Internationalization for the NFSv4 Protocols
draft-ietf-nfsv4-internationalization-07
| Document | Type | Active Internet-Draft (nfsv4 WG) | |
|---|---|---|---|
| Author | David Noveck | ||
| Last updated | 2023-11-20 | ||
| Replaces | draft-dnoveck-nfsv4-internationalization | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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draft-ietf-nfsv4-internationalization-07
NFSv4 D. Noveck
Internet-Draft NetApp
Updates: 8881, 7530 (if approved) 20 November 2023
Intended status: Standards Track
Expires: 23 May 2024
Internationalization for the NFSv4 Protocols
draft-ietf-nfsv4-internationalization-07
Abstract
This document describes the handling of internationalization for all
NFSv4 protocols, including NFSv4.0, NFSv4.1, NFSv4.2 and extensions
thereof, and future minor versions.
It updates RFC7530 and RFC8881.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 23 May 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language Definition . . . . . . . . . . . . 4
2.2. Requirements Language Derivation . . . . . . . . . . . . 5
3. Internationalization and Minor Versioning . . . . . . . . . . 7
4. Changes Relative to RFC7530 . . . . . . . . . . . . . . . . . 7
5. Limitations on Internationalization-Related Processing in the
NFSv4 Context . . . . . . . . . . . . . . . . . . . . . . 8
6. Summary of Server Behavior Types . . . . . . . . . . . . . . 9
6.1. Nomalization-related Behavior Types . . . . . . . . . . . 9
6.2. Case-related Behavior Types . . . . . . . . . . . . . . . 11
6.3. Interaction of Normalization and Case . . . . . . . . . . 12
6.4. Client Adaptations . . . . . . . . . . . . . . . . . . . 13
7. The Attribute Fs_charset_cap . . . . . . . . . . . . . . . . 14
7.1. The Attribute Fs_charset_cap in Published NFSv4.1
Specifications . . . . . . . . . . . . . . . . . . . . . 15
7.2. The Attribute Fs_charset_cap in Future NFSv4.1
Specifications . . . . . . . . . . . . . . . . . . . . . 18
8. String Encoding . . . . . . . . . . . . . . . . . . . . . . . 19
9. Normalization . . . . . . . . . . . . . . . . . . . . . . . . 20
10. Case-Insensitive Processing of File Names . . . . . . . . . . 21
10.1. Implementing Case-Insensitive Comparison of File
Names . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.2. Important Examples of Case-insensitive Handling of File
Names . . . . . . . . . . . . . . . . . . . . . . . . . 26
11. Internationalization-related Processing of File Names by
Clients . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.1. Server Restrictions to Deal with Lack of Client
Knowledge . . . . . . . . . . . . . . . . . . . . . . . 31
11.2. Client Processing of File Names for Current NFSv4
Protocols . . . . . . . . . . . . . . . . . . . . . . . 32
11.3. Client Processing of File Names for Future NFSv4
Protocols . . . . . . . . . . . . . . . . . . . . . . . 36
12. String Types with Processing Defined by Other Internet
Areas . . . . . . . . . . . . . . . . . . . . . . . . . . 38
12.1. Effect of IDNA Changes . . . . . . . . . . . . . . . . . 41
12.2. Potential Compatibility Issues Related to IDNA
Changes . . . . . . . . . . . . . . . . . . . . . . . . 42
13. Errors Related to UTF-8 . . . . . . . . . . . . . . . . . . . 43
14. Servers That Accept File Component Names That Are Not Valid
UTF-8 Strings . . . . . . . . . . . . . . . . . . . . . . 44
15. Future Minor Versions and Extensions . . . . . . . . . . . . 45
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
17. Security Considerations . . . . . . . . . . . . . . . . . . . 47
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
18.1. Normative References . . . . . . . . . . . . . . . . . . 47
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18.2. Informative References . . . . . . . . . . . . . . . . . 48
Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 50
Appendix B. Form-insensitive String Comparisons . . . . . . . . 54
B.1. Name Hashes . . . . . . . . . . . . . . . . . . . . . . . 56
B.2. Character Tables . . . . . . . . . . . . . . . . . . . . 59
B.3. Outline of comparison . . . . . . . . . . . . . . . . . . 60
B.4. Comparing Base Characters . . . . . . . . . . . . . . . . 61
B.5. Comparing Combining Characters . . . . . . . . . . . . . 63
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 65
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 65
1. Introduction
Internationalization is a complex topic with its own set of
terminology (see [RFC6365]). The topic is made more difficult to
understand for the NFSv4 protocols by the complicated history
described in Appendix A. In large part, this document is based on
the actual behavior of NFSv4 client and server implementations (for
all existing minor versions). It is intended to serve as a basis for
further implementations to be developed that can interact with
existing implementations. It is expected to enable interoperation
with implementations to be developed in the future.
Note that the set of behaviors on which this document is based are
each effected by a combination of an NFSv4 server implementation
proper and a server-side physical file system. It is common for
servers and physical file systems to be configurable as to the
behavior shown. In the discussion below, each configuration that
shows different behavior is considered separately.
As a consequence of this approach, normative terms defined in
[RFC2119] are often derived from implementation behavior, rather than
the other way around, as is more commonly the case. The specifics
are discussed in Section 2.
With regard to the question of interoperability with existing
specifications for NFSv4 minor versions, different minor versions
pose different issues, even though the actual behavior is the same
for all minor versions. This is because the specfications were often
adopted without the appropriate concern for usability,
implementability, or the expectations of existing NFS users.
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* With regard to NFSv4.0 as defined in [RFC7530], no significant
interoperability issues are expected to arise because the
discussion of internationalization in that specification, which is
the basis for this one, was also based on the behavior of existing
implementations. Although, in a formal sense, the treatment of
internationalization here supersedes that in [RFC7530], the
treatments are intended to be essentially the same, in order to
eliminate the possibility of interoperability issues.
Because of a change in the handling of Internationalized domain
names, there are some differences from the handling in [RFC7530],
as discussed in Appendix A. For a discussion of those differences
and potential compatibility issues, see Sections 12.1 and 12.2.
* With regard to NFSv4.1 as defined by [RFC8881], the situation is
quite different. The approach to internationalization specified
in that document, based in large part on that in RFC3530, was
never implemented, and implementers were either unaware of the
troublesome implications of that approach or chose to ignore the
existing specification as essentially unimplementable. An
internationalization approach compatible with that specified in
[RFC7530] tended to be followed, despite the fact that, in other
respects, NFSv4.1 was considered to be a separate protocol.
If there were NFSv4 servers who obeyed the internationalization
dictates within [RFC5661], or clients that expected servers to do
so, they would fail to interoperate with typical clients and
servers when dealing with non-UTF8 file names, which are quite
common. As no such implementations have come to our attention, it
has to be assumed that they do not exist and interoperability with
existing implementations as described here is an appropriate basis
for this document.
The same applies to all existing minor versions beyound NFSv4.1
(i.e. to NFSv4.2), which made no changes in the specification of
internationalization-related handling and maintained existing
implementation patterns,
2. Requirements Language
2.1. Requirements Language Definition
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 BCP 14 [RFC2119] [RFC8174] when,
and only when, they appear in all capitals, as shown here.
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2.2. Requirements Language Derivation
Although the key words "MUST", "SHOULD", and "MAY" retain their
normal meanings, as described above, we need to explain how the
statements involving these terms were arrived at:
* In the case of statements within Sections 12 and 15, these derive
from the requirements of other internet specifications.
* In the case of statements within Sections 7, 10, and 11 derive
from the author's view of the appropriate normative language to
use and will, when this document is advanced, represent the
working group's consensus on those same matters.
* However, in other cases, i.e. those in sections deriving from
RFC7530 [RFC7530] (i.e. Sections 5, 6, 8, 9, 13, 14, 16, 17) this
specification's descriptions were derived from existing
implementation patterns. Although this pattern is atypical, it is
needed to provide a description that satisfies the goal of
[RFC2119], providing a normative description to enable future
implementations to be compatible with existing ones. This
requires that we explain later in this section how the normative
terms used derive from the behavior of existing implementations,
in those situations in which existing implementation behavior
patterns can be determined.
Note that in introductory and explanatory sections of this document
(i.e. Sections 1 through 4 these terms do not appear except to
explain how they are used in this document. Also, they do not appear
in Appendix B which provides non-normative implementation guidance.
With regard to the parts of this document deriving from RFC7530, we
explain below how the normative terms used derive from the behavior
of existing implementations, in those situations in which existing
implementation behavior patterns can be determined.
* Behavior implemented by all existing clients or servers is
described using "MUST", since new implementations need to follow
existing ones to be assured of interoperability. While it is
possible that different behavior might be workable, we have found
no case where this seems reasonable.
The converse holds for "MUST NOT": if a type of behavior poses
interoperability problems, it MUST NOT be implemented by any
existing clients or servers.
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* Behavior implemented by most existing clients or servers, where
that behavior is more desirable than any alternative, is described
using "SHOULD", since new implementations need to follow that
existing practice unless there are strong reasons to do otherwise.
The converse holds for "SHOULD NOT".
* Behavior implemented by some, but not all, existing clients or
servers is described using "MAY", indicating that new
implementations have a choice as to whether they will behave in
that way. Thus, new implementations will have the same
flexibility that existing ones do.
* Behavior implemented by all existing clients or servers, so far as
is known -- but where there remains some uncertainty as to details
-- is described using "should". Such cases primarily concern
details of error returns. New implementations should follow
existing practice even though such situations generally do not
affect interoperability.
There are also cases in which certain server behaviors, while not
known to exist, cannot be reliably determined not to exist. In part,
this is a consequence of the long period of time that has elapsed
since the publication of the defining specifications, resulting in a
situation in which those involved in the implementation work may no
longer be involved in or be aware of working group activities.
In the case of possible server behavior that is neither known to
exist nor known not to exist, we use "SHOULD NOT" and "MUST NOT" as
follows, and similarly for "SHOULD" and "MUST".
* In some cases, the potential behavior is not known to exist but is
of such a nature that, if it were in fact implemented,
interoperability difficulties would be expected and reported,
giving us cause to conclude that the potential behavior is not
implemented. For such behavior, we use "MUST NOT". Similarly, we
use "MUST" to apply to the contrary behavior.
* In other cases, potential behavior is not known to exist but the
behavior, while undesirable, is not of such a nature that we are
able to draw any conclusions about its potential existence. In
such cases, we use "SHOULD NOT". Similarly, we use "SHOULD" to
apply to the contrary behavior.
In the case of a "MAY", "SHOULD", or "SHOULD NOT" that applies to
servers, clients need to be aware that there are servers that may or
may not take the specified action, and they need to be prepared for
either eventuality.
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3. Internationalization and Minor Versioning
Despite the fact that NFSv4.0 and subsequent minor versions have
differed in many ways, the actual implementations of
internationalization have remained the same and internationalized
file names have been handled without regard to the minor version
being used. Minor version specification documents contained
different treatments of internationalization as described in
Appendix A but of those only the implementation-based approach used
by [RFC7530], resulted in a workable description while a number of
attempts to specify an approach that implementors were to follow were
all ignored by implementers.
It is expected that any future minor versions will follow a similar
approach, even though there is nothing to prevent a future minor
version from adopting a different approach as long as the rules
within [RFC8178]) are adhered to. In any such case, the new minor
version would have to be marked as updating or obsoleting this
document. Issues relating to potential extensions within the
framework specified in this document are dealt with in Section 15.
4. Changes Relative to RFC7530
This document follows the internationalization approach defined in
RFC7530, with a number of significant necessary changes described
below.
* The handling of internationalization specified in [RFC7530] is
applied to all NFSv4 minor versions. No compatibility issues are
expected to arise because all existing implementations follow the
same approach to internationalization despite the large difference
between [RFC7530] and what was specified in [RFC5661]. Issues
relating to potential future minor versions and protocol
extensions are addressed in Section 15.
* Changes made necessary by the shift from IDNA2003 to IDNA2008 have
been made. The intention is to maintain compatibility with all
existing implementations of all NFSv4 minor versions. Potential
compatibility issues with regard to the IDNA shift are discussed
in Section 12.2.
* There is more detailed discussion of case-insensitive handling of
file names, with particular attention to the complexities that can
arise when multiple language conventions in these matters need to
be accommodated. The discussion in Section 10 applies to both
client and server, although issues relating to the client's
knowledge of server behavior are dealt with in Section 11.
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* There is additional material, dealing with the implications of
server-side internationalization-related file name processing for
clients that cache the results of READDIR's. This includes a
discussion of options to deal with the current lack of detailed
information about the server (in Section 11.2), and options for
handling when more detailed information can be made available (in
Section 11.3)."
5. Limitations on Internationalization-Related Processing in the NFSv4
Context
There are a number of noteworthy circumstances that limit the degree
to which internationalization-related encoding and normalization-
related restrictions can be made universal with regard to NFSv4
clients and servers:
* The NFSv4 client is part of an extensive set of client-side
software components whose design and internal interfaces are not
within the IETF's purview, limiting the degree to which a
particular character encoding might be made standard.
* Server-side handling of file component names is most often
implemented within a server-side physical file system, whose
handling of character encoding and normalization is not
specifiable by the IETF.
* Typical implementation patterns in UNIX systems result in the
NFSv4 client having no knowledge of the character encoding being
used, which might even vary between processes on the same client
system.
* Users may need access to files stored previously with non-UTF-8
encodings, or with UTF-8 encodings that are not in accord with any
particular normalization form.
Despite the above, there are cases in which UTF-8-related processing
can be provided by servers, as described in Section 6. These
behaviors in which the server is aware of UTF-8 encoding relate to
normalization and to case and are of two types:
* Equivalent strings are treated as identical in matching names with
associated files. This typically requires special code within the
server-side file system, rather than in the server proper.
* Name strings may be mapped to equivalent names resulting in file
having an equivalent name rather than the one specified by the
client. This approach is implementable within the server proper.
