequence of LOOKUP operations to achieive the same effect. o Clarification of the internationalization issues and adoption of the new stringprep profile framework. 1.2. NFS Version 4 Goals 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: 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 filesystem model that provides a useful, common set of features that does not unduly favor one filesystem or operating system over another. o Designed for protocol extensions. The protocol is designed to accept standard extensions that do not compromise backward compatibility. 1.3. Inconsistencies of this Document with Section 18 Section 18, RPC Definition File, contains the definitions in XDR description language of the constructs used by the protocol. Prior to Section 18, several of the constructs are reproduced for purposes Shepler, et al. Standards Track [Page 9]
RFC 3530 NFS version 4 Protocol April 2003 of explanation. The reader is warned of the possibility of errors in the reproduced constructs outside of Section 18. For any part of the document that is inconsistent with Section 18, Section 18 is to be considered authoritative. 1.4. 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 filesystems and distributed filesystems is expected as well. 1.4.1. 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 [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 filesystem resources. With this, the client can securely match the security mechanism that meets the policies specified at both the client and server. 1.4.2. 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. Shepler, et al. Standards Track [Page 10]
RFC 3530 NFS version 4 Protocol April 2003 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 filesystem 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 filesystem 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. 1.4.3. Filesystem Model The general filesystem model used for the NFS version 4 protocol is the same as previous versions. The server filesystem 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 filesystem tree provided by the server. The server provides multiple filesystems by gluing them together with pseudo filesystems. These pseudo filesystems provide for potential gaps in the path names between real filesystems. 1.4.3.1. 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 filesystem object to which it referred. For some server implementations, this persistence requirement has been difficult to Shepler, et al. Standards Track [Page 11]
RFC 3530 NFS version 4 Protocol April 2003 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 filesystem 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. 1.4.3.2. Attribute Types The NFS version 4 protocol introduces three classes of filesystem 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 filesystem 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. Mandatory attributes are the minimal set of file or filesystem attributes that must be provided by the server and must be properly represented by the server. Recommended attributes represent different filesystem 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 filesystem 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. Shepler, et al. Standards Track [Page 12]
RFC 3530 NFS version 4 Protocol April 2003 1.4.3.3. Filesystem Replication and Migration With the use of a special file attribute, the ability to migrate or replicate server filesystems is enabled within the protocol. The filesystem locations attribute provides a method for the client to probe the server about the location of a filesystem. In the event of a migration of a filesystem, the client will receive an error when operating on the filesystem 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 filesystem. From this information, the client can use its own policies to access the appropriate filesystem location. 1.4.4. 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. 1.4.5. 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 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). 1.4.6. 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 filesystem 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. Shepler, et al. Standards Track [Page 13]
RFC 3530 NFS version 4 Protocol April 2003 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. 1.5. 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 filesystem 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. Shepler, et al. Standards Track [Page 14]
RFC 3530 NFS version 4 Protocol April 2003 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 share reservations unless specifically stated otherwise. Server The "Server" is the entity responsible for coordinating client access to a set of filesystems. 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). 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 128-bit quantity returned by a server that uniquely defines the open and locking state provided by the server for a specific open or lock owner for a specific file. Shepler, et al. Standards Track [Page 15]
RFC 3530 NFS version 4 Protocol April 2003 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; 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 utf8str_cs component4; Represents path name components count4 typedef uint32_t count4; Various count parameters (READ, WRITE, COMMIT) length4 typedef uint64_t length4; Describes LOCK lengths Shepler, et al. Standards Track [Page 16]
RFC 3530 NFS version 4 Protocol April 2003 linktext4 typedef utf8str_cs 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 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 [RFC2743] for details. seqid4 typedef uint32_t seqid4; Sequence identifier used for file locking utf8string typedef opaque utf8string<>; UTF-8 encoding for strings utf8str_cis typedef opaque utf8str_cis; Case-insensitive UTF-8 string utf8str_cs typedef opaque utf8str_cs; Case-sensitive UTF-8 string Shepler, et al. Standards Track [Page 17]
RFC 3530 NFS version 4 Protocol April 2003 utf8str_mixed typedef opaque utf8str_mixed; UTF-8 strings with a case sensitive prefix and a case insensitive suffix. verifier4 typedef opaque verifier4[NFS4_VERIFIER_SIZE]; Verifier used for various operations (COMMIT, CREATE, OPEN, READDIR, SETCLIENTID, SETCLIENTID_CONFIRM, 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 filesystem object is 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 }; Shepler, et al. Standards Track [Page 18]
RFC 3530 NFS version 4 Protocol April 2003 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; /* major device number */ uint32_t specdata2; /* minor device number */ }; 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 filesystem identifier that is used as a mandatory attribute. fs_location4 struct fs_location4 { utf8str_cis server<>; pathname4 rootpath; }; fs_locations4 struct fs_locations4 { pathname4 fs_root; fs_location4 locations<>; }; Shepler, et al. Standards Track [Page 19]
RFC 3530 NFS version 4 Protocol April 2003 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; }; This structure is used with the CREATE, LINK, REMOVE, RENAME operations to let the client know the value of the change attribute for the directory in which the target filesystem 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 SETCLIENTID operation to either specify the address of the client that is using a clientid or as part of the callback registration. The Shepler, et al. Standards Track [Page 20]
RFC 3530 NFS version 4 Protocol April 2003 r_netid and r_addr fields are specified in [RFC1833], but they are underspecified in [RFC1833] as far as what they should look like for specific protocols. For TCP over IPv4 and for UDP over IPv4, the format of r_addr is the US-ASCII string: h1.h2.h3.h4.p1.p2 The prefix, "h1.h2.h3.h4", is the standard textual form for representing an IPv4 address, which is always four octets long. Assuming big-endian ordering, h1, h2, h3, and h4, are respectively, the first through fourth octets each converted to ASCII-decimal. Assuming big-endian ordering, p1 and p2 are, respectively, the first and second octets each converted to ASCII-decimal. For example, if a host, in big-endian order, has an address of 0x0A010307 and there is a service listening on, in big endian order, port 0x020F (decimal 527), then the complete universal address is "10.1.3.7.2.15". For TCP over IPv4 the value of r_netid is the string "tcp". For UDP over IPv4 the value of r_netid is the string "udp". For TCP over IPv6 and for UDP over IPv6, the format of r_addr is the US-ASCII string: x1:x2:x3:x4:x5:x6:x7:x8.p1.p2 The suffix "p1.p2" is the service port, and is computed the same way as with universal addresses for TCP and UDP over IPv4. The prefix, "x1:x2:x3:x4:x5:x6:x7:x8", is the standard textual form for representing an IPv6 address as defined in Section 2.2 of [RFC2373]. Additionally, the two alternative forms specified in Section 2.2 of [RFC2373] are also acceptable. For TCP over IPv6 the value of r_netid is the string "tcp6". For UDP over IPv6 the value of r_netid is the string "udp6". 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. Shepler, et al. Standards Track [Page 21]
RFC 3530 NFS version 4 Protocol April 2003 nfs_client_id4 struct nfs_client_id4 { verifier4 verifier; opaque id<NFS4_OPAQUE_LIMIT>; }; This structure is part of the arguments to the SETCLIENTID operation. NFS4_OPAQUE_LIMIT is defined as 1024. open_owner4 struct open_owner4 { clientid4 clientid; opaque owner<NFS4_OPAQUE_LIMIT>; }; This structure is used to identify the owner of open state. NFS4_OPAQUE_LIMIT is defined as 1024. lock_owner4 struct lock_owner4 { clientid4 clientid; opaque owner<NFS4_OPAQUE_LIMIT>; }; This structure is used to identify the owner of file locking state. NFS4_OPAQUE_LIMIT is defined as 1024. open_to_lock_owner4 struct open_to_lock_owner4 { seqid4 open_seqid; stateid4 open_stateid; seqid4 lock_seqid; lock_owner4 lock_owner; }; This structure is used for the first LOCK operation done for an open_owner4. It provides both the open_stateid and lock_owner such that the transition is made from a valid open_stateid sequence to that of the new lock_stateid sequence. Using this mechanism avoids the confirmation of the lock_owner/lock_seqid pair since it is tied to established state in the form of the open_stateid/open_seqid. Shepler, et al. Standards Track [Page 22]
RFC 3530 NFS version 4 Protocol April 2003 stateid4 struct stateid4 { uint32_t seqid; opaque other[12]; }; This structure is used for the various state sharing mechanisms between the client and server. For the client, this data structure is read-only. The starting value of the seqid field is undefined. The server is required to increment the seqid field monotonically at each transition of the stateid. This is important since the client will inspect the seqid in OPEN stateids to determine the order of OPEN processing done by the server. 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 [RFC3232] 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. Where an NFS version 4 implementation supports operation over the IP network protocol, the supported transports between NFS and IP MUST be among the IETF-approved congestion control transport protocols, which include TCP and SCTP. To enhance the possibilities for interoperability, an NFS version 4 implementation MUST support operation over the TCP transport protocol, at least until such time as a standards track RFC revises this requirement to use a different IETF-approved congestion control transport protocol. 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. Shepler, et al. Standards Track [Page 23]
