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Independent Submission                                         M. Kerwin
Internet-Draft                                                       QUT
Obsoletes: 1738 (if approved)                         September 26, 2014
Intended status: Standards Track
Expires: March 30, 2015

The file URI Scheme
draft-kerwin-file-scheme-13

Abstract

This document specifies the "file" Uniform Resource Identifier (URI)
scheme, replacing the definition in RFC 1738.

It attempts to document current practices, while at the same time
defining a common core which is intended to interoperate across the
broad spectrum of existing implementations.

Note to Readers (To be removed by the RFC Editor)

This draft should be discussed on the IETF Applications Area Working
Group discussion list <apps-discuss@ietf.org>.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF).  Note that other groups may also distribute
working documents as Internet-Drafts.  The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on March 30, 2015.

Copyright (c) 2014 IETF Trust and the persons identified as the

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents

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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document.  Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.  Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
1.1.  History . . . . . . . . . . . . . . . . . . . . . . . . .   3
1.2.  Similar Technologies  . . . . . . . . . . . . . . . . . .   3
1.3.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
3.  Methods on file URIs  . . . . . . . . . . . . . . . . . . . .   7
3.1.  Translating Local File Path to file URI . . . . . . . . .   7
3.2.  Translating UNC String to file URI  . . . . . . . . . . .  10
3.3.  Translating Non-local File Path to file URI . . . . . . .  11
3.4.  Incompatible File Paths . . . . . . . . . . . . . . . . .  11
3.4.1.  Namespaces  . . . . . . . . . . . . . . . . . . . . .  11
4.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
Appendix A.  UNC Syntax . . . . . . . . . . . . . . . . . . . . .  16
Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

A file URI identifies a file on a particular file system.  It can be
used in discussions about the file, and if other conditions are met
it can be dereferenced to directly access the file.

The file URI scheme is not coupled with a specific protocol.  As
such, there is no well-defined set of methods that can be performed
on file URIs, nor a media type associated with them.

This document defines a syntax that is compatible with most extant
implementations, while attempting to push towards a stricter subset
of "ideal" constructs.  In many cases it simultaneously acknowledges
and deprecates some less common or outdated constructs.

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1.1.  History

The file URI scheme was first defined in [RFC1630], which, being an
informational RFC, does not specify an Internet standard.  The
definition was standardised in [RFC1738], and the scheme was
registered with the Internet Assigned Numbers Authority (IANA);
however that definition omitted certain language included by former
that clarified aspects such as:

o  the use of slashes to denote boundaries between directory levels
of a hierarchical file system; and

o  the requirement that client software convert the file URI into a
file name in the local file name conventions.

The Internet draft [I-D.draft-hoffman-file-uri] was written in an
effort to keep the file URI scheme on standards track when [RFC1738]
was made obsolete, but that draft expired in 2005.  It enumerated
concerns arising from the various, often conflicting implementations
of the scheme.  It serves as the spiritual predecessor of this
document.

Additionally the WHATWG defines a living URL standard [WHATWG-URL],
which includes algorithms for interpreting file URIs (as URLs).

1.2.  Similar Technologies

The Universal Naming Convention (UNC) [MS-DTYP] defines a string
format that can perform a similar role to the file URI scheme in
describing the location of files.  A UNC filespace selector string
has three parts: host, share, and path; see: Appendix A.  This
document describes a means of translating between UNC filespace
selector strings and file URIs.

1.3.  Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

2.  Syntax

The file URI syntax is defined here in Augmented Backus-Naur Form
(ABNF) [RFC5234], including the core ABNF syntax rule "ALPHA" defined
by that specification, and importing the "userinfo", "host", "path-
absolute", and "query" rules from [RFC3986] (as updated by
[RFC6874].)