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6. Summary of Server Behavior Types
This section describes the language-related behaviors that are vaidly
manifested by NFSv4 servers together with the server-side file
systems made accessible by such servers. These relate to the
handling of normalization (see Section 6.1) and of case (see
Section 6.2).
How these two aspects of behavior interact is described in
Section 6.3 while the implications for client behavior are discussed
in Section 6.4.
For both of these aspects, there are three choices:
* UTF-8-unaware file systems MAY be used.
In this case, neither normalization-related nor case-related
behavior is possible.
Implementations need no special handling either in the server
proper or in the file system itself.
* UTF-8-aware file systems MAY treat as indistinguishable file name
strings that differ only as to nomalization form or only as to
case.
This choice may be made independently for normalization and for
case.
Implementations cannot be done in the server proper but would
typically be done in the file system itself.
* UTF-8-aware file systems MAY modify file name strings to a
corrsponding value of a particular nomalization form or of a
particular case.
Implementations can be done in the server proper and do not
require special character handling in the file system itself.
6.1. Nomalization-related Behavior Types
Servers, together with server-side file systems accessed using NFSv4,
MAY reject component name strings that are not valid UTF-8. This
choice leads to a number of types of valid server behavior, as
outlined below. When these are combined with the valid
normalization-related behaviors as described in Section 8, this leads
to the valid combined behaviors outlined below.
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* Servers that do not limit file component names on particular file
systems to UTF-8 strings are very common and are necessary to deal
with clients/applications not oriented to the use of UTF-8. Such
servers ignore normalization-related issues, and there is no way
for them to implement either a specfic normalization form or
representation-independent lookups. These are best described as
behaving as "UTF-8-unaware servers" for such file systems, since
they treat file component names as uninterpreted strings of bytes
and have no knowledge of the characters represented. See
Section 13 for details.
* Servers that limit file component names within a given file system
to UTF-8 strings exist with normalization-related handling as
described in Section 8. These are best described as behaving as
"UTF-8-only servers".
* It is possible for a server to allow component names that are not
valid UTF-8, while still being aware of the structure of UTF-8
strings. Such servers could, in theory, implement either
normalization or representation-independent lookups but apply
those techniques only to valid UTF-8 strings. Such servers are
not common, but it is possible to configure at least one known
server to have this behavior. This behavior SHOULD NOT be used,
due to the possibility that a file name using one encoding may, by
coincidence, have the appearance of a UTF-8 file name; the results
of UTF-8 normalization or representation-independent lookups are
unlikely to be correct in all cases, when considered from the
viewpoint of the other encoding. Such difficulties can be
compounded when case-insensitive name handling is in effect.
To summarize, the following are the valid behaviors for particular
server/file system combinations:
* Can behave without knowledge of UTF-8-based internationalization,
and behave as UTF8-unaware servers for some file systems.
This is a common implementation pattern and is compatible with the
expectations of those using NFSv3.
* Enforce use of names that are valid UTF-8 strings, without
manifesting any normalization-aware behavior. Such servers are
UTF-8-aware but treat different normalization forms of the same
name as different file names.
This behavior pattern is compatible with existing server and file
system implementations but might encounter difficulties with
appplications dealing with internationalization using encodings
other than UTF-8.
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* Enforce use of names that are valid UTF-8 strings and implement
representaion-independent name matching in order to treat
canonically equivalent strings as designating the same file.
This behavior cannot, as a practical matter, be implemented within
the server. Implementation requires implementing name matching
changes within the file system. See Appendix B for helpful
implementation guidance.
* Enforce use of names that are valid UTF-8 strings and deal with
normalization by normalizing all names to a common normalization
form.
This behavior can be implemented within the server proper but
might prove disconcerting to applications that create a file with
one name and encounter it with another, canonically equivalent
name.
6.2. Case-related Behavior Types
The following are the valid behaviors related to case for particular
server/file system combinations:
* Can behave without knowledge of case mappings, i.e. act as a case-
sensitive file system. All UTF-8-unaware file systems are case-
sensitive, although some UTF-8-aware file systems can be case-
insensitive as well.
This is a common implementation pattern and is compatible with the
expectations of those using NFSv3.
* Implement case-insensitive name matching in order to treat strings
that only differ as to case as designating the same name.
This behavior cannot, as a practical matter, be implemented within
the server. Implementation requires name matching changes within
the file system.
It is often the case that the case conversion of specfic
characters is not as straightforward as it might be expected to
be. See Section 10 for further dicussion.
* Implement case-insensitive naming by mapping all file names to a
common case, either upper or lower.
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This behavior can be implemented within the server proper but
might prove disconcerting to applications that create a file with
one name and encounter it with a dufferent name, with unexpected
case modifications.
6.3. Interaction of Normalization and Case
For the most part the behavior choices are as discussed in Sections
6.1 and 6.2. However the following issues need to be noted:
* UTF-8-unaware filesystems cannot implement normalization-related
behavior. Similarly, they cannot implement case-related behavior
and so are case-sensitive and case-preserving.
The reason for both of these restrictions is that a UTF-8-unaware
filesysstem is inherently unaware of the character encoding used
and so cannot modify characters specfied by the client it to
refect normalization behvior or case.
* UTF-8-aware filessystems may choose to implement normalization
handling or case handling independently.
When representation-independent name matching is combined with a
similar handling of case (i.e. case-insensitive case-preserving
handling), the matching of string name reflects the the fact that
two canonically equivalent names are treated as identical and
similarly for names differing only as to case. Implementation
issues for representation-independent name matching are discussed
in Appendix B.
* When case-related behavior is in effect, character-sequences that
differ only in case are treated as identical. In many cases, this
identification is straightforward.
Generallly, the case mapping of Roman, Cyrillic, and Greek
alphabets are well-understood and the addition of combining
characters to these mappings pose no additional issues.
There are cases, such as those involving the handling of dotted
and dotless i's in Turkish and Azeri and the German Sharp S
character, where different mappings could be used, requiring that
client and server do not make different assumptions See
Section 6.4.
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6.4. Client Adaptations
The handling of names by the server affects both the client proper
and client-side applications that deal with file names. When
normalization-handling and case-handling behaviors are implemented by
the server file system, these affect how file names are mapped to
specific files.
In the case of UTF-8-unaware file systems, each distinct name string
is mapped to a single corresponding file object. This applies even
if two strings are canonically equivalent or differ only in the case
of the corresponding string characters.
When behaviors related to normalization or case are in effect, there
are two possibilities which may be chosen for each behavior class:
* When canonically equivalent names, or names which only differ as
to case, are considered the same, there can only be a single such
file, that share the equivalent name.
In such situations, users normally expect such name identiication
to occur.
* When specified file names are normalized to a different form, or
converted to a different case, the file name will differ from that
specified by the user.
In such situations, users are typically prepared for this sort of
name variance.
When clients cache the existence or non-existence of files with
particuar names, as they often do, they need to take account of
servers handling of normalization and case.
This issue rarely affects cases of positive name caching in which the
client records the existence of a file with a particular name.
Clients have to avoid assuming that the name create is that povided
by the client, shich is nit universally true
In the case of negative name caching, clients might record the non-
existence of a file with a particular name and be unaware that the
creation of a file with a different, although equivalent, name by
another client. To prevent that, the client would have to forego
negative name caching or have a way of determinine the server's
notion of name equivalence. With such information, the client can
adapt its notion of name equivalence to be that used by the server.
See Section 11 for details.
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7. The Attribute Fs_charset_cap
This attribute, nominally "RECOMMENDED", appears to have been added
to NFSv4.1 to allow servers, while staying within the constraints of
the stringprep-based specification of internationalization, to allow
uses of UTF-8-unaware naming by clients. As a result, those NFSv4
servers implementing internationalization as NFSv3 had done, could be
considered spec-compliant, as long as a later "SHOULD" was ignored.
However, because use of UTF-8 was tied to existing stringprep
restrictions, implementations of internationalization, that were
aware of Unicode canonical equivalence issues were not provided for.
Although this attribute may have been implemented despite the
problems noted in Section 7.1, the overall scheme was never
implemented and NFSv4.1 implementations dealt with
internationalization in the same way as NFSv4.0 implementations had.
It is generally accepted that NFSv4 attributes designated elsewhere
as "RECOMMENDED" are essentially OPTIONAL with the client having the
responsibility to deal with server non-support of them. While
RFC7530 has gone so far as to explicitly exclude this use from the
general statement that these terms are to be used as defined by
RFC2119, no NFSv4.1 specification has done so, at least through
[RFC8881]. In this particular case, there are a number of
circumstances that makes this OPTIONAL status noteworthy:
* The statement "It is expected that servers will support all
attributes they comfortably can and only fail to support
attributes that are difficult to support in their operating
environments", appearing in Section 5.2 of [RFC8881] is
troublesome since it is hard to understand how a server could find
this read-only attribute "difficult to support" regardless of the
operating environment used.
* This was added in minor version one which added a number of
REQUIRED operations and could well have added a REQUIRED
attribute.
* The fact that the client is to be prepared for non-support of the
attribute would require specification of a default value to be
assumed by the client, yet none is provided.
The attribute contains two flag bits. As discussed below, in
Section 7.1, it is hard two see why two bits are required while the
implications of this issue for future NFSv4.1 specifications will be
discussed in Section 7.2
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7.1. The Attribute Fs_charset_cap in Published NFSv4.1 Specifications
We reproduce Section 14.4 of [RFC8881] below, with comments
interspersed trying to make sense of what is there, in order to
arrive at an appropriate replacement, to be presented in Section 7.2.
In that connection, we need to understand better a few issues:
* The use of two bits while one is clearly adequate, given the
subject matter actually mentioned.
* The mention of possible "capabilities" which could not possibly be
realized.
* The use of the RFC2119 keyword "SHOULD" in contexts in which this
term is clearly inappropriate.
Issues related to the confusion caused by mention of "UTF-8
characters" and the lack of mention of Unicode will be addressed in
the revision in Section 7.2 but will not be further discussed here.
const FSCHARSET_CAP4_CONTAINS_NON_UTF8 = 0x1;
const FSCHARSET_CAP4_ALLOWS_ONLY_UTF8 = 0x2;
typedef uint32_t fs_charset_cap4;
While it is made clear that two separate bits are to be provided,
their names seem to indicate that they should be complements of one
another. As a way of understanding why two bits were specified, it
is helpful to consider a possible boolean attribute as a potential
replacement. That attribute would clearly govern whether names that
do not conform to the rules of UTF-8 are to be rejected, which was a
"MUST" in [RFC3530]. Although conveying this information is clearly
part of the motivation, stating so explicitly might have been judged
by the authors as unnecessarily provocative, given the role of IESG
in arriving at the internationalization approach specified in
RFC3530.
Because some operating environments and file systems do not
enforce character set encodings,
It is clear that the ability of operating environments to enforce use
of UTF-8 encoding is not an issue, since RFC3530 made this the
responsibility of the server implementation. That mandate was never
followed because implementers chose not to follow it, and not because
they were unable to do so.
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The apparently confused statement above is best understood if one
notes that its essential job is to state that the "MUST" in RFC3530
referred to above is not reasonable. However, the authors might well
have felt unable to say so explicitly, in light of the potential IESG
reaction.
NFSv4.1 supports the fs_charset_cap attribute (Section 5.8.2.11)
that indicates to the client a file system's UTF-8 capabilities.
The problem with the mention of (plural) capabilities is that the
only capability mentioned which servers could implement is to accept
strings which are not valid UTF-8. There are other potential
capabilities having to do with the handling of canonically equivalent
file nsmes, but since they were not mentioned, they will not be
discussed further here.
The attribute is an integer containing a pair of flags. The first
flag is FSCHARSET_CAP4_CONTAINS_NON_UTF8, which, if set to one,
tells the client that the file system contains non-UTF-8
characters,
As stated, this would mean that a server would have to keep track of
a count of non-UTF-8-encoded names within the file system and change
the attribute value as that count varied between zero and non-zero.
Since it is most unlikely that any server would keep track of that or
that any client would find it useful, we will assume that the
capability to store such names is what is most likely intended.
and the server will not convert non-UTF characters to UTF-8 if the
client reads a symbolic link or directory,
There is no way for the server to convert non-UTF names to UTF-8 or
anything else, since it has no knowledge of the name encoding to
begin with. The alternative to treating names as UTF-8-encoded
Unicode strings is to treat them as POSIX does, as uninterpreted
strings of bytes. That makes it impossible to interpret strings that
do not follow the rules of UTF-8 at all, making it impossible to
convert the string to UTF-8.
neither will operations with component names or pathnames in the
arguments convert the strings to UTF-8.
As stated above, there is no way a server could ever do that.
The second flag is FSCHARSET_CAP4_ALLOWS_ONLY_UTF8, which, if set
to one, indicates that the server will accept (and generate) only
UTF-8 characters on the file system.
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That is clear and so it poses no problem for a revised treatment,
unlike the other flag.
If FSCHARSET_CAP4_ALLOWS_ONLY_UTF8 is set to one,
FSCHARSET_CAP4_CONTAINS_NON_UTF8 MUST be set to zero.
There is no problem with this statement. However, it does, by
implication, raise the issue of what values of
FSCHARSET_CAP4_CONTAINS_NON_UTF8 may be set in the case in which
FSCHARSET_CAP4_ALLOWS_ONLY_UTF8 is set to zero.
FSCHARSET_CAP4_ALLOWS_ONLY_UTF8 SHOULD always be set to one.
According to xref target="RFC2119"/>, "SHOULD" means that "there may
exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighing a different course". In this context, it is
unclear what these "full implications" might be, given the
introduction above. The clause, "because some operating environments
and file systems do not enforce character set encodings", gives one
no basis for treating this as other than an unproblematic behavioral
variant, calling into question the use of "SHOULD".
Also, the statement in RFC2119 that these terms (i.e. those like
"SHOULD") "only be used where it is actually required for
interoperation or to limit behavior which has the potential for
causing harm" raises the following issues:
* The whole purpose of this feature is to enable interoperation and
there is no basis for the implication that one particular flag
value is superior to another in allowing interoperation.
