Internet Engineering Task Force (IETF) T. Haynes
Request for Comments: 9754 T. Myklebust
Category: Standards Track Hammerspace
ISSN: 2070-1721 March 2025
Extensions for Opening and Delegating Files in NFSv4.2
Abstract
The Network File System v4 (NFSv4) allows a client to both open a
file and be granted a delegation of that file. This delegation
provides the client the right to authoritatively cache metadata on
the file locally. This document presents several extensions for both
opening the file and delegating it to the client. This document
extends NFSv4.2 (see RFC 7863).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9754.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Definitions
1.2. Requirements Language
2. Offline Files
2.1. XDR for the Offline Attribute
3. Determining OPEN Feature Support
3.1. XDR for Open Arguments
4. OPEN Grants Either an Open or a Delegation Stateid
4.1. Implementation Experience
5. Proxying of Times
5.1. Use Case for NFSv3 Client Proxy
5.2. XDR for Proxying of Times
6. Extraction of XDR
7. Security Considerations
8. IANA Considerations
9. Normative References
Acknowledgments
Authors' Addresses
1. Introduction
In the Network File System version 4 (NFSv4), a client may be granted
a delegation for a file (see Section 1.8.4 of [RFC8881]). This
allows the client to act as the authority for the file's data and
metadata. This document presents a number of extensions that enhance
the functionality of opens and delegations. These allow the client
to:
* detect an offline file, which may require significant effort to
obtain;
* determine which extensions of OPEN flags are supported by the
server;
* retrieve either the open or delegation stateid, but not both
simultaneously, during the OPEN procedure; and
* cache both the access and modify timestamps, thereby reducing the
frequency with which the client must query the server for this
information.
Using the process detailed in [RFC8178], the revisions in this
document become an extension of NFSv4.2 [RFC7862]. They are built on
top of the External Data Representation (XDR) [RFC4506] generated
from [RFC7863].
1.1. Definitions
This document uses the following terminology:
offline file: A file that exists on a device that is not connected
to the server. There is typically a cost associated with bringing
the file to an online status. Historically, this would be a file
on tape media, and the cost would have been finding and loading
the tape. A more modern interpretation is that the file is in the
cloud, and the cost is a monetary one in downloading the file.
proxy: The proxying of attributes occurs when a client has the
authority, as granted by the appropriate delegation, to represent
the attributes normally maintained by the server. For read
attributes, this occurs when the client has either a read or write
delegation for the file. For write attributes, this occurs when
the client has a write delegation for the file. The client having
this authority is the "proxy" for those attributes.
Further, the definitions of the following terms are referenced as
follows:
* CB_GETATTR (Section 20.1 of [RFC8881])
* change (Section 5.8.1.4 of [RFC8881])
* CLOSE (Section 18.2 of [RFC8881])
* compound (Section 2.3 of [RFC8881])
* DELEGRETURN (Section 18.6 of [RFC8881])
* GETATTR (Section 18.7 of [RFC8881])
* LAYOUTGET (Section 18.43 of [RFC8881])
* LOCK (Section 18.10 of [RFC8881])
* NFS4ERR_DELAY (Section 15.1.1.3 of [RFC8881])
* OPEN (Section 18.16 of [RFC8881])
* open_delegation_type4 (Section 18.16.1 of [RFC8881])
* READ (Section 18.22 of [RFC8881])
* READDIR (Section 18.23 of [RFC8881])
* SETATTR (Section 18.30 of [RFC8881])
* stateid (Section 8.2 of [RFC8881])
* time_access (Section 5.8.2.37 of [RFC8881])
* time_metadata (Section 5.8.2.42 of [RFC8881])
* time_modify (Section 5.8.2.43 of [RFC8881])
* WRITE (Section 18.32 of [RFC8881])
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Offline Files
If a file is offline, then the server has immediate high-performance
access to the file's attributes, but not to the file's content. The
action of retrieving the data content is expensive, to the extent
that the content should only be retrieved if it is going to be used.
For example, a graphical file manager (such as Finder in Mac OS X)
may want to access the beginning of the file to preview it for a user
who is hovering their pointer over the file name and not accessing it
otherwise. If the file is retrieved, it will most likely be either
immediately thrown away or returned.
