Internet DRAFT - draft-dnoveck-nfsv4-internationalization
draft-dnoveck-nfsv4-internationalization
NFSv4 D. Noveck
Internet-Draft NetApp
Updates: 5661, 7530 (if approved) March 7, 2021
Intended status: Standards Track
Expires: September 8, 2021
Internationalization for the NFSv4 Protocols
draft-dnoveck-nfsv4-internationalization-03
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 RFC5661.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 8, 2021.
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Copyright (c) 2021 IETF Trust and the persons identified as the
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
2.1. Requirements Language Definition . . . . . . . . . . . . 4
2.2. Requirements Language Derivation . . . . . . . . . . . . 4
3. Internationalization and Minor Versioning . . . . . . . . . . 6
4. Changes Relative to RFC7530 . . . . . . . . . . . . . . . . . 7
5. Limitations on Internationalization-Related Processing in the
NFSv4 Context . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Summary of Server Behavior Types . . . . . . . . . . . . . . 8
7. The Attribute Fs_charset_cap . . . . . . . . . . . . . . . . 9
7.1. The Attribute Fs_charset_cap in Published NFSv4.1
Specifications . . . . . . . . . . . . . . . . . . . . . 10
7.2. The Attribute Fs_charset_cap in Future NFSv4.1
Specifications . . . . . . . . . . . . . . . . . . . . . 12
8. String Encoding . . . . . . . . . . . . . . . . . . . . . . . 14
9. Normalization . . . . . . . . . . . . . . . . . . . . . . . . 15
10. Case-Insensitive Processing of File Names . . . . . . . . . . 15
10.1. Implementing Case-Insensitive Comparison of File Names . 19
10.2. Important Examples of Case-insensitive Handling of File
Names . . . . . . . . . . . . . . . . . . . . . . . . . 21
11. Internationalization-related Processing of File Names by
Clients . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Server Restrictions to Deal with Lack of Client
Knowledge . . . . . . . . . . . . . . . . . . . . . . . 25
11.2. Client Processing of File Names for Current NFSv4
Protocols . . . . . . . . . . . . . . . . . . . . . . . 26
11.3. Client Processing of File Names for Future NFSv4
Protocols . . . . . . . . . . . . . . . . . . . . . . . 30
12. String Types with Processing Defined by Other Internet Areas 31
12.1. Effect of IDNA Changes . . . . . . . . . . . . . . . . . 33
12.2. Potential Compatibility Issues Related to IDNA Changes . 34
13. Errors Related to UTF-8 . . . . . . . . . . . . . . . . . . . 36
14. Servers That Accept File Component Names That Are Not Valid
UTF-8 Strings . . . . . . . . . . . . . . . . . . . . . . . . 37
15. Future Minor Versions and Extensions . . . . . . . . . . . . 38
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
17. Security Considerations . . . . . . . . . . . . . . . . . . . 39
18. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
18.1. Normative References . . . . . . . . . . . . . . . . . . 40
18.2. Informative References . . . . . . . . . . . . . . . . . 41
Appendix A. History . . . . . . . . . . . . . . . . . . . . . . 42
Appendix B. Form-insensitive String Comparisons . . . . . . . . 47
B.1. Name Hashes . . . . . . . . . . . . . . . . . . . . . . . 49
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B.2. Character Tables . . . . . . . . . . . . . . . . . . . . 51
B.3. Outline of comparison . . . . . . . . . . . . . . . . . . 52
B.4. Comparing Base Characters . . . . . . . . . . . . . . . . 53
B.5. Comparing Combining Characters . . . . . . . . . . . . . 54
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 57
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 57
1. Introduction
Internationalization is a complex topic with its own set of
terminology (see [22]). The topic is made more complex for the NFSv4
protocols by the tangled 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) and is
intended to serve as a basis for further implementations to be
developed that can interact with existing implementations as well as
those to be developed in the future.
Note that the behaviors on which this document are based are each
demonstrated 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 choice, normative terms defined in RFC2119
[1] 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.
o With regard to NFSv4.0 as defined in RFC7530 [3], no significant
interoperability issues are expected to arise because the
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 [3], the
treatments are intended to be essentially the same, in order to
eliminate interoperability issues.
Because of a change in the handling of Internationalized domain
names, there are some differences from the handling in RFC7530
[3], as discussed in Appendix A. For a discussion of those
differences and potential compatibility issues, see Sections 12.1
and 12.2.