* There is no basis for assuming that accepting file names that are
not UTF-8-encoded Unicode has any potential for causing harm.
Despite the statement in RFC2119, that "they [i.e. terms such as
'SHOULD'] must not be used to impose a particular method on
implementors", it is hard to avoid the conclusion that this is in
fact the motivation for the "SHOULD". The authors might not have had
any such intention but it is posible they felt that the IESG might
well have had such an intention and used "SHOULD" for this reason.
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7.2. The Attribute Fs_charset_cap in Future NFSv4.1 Specifications
We provide a revised version of Section 14.4 of [RFC8881] below,
taking into account the issues noted in Section 7.1. Given there was
a working group consensus to adopt the confusing language discussed
there, we must now adopt, by consensus, a clearer replacement that
reflects the working group's intentions and is compatible with
existing implementations. Given the passage of time and the changed
context, it might not be possible to determine those intentions. In
any case, we will have to be aware of how this attribute was
implemented and used, particularly with regard to the first flag,
whose meaning remains obscure.
The following treatment is proposed as a basis for discussion, with
the understanding that it would need to be changed, if it could raise
interoperability issues.
const FSCHARSET_CAP4_CONTAINS_NON_UTF8 = 0x1;
const FSCHARSET_CAP4_ALLOWS_ONLY_UTF8 = 0x2;
typedef uint32_t fs_charset_cap4;
This attribute provides a simple way of determining whether a
particular file system behaves as a UTF-8-only server and rejects
file names which are not valid UTF-8 strings. When this attribute
is supported and the value returned has the
FSCHARSET_CAP4_ALLOWS_ONLY_UTF8 flag set, the error NFS4ERR_INVAL
MUST be returned if any file name argument contains a string which
is not a valid UTF-8 string.
When this attribute is supported and the value returned has the
FSCHARSET_CAP4_ALLOWS_ONLY_UTF8 flag clear, the error
NFS4ERR_INVAL will not be returned based on adherence to the rules
of UTF-8. While such file systems are generally UTF-8-unaware,
this cannot be assumed, since server are allowed (in some
circumstances; it is a "SHOULD NOT") to accept non-UTF-8 names
while being aware of the structure of UTF-8-conforming names, for
the purposes of determining canonical equivalence, for example.
See Section 6.
With regard to the flag FSCHARSET_CAP4_CONTAINS_NON_UTF8, it has
proved impossible to determine, from existing treatments of this
attribute, any value that might be helpful here. As a result, we
are forced to assume that this flag is always a complement of
FSCHARSET_CAP4_ALLOWS_ONLY_UTF8 and that any result in which it is
not so is to be ignored, with the appropriate handling being the
same as would apply if the attribute were not supported.
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When this attribute is not supported, the client can perform a
LOOKUP using a name not conforming to the rules of UTF-8 and use
the error returned to determine whether non-UTF-8 names are
accepted.
8. String Encoding
Strings that potentially contain characters outside the ASCII range
[RFC20] are generally represented in NFSv4 using the UTF-8 encoding
[RFC3629] of Unicode [UNICODE]. See [RFC3629] for precise encoding
and decoding rules.
Some details of the protocol treatment depend on the type of string:
* For strings that are component names, the preferred encoding for
any non-ASCII characters is the UTF-8 representation of Unicode.
In many cases, clients have no knowledge of the encoding being
used, with the encoding done at the user level under the control
of a per-process locale specification. As a result, it may be
impossible for the NFSv4 client to enforce the use of UTF-8. The
use of non-UTF-8 encodings can be problematic, since it may
interfere with access to files stored using other forms of name
encoding. Also, normalization-related processing (see Section 9)
of a string not encoded in UTF-8 could result in inappropriate
name modification or aliasing. In cases in which one has a non-
UTF-8 encoded name that accidentally conforms to UTF-8 rules,
substitution of canonically equivalent strings can change the non-
UTF-8 encoded name drastically.
For similar reasons, where non-UTF-8 encoded names are accepted,
case-related mappings cannot be relied upon. For this reason, the
attribute case_insensitive MUST NOT be returned as TRUE for file
systems which accept non-UTF-8 encoded file names.
The kinds of modification and aliasing mentioned here can lead to
both false negatives and false positives, depending on the strings
in question, which can result in security issues such as elevation
of privilege and denial of service (see [RFC6943] for further
discussion).
* For strings based on domain names, non-ASCII characters MUST be
represented using the UTF-8 encoding of Unicode or some encoding
based on that (e.g. xn-labels including Punycoe, and additional
string format restrictions will apply. See Section 12 for
details.
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* The contents of symbolic links (of type linktext4 in the XDR) MUST
be treated as opaque data by NFSv4 servers. Although UTF-8
encoding is often used, it need not be. In this respect, the
contents of symbolic links are like the contents of regular files
in that their encoding is not within the scope of this
specification.
* For other sorts of strings, any non-ASCII characters SHOULD be
represented using the UTF-8 encoding of Unicode.
9. Normalization
The client and server operating environments can potentially differ
in their policies and operational methods with respect to character
normalization (see [UNICODE] for a discussion of normalization
forms). This difference may also exist between multile applications
on the same client. This adds to the difficulty of providing a
single normalization policy for the protocol that allows for maximal
interoperability. This issue is similar to issues related to
character case, in which the server may or may not support case-
insensitive file name matching and may or may not preserve the
character case when storing file names. The protocol does not
mandate a particular behavior but allows for a range of useful
behaviors.
The NFSv4 protocol does not mandate the use of a particular
normalization form. A subsequent minor version of the NFSv4 protocol
might specify a particular normalization form, although there would
be difficulties in doing so (see Section 15 for details). In any
case, the server and client can expect that they might receive
unnormalized strings within protocol requests and responses. If the
operating environment requires normalization, then the implementation
will need to normalize the various UTF-8 encoded strings within the
protocol before presenting the information to an application (at the
client) or local file system (at the server).
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Server implementations MAY normalize file names to conform to a
particular normalization form before using the resulting string when
looking up or creating files. Servers MAY also perform
normalization-insensitive string comparisons without modifying the
names to match a particular normalization form. Except in cases in
which component names are excluded from normalization-related
handling because they are not valid UTF-8 strings, a server MUST make
the same choice (as to whether to normalize or not, the target form
of normalization, and whether to do normalization-insensitive string
comparisons) in the same way for all accesses to a particular file
system. Servers SHOULD NOT reject a file name because it does not
conform to a particular normalization form, as this would deny access
to clients that use a different normalization form or clients acting
on behalf of applications that use a different normalization form.
10. Case-Insensitive Processing of File Names
When the server is to process file names in a case-insensitive way in
a given file system, it may choose to do so in a number of ways.
* It can force all characters which have multiple forms to a common
case, whether upper case or lower case. Although this may cause
the file name shown in the directory to be different from that
specified when the file is created, these two names will be judged
as equivalent when a case-insensitive comparison is used. Such
file systems are case-insensitive but not case-preserving.
* It can preserve all names, presented as valid and not subject to
case-based modification, while treating two names that are
equivalent when a case-insensitive comparison is used as referring
to the same file. Such file systems are both case-insensitive and
case-preserving.
When a server implements case-insensitive file name handling, it is
necessary that clients do so as well. For example, if a client
possessing the cached contents of a directory, notes that the file
"a" does not exist, it cannot immediately act on that presumed non-
existence, without checking for the potential existence of "A" as
well. As a result, clients need to be able to provide case-
insensitive name comparisons, irrespective of whether the server
handling is case-preserving or not.
Because case-insensitive name comparisons are not always as
straightforward as the above example suggests, the client, if it is
to emulate the server's name handling, would need information about
how certain cases are to be dealt with. In cases in which that
information is unavailable, the client needs to avoid making
assumptions about the server's handling, since it will be unaware of
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the Unicode version implemented by the server, or many of the details
of specific issues that might need to be addressed differently by
different server file systems in implementing case-insensitive name
handling.
Many of the problematic issues with regard to the case-insensitive
handling of names are discussed in Section 5.18 of the Unicode
Standard [UNICODE-CASEM] which deals with case mapping. While we
need to address all of these issues as well, our approach will not be
exactly the same.
* Since the client will be doing case-insensitive comparisons,
issues that apply only to uppercasing or lowercasing do not have
the same significance.
* Many clients will have to operate correctly even in the absence of
detailed information about the specifics of server-side case-
mapping or the version of Unicode implemented by the server.
* Clients will have to accommodate server behaviors not anticipated
by the Unicode Specification since it might be that neither the
server nor the client would have any relavant locale knowledge
when file names are processed.
Another source of information about case-folding, and indirectly
about case-insensitive comparisons, is the case-folding text file
which is part of the Unicode Standard [UNICODE-CASEF]. This file
contains, for each Unicode character that can be uppercased or
lowercased, a single character, or, in some cases a string of
characters of the other case. For characters in capital case, the
lowercase counterpart is given. Each of the mappings is
characterized as of one of four types:
* Common case folding, denoted by a status field of "C". These are
used for mapping where a single character can be mapped to a
single character of another case. These are always valid with one
potential exception being the mappings of LATIN CAPITAL LETTER I
to LATIN SMALL LETTER I and vice versa, which might be superseded
by the T-type mappings associated with some Turkic languages when
written using Latin letters.
* Full case folding, denoted by a status field of "F". These are
used for mappings in which single character is mapped to a multi-
character string of a different case.
* Special case folding, denoted by a status field of "S". These
provide additional single-character-to-single-character which
might be used when there is also an F-type mapping of the same
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character. In the case of case folding, this is an alternative to
the corresponding F-type, although, for the purposes of case-
insensitive string comparison, it is possible for both to be
considered valid at the same time
* Special case foldings for Turkic languages, denoted by a status
field of "T". These consist of the invertible case mappings
between LATIN SMALL LETTER I (U+0069) and LATIN CAPITAL LETTER I
WITH DOT ABOVE (U+0130) and between LATIN CAPITAL LETTER I
(U+0049) and LATIN SMALL LETTER DOTLESS I (U+0131). The
relationship between these mappings and the C-type mappings for
LETTER I is discussed below in item EX8.
While the case mapping section does discuss case-insensitive string
comparisons, and describes a procedure for constructing equivalence
classes of Unicode characters, the description does not deal clearly
with the effect of F-type mappings. There are a number of problems
with dealing with F-type mappings for case folding and basing case-
insensitive string comparisons on those mappings, particularly in
situations, such as file systems, in which extensive processing of
strings is unlikely to be practical.
* Mappings from single characters to multi-character strings, are,
for case-folding purposes, not invertible. However, case-
insensitive name comparison, by its nature, requires invertible
mappings, in which a multi-character string is mapped to a single
character of a different case. This is not compatible with any
existing simple case-mapping models.
* Scanning of names for multi-character sequences might well be too
complicated, especially since such sequences might overlap in
complicated ways.
* Case foldings which map single characters to multi-character
sequences (see item EX4 below for an important example), would
give rise, because of the invertibility of case mappings when used
to determine case-insensitive string equivalence for very large
sets of strings. For example, a string of eight copies of the
letter S would give rise to an set of 256 equivalent strings plus
over two thousand others when the German SHARP S characters
discussed in item EX4 are included.
Despite these potential difficulties, case mappings involving multi-
character sequences can be reversed when used as a basis for case-
insensitive string comparisons and incorporated into a set of
equivalence classes on name strings, as descibed below.
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* Case-insensitive servers MAY do either case-mapping to a chosen
case or case-insensitive string comparisons when providing a case-
preserving implementation. In either case, the server MAY include
F-type mappings, which map a single character to a multi-character
string. However, only the case in which it is doing case-
insensitive string comparison will it use the inverse of F-type
mappings, in which a multi-character string is mapped to a single
character of a different case
In these cases, the server can choose to use either a C-type
mapping or an F-type mapping, or both, when both exist. Similarly
the server may choose to implement the C-type mappings of LATIN
CAPITAL LETTER I to LATIN SMALL LETTER I and vice versa, the
corresponding T-type mappings or both, although using only the
second of these is not allowed, unless there is a means of
informing the client that it has been chosen.
* The client, when informed of the details of the client's handling
of case, has the ability to efficiently implement an appropriate
case-insensitive name comparison compatible with that of the
server. This includes the ability to handle mappings between
single characters and multi-character strings.
* Implementation of case-insensitive name comparisons will typically
require a case-insensitive name hash.
10.1. Implementing Case-Insensitive Comparison of File Names
Implementing case-insensitive string comparisons based on equivalence
classes including multi-character strings can be performed as
described below. When such case-based equivalence classes contain
multi-character strings, there are potential complexities that derive
from the need to recognize such multi-character strings within the
inputs bein compares.
The algorithm presented in this section requires that if there is
more than one multi-character string within a given equivalence
class, they must all be equivalent, with any equivalences derivable
from case-insensitive string equivalence using single-character
equivalence classes.
Although other sources are possible (see items EX2 and EX3 in
Section 10.2), multi-character sequences often appear in case-
insensitive equivalence classes as the result of the canonical
decomposition of one or more precomposed characters as elements of a
case-insensitive equivalence class.
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While the algorithm presented in this section can deal with certain
case-based equivalences deriving from canonical decomposition, it is
not capable of providing general handling of the combination of
canonical equivalence and case-based equivalence. While this can be
addressed by normalizing strings before doing case-insensitive
comparison, it is more efficient to do a general form-insensitive and
case-insensitive string comparison in a single step as described in
Appendix B
The following tables would be used by the comparison algorithm
presented below.
* For each possible character value, the associated equivalence
class for case-insensitive comparison will be identified
* For each such equivalence class, the hash value contribution will
be provided. In the case of equivalence class that do not include
multi-character strings including equivalence classes that only
include a single member, this will be the hash value contribution
of one particular variant (usually lower case) of the character
* In the case of equivalence classes that do include multi-character
strings, the hash value contribution needs to be equivalent to the
combined contribution of each character within the multi-character
string. In addition, for each such equivalence class, the length
of the multicharacter string will be provided together with a
pointer to an array describing the multi-character string, most
probably presenting each character as an equivalence class
identifier.