RFC 3530 NFS version 4 Protocol April 2003 As noted in the Security Considerations section, the authentication model for NFS version 4 has moved from machine-based to principal- based. However, this modification of the authentication model does not imply a technical requirement to move the TCP connection management model from whole machine-based to one based on a per user model. In particular, NFS over TCP client implementations have traditionally multiplexed traffic for multiple users over a common TCP connection between an NFS client and server. This has been true, regardless whether the NFS client is using AUTH_SYS, AUTH_DH, RPCSEC_GSS or any other flavor. Similarly, NFS over TCP server implementations have assumed such a model and thus scale the implementation of TCP connection management in proportion to the number of expected client machines. It is intended that NFS version 4 will not modify this connection management model. NFS version 4 clients that violate this assumption can expect scaling issues on the server and hence reduced service. 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]. 3.1.1. Client Retransmission Behavior When processing a request received over a reliable transport such as TCP, the NFS version 4 server MUST NOT silently drop the request, except if the transport connection has been broken. Given such a contract between NFS version 4 clients and servers, clients MUST NOT retry a request unless one or both of the following are true: o The transport connection has been broken o The procedure being retried is the NULL procedure Since reliable transports, such as TCP, do not always synchronously inform a peer when the other peer has broken the connection (for example, when an NFS server reboots), the NFS version 4 client may want to actively "probe" the connection to see if has been broken. Use of the NULL procedure is one recommended way to do so. So, when a client experiences a remote procedure call timeout (of some arbitrary implementation specific amount), rather than retrying the remote procedure call, it could instead issue a NULL procedure call to the server. If the server has died, the transport connection break will eventually be indicated to the NFS version 4 client. The client can then reconnect, and then retry the original request. If the NULL procedure call gets a response, the connection has not broken. The client can decide to wait longer for the original request's response, or it can break the transport connection and reconnect before re-sending the original request. Shepler, et al. Standards Track [Page 24]
RFC 3530 NFS version 4 Protocol April 2003 For callbacks from the server to the client, the same rules apply, but the server doing the callback becomes the client, and the client receiving the callback becomes the server. 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 [RFC2743]. This allows for the use of various 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 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 a 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 Shepler, et al. Standards Track [Page 25]
RFC 3530 NFS version 4 Protocol April 2003 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]. Users and implementors are warned that 56 bit DES is no longer considered state of the art in terms of resistance to brute force attacks. Once a revision to [RFC1964] is available that adds support for AES, implementors are urged to incorporate AES into their NFSv4 over Kerberos V5 protocol stacks, and users are similarly urged to migrate to the use of AES. 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" 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 server. 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. Shepler, et al. Standards Track [Page 26]
RFC 3530 NFS version 4 Protocol April 2003 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. 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 filesystem 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. See the section "Security Considerations" for further discussion. Shepler, et al. Standards Track [Page 27]
RFC 3530 NFS version 4 Protocol April 2003 3.3.1. 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 operation 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. 3.3.2. 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 filesystem 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. See the section for the "SECINFO" operation for further discussion of how the client will respond to the NFS4ERR_WRONGSEC error and use SECINFO. 3.4. Callback RPC Authentication Except as noted elsewhere in this section, the callback RPC (described later) MUST mutually authenticate the NFS server to the principal that acquired the clientid (also described later), using the security flavor the original SETCLIENTID operation used. For AUTH_NONE, there are no principals, so this is a non-issue. AUTH_SYS has no notions of mutual authentication or a server principal, so the callback from the server simply uses the AUTH_SYS credential that the user used when he 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. Shepler, et al. Standards Track [Page 28]
RFC 3530 NFS version 4 Protocol April 2003 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. 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. This is the same principal the client acquired a GSS-API context for when it issued the SETCLIENTID operation, therefore, the realm name for the server principal must be the same for the callback as it was for the SETCLIENTID. 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 pair, and if the user that the NFS server is authenticating to has a public key certificate, then it works. In situations where the 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 Shepler, et al. Standards Track [Page 29]
RFC 3530 NFS version 4 Protocol April 2003 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 filesystem 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 filesystem object. 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 filesystem 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 operation 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. Shepler, et al. Standards Track [Page 30]
RFC 3530 NFS version 4 Protocol April 2003 4.1.1. Root Filehandle The first of the special filehandles is the ROOT filehandle. The ROOT filehandle is the "conceptual" root of the filesystem 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 operation. 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 filesystem 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 filesystem 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 filesystem object. The client may not make any assumptions about this binding. The client uses the PUBLIC filehandle via the PUTPUBFH operation. 4.2. Filehandle Types In the NFS version 2 and 3 protocols, there was one type of filehandle with a single set of semantics. This type of filehandle is termed "persistent" in NFS Version 4. The semantics of a persistent filehandle remain the same as before. A new type of filehandle introduced in NFS Version 4 is the "volatile" filehandle, which attempts to accommodate certain server environments. The volatile filehandle type was introduced to address server functionality or implementation issues which make correct implementation of a persistent filehandle infeasible. Some server environments do not provide a filesystem level invariant that can be used to construct a persistent filehandle. The underlying server filesystem may not provide the invariant or the server's filesystem 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 filesystem reorganization or migration. However, the volatile filehandle increases the implementation burden for the client. Shepler, et al. Standards Track [Page 31]
RFC 3530 NFS version 4 Protocol April 2003 Since the client will need to handle persistent and volatile filehandles 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. 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. 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 filesystem 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 filesystem object to which it refers. Once the server creates the filehandle for a filesystem 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 filesystem is migrated, the new NFS server must honor the same filehandle as the old NFS server. The persistent filehandle will be become stale or invalid when the filesystem 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 filesystem containing the object is no longer available. The file Shepler, et al. Standards Track [Page 32]
RFC 3530 NFS version 4 Protocol April 2003 system may become unavailable if it exists on removable media and the media is no longer available at the server or the filesystem in whole has been destroyed or the filesystem 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. The mandatory attribute "fh_expire_type" is used by the client to determine what type of filehandle the server is providing for a particular filesystem. 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 filesystem. 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_VOLATILE_ANY The filehandle may expire at any time, except as specifically excluded (i.e., FH4_NO_EXPIRE_WITH_OPEN). FH4_NOEXPIRE_WITH_OPEN May only be set when FH4_VOLATILE_ANY is set. If this bit is set, then the meaning of FH4_VOLATILE_ANY is qualified to exclude any expiration of the filehandle when it is open. FH4_VOL_MIGRATION The filehandle will expire as a result of migration. If FH4_VOL_ANY is set, FH4_VOL_MIGRATION is redundant. Shepler, et al. Standards Track [Page 33]
RFC 3530 NFS version 4 Protocol April 2003 FH4_VOL_RENAME The filehandle will expire during rename. This includes a rename by the requesting client or a rename by any other client. If FH4_VOL_ANY is set, FH4_VOL_RENAME is redundant. Servers which provide volatile filehandles that may expire while open (i.e., if FH4_VOL_MIGRATION or FH4_VOL_RENAME is set or if FH4_VOLATILE_ANY is set and FH4_NOEXPIRE_WITH_OPEN not set), should deny a RENAME or REMOVE that would affect an OPEN file of any of the components leading to the OPEN file. In addition, the server should deny all RENAME or REMOVE requests during the grace period upon server restart. Note that the bits FH4_VOL_MIGRATION and FH4_VOL_RENAME allow the client to determine that expiration has occurred whenever a specific event occurs, without an explicit filehandle expiration error from the server. FH4_VOL_ANY does not provide this form of information. In situations where the server will expire many, but not all filehandles upon migration (e.g., all but those that are open), FH4_VOLATILE_ANY (in this case with FH4_NOEXPIRE_WITH_OPEN) is a better choice since the client may not assume that all filehandles will expire when migration occurs, and it is likely that additional expirations will occur (as a result of file CLOSE) that are separated in time from the migration event itself. 4.2.4. One Method of Constructing a Volatile Filehandle 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 When the client presents a volatile filehandle, the server makes the following checks, which assume that the check for the volatile bit has passed. 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. Shepler, et al. Standards Track [Page 34]
RFC 3530 NFS version 4 Protocol April 2003 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 filesystem 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 filesystem name space. If the expired filehandle refers to an object that has been removed from the filesystem, 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 Note that the COMPOUND procedure does not provide atomicity. This example only reduces the overhead of recovering from an expired filehandle. 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 is able query what attributes the server supports and construct requests with only those supported attributes (or a subset thereof). Shepler, et al. Standards Track [Page 35]
RFC 3530 NFS version 4 Protocol April 2003 To this end, attributes are 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 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. A server should support as many of the recommended attributes as possible but by their definition, the server is not required to support all of them. 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. Note that the hidden directory returned by OPENATTR is a convenience for protocol processing. The client should not make any assumptions about the server's implementation of named attributes and whether the underlying filesystem at the server has a named attribute directory or not. Therefore, operations such as SETATTR and GETATTR on the named attribute directory are undefined. Shepler, et al. Standards Track [Page 36]