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o With regard to NFSv4.1 as defined RFC5661 [4], 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 [3] 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 [4], 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.
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 [1] [2] when, and only when,
they appear in all capitals, as shown here.
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:
o In the case of statements within Sections 12 and 15, these derive
from the requirements of other internet specifications.
o 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.
o However, in other cases, i.e. those in sections deriving from
RFC7530 [3] (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
[1], providing a normative description to enable future
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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.
o 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.
o 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".
o 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.
o 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,
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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 t implementation work may no
longer be involved in or 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".
o 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.
o 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.
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
names have been handled without regard to the minor version being
used. Minor version specification documents contained different
treatments of internationlization as described in Appendix A but of
those only the implementation-based approach used by RFC7530 [3],
resulted in a workable description while a number of attempts to
specify an approach that implementors were to follow were all
ignored.
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 [9]) 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 pecified
in this document are dealt with in Section 15.
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4. Changes Relative to RFC7530
This document follows the internationalization approach defined in
RFC7530, with a number of significant necessary changes.
o The handling of internationalization specified in [3] 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 [3] and what was specified in [4]. Issues relating to
potential future minor versions and protocol extensions are
addressed in Section 15.
o Some changes motivated by the shift from IDNA2003 to IDNA2008 have
been made. The intention is to maintain compatibility with all
existing NFSv4 minor versions. Potential compatibility issues
with regard to the IDNA shift are discussed in Section 12.2.
o There is more detailed discussion of case-insensitive handling of
file names, with particular attention to the complexities that can
arise when multiple language convention in these matters need to
be accommodated. The discussion in Section 10 applies to both
client or server, although issues relating to the client's
knowledge are dealt with in Section 11.
o 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 is 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:
o 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.
o Server-side handling of file component names is typically
implemented within a server-side physical file system, whose
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handling of character encoding and normalization is not
specifiable by the IETF.
o 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.
o 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.
6. Summary of Server Behavior Types
Servers MAY reject component name strings that are not valid UTF-8.
This 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 combined
behaviors outlined below.
o 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".
o 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 normalization 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.
o 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
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viewpoint of the other encoding. Such difficulties can be
compounded when case-insensitive name handling is in effect.
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 as NFSv4.0 implementations had.
It is generally accepted that attributes designated "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 [10]. In this
particular case, there are a number of circumstances that makes this
OPTIONAL status noteworthy:
o 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 [10] 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
o This was added in minor version one which added a number of
REQUIRED operations and could well have added a REQUIRED
attribute.
o The fact that the client is to be prepared for non-support of the
attribute would require specification of a default value, 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 [10] 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:
o The use of two bits while one is clearly adequate, given the
subject matter actually mentioned
o The mention of possible "capabilities" which could not possibly be
realized.
o 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 [21]. Although conveying this information is
clearly part of the motivation, stating so clearly might have been
judged by the authors as too 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. 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.
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However, the authors might well feel unable to say so clearly, 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 implementation of canonical
equivalence, 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.
That is clear and so it poses no problem for a revised treatment,
unlike the other flag.
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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 RFC2119 [1], "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 e environments and file systems do not
enforce character set encodings", gives one no basis for treating
this as other than an unproblematic behavior 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"
o 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.
o 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", although the authors might not
have had any such intention but felt that the IESG might well have
such an intention.
7.2. The Attribute Fs_charset_cap in Future NFSv4.1 Specifications
We provide a revised version of Section 14.4 of [10] 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
reclects the working group's intentions. 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
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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 needs to be changed, if it raises
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 is to be ignored, with the appropriate handling being the same
as would apply if the attribute were not supported.
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.
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8. String Encoding
Strings that potentially contain characters outside the ASCII range
[11] are generally represented in NFSv4 using the UTF-8 encoding [8]
of Unicode [12]. See [8] for precise encoding and decoding rules.
Some details of the protocol treatment depend on the type of string:
o 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 [23] for further
discussion).
o For strings based on domain names, non-ASCII characters MUST be
represented using the UTF-8 encoding of Unicode, and additional
string format restrictions may apply. See Section 12 for details.
o 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.
o For other sorts of strings, any non-ASCII characters SHOULD be
represented using the UTF-8 encoding of Unicode.
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9. Normalization
The client and server operating environments can potentially differ
in their policies and operational methods with respect to character
normalization (see [12] for a discussion of normalization forms).