Case-insensitive comparison proceeds as follows:
* Implementation of case-insensitive name comparisons will typically
require a case-insensitive name hash using the tables described
above. If such a hash value is kept for all cached names,
comparisons of hashes can be used instead of the detailed
comparison set forth below. Using such hash comparisons, a large
set of potentially equivalent names can be excluded based on the
occurrence of hash mismatches, since case-equivalent names would
have the same hash value. value.
* For names with matching hash values, a detailed case-insensitive
comparison will be necessary. This can proceed character-by-
character or byte-by-byte. However, in the byte-by-byte case,
processing in the event of a mismatch must start at the start of
the current character, rather than the byte at which the
difference was detected.
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* In cases in which there is a mismatch, the associated equivalence
classes will be compared. When these are identical, indicating
the case equivalence of the two characters, the comparison of the
two strings continues at the next character of each string.
* When the two equivalence classes are not identical, further
comparisons to determine if a single character within one string
matches (except for case) a multi-character string within the
other. For each of two equivalence classes being compared that
include a multi-character string, the check below must be made to
determine whether the multi-character string at the corresponding
position of the other string being compared, is within the current
equivalence class. If neither of the two equivalence classes
include multi-character strings, the comparison terminates with a
mismatch indication.
* For each equivalence class that does include a multi-character
string (there might be one or two), a scan needs to be made to see
of the characters at the current position if the other string
matches (except for case) the multi-character string which is
included in the current equivalence class. If this check
succeeds, for either equivalence class, the comparison of the two
strings continues at the next character of each string. In the
event of failure, the same sort of comparison is done using the
other current equivalence class, if it include multi-character
strings. Once this check fails for all equivalence classes that
include multi-character strings, the comparison terminates with a
mismatch indication.
10.2. Important Examples of Case-insensitive Handling of File Names
In this section, we discuss many of the interesting and/or
troublesome issues that the need for case-insensitive handling gives
rise to in fully internationalized environments. Many of these are
also discussed in [UNICODE-CASEM]. However, our treatment of these
issues, while not inconsistent with that in [UNICODE-CASEM], differs
significantly for a number of reasons:
* Our primary focus is on case-insensitive string comparison rather
than with case mapping per se. While such comparison is natural
for the client and allowed for servers, its greater flexibility
makes it important to understand its capabilities in dealing with
potentially troublesome issues in providing case-insensitive file
name handling.
* Because a case mapping model forces the specification of a single
case mapping result when there are multiple potentially valid
results, there are inevitably cases in which the result chosen is
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inappropriate for some users. These are cases in which F-type and
S-type mappings are present and in which C-type and T-type
mappings conflict. Normally, an appropriate choice is selected by
use of the locale, but in a file system environment, valid locale
information might not be present. As a result, case-insensitive
string comparison, which does not force such case mapping choices,
will be more desirable.
The examples below present common situations that go beyond the
simple invertible case mappings of Latin characters and the
straightforward adaptation of that model to Greek and Cyrillic. In
EX4 and EX5 we have case-based equivalence classes including multi-
character strings not derived from canonical equivalences while for
EX7 and EX8 all multi-character strings are derived from canonical
equivalences. In addition, EX1, EX2, EX3 and EX6 discuss other
situations in which an equivalence class has more than two elements.
EX1: Certain digraph characters such LATIN SMALL LETTER DZ (U+01F3)
have additional case variants to consider such as the titlecase
character LATIN CAPTAL LETTER D WITH SMALL LETTER Z (U+01F2) in
addition to the uppercase LATIN CAPITAL LETTER DZ (U+01F1).
While the titlecased variant would not appear in names in case-
insensitive non-case-preserving file systems, case-insensitive
string comparison has no problem in treating these three
characters as within the same equivalence class.
This equivalence class can be derived from only C-type
mappings. The possibility of mapping these characters to two-
character sequences they represent is not a troublesome issue
since that would be derived from a compatibility equivalence,
rather than a canonical equivalence, and there is no F-type
mapping making it an option.
EX2: To deal with the case of the OHM SIGN (U+2126) which is
essentially identical to the GREEK CAPITAL LETTER OMEGA
(U+03A9), one can construct an equivalence class consisting of
OHM SIGN (U+2126), GREEK CAPITAL LETTER OMEGA (U+03A9), and
GREEK SMALL LETTER OMEGA (U+03C9).
This equivalence class can be derived only from C-type
mappings. Both OHM SIGN (U+2126), and GREEK CAPITAL LETTER
OMEGA (U+03A9) lowercase to GREEK LETTER OMEGA (U+03C9), while
that character only uppercases to GREEK CAPITAL LETTER OMEGA
(U+03A9).
EX3: To deal with the case of the ANGSTROM SIGN (U+212B) which is
essentially identical to LATIN CAPITAL LETTER A WITH RING ABOVE
(U+00C5), one can construct an equivalence class consisting of
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ANGSTROM SIGN (U+212B), LATIN CAPITAL LETTER A WITH RING ABOVE
(U+00C5), LATIN SMALL LETTER A WITH RING ABOVE (U+00E5),
together with the two-character sequences involving LATIN
CAPITAL LETTER A (U+0041) or LATIN SMALL LETTER A (U+0061)
followed by COMBINING RING ABOVE (U+030A).
This equivalence class can be derived from C-type mappings
together with the ability to map characters to canonically
equivalent strings. Both ANGSTROM SIGN (U+212B), and LATIN
CAPITAL LETTER A WITH RING ABOVE (U+00C5) lowercase to LATIN
SMALL LETTER A WITH RING ABOVE (U+00E5), while that character
only uppercases to CAPITAL LETTER A WITH RING ABOVE (U+00C5).
EX4: In some cases, case mapping of a single character will result
in a multi-character string. For example, the German character
LATIN SMALL LETTER SHARP S (U+00DF) would be uppercased to
"SS", i.e. two copies of LATIN CAPITAL LETTER S (U+0053). On
the other hand, in some situations, it would be uppercased to
the character LATIN CAPITAL LETTER SHARP S (U+1E9E), using an
S-type mapping, referred to as an instance of "Tailored
Casing". Unfortunately, in the context of a file system, there
is unlikely to be available information that provides guidance
about which of these case mappings should be chosen. However,
the use of case-insensitive mappings with larger equivalence
classes often provides handling that is acceptable to a wider
variety of users. In this case, German-speakers get the
mapping they expect while those unfamiliar with these
characters only see them when they access a file whose name
contains such characters.
It appears that if the construction of case-based equivalence
classes were generalized to include multi-character sequences,
then all of LATIN SMALL LETTER SHARP S (U+00DF), LATIN CAPITAL
LETTER SHARP S (U+1E9E), "ss", "sS", "Ss", and "SS" would
belong to the same equivalence class and could be handled by
the general algorithm described in Section 10.1, as well by
code specifically written to deal with this particular issue.
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EX5: Other ligatures, such as LATIN SMALL LIGATURE FFL (U+FB04),
could be handled similarly by this algorithm, if there were
felt to be a need to do so. However, because the decomposition
of this character into the string consisting of the three
letters LATIN SMALL LETTER F (U+0066), LATIN SMALL LETTER F
(U+0066), LATIN SMALL LETTER L (U+006C), is a compatibility
equivalence, and the F-type mapping of this ligature to the
three constituent characters is to be treated as optional,
implementations can choose either to treat this character as
having no uppercase equivalent or treat it as part of larger
equivalence class including "ffl", "ffL", "fFl", etc.).
EX6: The character COMBINING GREEK YPOGEGRAMMENI (U+0345), also
known as "iota-subscript" requires special handling when
uppercasing and lowercasing. While the description of the
appropriate handling for this character, in the case mapping
section, is focused on multi- character sequences representing
diphthongs, case-insensitive comparisons can be performed
without consideration of multi-character sequences. This can
be done by assigning COMBINING GREEK YPOGEGRAMMENI (U+0345),
GREEK SMALL LETTER IOTA (U+03B9), and GREEK CAPITAL LETTER IOTA
(U+0399) to the same equivalence class, even though the first
of these is a combining character and the others are not.
EX7: In some cases, context-dependent case mapping is required. For
example, GREEK CAPITAL LETTER SIGMA (U+03A3) lowercases to
GREEK SMALL LETTER SIGMA (U+03C3) if it is followed by another
letter and to GREEK SMALL LETTER FINAL SIGMA (U+03C2) if it is
not.
Despite this, case-insensitive comparisons can be implemented,
by considering all of these characters as part of the same
equivalence class, without any context-dependence, and this
equivalence class can be derived using only C-type mappings.
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EX8: In most languages written using Latin characters, the uppercase
and lowercase varieties of the letter "I" map to one nother.
In a number of Turkic languages, there are two distinct
characters derived from "I" which differ only with regard to
the presence or absence of a dot so that there are both capital
and small i's with each having dotted and dotless variants.
Within such languages, the dotted and dotless I's represent
different vowel sounds and are treated as separate characters
with respect to case mapping. The uppercase of LATIN SMALL
LETTER I (U+0069) is LATIN CAPITAL LETTER I WITH DOT ABOVE
(U+0130), rather than LATIN CAPITAL LETTER I (U+0049).
Similarly the lowercase of LATIN CAPITAL LETTER I (U+0049) is
LATIN SMALL LETTER DOTLESS I (U+0131) rather than LATIN SMALL
LETTER I (U+0069).
When doing case mapping, the server must choose to uppercase
LATIN SMALL LETTER I (U+0069) to either LATIN CAPITAL LETTER I
(U+0049), based on a C-type mapping to LATIN CAPITAL LETTER I
WITH DOT ABOVE (U+0130), based on a T-type mapping. The former
is acceptable to most people but confusing to speakers of the
Turkic languages in question since the case mapping changes the
character to represent a different vowel sound. On the other
hand, the latter mapping seemingly inexplicably results in a
character many users have never seen before. Normally such
choices are dealt with based on a locale but, in a file system
environment, no locale information is likely to be available.
In the context of case-insensitive string comparison, it is
possible to create a larger equivalence class, including all of
the letters LATIN SMALL LETTER I (U+0069), LATIN CAPITAL LETTER
I (U+0049), LATIN CAPITAL LETTER I WITH DOT ABOVE (U+0130),
LATIN SMALL LETTER DOTLESS I (U+0131) together with the two-
character string consisting of LATIN CAPITAL LETTER I (U+0049)
followed by COMBINING DOT ABOVE (U+0307).
11. Internationalization-related Processing of File Names by Clients
Given the way that internationalization is addressed within the NFSv4
protocols, clients and applications accessing NFS files can generally
remain unaware of the specific type of internationalization-related
processing implemented by the server. For example, although a server
MAY store all file names according to the rules appropriate to a
particular normalization form, it MUST NOT reject names solely
because they are not encoded using this normalization form, allowing
the clients and applications to avoid knowledge of normalization
choices.
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However, as has been pointed out in [I-D.williams-filesystem-18n],
there are situations in which clients implementing local
optimizations use the saved contents of directories fetched from the
server, making it necessary that the client's and the server's
handling of internationalization-related name mapping issues be in
concord. There are two basic ways this issue can be addressed:
* Where the protocol has not defined a means whereby the client can
obtain information about the details of internationalized name
handling implemented within the server, the client can avoid
conflict with the server by limiting its use of local
optimizations. While positive name caching can be used without
adverse effects, negative name caching has to be limited to avoid
situations in which a given name is not present but an equivalent
one may exist, as far as the server is concerned. This situation,
which applies to all current NFSv4 protocols is discussed in
Section 11.2.
* The client could be provided complete information about the
server's internationalization-related name handling (typically
implemented within the server-based file system. This sort of
information, which could be implemented in later NFSv4 minor
versions, or in an extension to an existing extensible minor
version, is discussed in Section 11.3.
* Note that when case-insensitive handling of file names is
implemented by a server-side file system, further complications
can arise. For the most part, these are addressed in Sections
11.2 and 11.3 by treating the particulars of case-handling as
another element of the name handling implemented by the server.
However, some of the specific complexities are addressed
separately in Section 10.
11.1. Server Restrictions to Deal with Lack of Client Knowledge
There are a number of restrictions, not previously specified in
[RFC7530], on server implementation of internationalized file name
handling. These restrictions apply to both case-sensitive and case-
insensitive file systems and are designed to limit the options that
servers have in choosing server-side internationalized file name
handling so as to enable the clients to either duplicate that
handling or to limit it so as to avoid relying on cases in which the
proper handling cannot be determined or duplicated by the client.
* The canonical equivalence relation implemented by the server, for
each internationalization-aware file system MUST match that
defined by some particular UNICODE version equal to or later than
version 4.0.
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* The case-equivalence relationship implemented by the server, for
each case-insensitive file system MUST include all C-type case
mappings included by the particular UNICODE version whose
canonical equivalence relation is implemented by the server, with
the possible exception of those conflicting with T-type case
mappings. by some particular Unicode version equal to or later
than version 4.0.
* In cases in which the server provides no way of determining the
details of the case-equivalence relationship implemented by the
server for a particular file system, that mapping MUST include all
C-type case mappings included by the particular UNICODE version
whose canonical equivalence relation is implemented by the server,
e.g. it MUST map between LATIN SMALL LETTER I (U+0069)and LATIN
CAPITAL LETTER I (U+0049).
11.2. Client Processing of File Names for Current NFSv4 Protocols
The existing minor versions, NFSv4.0 [RFC7530], NFSv4.1 [RFC5661],
and NFSv4.2 [RFC7862], have very limited facilities allowing a client
to get information about the server's internationalization-related
file name handling. Because these protocols were all defined when it
was assumed that the server's internationalized file name handling
could be specified in great detail, there no provision was made for
attributes defining the server's choices. As a result, the
information available to the client is quite limited:
* The client can determine that the server is not performing
internationalized file name processing. It can do this by looking
up a file name using a string which is not valid UTF-8, concluding
that if the LOOKUP is not rejected on that basis, then the file
system is not internationalization-aware, allowing the client to
ignore the potential difficulties which server-based
internationalized file name processing might give rise to.