RFC 3530 NFS version 4 Protocol April 2003 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 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 the client is better positioned decide whether and how to fabricate or construct an attribute or whether to 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 filesystem 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. Shepler, et al. Standards Track [Page 37]
RFC 3530 NFS version 4 Protocol April 2003 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 filesystem. 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. Classification of Attributes Each of the Mandatory and Recommended attributes can be classified in one of three categories: per server, per filesystem, or per filesystem object. Note that it is possible that some per filesystem attributes may vary within the filesystem. See the "homogeneous" attribute for its definition. Note that the attributes time_access_set and time_modify_set are not listed in this section because they are write-only attributes corresponding to time_access and time_modify, and are used in a special instance of SETATTR. o The per server attribute is: lease_time o The per filesystem attributes are: supp_attr, fh_expire_type, link_support, symlink_support, unique_handles, aclsupport, cansettime, case_insensitive, case_preserving, chown_restricted, files_avail, files_free, files_total, fs_locations, homogeneous, maxfilesize, maxname, maxread, maxwrite, no_trunc, space_avail, space_free, space_total, time_delta o The per filesystem object attributes are: type, change, size, named_attr, fsid, rdattr_error, filehandle, ACL, archive, fileid, hidden, maxlink, mimetype, mode, numlinks, owner, owner_group, rawdev, space_used, system, time_access, time_backup, time_create, time_metadata, time_modify, mounted_on_fileid For quota_avail_hard, quota_avail_soft, and quota_used see their definitions below for the appropriate classification. Shepler, et al. Standards Track [Page 38]
RFC 3530 NFS version 4 Protocol April 2003 5.5. 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. The scope of this attribute applies to all objects with a matching fsid. type 1 nfs4_ftype READ The type of the object (file, directory, symlink, etc.) 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_metadata attribute for this attribute's value but only if the filesystem object can not be updated more frequently than the resolution of time_metadata. size 4 uint64 R/W The size of the object in bytes. Shepler, et al. Standards Track [Page 39]
RFC 3530 NFS version 4 Protocol April 2003 link_support 5 bool READ True, if the object's filesystem supports hard links. symlink_support 6 bool READ True, if the object's filesystem supports symbolic links. named_attr 7 bool READ True, if this object has named attributes. In other words, object has a non-empty named attribute directory. fsid 8 fsid4 READ Unique filesystem identifier for the filesystem holding this object. fsid contains major and minor components each of which are uint64. unique_handles 9 bool READ True, if two distinct filehandles guaranteed to refer to two different filesystem 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. filehandle 19 nfs_fh4 READ The filehandle of this object (primarily for readdir requests). Shepler, et al. Standards Track [Page 40]
RFC 3530 NFS version 4 Protocol April 2003 5.6. 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 filesystem. archive 14 bool R/W True, if this file has been archived since the time of last modification (deprecated in favor of time_backup). cansettime 15 bool READ True, if the server is able to change the times for a filesystem object as specified in a SETATTR operation. case_insensitive 16 bool READ True, if filename comparisons on this filesystem are case insensitive. case_preserving 17 bool READ True, if filename case on this filesystem are preserved. chown_restricted 18 bool 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 Windows 2000 the Shepler, et al. Standards Track [Page 41]
RFC 3530 NFS version 4 Protocol April 2003 "Take Ownership" privilege). fileid 20 uint64 READ A number uniquely identifying the file within the filesystem. files_avail 21 uint64 READ File slots available to this user on the filesystem containing this object - this should be the smallest relevant limit. files_free 22 uint64 READ Free file slots on the filesystem containing this object - this should be the smallest relevant limit. files_total 23 uint64 READ Total file slots on the filesystem containing this object. fs_locations 24 fs_locations READ Locations where this filesystem may be found. If the server returns NFS4ERR_MOVED as an error, this attribute MUST be supported. hidden 25 bool R/W True, if the file is considered hidden with respect to the Windows API. homogeneous 26 bool READ True, if this object's filesystem is homogeneous, i.e., are per filesystem attributes the same Shepler, et al. Standards Track [Page 42]
RFC 3530 NFS version 4 Protocol April 2003 for all filesystem's objects? maxfilesize 27 uint64 READ Maximum supported file size for the filesystem 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 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 mode and permission bits for this object. no_trunc 34 bool READ True, if a name longer than name_max is used, an error be returned and name is not truncated. Shepler, et al. Standards Track [Page 43]
RFC 3530 NFS version 4 Protocol April 2003 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. If the value of type is not NF4BLK or NF4CHR, the value return SHOULD NOT be considered useful. space_avail 42 uint64 READ Disk space in bytes available to this user on the filesystem containing this object - this should be the smallest relevant limit. space_free 43 uint64 READ Free disk space in bytes on the filesystem containing this object - this should Shepler, et al. Standards Track [Page 44]
RFC 3530 NFS version 4 Protocol April 2003 be the smallest relevant limit. space_total 44 uint64 READ Total disk space in bytes on the filesystem containing this object. space_used 45 uint64 READ Number of filesystem bytes allocated to this object. system 46 bool R/W True, if this file is a "system" file with respect to the Windows API. time_access 47 nfstime4 READ The time of last access to the object by a read that was satisfied by the server. 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". time_delta 51 nfstime4 READ Smallest useful server time granularity. Shepler, et al. Standards Track [Page 45]
RFC 3530 NFS version 4 Protocol April 2003 time_metadata 52 nfstime4 READ 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. mounted_on_fileid 55 uint64 READ Like fileid, but if the target filehandle is the root of a filesystem return the fileid of the underlying directory. 5.7. Time Access As defined above, the time_access attribute represents the time of last access to the object by a read that was satisfied by the server. The notion of what is an "access" depends on server's operating environment and/or the server's filesystem semantics. For example, for servers obeying POSIX semantics, time_access would be updated only by the READLINK, READ, and READDIR operations and not any of the operations that modify the content of the object. Of course, setting the corresponding time_access_set attribute is another way to modify the time_access attribute. Whenever the file object resides on a writable filesystem, the server should make best efforts to record time_access into stable storage. However, to mitigate the performance effects of doing so, and most especially whenever the server is satisfying the read of the object's content from its cache, the server MAY cache access time updates and lazily write them to stable storage. It is also acceptable to give administrators of the server the option to disable time_access updates. Shepler, et al. Standards Track [Page 46]
RFC 3530 NFS version 4 Protocol April 2003 5.8. Interpreting owner and owner_group The recommended attributes "owner" and "owner_group" (and also users and groups within the "acl" attribute) 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. Similarly, security principals may be represented in different ways by different security mechanisms. Servers normally translate these representations into a common format, generally that used by local storage, to serve as a means of identifying the users corresponding to these security principals. When these local identifiers are translated to the form of the owner attribute, associated with files created by such principals they identify, in a common format, the users associated with each corresponding set of security principals. The translation used to interpret owner and group strings 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. A server may provide a more general service, not limited by any particular translation (which would only translate a limited set of possible strings) by storing the owner and owner_group attributes in local storage without any translation or it may augment a translation method by storing the entire string for attributes for which no translation is available while using the local representation for those cases in which a translation is available. Servers that do not provide support for all possible values of the owner and owner_group attributes, should return an error (NFS4ERR_BADOWNER) when a string is presented that has no translation, as the value to be set for a SETATTR of the owner, owner_group, or acl attributes. When a server does accept an owner or owner_group value as valid on a SETATTR (and similarly for the owner and group strings in an acl), it is promising to return that same string when a corresponding GETATTR is done. Configuration changes and ill-constructed name translations (those that contain Shepler, et al. Standards Track [Page 47]
RFC 3530 NFS version 4 Protocol April 2003 aliasing) may make that promise impossible to honor. Servers should make appropriate efforts to avoid a situation in which these attributes have their values changed when no real change to ownership has occurred. The "dns_domain" portion of the owner string is meant to be a DNS domain name. For example, user@ietf.org. Servers should accept as valid a set of users for at least one domain. A server may treat other domains as having no valid translations. A more general service is provided when a server is capable of accepting users for multiple domains, or for all domains, subject to security constraints. 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 at the sender and that the receiver of the attribute should not use that string as a basis for translation into its own internal format. 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. To provide a greater degree of compatibility with previous versions of NFS (i.e., v2 and v3), which identified users and groups by 32-bit unsigned uid's and gid's, owner and group strings that consist of decimal numeric values with no leading zeros can be given a special interpretation by clients and servers which choose to provide such support. The receiver may treat such a user or group string as representing the same user as would be represented by a v2/v3 uid or gid having the corresponding numeric value. A server is not obligated to accept such a string, but may return an NFS4ERR_BADOWNER instead. To avoid this mechanism being used to subvert user and group translation, so that a client might pass all of the owners and groups in numeric form, a server SHOULD return an NFS4ERR_BADOWNER error when there is a valid translation for the user or owner designated in this way. In that case, the client must use the appropriate name@domain string and not the special form for compatibility. The owner string "nobody" may be used to designate an anonymous user, which will be associated with a file created by a security principal that cannot be mapped through normal means to the owner attribute. Shepler, et al. Standards Track [Page 48]
RFC 3530 NFS version 4 Protocol April 2003 5.9. 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". 5.10. Quota Attributes For the attributes related to filesystem 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.). 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". Shepler, et al. Standards Track [Page 49]