This difference may also exist between applications on the same
client. This adds to the difficulty of providing a single
normalization policy for the protocol that allows for maximal
interoperability. This issue is similar to the issues of character
case where the server may or may not support case-insensitive file
name matching and may or may not preserve the character case when
storing file names. The protocol does not mandate a particular
behavior but allows for 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 characters 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).
Server implementations MAY normalize file names to conform to a
particular normalization form before using the resulting string when
looking up or creating a file. 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 application 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.
o It can force all characters which have multiple forms to a common
case, whether uppercase of lowercase. Although this may cause the
file name shown in the directory to be different from that
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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.
o 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
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 name are discussed in Section 5.18 of the Unicode
Standard [13] which deals with case mapping. While we need to
address all of these issues as well, our approach will not be exactly
the same.
o Since the client will be doing case-insensitive comparisons,
issues that apply only to uppercasing or lowercasing do not have
the same significance.
o Many clients will have to operate correctly even in the absence of
detailed information about the specifics of server case-mapping or
the version on Unicode implemented by the server.
o Clients will have to accommodate server behaviors not anticipated
by the Unicode Specification since the neither the server nor the
client might have any locale knowledge when file names are
processed.
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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 [14]. 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:
o 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 of associated with some Turkic languages.
o 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.
o 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
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 in
considered valid at the same time
o 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 possible.
o 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
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mappings, in which a multi-character string is mapped to a single
character of a different case which not compatible with any
existing simple case-mapping models.
o Scanning of names for multi-character sequences might well be too
complicated, especially since such sequences might overlap in
complicated ways.
o 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.
o 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, it 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.
o 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.
o Implementation of case-insensitive name comparisons will typically
require a case-insensitive name hash.
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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. This algorithm 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.
While the algorithm described 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.
o For each possible character value, the associated equivalence
class for case-insensitive comparison will be identified
o For each such equivalence class, the hash value contribution will
be provided. In the case of equivalence class that do not include
multi-character 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
o In the case of equivalence classes that do include multi-character
strings, the hash value contribution needs to 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 id.
Case-insensitive comparison proceeds as follows:
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o Implementation of case-insensitive name comparisons will typically
require a case-insensitive name hash using the tables described
above. If such a hash vale is kept or 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.
o 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.
o 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.
o 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.
o 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.
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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 environment. Many of these are
also discussed in [13]. However, our treatment of these issues,
while not inconsistent with that in [13], differs significantly for a
number of reasons:
o 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.
o 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
is 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 filesystem 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
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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
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 them.
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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.
EX5: Other ligatures, such as LATIN SMALL LIGATURE FFL (U+FB04),
could be handled similarly by this algorithm, if there were
felt 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 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.
EX8: In most languages written using Latin characters, the uppercase
and lowercase varieties of the letter "I" differ in that only
the lowercase character. 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
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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 may 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.
However, as has been pointed out in [25], 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:
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o 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 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.
o The client can be provided complete information about the server's
internationalization-related name handling (typically implemented
within the server-based file system. This situation, 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.
o Note that when case-insensitive handling of file names is
implemented by a server-side filesystem, 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 a 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 [3], 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 limit it to avoid relying on cases in which the proper
handling cannot be determined or duplicated by the client.
o The canonical equivalence relation implemented by the server, for
each internationalization-aware filesystem MUST match that defined
by some particular UNICODE version equal to or later than version
4.0.
o The case-equivalence relationship implemented by the server, for
each case-insensitive filesystem 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
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mappings. by some particular Unicode version equal to or later
than version 4.0.
o 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,
i.e. 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 [3], NFSv4.1 [4], and NFSv4.2
[5], 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 was no provision for attributes
defining the server's choices. As a result, the information
available to the client is quite limited:
o 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.
o The client can use the optional per-fs attributes case_insensitive
and case_preserving to how the server deals with character case
for 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
a range of choices trading off forgone optimization opportunities
against the difficulty of implementation while avoiding negative
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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 follow possibilities, arranged in order of increasing
value (and difficulty of implementation) should be considered.
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 filesystems, or where, due to a lack
of directory delegation support, the client has no assurance
that 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 string 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.
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
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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 filesystem is unknown, strings for
which it is possible that the canonically equivalent string
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.
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.
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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 filesystems.
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 variant 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
used by the server filesystem 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
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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 of NFSv4 has an extension framework allowing the addition of
new attributes in later minor version 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:
o The Unicode version used to define the canonical equivalence
relation implemented by the server could be provided as an fs-
scope attribute.
o For case-insensitive filesystems, 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
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, 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 case-sensitive file system, the case-mapping attribute
is not relevant. In dealing with the non-support of the Unicode
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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.