* The client can use the optional per-fs attributes case_insensitive
and case_preserving to know how the server deals with character
case for a particular file system. When one of these attributes
is not supported by a particular file system, the client treats
the attribute as if it were false.
When a file system is internationalization-unaware, the client can
use both positive and negative name caching, without any issues
arising from the potential for conflict between distinct file names
that would be considered equivalent by the server. In other cases,
the handling is more restricted in the use of negative name caching.
The issue with regard to case-sensitive and case-insensitive file
systems are discussed separately below. In each case, the client has
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a range of choices trading off the possibility of forgone
optimization opportunities against the difficulty of implementation.
In doing so, it can avoid the negative consequences arising from the
fact that certain details of the server's name handling are not known
to it.
In the case of case-sensitive file systems, the uncertainty to be
dealt with concerns the version of Unicode implemented by the server,
given that different versions may have different canonical
equivalence relationships. However, whether the server implements a
particular normalization form or implements form-insensitive file
name matching has no effect on client behavior. In light of the
uncertainty created by the lack of knowledge of the precise Unicode
version used by the server to implement its canonical equivalence
relation, the following possibilities, arranged in order of
increasing value (and difficulty of implementation) need to be
considered by client implementers.
A1: The client can simply decline to implement optimizations based
on negative name caching on internationalization-aware file
systems.
While this might have a negative effect on performance, it might
be the best option for clients not heavily used to access
internationalization-aware file systems, or where, due to a lack
of directory delegation support, the client has no assurance
that it will be notified of the invalidation of a previous
assumption that a particular file does not exist.
A2: Relatively simple name filtering can exclude the names for which
negative name caching might cause difficulties. For example,
the client could scan file names for characters whose presence
might pose difficulties and allow negative name caching only for
strings known not to contain such characters. Because the
Unicode version used by the server file system is not known,
this treatment would be limited to strings only containing
characters defined in the earliest version of Unicode which
could be supported, that is, Unicode 4.0.
One simple way for a client to provide such filtering would be
to establish an upper limit (e.g. U+00FF) and disallow negative
name caching for strings containing characters above that value
or characters below that value that might cause there to be
canonically equivalent strings on the server. A simple mask
could be used to allow each character to be examined allowing
composed and combining characters to be identified together with
code points unassigned in Unicode 4.0.
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This approach would allow negative name caching to be disallowed
for strings containing those characters while allowing it for
other strings that do not. A larger limit (and a corresponding
mask) would make sense for clients used to access many file
names containing characters from non-Latin alphabets.
A3: A client might implement its own internationalized file name
handling paralleling that of the server. Because the Unicode
version used by the server file system is not known, strings for
which it is possible that the set of canonically equivalent
strings might be different depending on the version of Unicode
implemented by the server will have to be identified and
excluded from using negative name caching. This would require
that strings containing code points unassigned in Unicode
version 4.0, and those denoting combining characters that could
be parts of precomposed character added to later versions of
Unicode be excluded from negative name caching. The necessary
filtering could apply to all potential code points although
clients might choose to simplify implementation by excluding
strings containing code points beyond a certain point, e.g.
(U+0FFFF).
When a client implements internationalized name handling, it
needs to be able to detect when the apparent absence of a file
within a directory is contradicted by the occurrence of a file
with a distinct, but canonically equivalent, name. In order to
efficiently find such names, when they exist, a client typically
needs to implement a form of name hashing which always produces
the same result for two canonically equivalent names. This can
be done by making the contribution of any character to the name
hash, equal to the contribution of the corresponding canonical
decomposition string.
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In the case of case-insensitive file systems, the uncertainty to be
dealt with includes the version of Unicode implemented by the server
as well as the details of the possible case-handling implemented by
the server. In addition to the fact that different Unicode versions
may have different canonical equivalence relationships, the server
may implement different approaches to the handling of issues related
to the handling of dotted and dotless i, in Turkish and Azeri.
However, the question of whether the server's handling is case-
preserving has no effect on client behavior, as is the question of
whether the server implements a particular normalization form or
implements form-insensitive file name matching. In light of the
uncertainty created by the lack of knowledge of the details of the
case-related equivalence relation together with the precise Unicode
version used by the server to implement its canonical equivalence
relation, the following possibilities, arranged in order of
increasing value (and difficulty of implementation) should be
considered.
B1: The client can simply decline to implement optimizations based
on negative name caching on case-insensitive file systems.
While this might have a negative effect on performance where
significant benefits from negative name caching might be
expected, it might be the best option for clients not heavily
used to access case-insensitive file systems.
B2: Filtering similar to that discussed in item A2 could be
implemented, although a higher limit is likely to be chosen
(e.g. U+07FF) if significant use of non-Latin scripts is
expected. Because of the uncertainty regarding the handling of
case relationship among characters used for the variants of "I"
used by Turkic languages, this filtering would have to exclude
names containing LATIN CAPITAL LETTER I WITH DOT ABOVE and LATIN
SMALL LETTER DOTLESS I together with precomposed characters
derived from them.
In cases in which such filtering did not exclude the item from
consideration, it would need to search for files with possibly
equivalent names, including those equivalent by canonical
equivalence, case-insensitive equivalence, or a combination of
the two. This will typically require a form of name hashing
which always produces the same hash for equivalent names,
similar to that discussed in item A3 but including case-
insensitive equivalence as well.
B3: A client might implement its own internationalized, case-
insensitive file name handling paralleling that of the server.
Because the case mappings are uncertain and the Unicode version
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used by the server file system is unknown, strings for which it
is possible that the equivalent string might be different
depending on the version of Unicode implemented by the server or
the choice of case mappings would have to be identified and
excluded from using negative name caching. This would require
that strings containing code points unassigned in Unicode
version 4.0, and those denoting combining characters that could
be parts of precomposed characters added to later versions of
Unicode be excluded from negative name caching. The necessary
filtering could apply to all potential code points although
clients might choose to simplify implementation by excluding
strings containing code points beyond a certain point (e.g.
U+00FFFF).
When a client implements internationalized name handling, it
needs to be able to detect when the apparent absence of a file
within a directory is contradicted by the occurrence of a file
with a distinct, but canonically equivalent name. In order to
efficiently find such names, when they exist, a client typically
needs to implements a form of name hashing which always produces
the same result for two canonically equivalent names. This can
be done by making the contribution of any character to the name
hash, equal to contribution of the correspond canonical
decomposition string.
11.3. Client Processing of File Names for Future NFSv4 Protocols
Because NFSv4 has an extension framework allowing the addition of new
attributes in later minor versions or in extensions to extensible
minor versions. Such new attributes are likely to be OPTIONAL. They
could include a number of useful per-fs attributes to deal with the
information gaps discussed in Section 11.2:
* The Unicode version used to define the canonical equivalence
relation implemented by the server could be provided as an fs-
scope attribute.
* For case-insensitive file systems, details regarding the actual
case mapping used could be provided as an fs-scope attribute.
These details would include the case mapping associated with LATIN
LETTER I (i.e. whether the C-type or T-type case mappings or both
are to be used). Similarly for characters having F-type case
mappings, information needs to be provided about whether the
F-type, mapping, the S-type mapping, or both, are to be used.
There is little prospect of such additional attributes being
REQUIRED. Although the term "RECOMMENDED" has been used to describe
NFSv4 attributes that are not REQUIRED, any such attributes are best
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considered OPTIONAL for the server to support with the client
required to deal with the case in which the attribute is not
supported.
When such attributes are defined and implemented, it would be
possible for the client and server to implement compatible
internationalization-related file name handling. However, as a
practical matter, providing such compatibility would be considerably
eased if there existed unencumbered open-source implementations of
the algorithm and tables described in Appendix B. This would allow
clients, servers, and server-based file systems, to easily adopt
compatible approaches to these issues, each calling a common set of
primitives, even though each might have a different execution
environment and might be processing file names for different
purposes.
In the case of a case-sensitive file system, the case-mapping
attribute is not relevant. In dealing with the non-support of the
Unicode version attribute, the client is in the same position as that
of clients described in Section 11.2. In the case in which the
Unicode version is supported, the client would be able to implement
the same version of the canonical equivalence relation implemented by
the server, thus avoiding the need for the sort of overbroad
filtering mentioned in items A2 and A3 within Section 11.2
The case of case-insensitive file systems is more complicated, since
there are two OPTIONAL attributes to deal with:
C1: When neither of these OPTIONAL attributes is supported, the
client is in the same position as that of clients described in
Section 11.2 in dealing with a case-insensitive file system.
C2: When the Unicode version is available but the details of case
mapping are not, the client handling will be similar to that
specified the options B1 through B3 defined in Section 11.2.
However, in cases B2 and B3, it will be possible to reduce the
scope of the character filtering applied, by enabling names
containing characters defined after Unicode version 4.0 to be
processed, as long as none of the case mapping options for those
characters is at all problematic.
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C3: When the details of case mapping are available but Unicode
version is not, the client handling will be similar to that
specified the options B1 through B3 defined in Section 11.2.
However, in cases B2 and B3 However, in cases B2 and B3, it will
be possible to reduce the scope of the character filtering by
enabling names containing characters of uncertain case mapping
to be processed as long as those character were defined in
Unicode version 4.0.
C4: When both of these OPTIONAL attributes are supported, the client
has the ability, at least theoretically, to reproduce the
internationalization-related file name handling implemented by a
server for a case-insensitive file system. However, when the
client is unable to provide such an implementation, it is free
to ignore the attribute and implement one of the options B1
through B3 defined in Section 11.2.
12. String Types with Processing Defined by Other Internet Areas
There are two types of strings that NFSv4 deals with that are based
on domain names. Processing of such strings is defined by other
standards-track documents, and hence the processing behavior for such
strings should be consistent across all server and client operating
systems and server file systems.
This section differs from other sections of this document in two
respects:
* Although the normative statements within this section are derived
from the behavior of existing NFSv4 implementations, they need to
be consistent with existing RFCs regarding domain handling.
* Because of the switch from IDNA2003 [RFC3490] [RFC3491] to
IDNA2008 [RFC5890], this section is necessarily different from the
corresponding section (i.e. Section 12.6) of [RFC7530]. The
differences are discussed in Section 12.1.
Because of this shift, there could be compatibility issues to be
expected between implementations obeying Section 12.6 of [RFC7530],
if any such implementations exist, and those following this document.
Whether such compatibility issues actually exist depends on the
behavior of NFSv4 implementations and how domain names are actually
used in existing implementations. These matters will be discussed in
Section 12.2.
The types of strings referred to above are as follows:
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* Server names as they appear in the fs_locations and
fs_locations_info attribute. Note that for most purposes, such
server names will only be sent by the server to the client. The
exception is the use of these attributes in a VERIFY or NVERIFY
operation.
* Principal suffixes that are used to denote sets of users and
groups, and are in the form of domain names. These may apprear in
the owner and group attributes and as ace_who values within ACL
attributes. Such values are sent by the client to the server in
performing SETATTR, VERIFY, and NVERIFY operations and returned to
the client in performing GETATTR opererations.
There is likely to be few or no implementations conformng to
Section 12.6) of [RFC7530] as a result of how internationalization
was supported previously.
* When [RFC3530] was published, its discussion of
internationalization was ignored as unimplementable and
inappropriate. This included the handling of domain names,
although the reasons for ignoring the specifcation might have been
different.
* When [RFC7530] was published, implementors saw no reason to modify
the existing domain-handling code which worked adequately for
valid domain names.
These strings can be expressed in two ways:
* As the UTF-8 representation of the string represented. This
includes cases in which all of the characters are within the Ascii
range. We refer to such representations as the U-label form.
* As the string "xn--" followed by the text of the string
transformed using the Punycode encoding described in [RFC3492].
We refer to such representations as the xn-label form.
In cases in which such strings are sent by the client to the server:
* The server MUST accept such strings in xn-label form.
When it does so, MAY reject, using the error NFS4ERR_INVAL, any of
the following:
- a string for which the characters after "xn--" are not valid
output of the Punycode algorithm [RFC3492].
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- a string that contains a reserved LDH label which is not an
XN-label.
* The server MAY accept such strings in U-label form and is REQUIRED
to do so only in the case in which the string conists only of
ascii characters.
The server MAY reject, using the error NFS4ERR_INVAL, strings
which are not valid UTF-8 or do not form a valid U-label for other
reasons.
When the server does not make the validity checks mentioned above,
the result will be use of an invalid domain name. Since such domains
do not exist, clients are unlikely to use them and servers will be
unable to access such domains.
Servers MUST NOT modify the string to a canonically equivalent one
(e.g. as part of normalization-related processing). Further, changes
of case SHOULD NOT be done and MUST NOT be done for strings that
contain multi-byte Unicode characters.
In cases in which such strings are sent by the server to the client,
they MAY be presented in either form. In view of this, clients that
anticipate receiving internationalized domain names will find it
advisable to convert such strings to a common form, preferred by the
client's users.
A domain name returned by GETATTR will generally be exactly the same
as that presented by SETATTR. The following exceptions are possible:
* There is a change of case when the domain string does not contain
any multi-byte Unicode characters.
* The server converts an xn-label string to the corresponding
U-label string or vice versa..
For VERIFY and NVERIFY, additional string processing requirements
apply to verification of the owner and owner_group attributes; see
the section entitled "Interpreting owner and owner_group" for the
document specifying the minor version in question (RFC7530 [RFC7530],
RFC5661 [RFC5661])
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12.1. Effect of IDNA Changes
Overall, the effect of the shift to IDNA2008 is to limit the degree
of understanding of the IDNA-based restrictions on domain names that
were expected of NFSv4 in RFC7530 [RFC7530]. Despite this
specification, the degree to which implementations actually
implemented such restrictions is open to question. The consequences
of this uncertainty will be discussed in detail in Section 12.2.