RFC 3530 NFS version 4 Protocol April 2003 5.11. Access Control Lists The NFS version 4 ACL attribute is an array of access control entries (ACE). Although, the client can read and write the ACL attribute, the NFSv4 model is the server does all access control based on the server's interpretation of the ACL. If at any point the client wants to check access without issuing an operation that modifies or reads data or metadata, the client can use the OPEN and ACCESS operations to do so. There are various access control entry types, as defined in the Section "ACE type". The server is able to communicate which ACE types are supported by returning the appropriate value within the aclsupport attribute. Each ACE covers one or more operations on a file or directory as described in the Section "ACE Access Mask". It may also contain one or more flags that modify the semantics of the ACE as defined in the Section "ACE flag". 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; utf8str_mixed who; }; To determine if a 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 is no longer considered in the processing of later ACEs. If an ACCESS_DENIED_ACE is encountered where the requester's access still has unALLOWED bits in common with the "access_mask" of the ACE, the request is denied. However, unlike the ALLOWED and DENIED ACE types, the ALARM and AUDIT ACE types do not affect a requester's access, and instead are for triggering events as a result of a requester's access attempt. Therefore, all AUDIT and ALARM ACEs are processed until end of the ACL. When the ACL is fully processed, if there are bits in requester's mask that have not been considered whether the server allows or denies the access is undefined. If there is a mode attribute on the file, then this cannot happen, since the mode's MODE4_*OTH bits will map to EVERYONE@ ACEs that unambiguously specify the requester's access. Shepler, et al. Standards Track [Page 50]
RFC 3530 NFS version 4 Protocol April 2003 The NFS version 4 ACL model is quite rich. Some server platforms may provide access control functionality that goes beyond the UNIX-style mode attribute, but which is not as rich as the NFS ACL model. So that users can take advantage of this more limited functionality, the server may indicate that it supports ACLs as long as it follows the guidelines for mapping between its ACL model and the NFS version 4 ACL model. The situation is complicated by the fact that a server may have multiple modules that enforce ACLs. For example, the enforcement for NFS version 4 access may be different from the enforcement for local access, and both may be different from the enforcement for access through other protocols such as SMB. So it may be useful for a server to accept an ACL even if not all of its modules are able to support it. The guiding principle in all cases is that the server must not accept ACLs that appear to make the file more secure than it really is. 5.11.1. ACE type 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. A server need not support all of the above ACE types. 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; Shepler, et al. Standards Track [Page 51]
RFC 3530 NFS version 4 Protocol April 2003 The semantics of the "type" field follow the descriptions provided above. The constants used for the type field (acetype4) 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; Clients should not attempt to set an ACE unless the server claims support for that ACE type. If the server receives a request to set an ACE that it cannot store, it MUST reject the request with NFS4ERR_ATTRNOTSUPP. If the server receives a request to set an ACE that it can store but cannot enforce, the server SHOULD reject the request with NFS4ERR_ATTRNOTSUPP. Example: suppose a server can enforce NFS ACLs for NFS access but cannot enforce ACLs for local access. If arbitrary processes can run on the server, then the server SHOULD NOT indicate ACL support. On the other hand, if only trusted administrative programs run locally, then the server may indicate ACL support. 5.11.2. 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 Shepler, et al. Standards Track [Page 52]
RFC 3530 NFS version 4 Protocol April 2003 (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; const ACE4_DELETE = 0x00010000; const ACE4_READ_ACL = 0x00020000; const ACE4_WRITE_ACL = 0x00040000; const ACE4_WRITE_OWNER = 0x00080000; const ACE4_SYNCHRONIZE = 0x00100000; Server implementations need not provide the granularity of control that is implied by this list of masks. For example, POSIX-based systems might not distinguish APPEND_DATA (the ability to append to a file) from WRITE_DATA (the ability to modify existing contents); both masks would be tied to a single "write" permission. When such a server returns attributes to the client, it would show both APPEND_DATA and WRITE_DATA if and only if the write permission is enabled. If a server receives a SETATTR request that it cannot accurately implement, it should error in the direction of more restricted access. For example, suppose a server cannot distinguish overwriting data from appending new data, as described in the previous paragraph. If a client submits an ACE where APPEND_DATA is set but WRITE_DATA is not (or vice versa), the server should reject the request with NFS4ERR_ATTRNOTSUPP. Nonetheless, if the ACE has type DENY, the server may silently turn on the other bit, so that both APPEND_DATA and WRITE_DATA are denied. Shepler, et al. Standards Track [Page 53]
RFC 3530 NFS version 4 Protocol April 2003 5.11.3. 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 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 The ACE4_SUCCESSFUL_ACCESS_ACE_FLAG (SUCCESS) and ACE4_FAILED_ACCESS_ACE_FLAG (FAILED) flag bits relate only to ACE4_SYSTEM_AUDIT_ACE_TYPE (AUDIT) and ACE4_SYSTEM_ALARM_ACE_TYPE (ALARM) ACE types. If during the processing of the file's ACL, the server encounters an AUDIT or ALARM ACE that matches the principal attempting the OPEN, the server notes that fact, and the presence, if any, of the SUCCESS and FAILED flags encountered in the AUDIT or ALARM ACE. Once the server completes the ACL processing, and the share reservation processing, and the OPEN call, it then notes if the OPEN succeeded or failed. If the OPEN succeeded, and if the SUCCESS flag was set for a matching AUDIT or ALARM, then the appropriate AUDIT or ALARM event occurs. If the OPEN failed, and if the FAILED flag was set for the matching AUDIT or ALARM, then the appropriate AUDIT or ALARM event occurs. Clearly either or both of the SUCCESS or FAILED can be set, but if neither is set, the AUDIT or ALARM ACE is not useful. Shepler, et al. Standards Track [Page 54]
RFC 3530 NFS version 4 Protocol April 2003 The previously described processing applies to that of the ACCESS operation as well. The difference being that "success" or "failure" does not mean whether ACCESS returns NFS4_OK or not. Success means whether ACCESS returns all requested and supported bits. Failure means whether ACCESS failed to return a bit that was requested and supported. 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; A server need not support any of these flags. If the server supports flags that are similar to, but not exactly the same as, these flags, the implementation may define a mapping between the protocol-defined flags and the implementation-defined flags. Again, the guiding principle is that the file not appear to be more secure than it really is. For example, suppose a client tries to set an ACE with ACE4_FILE_INHERIT_ACE set but not ACE4_DIRECTORY_INHERIT_ACE. If the server does not support any form of ACL inheritance, the server should reject the request with NFS4ERR_ATTRNOTSUPP. If the server supports a single "inherit ACE" flag that applies to both files and directories, the server may reject the request (i.e., requiring the client to set both the file and directory inheritance flags). The server may also accept the request and silently turn on the ACE4_DIRECTORY_INHERIT_ACE flag. 5.11.4. ACE who There are several special identifiers ("who") which need to be understood universally, rather than in the context of a particular DNS domain. 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, even if none of the access methods on the server understands the identifiers. Shepler, et al. Standards Track [Page 55]
RFC 3530 NFS version 4 Protocol April 2003 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@. 5.11.5. Mode Attribute The NFS version 4 mode attribute is based on the UNIX mode bits. The following bits are defined: const MODE4_SUID = 0x800; /* set user id on execution */ const MODE4_SGID = 0x400; /* set group id on execution */ const MODE4_SVTX = 0x200; /* save text even after use */ const MODE4_RUSR = 0x100; /* read permission: owner */ const MODE4_WUSR = 0x080; /* write permission: owner */ const MODE4_XUSR = 0x040; /* execute permission: owner */ const MODE4_RGRP = 0x020; /* read permission: group */ const MODE4_WGRP = 0x010; /* write permission: group */ const MODE4_XGRP = 0x008; /* execute permission: group */ const MODE4_ROTH = 0x004; /* read permission: other */ const MODE4_WOTH = 0x002; /* write permission: other */ const MODE4_XOTH = 0x001; /* execute permission: other */ Bits MODE4_RUSR, MODE4_WUSR, and MODE4_XUSR apply to the principal identified in the owner attribute. Bits MODE4_RGRP, MODE4_WGRP, and MODE4_XGRP apply to the principals identified in the owner_group attribute. Bits MODE4_ROTH, MODE4_WOTH, MODE4_XOTH apply to any principal that does not match that in the owner group, and does not have a group matching that of the owner_group attribute. The remaining bits are not defined by this protocol and MUST NOT be used. The minor version mechanism must be used to define further bit usage. Shepler, et al. Standards Track [Page 56]
RFC 3530 NFS version 4 Protocol April 2003 Note that in UNIX, if a file has the MODE4_SGID bit set and no MODE4_XGRP bit set, then READ and WRITE must use mandatory file locking. 5.11.6. Mode and ACL Attribute The server that supports both mode and ACL must take care to synchronize the MODE4_*USR, MODE4_*GRP, and MODE4_*OTH bits with the ACEs which have respective who fields of "OWNER@", "GROUP@", and "EVERYONE@" so that the client can see semantically equivalent access permissions exist whether the client asks for owner, owner_group and mode attributes, or for just the ACL. Because the mode attribute includes bits (e.g., MODE4_SVTX) that have nothing to do with ACL semantics, it is permitted for clients to specify both the ACL attribute and mode in the same SETATTR operation. However, because there is no prescribed order for processing the attributes in a SETATTR, the client must ensure that ACL attribute, if specified without mode, would produce the desired mode bits, and conversely, the mode attribute if specified without ACL, would produce the desired "OWNER@", "GROUP@", and "EVERYONE@" ACEs. 5.11.7. mounted_on_fileid UNIX-based operating environments connect a filesystem into the namespace by connecting (mounting) the filesystem onto the existing file object (the mount point, usually a directory) of an existing filesystem. When the mount point's parent directory is read via an API like readdir(), the return results are directory entries, each with a component name and a fileid. The fileid of the mount point's directory entry will be different from the fileid that the stat() system call returns. The stat() system call is returning the fileid of the root of the mounted filesystem, whereas readdir() is returning the fileid stat() would have returned before any filesystems were mounted on the mount point. Unlike NFS version 3, NFS version 4 allows a client's LOOKUP request to cross other filesystems. The client detects the filesystem crossing whenever the filehandle argument of LOOKUP has an fsid attribute different from that of the filehandle returned by LOOKUP. A UNIX-based client will consider this a "mount point crossing". UNIX has a legacy scheme for allowing a process to determine its current working directory. This relies on readdir() of a mount point's parent and stat() of the mount point returning fileids as previously described. The mounted_on_fileid attribute corresponds to the fileid that readdir() would have returned as described previously. Shepler, et al. Standards Track [Page 57]