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file-URI       = f-scheme ":" f-hier-part [ "?" query ]

f-scheme       = "file"

f-hier-part    = "//" auth-path
/ local-path

auth-path      = [ f-auth ] path-absolute
/ unc-path
/ windows-path

f-auth         = [ userinfo "@" ] host

local-path     = path-absolute
/ windows-path

unc-path       = 2*3"/" authority path-absolute

windows-path   = drive-letter path-absolute
drive-letter   = ALPHA [ drive-marker ]
drive-marker   = ":" / "|"

Note well: the "drive-marker" rule intentionally includes a bar
character "|" even though that character is not part of either the
unreserved or reserved character sets in [RFC3986], and thus would
normally have to be percent-encoded to be included in a URI.  This
specification explicitly supports the parsing of otherwise invalid
URIs - those with an unencoded bar character forming part of a DOS or
Windows drive letter identifier - to facilitate parsing extant
historical URIs, but new URIs of this form MUST NOT be generated.

The query field contains non-hierarchical data that, along with data
in the path components (path-absolute, unc-path, or windows-path)
serves to identify a resource.  This is not commonly used in
practice, but could be used to refer to a specific version of a file
in a versioning file system, for example.

Systems exhibit different levels of case-sensitivity.  Unless the
file system is known to be case-insensitive, implementations MUST
maintain the case of file and directory names when translating file
URIs to and from the local system's representation of file paths, and
any systems or devices that transport file URIs MUST NOT alter the
case of file URIs they transport.

The syntax definition above is necessarily different from those given
in [RFC1630] and [RFC1738] because it depends on the generic syntax
from [RFC3986] that post-dates all previous specifications.

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It is intended to support file URIs that take the following forms:

Local files:

o  "file:///path/to/file"

A "traditional" file URI for a local file, with an empty
authority.  This is the most common format in use today,
despite being technically incompatible with the definition in
[RFC1738].

o  "file:///c:/path/to/file"

The traditional representation of a local file in a DOS- or
Windows-based environment.

o  "file:/path/to/file"

A "modern" minimal representation of a local file in a UNIX-
like environment, with no authority field and an absolute path
that begins with a slash "/".

o  "file:c:/path/to/file"

The minimal representation of a local file in a DOS- or
Windows-based environment, with no authority field and an
absolute path that begins with a drive letter.

o  "file:///c/path/to/file"

o  "file:/c/path/to/file"

o  "file:c/path/to/file"

Representations of a local file in a DOS- or Windows-based
environment, using alternative representations of drive
letters.  These are supported for compatibility with historical
implementationsm, but deprecated by this specification.

o  "file:/c:/path/to/file"

A representation of a local file in a DOS- or Windows-based
environment, with no authority field and a slash preceding the
drive letter.  This representation is less common than those
above, and is deprecated by this specification.

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Non-local files:

o  "file://host.example.com/path/to/file"

The ideal representation of a non-local file, with an explicit
authority.

o  "file:////host.example.com/path/to/file"

The "traditional" representation of a non-local file, with an
empty authority and a complete (transformed) UNC string in the
path.

o  "file://///host.example.com/path/to/file"

As above, with an extra slash between the empty authority and
the transformed UNC string, conformant with the definition from
[RFC1738]; see: exceptions in Section 3.2.  This representation
is deprecated by this specification.  It is notably used by the
Firefox web browser.

Dubious encodings:

o  "file://c:/path/to/file"

o  "file://c/path/to/file"

An encoding that includes a Windows drive letter as the
authority field.  This encoding exists in some extant
implementations, and is supported by the grammar for historical
reasons.  New URIs of this form SHOULD NOT be generated.

o  "file:///c|/path/to/file"

o  "file://c|/path/to/file"

o  "file:/c|/path/to/file"

o  "file:c|/path/to/file"

Various generally invalid URIs that include a disallowed bar
character "|" in the drive letter identifier.  These encodings
are supported by the grammar for historical reasons.  As noted
above, new URIs of this form MUST NOT be generated.

It also intentionally excludes URIs of the form:

o  "file://auth.example.com//host.example.com/path/to/file"

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An encoding that includes both a non-local authority, and a UNC
string.  The traditional implication was that the shared object
described by the UNC string may only be accessed from the
machine "auth.example.com".