A compound with a GETATTR or READDIR can report the file's attributes
without bringing the file online. However, either an OPEN or a
LAYOUTGET might cause the file server to retrieve the archived data
contents, bringing the file online. For non-parallel NFS systems
(see Section 12 of [RFC8881]), the OPEN operation requires a
filehandle to retrieve the data content. For parallel NFS (pNFS)
systems, the filehandle retrieved from an OPEN need not cause the
data content to be retrieved. However, when the LAYOUTGET operation
is processed, a layout-type-specific mapping will cause the data
content to be retrieved from offline storage.
If the client is not aware that the file is offline, it might
inadvertently open the file to determine what type of file it is
accessing. By interrogating the new attribute fattr4_offline, a
client can predetermine the availability of the file, avoiding the
need to open it at all. Being offline might also involve situations
in which the file is archived in the cloud, i.e., there can be an
expense in both retrieving the file to bring it online and in sending
the file back to offline status.
2.1. XDR for the Offline Attribute
<CODE BEGINS>
///
/// typedef bool fattr4_offline;
///
///
/// const FATTR4_OFFLINE = 83;
///
<CODE ENDS>
3. Determining OPEN Feature Support
Section 4.4.2 of [RFC8178] allows for extending a particular minor
version of the NFSv4 protocol without requiring the definition of a
new minor version. The client can probe the capabilities of the
server and, based on the result, determine if both it and the server
support optional features not previously specified as part of the
minor version.
The fattr4_open_arguments attribute is a new XDR extension that
provides helpful support when the OPEN procedure is extended in such
a fashion. It models all of the parameters via bitmap4 data
structures, which allows for the addition of a new flag to any of the
OPEN arguments. The scope of this attribute applies to all objects
with a matching fsid.
Two new flags are provided:
* OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION (see Section 4)
* OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS (see Section 5)
Subsequent extensions can use this framework when introducing new
OPTIONAL functionality to OPEN by creating a new flag for each
OPTIONAL parameter.
Since fattr4_open_arguments is a RECOMMENDED attribute, if the server
informs the client via NFS4ERR_ATTRNOTSUPP that it does not support
this new attribute, the client MUST take this to mean that the
additional new OPTIONAL functionality to OPEN is also not supported.
Some other concerns are how to process both currently REQUIRED flags
and OPTIONAL flags that become REQUIRED in the future. The server
MUST mark REQUIRED flags as being supported. Note that these flags
MUST only change from OPTIONAL to REQUIRED when the NFSv4 minor
version is incremented.
3.1. XDR for Open Arguments
<CODE BEGINS>
///
/// struct open_arguments4 {
/// bitmap4 oa_share_access;
/// bitmap4 oa_share_deny;
/// bitmap4 oa_share_access_want;
/// bitmap4 oa_open_claim;
/// bitmap4 oa_create_mode;
/// };
///
///
/// enum open_args_share_access4 {
/// OPEN_ARGS_SHARE_ACCESS_READ = 1,
/// OPEN_ARGS_SHARE_ACCESS_WRITE = 2,
/// OPEN_ARGS_SHARE_ACCESS_BOTH = 3
/// };
///
///
/// enum open_args_share_deny4 {
/// OPEN_ARGS_SHARE_DENY_NONE = 0,
/// OPEN_ARGS_SHARE_DENY_READ = 1,
/// OPEN_ARGS_SHARE_DENY_WRITE = 2,
/// OPEN_ARGS_SHARE_DENY_BOTH = 3
/// };
///
///
/// enum open_args_share_access_want4 {
/// OPEN_ARGS_SHARE_ACCESS_WANT_ANY_DELEG = 3,
/// OPEN_ARGS_SHARE_ACCESS_WANT_NO_DELEG = 4,
/// OPEN_ARGS_SHARE_ACCESS_WANT_CANCEL = 5,
/// OPEN_ARGS_SHARE_ACCESS_WANT_SIGNAL_DELEG_WHEN_RESRC_AVAIL
/// = 17,
/// OPEN_ARGS_SHARE_ACCESS_WANT_PUSH_DELEG_WHEN_UNCONTENDED
/// = 18,
/// OPEN_ARGS_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS = 20,
/// OPEN_ARGS_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION = 21
/// };
///
///
/// enum open_args_open_claim4 {
/// OPEN_ARGS_OPEN_CLAIM_NULL = 0,
/// OPEN_ARGS_OPEN_CLAIM_PREVIOUS = 1,
/// OPEN_ARGS_OPEN_CLAIM_DELEGATE_CUR = 2,
/// OPEN_ARGS_OPEN_CLAIM_DELEGATE_PREV = 3,
/// OPEN_ARGS_OPEN_CLAIM_FH = 4,
/// OPEN_ARGS_OPEN_CLAIM_DELEG_CUR_FH = 5,
/// OPEN_ARGS_OPEN_CLAIM_DELEG_PREV_FH = 6
/// };
///
///
/// enum open_args_createmode4 {
/// OPEN_ARGS_CREATEMODE_UNCHECKED4 = 0,
/// OPEN_ARGS_CREATE_MODE_GUARDED = 1,
/// OPEN_ARGS_CREATEMODE_EXCLUSIVE4 = 2,
/// OPEN_ARGS_CREATE_MODE_EXCLUSIVE4_1 = 3
/// };
///
///
/// typedef open_arguments4 fattr4_open_arguments;
///
///
/// %/*
/// % * Determine what OPEN supports.