RFC 3530 NFS version 4 Protocol April 2003 While the NFS version 4 client could simply fabricate a fileid corresponding to what mounted_on_fileid provides (and if the server does not support mounted_on_fileid, the client has no choice), there is a risk that the client will generate a fileid that conflicts with one that is already assigned to another object in the filesystem. Instead, if the server can provide the mounted_on_fileid, the potential for client operational problems in this area is eliminated. If the server detects that there is no mounted point at the target file object, then the value for mounted_on_fileid that it returns is the same as that of the fileid attribute. The mounted_on_fileid attribute is RECOMMENDED, so the server SHOULD provide it if possible, and for a UNIX-based server, this is straightforward. Usually, mounted_on_fileid will be requested during a READDIR operation, in which case it is trivial (at least for UNIX- based servers) to return mounted_on_fileid since it is equal to the fileid of a directory entry returned by readdir(). If mounted_on_fileid is requested in a GETATTR operation, the server should obey an invariant that has it returning a value that is equal to the file object's entry in the object's parent directory, i.e., what readdir() would have returned. Some operating environments allow a series of two or more filesystems to be mounted onto a single mount point. In this case, for the server to obey the aforementioned invariant, it will need to find the base mount point, and not the intermediate mount points. 6. Filesystem Migration and Replication With the use of the recommended attribute "fs_locations", the NFS version 4 server has a method of providing filesystem migration or replication services. For the purposes of migration and replication, a filesystem 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 filesystem 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 filesystem. Depending on the type of service being provided, the list will provide a new location or a set of alternate locations for the filesystem. The client will use this information to redirect its requests to the new server. 6.1. Replication It is expected that filesystem replication will be used in the case of read-only data. Typically, the filesystem will be replicated on two or more servers. The fs_locations attribute will provide the Shepler, et al. Standards Track [Page 58]
RFC 3530 NFS version 4 Protocol April 2003 list of these locations to the client. On first access of the filesystem, 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 Filesystem migration is used to move a filesystem from one server to another. Migration is typically used for a filesystem 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 filesystem 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 filesystem, 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 PUTFH and 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 filesystem 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 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. Shepler, et al. Standards Track [Page 59]
RFC 3530 NFS version 4 Protocol April 2003 6.3. Interpretation of the fs_locations Attribute The fs_location attribute is structured in the following way: struct fs_location { utf8str_cis server<>; pathname4 rootpath; }; struct fs_locations { pathname4 fs_root; fs_location locations<>; }; The fs_location struct is used to represent the location of a filesystem by providing a server name and the path to the root of the filesystem. 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 filesystem 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 filesystem at the various servers listed. As an example, there is a replicated filesystem located at two servers (servA and servB). At servA the filesystem is located at path "/a/b/c". At servB the filesystem is located at path "/x/y/z". In this example the client accesses the filesystem 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 filesystem'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 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 Shepler, et al. Standards Track [Page 60]
RFC 3530 NFS version 4 Protocol April 2003 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. See the section "Security Considerations" for a discussion on the recommendations for the security flavor to be used by any GETATTR operation that requests the "fs_locations" attribute. 6.4. Filehandle Recovery for Migration or Replication Filehandles for filesystems that are replicated or migrated generally have the same semantics as for filesystems that are not replicated or migrated. For example, if a filesystem has persistent filehandles and it is migrated to another server, the filehandle values for the filesystem 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 filehandle 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 filesystem 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. Shepler, et al. Standards Track [Page 61]
RFC 3530 NFS version 4 Protocol April 2003 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 filesystem. 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 filesystem" that allows the user to browse from one mounted filesystem 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 Filesystem 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 filesystem" that provides a view of exported directories only. A pseudo filesystem has a unique fsid and behaves like a normal, read only filesystem. Based on the construction of the server's name space, it is possible that multiple pseudo filesystems may exist. For example, /a pseudo filesystem /a/b real filesystem /a/b/c pseudo filesystem /a/b/c/d real filesystem Each of the pseudo filesystems are considered separate entities and therefore will have a unique fsid. Shepler, et al. Standards Track [Page 62]
RFC 3530 NFS version 4 Protocol April 2003 7.4. Multiple Roots The DOS and Windows operating environments are sometimes described as having "multiple roots". Filesystems are commonly represented as disk letters. MacOS represents filesystems 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 filesystem is that it is a logical representation of filesystem(s) available from the server. Therefore, the pseudo filesystem is most likely constructed dynamically when the server is first instantiated. It is expected that the pseudo filesystem 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 filesystem, 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 filesystem is exported, one might conclude that a pseudo-filesystem is not needed. This would be wrong. Assume the following filesystems 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-filesystem. 7.7. Mount Point Crossing The server filesystem environment may be constructed in such a way that one filesystem contains a directory which is 'covered' or mounted upon by a second filesystem. For example: /a/b (filesystem 1) /a/b/c/d (filesystem 2) Shepler, et al. Standards Track [Page 63]
RFC 3530 NFS version 4 Protocol April 2003 The pseudo filesystem for this server may be constructed to look like: / (place holder/not exported) /a/b (filesystem 1) /a/b/c/d (filesystem 2) It is the server's responsibility to present the pseudo filesystem 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 filesystem "/a/b/c/d". In previous versions of the NFS protocol, the server would respond with the filehandle of directory "/a/b/c/d" within the filesystem "/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 filesystem 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 filesystem, the server may effectively hide filesystems from a client that may otherwise have legitimate access. As suggested practice, the server should apply the security policy of a shared resource in the server's namespace to the components of the resource's ancestors. For example: / /a/b /a/b/c The /a/b/c directory is a real filesystem and is the shared resource. The security policy for /a/b/c is Kerberos with integrity. The server should apply the same security policy to /, /a, and /a/b. This allows for the extension of the protection of the server's namespace to the ancestors of the real shared resource. Shepler, et al. Standards Track [Page 64]
RFC 3530 NFS version 4 Protocol April 2003 For the case of the use of multiple, disjoint security mechanisms in the server's resources, the security for a particular object in the server's namespace should be the union of all security mechanisms of all direct descendants. 8. File Locking and Share Reservations Integrating locking into the NFS protocol necessarily causes it to be stateful. With the inclusion of share reservations 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 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 reservations 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 NFS version 3 methodology 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. Shepler, et al. Standards Track [Page 65]
RFC 3530 NFS version 4 Protocol April 2003 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. A sequence of a SETCLIENTID operation followed by a SETCLIENTID_CONFIRM operation is required to establish the identification onto the server. Establishment of identification by a new incarnation of the client also has the effect of immediately breaking any leased state that a previous incarnation of the client might have had on the server, as opposed to forcing the new client incarnation to wait for the leases to expire. Breaking the lease state amounts to the server removing all lock, share reservation, and, where the server is not supporting the CLAIM_DELEGATE_PREV claim type, all delegation state associated with same client with the same identity. For discussion of delegation state recovery, see the section "Delegation Recovery". Client identification is encapsulated in the following structure: struct nfs_client_id4 { verifier4 verifier; opaque id<NFS4_OPAQUE_LIMIT>; }; The first field, verifier is a client incarnation verifier that is used to detect client reboots. Only if the verifier is different from that which the server has previously recorded the client (as identified by the second field of the structure, id) does the server start the process of canceling the client's leased state. The second field, id is a variable length string that uniquely defines the client. There are several considerations for how the client generates the id string: o The string should be unique so that multiple clients do not present the same string. The consequences of two clients presenting the same string range from one client getting an error to one client having its leased state abruptly and unexpectedly canceled. Shepler, et al. Standards Track [Page 66]
RFC 3530 NFS version 4 Protocol April 2003 o The string should be selected so the subsequent incarnations (e.g., reboots) of the same client cause the client to present the same string. The implementor is cautioned against an approach that requires the string to be recorded in a local file because this precludes the use of the implementation in an environment where there is no local disk and all file access is from an NFS version 4 server. o The string should be different for each server network address that the client accesses, rather than common to all server network addresses. The reason is that it may not be possible for the client to tell if the same server is listening on multiple network addresses. If the client issues SETCLIENTID with the same id string to each network address of such a server, the server will think it is the same client, and each successive SETCLIENTID will cause the server to begin the process of removing the client's previous leased state. o The algorithm for generating the string should not assume that the client's network address won't change. This includes changes between client incarnations and even changes while the client is stilling running in its current incarnation. This means that if the client includes just the client's and server's network address in the id string, there is a real risk, after the client gives up the network address, that another client, using a similar algorithm for generating the id string, will generate a conflicting id string. Given the above considerations, an example of a well generated id string is one that includes: o The server's network address. o The client's network address. o For a user level NFS version 4 client, it should contain additional information to distinguish the client from other user level clients running on the same host, such as a process id or other unique sequence. o Additional information that tends to be unique, such as one or more of: - The client machine's serial number (for privacy reasons, it is best to perform some one way function on the serial number). - A MAC address. Shepler, et al. Standards Track [Page 67]