In analyzing how various cases are to be dealt with according to
RFC7530, there a number of troubling uncertainties that arise in
trying to interpret the existing specification:
* There are a number of cases in which "SHOULD" is used that are
confusing. According to RFC2119 [RFC2119], "SHOULD" means that
"there may exist valid reasons in particular circumstances to
ignore a particular item, but the full implications must be
understood and carefully weighed before choosing a different
course". To fully understand a particular "SHOULD", there needs
to be enough context to determine whether particular reasons for
ignoring the item are in fact valid, and sufficient guidance to
understand the implication of ignoring the item. In the absence
of such information, the relevant fact is that the peer needs to
deal with the item being ignored, making the implications of a
"SHOULD" hard to distinguish from those of "MAY".
* While the document states, "the general rules for handling all of
these domain-related strings are similar and independent of the
role of the sender or receiver as client or server", all of the
following text is explicitly about the server's options, choices
and responsibilities, leaving the client case unclear.
* In a number of places within the paragraph describing server
approach #1, the word "can" is used as in the text "the server can
use the ToUnicode function", leaving it unclear whether the server
can choose to do anything else and if so what.
The following cases are those where RFC7530 requires use of IDNA
handling and this requirement could, if implementations follow them,
create potential compatibility issues, which need to be understood.
* The degree to which RFC3490 [RFC3490] requires that characters
other than U+002E (full stop) be treated as label separators,
including U+3002 (ideographic full stop), U+FF0E (fullwidth full
stop), U+FF61 (halfwidth ideographic full stop).
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* The degree to which RFC3490 [RFC3490] might require that server or
client needs to validate a putative A-label or U-label or to
rectify it if it is not valid.
12.2. Potential Compatibility Issues Related to IDNA Changes
There are a number of factors relating to the handling of domain
names within NFSv4 implementations that are important in
understanding why any compatibility issues might be less troubling
than a comparison of the two IDNA approaches might suggest:
* Much of the potentially conflicting IDNA-related behavior required
or recommended for the server by RFC7530 [RFC7530] appears to not
be actually implemented, limiting the potential harmful effects of
ceasing to mandate it.
* Even if such behavior were implemented by servers, no
compatibility issue would arise unless clients actually relied on
the server to implement it. Given that none of this behavior is
made required, the chances of that occurring is quite small.
* The range of potential values for user and group attributes sent
by clients are often quite small with implementations commonly
restricting all such values to a single domain string. This is
even though RFCs 7530 [RFC7530] and 5661 [RFC5661] are written
without mention of such restrictions.
Specification of users and groups in the "id@domain" format within
NFSv4 was adopted to enable expansion of the spaces of users and
groups beyond the 32-bit id spaces mandated in NFSv3 [RFC1813] and
NFsv2 [RFC1094]. While one obstacle to expansion was eliminated,
most implementations were unable to actually effect that
expansion, principally because the physical file systems used
assume that user and group identifiers fit in 32 bits each and the
vnode interfaces used by server implementations make similar
assumptions.
Given these restrictions, the typical implementation pattern is
for servers to accept only a single domain, specified as part of
the server configuration, together with information necessary to
effect the appropriate name-to-id mappings.
* For the other uses of domain names in NFSv4, to represent host
names in location attributes, the values are generated by the
server and will normally only include host names within DNS-
registered domains.
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Keeping the above in mind, we can see that interoperability issues,
while they might exist, are unlikely to raise major challenges as
looking to the following specific cases shows.
* When an internationalized domain name is used as part of a user or
group, it would need to be configured as such, with the domain
string known to both client and server.
While it is theoretically possible that a client might work with
an invalid domain string and rely on the server to correct it to
an IDNA-acceptable one, such a scenario has to be considered
extremely unlikely, since it would depend on multiple servers
implementing the same correction, especially since there is no
evidence of such corrections ever having been implemented by NFSv4
servers.
* When an internationalized domain in a location string is meant to
specify a registered domain, similar considerations apply.
While it is theoretically possible that a client might work with
an invalid domain string and rely on the server to correct it to
an appropriate registered one, such a scenario has to be
considered extremely unlikely, since it would depend on multiple
servers implementing the same correction, especially since there
is no evidence of such corrections ever having been implemented by
NFSv4 servers.
* When an internationalized domain in a location string is meant to
specify a non-registered domain, any such server-applied
corrections would be useless.
In this situation, any potential interoperability issue would
arise from rejecting the name, which has to be considered as what
should have been done in the first place.
13. Errors Related to UTF-8
Where the client sends an invalid UTF-8 string, the server MAY return
an NFS4ERR_INVAL error. This includes cases in which inappropriate
prefixes are detected and where the count includes trailing bytes
that do not constitute a full Multiple-Octet Coded Universal
Character Set (UCS) character.
Requirements for server handling of component names that are not
valid UTF-8, when a server does not return NFS4ERR_INVAL in response
to receiving them, are described in Section 14.
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Where the string supplied by the client is not rejected with
NFS4ERR_INVAL but contains characters that are not supported by the
server as a value for that string (e.g., names containing slashes, or
characters that do not fit into 16 bits when converted from UTF-8 to
a Unicode codepoint), the server MUST return an NFS4ERR_BADCHAR
error.
Where a UTF-8 string is used as a file name, and the file system,
while supporting all of the characters within the name, does not
allow that particular name to be used, the server will return the
error NFS4ERR_BADNAME. This includes such situations as file system
prohibitions of "." and ".." as file names for certain operations,
and similar constraints.
14. Servers That Accept File Component Names That Are Not Valid UTF-8
Strings
As stated previously, servers MAY accept, on all or on some subset of
the physical file systems exported, component names that are not
valid UTF-8 strings. A typical pattern is for a server to use
UTF-8-unaware physical file systems that treat component names as
uninterpreted strings of bytes, rather than having any awareness of
the character set being used.
Such servers SHOULD NOT change the stored representation of component
names from those received on the wire and SHOULD use an octet-by-
octet comparison of component name strings to determine equivalence
(as opposed to any broader notion of string comparison). This is
because the server has no knowledge of the specific character
encoding being used.
Nonetheless, when such a server uses a broader notion of string
equivalence than what is recommended in the preceding paragraph, the
following considerations apply:
* Outside of 7-bit ASCII, string processing that changes string
contents is usually specific to a character set and hence is
generally unsafe when the character set is unknown. This
processing could change the file name in an unexpected fashion,
rendering the file inaccessible to the application or client that
created or renamed the file and to others expecting the original
file name. Hence, such processing is best not performed, because
doing so is likely to result in incorrect string modification or
aliasing.
* Unicode normalization is particularly dangerous, as such
processing assumes that the string is UTF-8. When that assumption
is false because a different character set was used to create the
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file name, normalization may corrupt the file name with respect to
that character set, rendering the file inaccessible to the
application that created it and others expecting the original file
name. Hence, Unicode normalization SHOULD NOT be performed,
because it might cause incorrect string modification or aliasing.
When the above recommendations are not followed, the resulting string
modification and aliasing can lead to both false negatives and false
positives, depending on the strings in question, which can result in
security issues such as elevation of privilege and denial of service
(see [RFC6943] for further discussion).
15. Future Minor Versions and Extensions
As stated above, all current NFSv4 minor versions allow use of non-
UTF-8 encodings, allow servers a choice of whether to be aware of
normalization issues or not, and allow servers a number of choices
about how to address normalization issues. This range of choices
reflects the need to accommodate existing file systems and user
expectations about character handling which in turn reflect the
assumptions of the POSIX model for the handling file names.
While it is theoretically possible for a subsequent minor version to
change these aspects of the protocol (see [RFC8178]), this section
will explain why any such change is highly unlikely, making it
expected that these aspects of NFSv4 internationalization handling
will be retained indefinitely. As a result, any new minor version
specification document that made such a change would have to be
marked as updating or obsoleting this document
No such change could be done as an extension to an existing minor
version or in a new minor version consisting only of OPTIONAL
features. Such a change could only be done in a new minor version,
which, like minor version one, was prepared to be incompatible to
some degree with the previous minor versions. While it appears
unlikely that such minor versions will be adopted, the possibility
cannot be excluded, so we need to explore the difficulties of
changing the aspects of internationalization handling mentioned
above.
* Establishing UTF-8 as the sole means of encoding for
internationalized characters, would make inaccessible existing
files stored with other encodings. Further, unless there were a
corresponding change in the UNIX file interface model, it would
cause the set of valid names for local and remote files to
diverge.
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* Imposing a particular normalization form, in the sense of refusing
to create to allow access to files whose UTF-8-encoded names are
not of the selected normalization form would give rise to similar
difficulties.
* Defining a preferred normalization form to be returned as the
names of all internationalized files, would result in applications
having to deal with sudden unexplained changes of file names for
existing files.
None of the above appears likely since there does not seem to be any
corresponding benefits to justify the difficulties that adopting them
would create.
There would also be difficulties in otherwise reducing the set of
three acceptable normalization handling options, without reducing it
to a single option by imposing a specific normalization form.
* Eliminating the possibility of a single possible normalization
form, would pose similar difficulties to imposing the other one,
even if representation-independent comparisons were also allowed.
In either case, a specific normalization form would be disfavored,
with no corresponding benefit.
* Allowing only representation-independent lookups would not impose
difficulties for clients, but there are reasons to doubt it could
be universally implemented, since such name comparisons would have
to be done within the file system itself.
Such a change could only be made once file system support for
representation-independent file lookups would become commonly
available. As long as the POSIX file naming model continues its
sway, that would be unlikely to happen.
One possible internationalization-related extension that the working
could adopt would be definition of OPTIONAL per-fs attributes
defining the internationalization-related handling for that file
system. That would allow clients to be aware of server choices in
this area and could be adopted without disrupting existing clients
and servers.
16. IANA Considerations
The current document does not require any actions by IANA.
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17. Security Considerations
Unicode in the form of UTF-8 is generally used for file component
names (i.e., both directory and file components). However, other
character sets may also be allowed for these names. For the owner
and owner_group attributes and other sorts strings whose form is
affected by standards outside NFSv4 (see Section 12.) are always
encoded as UTF-8. String processing (e.g., Unicode normalization)
raises security concerns for string comparison. See Sections 12 and
9 as well as the respective Sections 5.9 of RFC7530 [RFC7530] and
RFC5661 [RFC5661] for further discussion. See [RFC6943] for related
identifier comparison security considerations. File component names
are identifiers with respect to the identifier comparison discussion
in [RFC6943] because they are used to identify the objects to which
ACLs are applied (See the respective Sections 6 of RFC7530 [RFC7530]
and RFC5661 [RFC5661]).
18. References
18.1. Normative References
[RFC20] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, October 1969,
<http://www.rfc-editor.org/info/rfc20>.
[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>.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<https://www.rfc-editor.org/info/rfc3492>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.
[RFC7530] Haynes, T., Ed. and D. Noveck, Ed., "Network File System
(NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
March 2015, <https://www.rfc-editor.org/info/rfc7530>.
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[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>.
[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>.
[UNICODE] The Unicode Consortium, "The Unicode Standard, Version
7.0.0", (Mountain View, CA: The Unicode Consortium,
2014 ISBN 978-1-936213-09-2), June 2014,
<http://www.unicode.org/versions/Unicode7.0.0/>.
[UNICODE-CASEF]
The Unicode Consortium, "CaseFolding-13.0.0.txt",
(Mountain View, CA: The Unicode Consortium, 2014 ISBN
978-1-936213-26-9), March 2020,
<https://www.unicode.org/Public/13.0.0/ucd/
CaseFolding.txt>.
[UNICODE-CASEM]
The Unicode Consortium, "The Unicode Standard, Version
13.0.0, Section 5.18 Case Mappings", (Mountain View, CA:
The Unicode Consortium, 2014 ISBN 978-1-936213-26-9),
March 2020,
<http://www.unicode.org/versions/Unicode13.0.0/
ch05.pdf#G21180>.
18.2. Informative References
[I-D.ietf-nfsv4-rfc3010bis]
Beame, C., Thurlow, R., Callaghan, B., Robinson, D.,
Noveck, D., Eisler, M., and S. Shepler, "Network File
System (NFS) version 4 Protocol", Work in Progress,
Internet-Draft, draft-ietf-nfsv4-rfc3010bis-05, 7 November
2002, <https://datatracker.ietf.org/doc/html/draft-ietf-
nfsv4-rfc3010bis-05>.
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[I-D.williams-filesystem-18n]
Williams, N., "Internationalization Considerations for
Filesystems and Filesystem Protocols", Work in Progress,
Internet-Draft, draft-williams-filesystem-18n-00, 6 July
2020, <https://datatracker.ietf.org/doc/html/draft-
williams-filesystem-18n-00>.
[RFC1094] Nowicki, B., "NFS: Network File System Protocol
specification", RFC 1094, DOI 10.17487/RFC1094, March
1989, <https://www.rfc-editor.org/info/rfc1094>.
[RFC1813] Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
Version 3 Protocol Specification", RFC 1813,
DOI 10.17487/RFC1813, June 1995,
<https://www.rfc-editor.org/info/rfc1813>.
[RFC3010] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "NFS version 4
Protocol", RFC 3010, DOI 10.17487/RFC3010, December 2000,
<https://www.rfc-editor.org/info/rfc3010>.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
DOI 10.17487/RFC3454, December 2002,
<https://www.rfc-editor.org/info/rfc3454>.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, DOI 10.17487/RFC3490, March 2003,
<https://www.rfc-editor.org/info/rfc3490>.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, DOI 10.17487/RFC3491, March 2003,
<https://www.rfc-editor.org/info/rfc3491>.
[RFC3530] Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File System
(NFS) version 4 Protocol", RFC 3530, DOI 10.17487/RFC3530,
April 2003, <https://www.rfc-editor.org/info/rfc3530>.
[RFC5661] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"Network File System (NFS) Version 4 Minor Version 1
Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
<https://www.rfc-editor.org/info/rfc5661>.