/// % */
/// const FATTR4_OPEN_ARGUMENTS = 86;
///
///
/// const OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION = 0x200000;
///
///
/// const OPEN4_RESULT_NO_OPEN_STATEID = 0x00000010;
///
<CODE ENDS>
4. OPEN Grants Either an Open or a Delegation Stateid
The OPEN procedure returns an open stateid to the client to reference
the state of the file. The client could also request a delegation
stateid in the OPEN arguments. The file can be considered open for
the client as long as the count of open and delegated stateids is
greater than 0. Either type of stateid is sufficient to enable the
server to treat the file as if it were open, which allows READ,
WRITE, LOCK, and LAYOUTGET operations to proceed. If the client gets
both an open and a delegation stateid as part of the OPEN, then it
has to return them both to the server. A further consideration is
that during each operation, the client can send a costly GETATTR.
If the client knows that the server supports the
OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION flag (as determined by an
earlier GETATTR operation that queried for the fattr4_open_arguments
attribute), then the client can supply that flag during the OPEN and
get either an open or a delegation stateid.
The client is already prepared to not get a delegation stateid, even
if requested. In order to not send an open stateid, the server MUST
indicate that fact with the result flag of
OPEN4_RESULT_NO_OPEN_STATEID. The open stateid field,
OPEN4resok.stateid, MUST be set to the special all-zero stateid in
this case.
Note that the OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION flag is a
hint. The server might return both stateids. Consider the scenario
in which the client opens a file for read-only (with
OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION set) and only gets an
open stateid. If the client then opens the file for read-write (with
OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION set), the server can
return one of the following three options:
1. Only an open stateid with the correct seqid.
2. Only a delegation stateid with the open stateid now having an
incorrect seqid as it needs to be upgraded.
3. Both an open stateid (which will be upgraded) and a delegation
stateid.
In this scenario, returning just a delegation stateid would hide
information from the client. If the client already has an open
stateid, then the server SHOULD ignore the
OPEN4_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION flag and return both the
open and delegation stateids.
4.1. Implementation Experience
The CLOSE operation neither explicitly nor implicitly releases any
delegation stateids. This is not symmetrical with the OPEN
operation, which can grant both an open and a delegation stateid.
This specification could have tried to extend the CLOSE operation to
release both stateids, but implementation experience shows that is
more costly than the approach that has been proposed.
Consider a small workload of creating a file with content. This
involves three synchronous operations and one asynchronous operation
with existing implementations:
* The first synchronous operation has to OPEN the file.
* The second synchronous operation performs the WRITE to the file.
* The third synchronous operation has to CLOSE the file.
* The asynchronous operation uses DELEGRETURN to return the
delegation stateid.
<CODE BEGINS>
SEQ PUTFH OPEN GETFH GETATTR
SEQ PUTFH WRITE GETATTR
SEQ PUTFH CLOSE
...
SEQ PUTFH DELEGRETURN
<CODE ENDS>
With the proposed approach of setting the
OPEN_ARGS_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION flag during the OPEN,
the number of operations is always three. The first two compounds
are still synchronous, but the last is asynchronous. That is, since
the client no longer has to send a CLOSE operation, it can delay the
DELEGRETURN until either the server requests it back via delegation
recall or garbage collection causes the client to return the stateid.
<CODE BEGINS>
SEQ PUTFH OPEN(OPEN_ARGS_SHARE_ACCESS_WANT_OPEN_XOR_DELEGATION)
GETFH GETATTR
SEQ PUTFH WRITE GETATTR
...