RFC 3530 NFS version 4 Protocol April 2003 - The timestamp of when the NFS version 4 software was first installed on the client (though this is subject to the previously mentioned caution about using information that is stored in a file, because the file might only be accessible over NFS version 4). - A true random number. However since this number ought to be the same between client incarnations, this shares the same problem as that of the using the timestamp of the software installation. As a security measure, the server MUST NOT cancel a client's leased state if the principal established the state for a given id string is not the same as the principal issuing the SETCLIENTID. Note that SETCLIENTID and SETCLIENTID_CONFIRM has a secondary purpose of establishing the information the server needs to make callbacks to the client for purpose of supporting delegations. It is permitted to change this information via SETCLIENTID and SETCLIENTID_CONFIRM within the same incarnation of the client without removing the client's leased state. Once a SETCLIENTID and SETCLIENTID_CONFIRM sequence has successfully completed, the client uses the shorthand client identifier, of type clientid4, instead of the longer and less compact nfs_client_id4 structure. This shorthand client identifier (a clientid) is assigned by the server and 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 "lock_owner and stateid Definition" for details). See the detailed descriptions of SETCLIENTID and SETCLIENTID_CONFIRM for a complete specification of the operations. Shepler, et al. Standards Track [Page 68]
RFC 3530 NFS version 4 Protocol April 2003 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. Note that if the id string in a SETCLIENTID request is properly constructed, and if the client takes care to use the same principal for each successive use of SETCLIENTID, then, barring an active denial of service attack, NFS4ERR_CLID_INUSE should never be returned. However, client bugs, server bugs, or perhaps a deliberate change of the principal owner of the id string (such as the case of a client that changes security flavors, and under the new flavor, there is no mapping to the previous owner) will in rare cases result in NFS4ERR_CLID_INUSE. In that event, when the server gets a SETCLIENTID for a client id that currently has no state, or it has state, but the lease has expired, rather than returning NFS4ERR_CLID_INUSE, the server MUST allow the SETCLIENTID, and confirm the new clientid if followed by the appropriate SETCLIENTID_CONFIRM. 8.1.3. lock_owner 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 lock_owner 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. Shepler, et al. Standards Track [Page 69]
RFC 3530 NFS version 4 Protocol April 2003 When the server grants the lock, it responds with a unique stateid. The stateid is used as a shorthand reference to the lock_owner, 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. 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, o divide the "other" field of each stateid into two fields: - A server verifier which uniquely designates a particular server instantiation. - An index into a table of locking-state structures. Shepler, et al. Standards Track [Page 70]
RFC 3530 NFS version 4 Protocol April 2003 o utilize the "seqid" field of each stateid, such that seqid is monotonically 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 and Locking All READ, WRITE and SETATTR operations contain a stateid. For the purposes of this section, SETATTR operations which change the size attribute of a file are treated as if they are writing the area between the old and new size (i.e., the range truncated or added to the file by means of the SETATTR), even where SETATTR is not explicitly mentioned in the text. If the lock_owner performs a READ or WRITE in a situation in which it has established a lock or share reservation on the server (any OPEN constitutes a share reservation) the stateid (previously returned by the server) must be used to indicate what locks, including both record locks and share reservations, are held by the lockowner. If no state is established by the client, either record lock or share reservation, a stateid of all bits 0 is used. Regardless whether a stateid of all bits 0, or a stateid returned by the server is used, if there is a conflicting share reservation or mandatory record lock held on the file, the server MUST refuse to service the READ or WRITE operation. Share reservations are established by OPEN operations and by their nature are mandatory in that when the OPEN denies READ or WRITE operations, that denial results in such operations being rejected with error NFS4ERR_LOCKED. Record locks may be implemented by the server as either mandatory or advisory, or the choice of mandatory or advisory behavior may be determined by the server on the basis of the file being accessed (for example, some UNIX-based servers support a "mandatory lock bit" on the mode attribute such that if set, record locks are required on the file before I/O is possible). When record locks are advisory, they only prevent the granting of conflicting lock requests and have no effect on READs or WRITEs. Mandatory record locks, however, prevent conflicting I/O operations. When they are attempted, they are rejected with NFS4ERR_LOCKED. When the client gets NFS4ERR_LOCKED on a file it knows it has the proper share reservation for, it will need to issue a LOCK request on the region Shepler, et al. Standards Track [Page 71]
RFC 3530 NFS version 4 Protocol April 2003 of the file that includes the region the I/O was to be performed on, with an appropriate locktype (i.e., READ*_LT for a READ operation, WRITE*_LT for a WRITE operation). With NFS version 3, there was no notion of a stateid so there was no way to tell if the application process of the client sending the READ or WRITE operation had also acquired the appropriate record lock on the file. Thus there was no way to implement mandatory locking. With the stateid construct, this barrier has been removed. Note that for UNIX environments that support mandatory file locking, the distinction between advisory and mandatory locking is subtle. In fact, advisory and mandatory record locks are exactly the same in so far as the APIs and requirements on implementation. If the mandatory lock attribute is set on the file, the server checks to see if the lockowner has an appropriate shared (read) or exclusive (write) record lock on the region it wishes to read or write to. If there is no appropriate lock, the server checks if there is a conflicting lock (which can be done by attempting to acquire the conflicting lock on the behalf of the lockowner, and if successful, release the lock after the READ or WRITE is done), and if there is, the server returns NFS4ERR_LOCKED. For Windows environments, there are no advisory record locks, so the server always checks for record locks during I/O requests. Thus, the NFS version 4 LOCK operation does not need to distinguish between advisory and mandatory record locks. It is the NFS version 4 server's processing of the READ and WRITE operations that introduces the distinction. Every stateid other than the special stateid values noted in this section, whether returned by an OPEN-type operation (i.e., OPEN, OPEN_DOWNGRADE), or by a LOCK-type operation (i.e., LOCK or LOCKU), defines an access mode for the file (i.e., READ, WRITE, or READ- WRITE) as established by the original OPEN which began the stateid sequence, and as modified by subsequent OPENs and OPEN_DOWNGRADEs within that stateid sequence. When a READ, WRITE, or SETATTR which specifies the size attribute, is done, the operation is subject to checking against the access mode to verify that the operation is appropriate given the OPEN with which the operation is associated. In the case of WRITE-type operations (i.e., WRITEs and SETATTRs which set size), the server must verify that the access mode allows writing and return an NFS4ERR_OPENMODE error if it does not. In the case, of READ, the server may perform the corresponding check on the access mode, or it may choose to allow READ on opens for WRITE only, to accommodate clients whose write implementation may unavoidably do Shepler, et al. Standards Track [Page 72]
RFC 3530 NFS version 4 Protocol April 2003 reads (e.g., due to buffer cache constraints). However, even if READs are allowed in these circumstances, the server MUST still check for locks that conflict with the READ (e.g., another open specify denial of READs). Note that a server which does enforce the access mode check on READs need not explicitly check for conflicting share reservations since the existence of OPEN for read access guarantees that no conflicting share reservation can exist. A stateid of all bits 1 (one) MAY allow READ operations to bypass locking checks at the server. However, WRITE operations with a stateid with bits all 1 (one) MUST NOT bypass locking checks and are treated exactly the same as if a stateid of all bits 0 were used. A lock may not be granted while a READ or WRITE operation using one of the special stateids is being performed and the range of the lock request conflicts with the range of the READ or WRITE operation. For the purposes of this paragraph, a conflict occurs when a shared lock is requested and a WRITE operation is being performed, or an exclusive lock is requested and either a READ or a WRITE operation is being performed. A SETATTR that sets size is treated similarly to a WRITE as discussed above. 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 lock_owners have different sequences. The server maintains the last sequence number (L) received and the response that was returned. The first request issued for any given lock_owner is issued with a sequence number of zero. Note that for requests that contain a sequence number, for each lock_owner, there should be no more than one outstanding request. If a request (r) 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 SETCLIENTID/SETCLIENTID_CONFIRM sequence changes the client verifier. Shepler, et al. Standards Track [Page 73]
RFC 3530 NFS version 4 Protocol April 2003 Since the sequence number is represented with an unsigned 32-bit integer, the arithmetic involved with the sequence number is mod 2^32. For an example of modulo arithmetic involving sequence numbers see [RFC793]. 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 lock_owner must be cached as long as the lock state exists on the server. The client MUST monotonically increment the sequence number for the CLOSE, LOCK, LOCKU, OPEN, OPEN_CONFIRM, and OPEN_DOWNGRADE operations. This is true even in the event that the previous operation that used the sequence number received an error. The only exception to this rule is if the previous operation received one of the following errors: NFS4ERR_STALE_CLIENTID, NFS4ERR_STALE_STATEID, NFS4ERR_BAD_STATEID, NFS4ERR_BAD_SEQID, NFS4ERR_BADXDR, NFS4ERR_RESOURCE, NFS4ERR_NOFILEHANDLE. 8.1.6. Recovery from Replayed Requests As described above, the sequence number is per lock_owner. 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 (lock_owner, sequence number) state as long as there are open files or closed files with locks outstanding. 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 lock_owner state. 8.1.7. Releasing lock_owner State When a particular lock_owner no longer holds open or file locking state at the server, the server may choose to release the sequence number state associated with the lock_owner. 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 lock_owner no longer is being utilized by the client. The server may choose to hold the lock_owner state in the event that retransmitted requests are received. However, the period to hold this state is implementation specific. Shepler, et al. Standards Track [Page 74]