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[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
DOI 10.17487/RFC6365, September 2011,
<https://www.rfc-editor.org/info/rfc6365>.
[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, <https://www.rfc-editor.org/info/rfc6943>.
Appendix A. History
This section describes the history of internationalization within
NFSv4. Despite the fact that NFSv4.0 and subsequent minor versions
have differed in many ways, the actual implementations of
internationalization have remained the same and internationalized
names have been handled without regard to the minor version being
used. This is the reason the document is able to treat
internationalization for all NFSv4 minor versions together.
During the period from the publication of RFC3010 [RFC3010] until
now, two different perspectives with regard to internationalization
have been held and represented, to varying degrees, in specifications
for NFSv4 minor versions.
* The perspective held by NFSv4 implementers treated most aspects of
internationalization as basically outside the scope of what NFSv4
client and server implementers could deal with. This was because
the POSIX interface treated file names as uninterpreted strings of
bytes, because the file systems used by NFSv4 servers treated file
names similarly, and because those file systems contained files
with internationalized names using a number of different encoding
methods, chosen by the users of the POSIX interface. From this
perspective, wider support for internationalized names and general
use of universal encodings was a matter for users and applications
and not for protocol implementers or designers.
* Within the IETF in general and in the IESG, there was a feeling
that new protocols, such as NFSv4, could not avoid dealing with
internationalization issues, making it difficult to treat these
matters, as the implementers' perspective would have it, as
essentially out of scope.
As specifications were developed, approved, and at times rewritten,
this fundamental difference of approach was never fully resolved,
although, with the publication of RFC7530 [RFC7530], a satisfactory
modus vivendi may have been arrived at.
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Although many specifications were published dealing with NFSv4
internationalization, all minor versions used the same implementation
approach, even when the current specification for that minor version
specified an entirely different approach. As a result, we need to
treat the history of NFSv4 internationalization below as an
integrated whole, rather than treating individual minor versions
separately.
* The approach to internationalization specified in RFC3010
[RFC3010] sidestepped the conflict of approaches cited above by
discussing the reasons that UTF-8 encoding was desirable while
leaving file names as uninterpreted strings of bytes. The issue
of string normalization was avoided by saying "The NFS version 4
protocol does not mandate the use of a particular normalization
form at this time."
Despite this approach's inconsistency with general IETF
expectations regarding internationalization, RFC3010 was published
as a Proposed Standard. NFSv4.0 implementation related to
internationalization of file names followed the same paradigm used
by NFSv3, assuring interoperability with files created using that
protocol, as well as with those created using local means of file
creation.
* When it became necessary, because of issues with byte-range
locking, to create an rfc3010bis, no change to the previously
approved approach seemed indicated and the drafts submitted up
until [I-D.ietf-nfsv4-rfc3010bis] closely followed RFC3010 as
regards internationalization. The IESG then decided that a
different approach to internationalization was required, to be
based on stringprep [RFC3454] and rfc3010bis was accordingly
revised, replacing all of the Internationalization section, before
being published as RFC3530 [RFC3530].
These changes required the rejection of file names that were not
valid UTF-8, file names that included code points not, at the time
of publication, assigned a Unicode character (e.g. capital eszett)
or that were not allowed by stringprep (e.g. Zero-width joiner
and non-joiner characters). Because these restrictions would have
caused the set of valid file names to be different on NFS-mounted
and local file systems there was no chance of them ever being
implemented.
Because these specification changes were made without working
group involvement, most implementers were unaware of them while
those who were aware of the changes ignored them and continued to
develop implementations based on the internationalization approach
specified in RFC3010.
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* When NFsv4.1 was being developed, it seemed that no changes in
internationalization would be needed. Many working group
partiipants were unaware of the stringprep-based requirements
which made the NFSv4.0 internationalization specified in RFC3530
unimplementable. As a result, the internationalization specified
in RFC5661 [RFC5661] was based on that in RFC3530 [RFC3530],
although the addition of the attribute fs_charset_cap, discussed
below, provided additional flexibility.
The attribute fs_charset_cap, discussed below in Section 7
provides flags allowing the server to indicate that it accepts and
processes non-UTF-8 file names. Rejecting them was a "MUST" in
RFC3530 and became a "SHOULD" in RFC5661, although there is no
evidence that any of these designations ever affected server
behavior.
Even though NFSv4.1 was a separate protocol and could have had a
different approach to internationalization, for a considerable
time, the internationalization specification for both protocols
was based on stringprep (in RFC3530 and RFC5661) while the actual
implementations of the two minor versions both followed the
approach specified in RFC3010, despite its obsoleted status. This
happened since most working group members were aware of the
treatment internationalization by the various minor version RFCs.
* When work started on rfc3530bis it was clear that issues related
to internationalization had to be addressed. When the
implications of the stringprep references in RFC3530 were
discussed with implementers it became clear that mandating that
NFSv4.0 file names conform to stringprep was not appropriate.
While some working group members articulated the view that,
because of the need to maintain compatibility with the POSIX
interface and existing file systems, internationalization for
NFSv4 could not be successfully addressed by the IETF, the
rfc3530bis draft submitted to the IESG did not explicitly embrace
the implementers' perspective as set forth above.
The draft submitted to the IESG and RFC7530 [RFC7530] as published
provided an explanation (see Section 5) as to why restrictions on
character encodings were not viable. It allowed non-UTF-8
encodings to be used for internationalized file names while
defining UTF-8 as the preferred encoding and allowing servers to
reject non-UTF-8 string as invalid. Other stringprep-based string
restrictions were eliminated. With regard to normalization, it
continued to defer the matter, leaving open the possibility that
one might be chosen later.
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This approach is compatible, in implementation terms, with that
specified in the obsolete document RFC3010 [RFC3010], allowing it
to be used compatibly with existing implementations for all
existing minor versions. This is despite the fact that RFC5661
[RFC5661] specifies an entirely different approach.
As a result of discussions leading up to the publishing of
RFC7530, it was discovered that some local file systems used with
NFSv4 were configured to be both normalization-aware and
normalization-preserving, mapping all canonically equivalent file
names to the same file while preserving the form actually used to
create the file, of whatever form, normalized or not. This
behavior, which is legal according to RFC3010, which says little
about name mapping is probably illegal according to stringprep.
Nevertheless, it was expressly pointed out in RFC7530 as a valid
choice to deal with normalization issues, since it allows
normalization-aware processing without the difficulties that arise
in imposing a particular normalization form, as described in
Section 9.
In its discussion of internationalized domain names, RFC7530
[RFC7530] adopted an approach compatible with IDNA2003, rather
than attempting to derive the specification from the behavior of
existing implementations.
* When IDNA2003 was replaced by IDNA2008, the internationalization
specified by [RFC7530] was not changed. Also, it appears unlikely
that implementations were changed to reflect that shift.
* NFSv4.2 made no changes to internationalization. As a result,
RFC7862 [RFC7862] which made no mention of internationalization,
implicitly aligned internationalization in NFSv4.2 with that in
NFSv4.1, as specified by RFC5661 [RFC5661].
As a result of this implicit alignment, there is no need for this
document to specifically address NFSv4.2 or be marked as updating
RFC7862. It is sufficient that it updates RFC5661, which
specifies the internationalization for NFSv4.1, inherited by
NFSv4.2.
* Later, as work on the predecessors of this document was underway,
[I-D.williams-filesystem-18n] was submitted, making it necessary
that some gaps the discussion of internationalization in [RFC7530]
be filled in. These gaps primarily concerned the need for NFSv4
clients to match the handling of the corresponding server when
using cached file name data locally, or to avoid making invalid
assumptions about that handling, when information on the details
of such handling was not available.
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The above history, can, for the purposes of the rest of this document
be summarized in the following statements:
* The actual treatment of internationalization within NFSv4 has not
been affected by the particular minor version used, despite the
fact that the specifications for the minor versions have often
differed in their treatment of internationalization.
* With regard to file names, most implementations have followed the
internationalization approach specified in RFC3010, which is
compatible with the treatment in RFC7530.
* With regard to internationalized domain names, RFC7530 [RFC7530]
specified an approach compatible with IDNA at the time of
publication. However, no detailed analysis was done to determine
whether NFSv4 implementations actually followed that approach and
it appears that many implementations used approaches that were
much simpler.
* Because [RFC7530] did not specifically address the special issues
that clients would face, relying on the assumption that each file
is accessible only by its name. As this assumption is no longer
true when internationalized name handling is in effect, the
appropriate handling is discusssed below. Section 11.2 explains
the options for handling in the case in which the client has very
limited information about the details about the server's
internationalization-related handling of file names while
Section 11.3 discusses how a client might use more complete
information provided by new attributes.
In order to deal with all NFSv4 minor versions, this document follows
the internationalization approach defined in RFC7530, with some
changes discussed in Section 4 and applies that approach to all NFSv4
minor versions.
Appendix B. Form-insensitive String Comparisons
This section deals with two varieties of form-insensitive string
comparison:
* Providing a comparison function which is form-insensitive only.
For any string, whether normalized or not, this function will
determine it to be equivalent to all canonically equivalent
strings, including but not limited, to the normalized forms NFC
and NFD
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* Providing a comparison function which is both form-insensitive and
case-insensitive. This function will determine strings that only
differ in case to be equal but will also be form-insensitive, as
described above.
The non-normative guidance provided in this Appendix is intended to
be helpful in dealing with two distinct implementation areas:
* Implementation of server-side file systems intended to be accessed
using NFSv4 protocols. While it is often the case that such file
systems are developed by separate organizations from those
concerned with NFSv4 server development, the internationalization-
related requirements specified in this document must be adhered to
for successful inter-operation, making this implementation
guidance apropos despite any potential organizational barriers.
* Implementation of NFSv4 clients that need to provide matching
internationalization-related handling for reason discussed in
Section 11.
There are three basic reasons that two strings being compared might
be canonically equivalent even though not identical. For each such
reason, the implementation will be similar in the cases in which
form-insensitive comparison (only) is being done and in which the
comparison is both case-insensitive and form- insensitive.
* Two strings may differ only because each has a different one of
two code points that are essentially the same. Three code points
assigned to represent units, are essentially equivalent to the
character denoting those units. For example, the OHM SIGN
(U+2126) is essentially identical to the GREEK CAPITAL LETTER
OMEGA (U+03A9) as MICRO SIGN (U+00B5) is to GREEK SMALL LETTER MU
(U+03BC) and ANGSTROM SIGN (U+212B) is to LATIN CAPITAL LETTER A
WITH RING ABOVE (U+00C5).
As discussed in items EX2 and EX3 in Section 10.2, it is possible
to adjust for this situation using tables designed to resolve
case-insensitive equivalence, essentially treating the unit
symbols as an additional case variant, essentially ignoring the
fact that the graphic representation is the same. As a result,
those doing string comparisons that are both form-insensitive and
case-insensitive do not need to address this issue as part of
form-insensitivity, since it would be dealt with by existing case-
insensitive comparison logic.
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Where there is no case-insensitive comparison logic, this function
needs to be performed using similar tables whose primary function
is to provide the decomposition of precomposed characters, as
described in Appendix B.2.
* Two strings may differ in that one has the decomposed form
consisting of a base character and an associated combining
character while the other has a precomposed character equivalent.
Although, as discussed in items EX3 in Section 10.2, it is
possible to use tables designed to resolve case-insensitive
equivalence by providing as possible case-insensitively equivalent
string, multi-character string providing the decomposition of
precomposed characters, special logic to do so is only necessary
when the decomposition is not a canonical one, i.e. it is a
compatibility equivalence.
In general, the table used to do comparisons, whether case-
sensitive or not, need to provide information about the canonical
decomposition of precomposed characters. See Appendix B.2 for
details.
* Two strings may differ in that the strings consist of combining
characters that have the same effect differ as to the order in
which the characters appear.
There is no way this function could be performed within code
primarily devoted to case-insensitive equivalence. However, this
function could be added to implementations, providing both sorts
of equivalence once it is determined that the base characters are
case-equivalent while there is a difference of combining
characters in to be resolved. (See Appendix B.5 for a discussion
of how sets of combining characters can be compared).
B.1. Name Hashes
We discussed in Section 10.1 the construction of a case-insensitive
file name hash. While such a hash could also be form-insensitive if
the hash contribution of every pre-composed character matched the
combined contribution of the characters that it decomposes into.
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However, there is no obvious way that sort of hash could respect the
canonical equivalence of multiple combining characters modifying the
same base character, when those combining characters appear in
different orders. Addressing that issue would require a
significantly different sort of hash, in which combining characters
are treated differently from others, so that the re-ordering of a
string of combining characters applying to the same base character
will not affect the hash.
In the hash discussed in Section 10.1, there is no guarantee that the
hash for multiple combining characters presented in different orders
will be the same. This is because typically such hashes implement
some transformation on the existing hash, together with adding the
new character to the hash being accumulated. Such methods of hash
construction will arrive at different values if the ordering of
combining characters changes.
In order to create a hash with the necessary characteristics, one can
construct a separate sub-hash for composite character, consisting of
one non-combining character (may be pre-composed) together with the
set (possibly null) of combining characters immediately following it.
Each such composed character, whether precomposed or not, will have
its own sub-hash, which will be the same regardless of the order of
the combining characters.
If the hash is to include case-insensitivity, special handling is
needed to deal with issues arising from the handling of COMBINING
GREEK YPOGEGRAMMENI (U+0345). That combining character, as discussed
in item EX6 of Section 10.2 is uppercased to the non-combining
character GREEK CAPITAL LETTER IOTA (U+0399) which is in turn
lowercased to the non-combining character GREEK SMALL LETTER IOTA
(U+03B9). As a result, when computing a case-insensitive hash, when
a base character is IOTA (of either case) and the previous base
character is ALPHA, ETA, or OMEGA (of the same case as the IOTA),
that IOTA is treated, for the purpose of defining the composite
characters for which to generate sub-hashes as if it were a combining
character. As a result, in this case a string of containing two
composite characters will be treated as were a single composite
character since the iota will be treated as if it were a combining
character. This string will have its own sub-hash, which will be the
same regardless of the order of combining characters.