SEQ PUTFH DELEGRETURN
<CODE ENDS>
This approach reduces the cost of synchronous operations by 33% and
the total number of operations by 25%. Contrast that with the
alternative proposal of having CLOSE return both stateids, which
would not reduce the number of synchronous operations.
5. Proxying of Times
When a client is granted a write delegation on a file, it becomes the
authority for the file contents and associated attributes. If the
server queries the client as to the state of the file via a
CB_GETATTR, then according to the unextended NFSv4 protocol, it can
only determine the size of the file and the change attribute. In the
case of the client holding the delegation, it has the current values
of the access and modify times. There is no way that other clients
can have access to these values. To notify the server of the proxied
values, the client could send a compound of the form SEQ, PUTFH,
SETATTR (time_modify | time_access), DELEGRETURN; however, the
SETATTR operation would cause either or both of the change attribute
or time_metadata attribute to be modified to the current time on the
server. There is no current provision to obtain these values before
delegation return using CB_GETATTR. As a result, it cannot pass on
these times to an application expecting POSIX compliance, as is often
necessary for correct operation.
With the addition of the new OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS
flag, the client and server can negotiate that the client will be the
authority for these values, and upon return of the delegation stateid
via a DELEGRETURN, the times will be passed back to the server. If
the server is queried by another client for either the size or the
times, it will need to use a CB_GETATTR to query the client that
holds the delegation.
If a server informs the client via the fattr4_open_arguments
attribute that it supports
OPEN_ARGS_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS and it returns a valid
delegation stateid for an OPEN operation that sets the
OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS flag, then it MUST query the
client via a CB_GETATTR for the fattr4_time_deleg_access attribute
(see Section 5.2) and the fattr4_time_deleg_modify attribute (see
Section 5.2). (Note that the change time can be derived from the
modify time.) Further, when a server gets a SETATTR with those
attributes set, then it MUST accept those changes in the
fattr4_time_deleg_access and fattr4_time_deleg_modify attributes and
derive the change time, or it MUST reject the changes with
NFS4ERR_DELAY.
When the server grants a delegation stateid, it MUST inform the
client by setting the approriate appropriate flag in the open_delegation_type4
response. The server MUST set OPEN_DELEGATE_READ_ATTRS_DELEG when it
grants a read attribute delegation and MUST set
OPEN_DELEGATE_WRITE_ATTRS_DELEG when it grants a write attribute
delegation.
These new attributes are invalid to be used with GETATTR, VERIFY, and
NVERIFY, and they can only be used with CB_GETATTR and SETATTR by a
client holding an appropriate delegation. The SETATTR SHOULD be
either 1) in a separate compound before the one containing the
DELEGRETURN or 2) in the same compound as an operation before the
DELEGRETURN. Failure to properly sequence the operations may lead to
race conditions.
A key prerequisite of this approach is that the server and client are
in time synchronization with each other. Note that while the base
NFSv4.2 does not require such synchronization, the use of RPCSEC_GSS
typically makes such a requirement. When the client presents either
the fattr4_time_deleg_access or the fattr4_time_deleg_modify
attribute to the server, the server MUST decide for both of them
whether the time presented is:
* before the corresponding time_access attribute or time_modify
attribute on the file, or
* past the current server time.
When the time presented is before the original time, then the update
is ignored. When the time presented is in the future, the server can
either clamp the new time to the current time or return NFS4ERR_DELAY
to the client, allowing it to retry. Note that if the clock skew is
large, the delay approach would result in access to the file being
denied until the clock skew is exceeded.
A change in the access time MUST NOT advance the change time, also
known as the time_metadata attribute. However, a change in the
modify time might advance the change time (and in turn, the change
attribute). If the modify time is greater than the change time and
before the current time, then the change time is adjusted to the
modify time and not the current time (as is most likely done on most
SETATTR calls that change the metadata). If the modify time is in
the future, it will be clamped to the current time.
Note that each of the possible times (access, modify, and change) are
compared to the current time. They should all be compared against
the same time value for the current time (i.e., they do not retrieve
a different value of the current time for each calculation).
If the client sets the OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS flag
in an OPEN operation, then it MUST support the
fattr4_time_deleg_access and fattr4_time_deleg_modify attributes in
both the CB_GETATTR and SETATTR operations.