RFC 3530 NFS version 4 Protocol April 2003 In the case that a LOCK, LOCKU, OPEN_DOWNGRADE, or CLOSE is retransmitted after the server has previously released the lock_owner state, the server will find that the lock_owner has no files open and an error will be returned to the client. If the lock_owner does have a file open, the stateid will not match and again an error is returned to the client. 8.1.8. Use of Open Confirmation In the case that an OPEN is retransmitted and the lock_owner is being used for the first time or the lock_owner 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 lock_owner 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 lock_owner and associated sequence number. Since the OPEN_CONFIRM sequence connects a new open_owner on the server with an existing open_owner on a client, the sequence number may have any value. The OPEN_CONFIRM step assures the server that the value received is the correct one. See the section "OPEN_CONFIRM - Confirm Open" for further details. There are a number of situations in which the requirement to confirm an OPEN would pose difficulties for the client and server, in that they would be prevented from acting in a timely fashion on information received, because that information would be provisional, subject to deletion upon non-confirmation. Fortunately, these are situations in which the server can avoid the need for confirmation when responding to open requests. The two constraints are: o The server must not bestow a delegation for any open which would require confirmation. o The server MUST NOT require confirmation on a reclaim-type open (i.e., one specifying claim type CLAIM_PREVIOUS or CLAIM_DELEGATE_PREV). These constraints are related in that reclaim-type opens are the only ones in which the server may be required to send a delegation. For CLAIM_NULL, sending the delegation is optional while for CLAIM_DELEGATE_CUR, no delegation is sent. Delegations being sent with an open requiring confirmation are troublesome because recovering from non-confirmation adds undue complexity to the protocol while requiring confirmation on reclaim- type opens poses difficulties in that the inability to resolve Shepler, et al. Standards Track [Page 75]
RFC 3530 NFS version 4 Protocol April 2003 the status of the reclaim until lease expiration may make it difficult to have timely determination of the set of locks being reclaimed (since the grace period may expire). Requiring open confirmation on reclaim-type opens is avoidable because of the nature of the environments in which such opens are done. For CLAIM_PREVIOUS opens, this is immediately after server reboot, so there should be no time for lockowners to be created, found to be unused, and recycled. For CLAIM_DELEGATE_PREV opens, we are dealing with a client reboot situation. A server which supports delegation can be sure that no lockowners for that client have been recycled since client initialization and thus can ensure that confirmation will not be required. 8.2. Lock Ranges The protocol allows a lock owner to request a lock with a 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 filesystems 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. 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. Upgrading and Downgrading Locks If a client has a write lock on a record, it can request an atomic downgrade of the lock to a read lock via the LOCK request, by setting the type to READ_LT. If the server supports atomic downgrade, the request will succeed. If not, it will return NFS4ERR_LOCK_NOTSUPP. The client should be prepared to receive this error, and if appropriate, report the error to the requesting application. Shepler, et al. Standards Track [Page 76]
RFC 3530 NFS version 4 Protocol April 2003 If a client has a read lock on a record, it can request an atomic upgrade of the lock to a write lock via the LOCK request by setting the type to WRITE_LT or WRITEW_LT. If the server does not support atomic upgrade, it will return NFS4ERR_LOCK_NOTSUPP. If the upgrade can be achieved without an existing conflict, the request will succeed. Otherwise, the server will return either NFS4ERR_DENIED or NFS4ERR_DEADLOCK. The error NFS4ERR_DEADLOCK is returned if the client issued the LOCK request with the type set to WRITEW_LT and the server has detected a deadlock. The client should be prepared to receive such errors and if appropriate, report the error to the requesting application. 8.4. 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. 8.5. 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. Shepler, et al. Standards Track [Page 77]
RFC 3530 NFS version 4 Protocol April 2003 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 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, DELEGPURGE, DELEGRETURN, LOCK, LOCKU, OPEN, OPEN_CONFIRM, OPEN_DOWNGRADE, 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/SETCLIENTID_CONFIRM sequence. The use of the SETCLIENTID/SETCLIENTID_CONFIRM sequence (one that changes the client verifier) notifies the server to drop the locking state associated with the client. SETCLIENTID/SETCLIENTID_CONFIRM never renews a lease. 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. 8.6. 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. Shepler, et al. Standards Track [Page 78]
RFC 3530 NFS version 4 Protocol April 2003 8.6.1. 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 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 clientid 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. Note that the verifier must have the same uniqueness properties of the verifier for the COMMIT operation. 8.6.2. 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 Shepler, et al. Standards Track [Page 79]
RFC 3530 NFS version 4 Protocol April 2003 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. 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. Shepler, et al. Standards Track [Page 80]
RFC 3530 NFS version 4 Protocol April 2003 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 issue is included in [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. A server may, upon restart, establish a new value for the lease period. Therefore, clients should, once a new clientid is established, refetch the lease_time attribute and use it as the basis for lease renewal for the lease associated with that server. However, the server must establish, for this restart event, a grace period at least as long as the lease period for the previous server instantiation. This allows the client state obtained during the previous server instance to be reliably re-established. 8.6.3. 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. When a network partition is combined with a server reboot, there are edge conditions that place requirements on the server in order to avoid silent data corruption following the server reboot. Two of these edge conditions are known, and are discussed below. Shepler, et al. Standards Track [Page 81]
RFC 3530 NFS version 4 Protocol April 2003 The first edge condition has the following scenario: 1. Client A acquires a lock. 2. Client A and server experience mutual network partition, such that client A is unable to renew its lease. 3. Client A's lease expires, so server releases lock. 4. Client B acquires a lock that would have conflicted with that of Client A. 5. Client B releases the lock 6. Server reboots 7. Network partition between client A and server heals. 8. Client A issues a RENEW operation, and gets back a NFS4ERR_STALE_CLIENTID. 9. Client A reclaims its lock within the server's grace period. Thus, at the final step, the server has erroneously granted client A's lock reclaim. If client B modified the object the lock was protecting, client A will experience object corruption. The second known edge condition follows: 1. Client A acquires a lock. 2. Server reboots. 3. Client A and server experience mutual network partition, such that client A is unable to reclaim its lock within the grace period. 4. Server's reclaim grace period ends. Client A has no locks recorded on server. 5. Client B acquires a lock that would have conflicted with that of Client A. 6. Client B releases the lock. 7. Server reboots a second time. 8. Network partition between client A and server heals. Shepler, et al. Standards Track [Page 82]
RFC 3530 NFS version 4 Protocol April 2003 9. Client A issues a RENEW operation, and gets back a NFS4ERR_STALE_CLIENTID. 10. Client A reclaims its lock within the server's grace period. As with the first edge condition, the final step of the scenario of the second edge condition has the server erroneously granting client A's lock reclaim. Solving the first and second edge conditions requires that the server either assume after it reboots that edge condition occurs, and thus return NFS4ERR_NO_GRACE for all reclaim attempts, or that the server record some information stable storage. The amount of information the server records in stable storage is in inverse proportion to how harsh the server wants to be whenever the edge conditions occur. The server that is completely tolerant of all edge conditions will record in stable storage every lock that is acquired, removing the lock record from stable storage only when the lock is unlocked by the client and the lock's lockowner advances the sequence number such that the lock release is not the last stateful event for the lockowner's sequence. For the two aforementioned edge conditions, the harshest a server can be, and still support a grace period for reclaims, requires that the server record in stable storage information some minimal information. For example, a server implementation could, for each client, save in stable storage a record containing: o the client's id string o a boolean that indicates if the client's lease expired or if there was administrative intervention (see the section, Server Revocation of Locks) to revoke a record lock, share reservation, or delegation o a timestamp that is updated the first time after a server boot or reboot the client acquires record locking, share reservation, or delegation state on the server. The timestamp need not be updated on subsequent lock requests until the server reboots. The server implementation would also record in the stable storage the timestamps from the two most recent server reboots. Assuming the above record keeping, for the first edge condition, after the server reboots, the record that client A's lease expired means that another client could have acquired a conflicting record lock, share reservation, or delegation. Hence the server must reject a reclaim from client A with the error NFS4ERR_NO_GRACE. Shepler, et al. Standards Track [Page 83]
RFC 3530 NFS version 4 Protocol April 2003 For the second edge condition, after the server reboots for a second time, the record that the client had an unexpired record lock, share reservation, or delegation established before the server's previous incarnation means that the server must reject a reclaim from client A with the error NFS4ERR_NO_GRACE. Regardless of the level and approach to record keeping, the server MUST implement one of the following strategies (which apply to reclaims of share reservations, record locks, and delegations): 1. Reject all reclaims with NFS4ERR_NO_GRACE. This is superharsh, but necessary if the server does not want to record lock state in stable storage. 2. Record sufficient state in stable storage such that all known edge conditions involving server reboot, including the two noted in this section, are detected. False positives are acceptable. Note that at this time, it is not known if there are other edge conditions. In the event, after a server reboot, the server determines that there is unrecoverable damage or corruption to the the stable storage, then for all clients and/or locks affected, the server MUST return NFS4ERR_NO_GRACE. A mandate for the client's handling of the NFS4ERR_NO_GRACE error is outside the scope of this specification, since the strategies for such handling are very dependent on the client's operating environment. However, one potential approach is described below. When the client receives NFS4ERR_NO_GRACE, it could examine the change attribute of the objects the client is trying to reclaim state for, and use that to determine whether to re-establish the state via normal OPEN or LOCK requests. This is acceptable provided the client's operating environment allows it. In otherwords, the client implementor is advised to document for his users the behavior. The client could also inform the application that its record lock or share reservations (whether they were delegated or not) have been lost, such as via a UNIX signal, a GUI pop-up window, etc. See the section, "Data Caching and Revocation" for a discussion of what the client should do for dealing with unreclaimed delegations on client state. For further discussion of revocation of locks see the section "Server Revocation of Locks". Shepler, et al. Standards Track [Page 84]