The same outline will be followed for generating hashes which are to
be form-insensitive (only) and for those which are to be both form-
insensitive and case-insensitive. The initial value, representing
the base character, will differ based on the type of hash, as
discussed below.
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* In the case-sensitive case, the initial value of the sub-hash will
reflect the value of the base character with the only possible
need to map to a different value deriving from the existence of
OHM SIGN (U+2126), ANGSTROM SIGN (U+212B), and MICRO SIGN (U+00B5)
as characters distinct from the letters that represent these code
points. This could be done with a mapping table but most
implementations would probably choose to implement special-purpose
code to do this.
* In the case-insensitive case, the initial value of the sub-hash
will reflect the case-based equivalence class to which the
character (the lower-case equivalent is generally suitable). In
this context a table-based mapping is required and this mapping
can shift OHM SIGN, ANGSTROM SIGN, and MICRO SIGN to the case-
based equivalence class for the corresponding character.
Regardless of the type of hash to be produced, values based on the
following combining characters need to reflected in the sub-hash. In
order to make the sub-hash invariant to changes in the order of
combining characters, values based on the particular combining
character are combined with the hash being computed using a
commutative associative operation, such as addition.
To reduce false-positives it is desirable to make the hash relatively
wide (i.e. 32-64 bits) with the value based on base character in the
upper portion of the word with the values for the combining
characters appearing in a wide range of bit positions in the rest of
the word to limit the degree that multiple distinct sets of combining
characters have value that are the same. Although the details will
be affected by processor cache structure and the distribution of
names processed, a table of values will be used but typical
implementations will be different in the two cases we are dealing as
described in Appendix B.2.
As each sub-hash is computed, it is combined into a name-wide hash.
There is no need for this computation to be order-independent and it
will probably include a circular shift of the hash computed so far to
be added to the contribution of the sub-hash for the new base or
composed character.
As described in Appendix B.3 the appropriate full name hash will have
the major role in excluding potential matches efficiently. However,
in some small number of cases, there will be a hash match in which
the names to be compared are not equivalent, requiring more involved
processing. It is assumed below that a given name will be searching
for potential cached matches within the directory so that for that
name, on will be able retain information used to construct the full
name hash (e.g. individual sub-hashes plus the bounds of each
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composite character. These will be compared against cached entries
where only the full (e.g. 64-bit) name hash and the name itself will
be available for comparison.
B.2. Character Tables
The per-character tables used in these algorithms have a number of
type of entries for different types of characters. In some cases,
information for a given character type will be essentially the same
whether the comparison is to be form-insensitive or case-
insensitive. In others, there will be differences. Also, there may
be entry types that only exist for particular types of comparisons.
In any case, some bits within the table entry will be devoted to
representing the type of character and entry:
* For combining characters, the entry will provide information about
the character's contribution to the composite character sub-hash
in which it appears.
* For case-insensitive comparisons, there need to be special entries
for characters, which, while not themselves combining characters,
are the case-insensitive equivalents of combining characters. An
example of this situation is provided in item EX6 within
Section 10.2.
* For pre-composed characters, the entry needs to provide the
initial hash value which is to be the basis for the sub-hash for
the name substring including contributions for the base character
together with contribution of included combining characters. In
addition, such entries will provide, separately, information about
the character's canonical decomposition.
* For case-insensitive comparisons, there needs to be, for base
characters, entries assigning each base character to the case-
based equivalence class to which it belongs, although such entries
can be avoided if the equivalence class matches the character
(usually caseless and lowercase characters.
* Also, for case-insensitive comparisons, there will need to be
special entries for characters which multi-character string as
case-insensitive equivalent of the base character. Examples of
this situation are provided in items EX4 and EX5 within
Section 10.2. Such entries will need to have a hash-contribution
that reflects the hash that would be computed for the multi-
character string.
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* For form-insensitive comparisons, there will be special entries to
provide special handling for those cases in which there are two
canonically equivalent single characters. Such entries do not
exist for case-insensitive comparison since this situation can be
handled by a non-standard use of case mapping for base characters
by placing these two characters in the same case-based equivalence
In the common case in which a two-stage mapping will be used, there
will be common groups of characters in which no table entry will be
required, allowing a default entry type to be used for some character
groups with entry contents easily calculable from the code point.
* In the case form-insensitive comparison, this consists of all base
characters, with the hash contribution of the character derivable
by a pre-specified transformation of the code point value.
* In the case case-insensitive comparison, this consists of all base
character which are either caseless or equivalence class is the
same as the code point, typically lowercase characters. As in the
form-insensitive case, the hash contribution of the character is
derivable by a pre-specified transformation of the code point
value, which matches, in this case, the id assigned to the case-
based equivalence class.
B.3. Outline of comparison
We are assuming that comparisons will be based on the hash values
computed as described in Appendix B.1, whether the comparison is to
be form-insensitive or both case-insensitive and form-insensitive.
To facilitate this comparison, the name hash will be stored with the
names to be compared. As a result, when there is a need to
investigate a new name and whether there are existing matches, it
will be possible to search for matches with existing names cached for
that directory, using a hash for the new name which is computed and
compared to all the existing names, with the result that the detailed
comparisons described in Appendices B.4 and B.5 have to be done
relatively rarely, since non-matching names together with matching
hashes are likely to be atypical.
Given the above, it is a reasonable assumption, which we will take
note of in the sections below, that for one of the names to be
compared, we will have access to data generated in the process of
computing the name hash while for the other names, such data would
have to be generated anew, when necessary. When that data includes,
as we expect it will, the offset and length of the string regions
covered by each sub-hash, direct byte-by-byte comparisons between
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corresponding regions of the two strings can exclude the possibility
of difference without invoking any detailed logic to deal with the
possibility of canonical equivalence or case-based equivalence in the
absence of identical name segment.
In the case in which the byte-by-byte comparisons fail, further
analysis is necessary:
* First, the associated base characters are compared, as is
discussed in Appendix B.4. When doing form-insensitive comparison
this is straightforward. However, when case-insensitive
comparison is to be done, there is the possibility that the sub-
hash boundaries of the two comparands are different, requiring
that a common point in both comparands be found to resume
comparison after a successful match. For either form of
comparison, if a mismatch is found at this point then the
comparison fails, while, if there is match, there must be a
comparison of any following combining characters, as described
below, before moving on to the region covered by the appropriate
sub-string covered by the appropriate next sub-hash for each
comparand.
* If there is no mismatch as to the base characters, the set of
associated combining characters (might be null) must be compared,
as is discussed in Appendix B.5. If a mismatch is found at this
point then the comparison fails. This may be because the sets of
combining characters are different, because there are multiple
copies of the same combining character in one of the string, or
because the difference in combining character is not one that
maintains canonical equivalence (due to combining classes).
* When both comparisons show a match, the comparison resumes at the
next substring, using a byte-by-byte comparison initially. If the
comparison cannot be resumed because one of the strings is
exhausted, the comparison terminate, succeeding only if both
strings are exhausted while failing if only one of the strings is
exhausted.
B.4. Comparing Base Characters
In general, the task of comparing based characters is simple, using a
table lookup using the numeric value of the initial character in the
substring. When doing form-insensitive comparison this is the base
character associated with the initial (possibly pre-composed)
character, while for case-insensitive comparison it is the case-based
equivalence class associated with that character.
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When doing case-insensitive comparison, issues may arise that result
when there is a multi-character string that as the case- insensitive
equivalent of a single base character, as discussed in items EX4 and
EX5 within Section 10.2. These are best dealt with using the
approach outlined in Section 10.1. When it is noted that the current
base character (for either comparand) is a character whose associated
equivalence class contains one or more multi-character strings, then
these comparisons, normally requiring that each base character be
mapped to the same case-based equivalence class by modified to allow
equivalences allowed by these multi-character sequences.
In such cases, there may need to be comparisons involving the multi-
character string, in addition to the normal comparisons using the
base characters' equivalence class. As an illustration, we will
consider possible comparison results that involve characters string
within the equivalence class mentioned in item EX4 within
Section 10.2.
* When the base character for both comparands are either LATIN SMALL
LETTER SHARP S (U+00DF) or LATIN CAPITAL LETTER SHARP S (U+1E9E),
then a match is recognized.
* When the base character for one comparand is either LATIN SMALL
LETTER SHARP S (U+00DF) or LATIN CAPITAL LETTER SHARP S (U+1E9E),
while the other is not, each character in the that other comparand
is case-insensitively compared to the corresponding character of
the string "ss" with a match being signaled when all such
subsequent characters match, except for possibly being of a
different case. Because that comparison will involve multiple
base characters, the overall comparison point for that comparand
will have to be adjusted to reflect character already processed as
part of the comparison.
* When the base character for neither comparands is either LATIN
SMALL LETTER SHARP S (U+00DF) or LATIN CAPITAL LETTER SHARP S
(U+1E9E), then matching proceeds normally. As a result, the only
cases in which character strings within the equivalence class
being discussed will result is where both comparands have one of
the strings "ss", "sS", "Ss", or "SS" at the current comparison
point.
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B.5. Comparing Combining Characters
In order to effect the necessary comparison, one needs to assemble,
for each comparand, the set of combining characters within the
current substring. The means used might be different for different
comparands since there might be useful information retained from the
generation of the associated string hash for one of the comparands.
In any case, there are two potential sources for these characters:
* Those deriving from the canonical decomposition of a pre-composed
character, treated as a null set of if the base character is not a
precomposed one.
* Those combining characters that immediate following the base
character, which will be a null set if the immediately following
character is not a combining character. Note that it is possible,
when doing case-insensitive comparison to treat certain character,
not normally combining characters, as if they are. Such
situations can arise, when, as described in item EX6 within
Section 10.2, such non-combining character are the uppercase or
lowercase equivalents of combining characters.
Although, the two sets of character can be checked to see if they are
identical, this is a sufficient but not a necessary condition for
equivalence since some permutations of a set of combining characters
are considered canonically equivalent. To summarize the appropriate
equivalence rules:
* Combining characters of different combining classes may be freely
reordered.
* If combining characters of the same combining class are reordered,
then result is not canonically equivalent
The rules above do not directly apply to the case, discussed above,
in which some non-combining characters are the case-based equivalents
of combining characters such as COMBINING GREEK YPOGEGRAMMENI
(U+0345). Nevertheless, because of this equivalence, those
implementing case-insensitive comparisons do have to deal with this
potential equivalence when considering whether two strings containing
combining characters or their case-based equivalents match. As a
result when comparing strings of combining characters, we need to
implement the following modified rules.
* When one comparand has a true combining character and the other
comparand has an identical one, they may differ in location as
long as there is no permutation of combining characters of the
same combining class.
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* When one comparand has a true combining character and the other
has a case-insensitive equivalent which is not a combining
character, that character must appear last in its string while the
combining may character appear in its string in any position
except the last. In this case, there are no restrictions based on
combining classes.
* When both comparands contain a non-combining character case-
insensitively equivalent to a combining character, these character
must appear last in their respective strings.
Although it is possible to divide combining characters based on their
combining classes, sort each of the list and compare, that approach
will not be discussed here. Even though the use of sorts might allow
use of an overall N log N algorithm, the number of combining
characters is likely to be too low for this to be a practical
benefit. Instead, we present below an order N-squared algorithm
based on searches.
In this algorithm, one string, chosen arbitrarily id designated the
"source string" and successive character from it, are searched for in
the other, designated the "target string". Associated with the
target string is a mask to allow characters search for a found to be
marked so that they will not be found a second time. In the
treatment below, when a character is "searched for" only characters
not yet in the mask are examined and the character sought has its
associated mask bit set when it is found.
Each character in the source string is processed in turn with the
actual processing depending on particular character being processed,
with the following three possibilities to be dealt with.
1. For the typical case (i.e. a combining character with no case-
insensitive equivalents), the character is searched for in the
target string with the compare failing if it is not found.
If it is found, then the region of the target string between the
point corresponding to the current position in the source string
and the character found is examined to check for characters of
the same combining class. If any are found, the overall
comparison fails.
2. For the case of a combining character with a case- insensitive
equivalents, the character is searched for as described in the
first paragraph of item 1. However, the compare does not fail if
it is not found. Instead, a case-insensitive equivalent
character is searched for at the final position of the string and
the compare fails if that is not found.
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3. For the case of a non-combining character that has a combining
character as a case-insensitive equivalents, the overall
comparison fail if the character is not in the final position
within the source string or has already been successfully
searched for. Otherwise, the corresponding combining character
is searched for in the target as described in in the first
paragraph of item 1. The overall compare fails if it is not
found.
Once all characters in the source string has been processed, the mask
associated is examined to see if there are combining character that
were not found in the matching process described above. Normally, if
there are such characters, the overall comparison fails. However, if
the last character of the target was not matched and if it is a non-
combining character that is case-insensitively equivalent to a
combining character, then comparison succeeds and the remaining
character needs to be matched with the next substring in the source.
Acknowledgements
This document is based, in large part, on Section 12 of [RFC7530] and
all the people who contributed to that work, have helped make this
document possible, including David Black, Peter Staubach, Nico
Williams, Mike Eisler, Trond Myklebust, James Lentini, Mike Kupfer
and Peter Saint-Andre.
The author wishes to thank Tom Haynes for his timely suggestion to
pursue the task of dealing with internationalization on an NFSv4-wide
basis.
The author wishes to thank Nico WIlliams for his insights regarding
the need for clients implementing file access protocols to be aware
of the details of the server's internationalization-related name
processing, particularly when case-insensitive file systems are being
accessed.
The author wishes to thank Christoph Helwig for his insightful
comments regarding the implementation constraints that
internationalization-aware servers have to deal with to support
normalization and case-insensitivity
Author's Address
David Noveck
NetApp
201 Jones Road
Waltham, MA 02451
United States of America
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Phone: +1 781 572 8038
Email: davenoveck@gmail.com
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