5.1. Use Case for NFSv3 Client Proxy
Consider an NFSv3 client that wants to access data on a server that
only supports NFSv4.2. An implementation may introduce an NFSv3
server that functions as an NFSv4.2 client, serving as a gateway
between the two otherwise incompatible systems. As NFSv3 is a
stateless protocol, the state is not kept on the client, but rather
on the NFSv3 server. As the NFSv3 server is already managing the
state, it can proxy file delegations to avoid spurious GETATTRs.
That is, as the client queries the NFSv3 server for the attributes,
they can be served without the NFSv3 server sending a GETATTR to the
NFSv4.2 server.
5.2. XDR for Proxying of Times
<CODE BEGINS>
///
/// /*
/// * attributes for the delegation times being
/// * cached and served by the "client"
/// */
/// typedef nfstime4 fattr4_time_deleg_access;
/// typedef nfstime4 fattr4_time_deleg_modify;
///
///
/// %/*
/// % * New RECOMMENDED Attribute for
/// % * delegation caching of times
/// % */
/// const FATTR4_TIME_DELEG_ACCESS = 84;
/// const FATTR4_TIME_DELEG_MODIFY = 85;
///
///
/// const OPEN4_SHARE_ACCESS_WANT_DELEG_TIMESTAMPS = 0x100000;
///
/// enum open_delegation_type4 {
/// OPEN_DELEGATE_NONE = 0,
/// OPEN_DELEGATE_READ = 1,
/// OPEN_DELEGATE_WRITE = 2,
/// OPEN_DELEGATE_NONE_EXT = 3, /* new to v4.1 */
/// OPEN_DELEGATE_READ_ATTRS_DELEG = 4,
/// OPEN_DELEGATE_WRITE_ATTRS_DELEG = 5
/// };
<CODE ENDS>
6. Extraction of XDR
This document contains the XDR [RFC4506] description of the new open
flags for delegating the file to the client. The XDR description is
embedded in this document in a way that makes it simple for the
reader to extract into a ready-to-compile form. The reader can feed
this document into the following shell script to produce the machine-
readable XDR description of the new flags:
<CODE BEGINS>
#!/bin/sh
grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'
<CODE ENDS>
That is, if the above script is stored in a file called "extract.sh"
and this document is in a file called "spec.txt", then the reader can
do the following:
<CODE BEGINS>
sh extract.sh < spec.txt > delstid_prot.x
<CODE ENDS>
The effect of the script is to remove leading blank space from each
line, plus a sentinel sequence of "///". XDR descriptions with the
sentinel sequence are embedded throughout the document.
Note that the XDR code contained in this document depends on types
from the NFSv4.2 nfs4_prot.x file (generated from [RFC7863]). This
includes both nfs types that end with a 4 (such as offset4 and
length4) as well as more generic types (such as uint32_t and
uint64_t).
While the XDR can be appended to that from [RFC7863], the various
code snippets belong in their respective areas of that XDR.
7. Security Considerations
While this document extends some capabilities for client delegation,
there are no new security concerns. The client cannot be queried by
other clients as to the cached attributes. The client could report
false data for the cached attributes, but it already has this ability
via a SETATTR operation.
8. IANA Considerations
This document has no IANA actions.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4506] Eisler, M., Ed., "XDR: External Data Representation
Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506, May
2006, <https://www.rfc-editor.org/info/rfc4506>.
[RFC7862] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
November 2016, <https://www.rfc-editor.org/info/rfc7862>.
[RFC7863] Haynes, T., "Network File System (NFS) Version 4 Minor
Version 2 External Data Representation Standard (XDR)
Description", RFC 7863, DOI 10.17487/RFC7863, November
2016, <https://www.rfc-editor.org/info/rfc7863>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8178] Noveck, D., "Rules for NFSv4 Extensions and Minor
Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
<https://www.rfc-editor.org/info/rfc8178>.
[RFC8881] Noveck, D., Ed. and C. Lever, "Network File System (NFS)
Version 4 Minor Version 1 Protocol", RFC 8881,
DOI 10.17487/RFC8881, August 2020,
<https://www.rfc-editor.org/info/rfc8881>.
Acknowledgments
Trond Myklebust, Tom Haynes, and David Flynn all worked on the
prototype at Hammerspace.
Dave Noveck, Chuck Lever, Rick Macklem, and Zaheduzzaman Sarker
provided reviews of the document.
Jeff Layton provided experience from an implementation he authored.
Authors' Addresses
Thomas Haynes
Hammerspace
Email: loghyr@hammerspace.com
Trond Myklebust
Hammerspace
Email: trondmy@hammerspace.com