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
Internet standards, and hence the processing behavior for such
strings should be consistent across all server operating systems and
server file systems.
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This section differs from other sections of this document in two
respects:
o The normative statements within this section are not derived from
the behavior from existing NFSv4 implementations, but derive
instead from existing RFCs.
o Because of the switch from IDNA2003 [19] [20] to IDNA2008 [6],
this section is necessarily different from the corresponding
section (i.e. Section 12.6) of [3]. 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 [3] 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:
o Server names as they appear in the fs_locations and
fs_locations_info attribute. Notes 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.
o Principal suffixes that are used to denote sets of users and
groups, and are in the form of domain names.
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, although the consequences of failure to obey these
rules may be different for client or server. The server can report
errors when it is sent invalid strings, whereas the client will
simply ignore an invalid string or use a default value in its place.
The string sent SHOULD be in the form of one or more unvalidated
U-labels as defined by [6]. In cases where this cannot be done, the
string will instead be in the form of one or more LDH labels [6].
The receiver needs to be able to accept domain and server names in
any of the formats allowed. The server MUST reject, using the error
NFS4ERR_INVAL, any of the following:
o a string that is not valid UTF-8.
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o a string that contains an XN-label (begins with "xn--") for which
the characters after "xn--" are not valid output of the Punycode
algorithm [7].
o a string that contains a reserved LDH label which is not an
XN-label.
When a domain string is part of id@domain or group@domain, there are
two possible approaches:
1. The server generally treats the domain string as a series of
unvalidated U-labels. In cases where the domain string is a
series of unvalidated A-labels or Non-Reserved LDH (NR-LDH)
labels, it converts them to U-labels using the Punycode algorithm
[7]. As a result, the domain string returned within a user id on
a GETATTR may not match that sent when the user id is set using
SETATTR, although when this happens, the domain will be in the
form of an unvalidated U-label.
2. The server treats the domain string as a series of unvalidated
U-labels. Specifically, it does not map a domain string that is
not a U-label into a U-label using the methods described above.
As a result, the domain string returned on a GETATTR of the user
id MUST be the same as that used when setting the user id by the
SETATTR.
A server SHOULD use the first method.
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 (RFC750 [3],
RFC5661 [4])
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 [3]. Despite this specification,
the degree to which implementations actually implemented such
restrictions is open to question and 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:
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o There are a number of cases in which "SHOULD" is used that are
confusing. According to RFC2119 [1], "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".
o 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.
o 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.
o The degree to which RFC3490 [19] 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).
o The degree to which RFC3490 [19] 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:
o Much of the potentially conflicting IDNA-related behavior required
or recommended for the server by RFC7530 [3] might not actually be
implemented, limiting the potential harmful effects of ceasing to
mandate it.
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o 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.
o 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 [3] and 5661 [4] 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 [16] and
NFsv2 [15]. 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.
o The other uses of domain names in NFSv4, to represent hostnames in
location attributes, the values are generated by the server and
will normally include only include hostnames within DNS-registered
domains.
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
o 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.
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o 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
the 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.
o 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.
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 should 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 should return the
error NFS4ERR_BADNAME. This includes such situations as file system
prohibitions of "." and ".." as file names for certain operations,
and similar constraints.
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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 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:
o 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 should not be performed,
because doing so is likely to result in incorrect string
modification or aliasing.
o 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
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 may 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 [23] for further discussion).
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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 allows 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 of handling file names.
While it is theoretically possible for a subsequent minor version to
change these aspects of the protocol (see [9]), 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.
o 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.
o 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.
o 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 they would
create.
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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.
o 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.
o 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 an OPTIONAL per-fs attribute
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.
17. Security Considerations
Unicode in the form of UTF-8 is generally is 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 standard 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 [3] and RFC5661
[4] for further discussion. See [23] for related identifier
comparison security considerations. File component names are
identifiers with respect to the identifier comparison discussion in
[23] because they are used to identify the objects to which ACLs are
applied (See the respective Sections 6 of RFC7530 [3] and RFC5661
[4]).
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18. References
18.1. Normative References
[1] 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>.
[2] 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>.
[3] 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>.
[4] 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>.
[5] 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>.
[6] 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>.
[7] 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>.
[8] 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>.
[9] Noveck, D., "Rules for NFSv4 Extensions and Minor
Versions", RFC 8178, DOI 10.17487/RFC8178, July 2017,
<https://www.rfc-editor.org/info/rfc8178>.