RFC 3530 NFS version 4 Protocol April 2003 8.7. 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 lock_owner. This is straightforward to do without a special re- synchronize operation. Since the server maintains the last lock request and response received on the lock_owner, for each lock_owner, 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 lock_owner, 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 server will return the cached result or if never saw the request, perform it. 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 lock_owner will re-synchronize and in turn the lock state will re-synchronize. 8.8. 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. 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 before expiration. 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 Shepler, et al. Standards Track [Page 85]
RFC 3530 NFS version 4 Protocol April 2003 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_ADMIN_REVOKED. In this instance the client may assume that only the lock_owner'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. 8.9. 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 == 0) return (NFS4ERR_INVAL) Shepler, et al. Standards Track [Page 86]
RFC 3530 NFS version 4 Protocol April 2003 else if ((request.access & file_state.deny)) || (request.deny & file_state.access)) return (NFS4ERR_DENIED) This checking of share reservations on OPEN is done with no exception for an existing OPEN for the same open_owner. 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; 8.10. 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 reservations held by the lock_owner 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, NFS4ERR_LOCKS_HELD, if any locks would exist after the CLOSE. 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 Shepler, et al. Standards Track [Page 87]
RFC 3530 NFS version 4 Protocol April 2003 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). 8.10.1. Close and Retention of State Information Since a CLOSE operation requests deallocation of a stateid, dealing with retransmission of the CLOSE, may pose special difficulties, since the state information, which normally would be used to determine the state of the open file being designated, might be deallocated, resulting in an NFS4ERR_BAD_STATEID error. Servers may deal with this problem in a number of ways. To provide the greatest degree assurance that the protocol is being used properly, a server should, rather than deallocate the stateid, mark it as close-pending, and retain the stateid with this status, until later deallocation. In this way, a retransmitted CLOSE can be recognized since the stateid points to state information with this distinctive status, so that it can be handled without error. When adopting this strategy, a server should retain the state information until the earliest of: o Another validly sequenced request for the same lockowner, that is not a retransmission. o The time that a lockowner is freed by the server due to period with no activity. o All locks for the client are freed as a result of a SETCLIENTID. Servers may avoid this complexity, at the cost of less complete protocol error checking, by simply responding NFS4_OK in the event of a CLOSE for a deallocated stateid, on the assumption that this case must be caused by a retransmitted close. When adopting this approach, it is desirable to at least log an error when returning a no-error indication in this situation. If the server maintains a reply-cache mechanism, it can verify the CLOSE is indeed a retransmission and avoid error logging in most cases. 8.11. 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 Shepler, et al. Standards Track [Page 88]
RFC 3530 NFS version 4 Protocol April 2003 deny bits for all of the OPEN requests completed. Only a single CLOSE will be done to reset the effects of both OPENs. 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 OPENs result in the OPENed 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 OPENs 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 OPENs with separate stateids and will require separate CLOSEs 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 opens 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. 8.12. 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 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 (the server must wait for the leases to expire and the grace period to elapse before 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 would not be an issue. 8.13. Clocks, Propagation Delay, 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 Shepler, et al. Standards Track [Page 89]
RFC 3530 NFS version 4 Protocol April 2003 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. The server's lease period configuration should take into account the network distance of the clients that will be accessing the server's resources. It is expected that the lease period will take into account the network propagation delays and other network delay factors for the client population. Since the protocol does not allow for an automatic method to determine an appropriate lease period, the server's administrator may have to tune the lease period. 8.14. 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, stateids, and clientids) 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". If server replica or a server immigrating a filesystem agrees to, or is expected to, accept opaque values from the client that originated from another server, then it is a wise implementation practice for the servers to encode the "opaque" values in network byte order. This way, servers acting as replicas or immigrating filesystems will be able to parse values like stateids, directory cookies, filehandles, etc. even if their native byte order is different from other servers cooperating in the replication and migration of the filesystem. 8.14.1. Migration and State In the case of migration, the servers involved in the migration of a filesystem 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 Shepler, et al. Standards Track [Page 90]
RFC 3530 NFS version 4 Protocol April 2003 filesystem migration occurs. If the servers are successful in transferring all state, the client will continue to use stateids assigned by the original server. Therefore the new server must recognize these stateids as valid. This holds true for the clientid as well. Since responsibility for an entire filesystem 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 old 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. 8.14.2. 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, stateids and clientids 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. Shepler, et al. Standards Track [Page 91]
RFC 3530 NFS version 4 Protocol April 2003 8.14.3. Notification of Migrated Lease In the case of lease renewal, the client may not be submitting requests for a filesystem that has been migrated to another server. This can occur because of the implicit lease renewal mechanism. The client renews leases for all filesystems when submitting a request to any one filesystem 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 filesystem for which a lease has been moved to a new server. When a client receives an NFS4ERR_LEASE_MOVED error, it should perform an operation on each filesystem 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, the client will receive either NFS4ERR_STALE_CLIENTID or NFS4ERR_STALE_STATEID from the new server, as described above, and the client can then recover state information as it does in the event of server failure. 8.14.4. Migration and the Lease_time Attribute In order that the client may appropriately manage its leases in the case of migration, the destination server must establish proper values for the lease_time attribute. When state is transferred transparently, that state should include the correct value of the lease_time attribute. The lease_time attribute on the destination server must never be less than that on the source since this would result in premature expiration of leases granted by the source server. Upon migration in which state is transferred transparently, the client is under no obligation to re- fetch the lease_time attribute and may continue to use the value previously fetched (on the source server). If state has not been transferred transparently (i.e., the client sees a real or simulated server reboot), the client should fetch the value of lease_time on the new (i.e., destination) server, and use it Shepler, et al. Standards Track [Page 92]
RFC 3530 NFS version 4 Protocol April 2003 for subsequent locking requests. However the server must respect a grace period at least as long as the lease_time on the source server, in order to ensure that clients have ample time to reclaim their locks before potentially conflicting non-reclaimed locks are granted. The means by which the new server obtains the value of lease_time on the old server is left to the server implementations. It is not specified by the NFS version 4 protocol. 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. 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. Shepler, et al. Standards Track [Page 93]
RFC 3530 NFS version 4 Protocol April 2003 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. 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 Shepler, et al. Standards Track [Page 94]
RFC 3530 NFS version 4 Protocol April 2003 used on behalf of all the open_owners 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 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. Shepler, et al. Standards Track [Page 95]
RFC 3530 NFS version 4 Protocol April 2003 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. 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). A server MAY support a claim type of CLAIM_DELEGATE_PREV, but if it does, it MUST NOT remove delegations upon SETCLIENTID_CONFIRM, and instead MUST, for a period of time no less than that of the value of the lease_time attribute, maintain the client's delegations to allow time for the client to issue CLAIM_DELEGATE_PREV requests. The server that supports CLAIM_DELEGATE_PREV MUST support the DELEGPURGE operation. Shepler, et al. Standards Track [Page 96]
RFC 3530 NFS version 4 Protocol April 2003 When the server reboots or restarts, delegations are reclaimed (using the OPEN operation with CLAIM_PREVIOUS) 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. 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. Shepler, et al. Standards Track [Page 97]
RFC 3530 NFS version 4 Protocol April 2003 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. Revalidating means that the client fetches the change attribute from the server, compares it with the cached change attribute, and if different, declares the cached data (as well as the cached attributes) as invalid. 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 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. Since the change attribute is updated for data and metadata modifications, some client implementors may be tempted to use the time_modify attribute and not change to validate cached data, so that metadata changes do not spuriously invalidate clean data. The implementor is cautioned in this approach. The change attribute is guaranteed to change for each update to the file, Shepler, et al. Standards Track [Page 98]
RFC 3530 NFS version 4 Protocol April 2003 whereas time_modify is guaranteed to change only at the granularity of the time_delta attribute. Use by the client's data cache validation logic of time_modify and not change runs the risk of the client incorrectly marking stale data as valid. 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 filesystem. 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 cached 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. Shepler, et al. Standards Track [Page 99]