[10] 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>.
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[11] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, October 1969,
<http://www.rfc-editor.org/info/rfc20>.
[12] 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/>.
[13] 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>.
[14] 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>.
18.2. Informative References
[15] Nowicki, B., "NFS: Network File System Protocol
specification", RFC 1094, DOI 10.17487/RFC1094, March
1989, <https://www.rfc-editor.org/info/rfc1094>.
[16] 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>.
[17] 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>.
[18] 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>.
[19] 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>.
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[20] 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>.
[21] 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>.
[22] 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>.
[23] 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>.
[24] Shepler, S., "NFS version 4 Protocol", draft-ietf-
nfsv4-rfc3010bis-04 (work in progress), October 2002.
[25] Williams, N., "Internationalization Considerations for
Filesystems and Filesystem Protocols", draft-williams-
filesystem-18n-00 (work in progress), July 2020.
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 [17] until now, two
different perspectives with regard to internationalization have been
held and represented, to varying degrees, in specifications for NFSv4
minor versions.
o 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
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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.
o 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 [3], a satisfactory modus
vivendi may have been arrived at.
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.
o The approach to internationalization specified in RFC3010 [17]
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.
o 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 [24] closely followed RFC3010 as regards
internationalization. The IESG then decided that a different
approach to internationalization was required, to be based on
stringprep [18] and rfc3010bis was accordingly revised, replacing
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all of the Internationalization section, before being published as
RFC3530 [21].
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.
o When NFsv4.1 was being developed, it seemed that no changes in
internationalization would be required. Many people 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 [4] was
based on that in RFC3530 [21], 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.
As a result of this treatment of internationalization, 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.
o 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,
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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 set forth above.
The draft submitted to the IESG and RFC7530 [3] 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.
This approach is compatible, in implementation terms, with that
specified in RFC3010 [17], allowing it to be used compatibly with
existing implementations for all existing minor versions. This is
despite the fact that RFC5661 [4] 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 [3]
adopted an approach compatible with IDNA2003, rather than
attempting to derive the specification from the behavior of
existing implementations.
o When IDNA2003 was replaced by IDNA2008, the internationalization
specified by [3] was not changed. Also, it appears unlikely that
implementations were changed to reflect that shift.
o NFSv4.2 made no changes to internationalization. As a result,
RFC7862 [5] which made no mention of internationalization,
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implicitly aligned internationalization in NFSv4.2 with that in
NFSv4.1, as specified by RFC5661 [4].
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.
o Later, as work on the predecessors of this document was underway,
[25] was submitted, making it necessary that some gaps the
discussion of internationalization in [3] be filled in. These
gaps primarily concerned the need of 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.
The above history, can, for the purposes of the rest of this document
be summarized in the following statements:
o 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.
o With regard to file names, implementations have followed the
internationalization approach specified in RFC3010, which is
compatible with the treatment in RFC7530.
o With regard to internationalized domain names, RFC7530 [3]
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
o Because [3] 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
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changes discussed in Section 4 and applies that approach to all NFSv4
minor versions.
Appendix B. Form-insensitive String Comparisons
This section deal with two varieties of form-insensitive string
comparison:
o 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
o 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 to two distinct implementation areas:
o Implementation of server-side file systems intended to be accessed
using NFSv4 protocols. While it is often the case that such
filesystems 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.
o 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.
o 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).
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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.
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.
o 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.
o 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).
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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.
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.
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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.
o 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.
o 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
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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
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:
o For combining characters, the entry will provide information about
the character's contribution to the composite character sub-hash
in which it appears.
o 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
o 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.
o 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.
o 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
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that reflects the hash that would be computed for the multi-
character string.
o 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.
o 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.
o 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,
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as we expect it will, the offset and length of the string regions
covered by each sub-hash, direct byte-by-byte comparisons between
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:
o 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.
o 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).
o 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
o 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.
o 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.
o 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.
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
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generation of the associated string hash for one of the comparands.
In any case, there are two potential sources for these characters:
o 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.
o 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:
o Combining characters of different combining classes may be freely
reordered.
o 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.
o 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.
o 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
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except the last. In this case, there are no restrictions based on
combining classes.
o 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.
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
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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 [3] 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.
Author's Address
David Noveck
NetApp
1601 Trapelo Road
Waltham, MA 02451
United States of America
Phone: +1 781 572 8038
Email: davenoveck@gmail.com
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