3.  Methods on file URIs

In the strictest terms, the only operations that can be performed on
a file URI are translating it to and from a file path; subsequent
methods are performed on the resulting file path, and depend entirely
on the file system's APIs.

For example, consider the POSIX "open()", "read()", and "close()"
methods [POSIX] for reading a file's contents into memory.

Some APIs allow file system methods to be invoked directly on file
URIs, while others provide mappings to other similar methods, such as
GET and PUT from the Hyptertext Transfer Protocol (HTTP) [RFC7231].

The local file system API can only be used if the file URI has a
blank (or absent) authority and the path, when transformed to the
local system's conventions, is not a UNC string.  Note that this
differs from the definition in [RFC1738] in that previously an
authority containing the text "localhost" was used to refer to the
local file system, but in this specification it translates to a UNC
string referring to the host "localhost".

This specification does not define a mechanism for accessing files
stored on non-local file systems.

3.1.  Translating Local File Path to file URI

Below is an algorithmic description of the process used to convert a
file path to an Internationalized Resource Identifier (IRI)
[RFC3987], which can then be translated to a URI as per Section 3.1
of [RFC3987]; see: Section 4.

1.  Resolve the file path to its fully qualified absolute form.

2.  Initialise the URI with the "file:" scheme identifier.

3.  If including an empty authority field, append the "//" sigil to
the URI.

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4.  Append the root directory:

*  On a DOS- or Windows-based system, assign the drive letter
(e.g. "c:") as the first path segment, and append it to the
URI, followed by a slash character "/".

+  If an empty authority was included at step 3, a slash "/"
is prepended to the drive letter (e.g. "/c:") to
distinguish it from the authority.

*  On an OpenVMS Files-11 system, append a slash "/" to the URI,
and encode the device name as the first segment as per step 5,
below, except that the dollars sign character "\$" is not
treated as a reserved character in this segment.

*  On a UNIX-like system, append a slash "/" to the URI, to
denote the root directory.

5.  For each directory in the path after the root:

1.  Transform the directory name to a path segment ([RFC3986],
Section 3.3) as per Section 2 of [RFC3986].

2.  Append the transformed segment and a delimiting slash
character "/" to the URI.

6.  If the path includes a file name:

1.  Transform the file name to a path segment as above.

2.  Append the transformed segment to the URI.

7.  If any non-hierarchical data is required to identify the file
(for example a version number in a versioning file system):

1.  Append a question mark character "?" to the URI.

2.  Transform the non-hierarchical data to a query field
([RFC3986], Section 3.4) as per Section 2 of [RFC3986].

3.  Append the transformed query field to the URI.

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Examples:

File Path                      | URIs (minimal, traditional)
-------------------------------+--------------------------------
UNIX-like:                     |
/path/to/file                | file:/path/to/file
| file:///path/to/file
|
/path/to/dir/                | file:/path/to/dir/
| file:///path/to/dir/
|
DOS- or Windows-based:         |
c:\path\to\file.txt          | file:c:/path/to/file.txt
| file:///c:/path/to/file.txt
|
c:\path\to\dir\              | file:c:/path/to/dir/
| file:///c:/path/to/dir/
VMS Files-11:                  |
::DISK1:[PATH.TO]FILE.TXT;2  | file:/DISK1/PATH/TO/FILE.TXT?2
| file:///DISK1/PATH/TO/FILE.TXT?2
|

Differences from RFC 1738

In [RFC1738] a file URL always started with the token "file://",
followed by an authority and a "/".  That "/" was not considered part
of the path.  This implies that the correct encoding for the above
example file path in a UNIX-like environment would have been:

token     + authority + slash + path
= "file://" + ""        + "/"   + "/path/to/file.txt"
= "file:////path/to/file.txt"

However that construct was never used in practice, and in fact would
have collided with the eventual encoding of UNC strings in URIs.

Exceptions

DOS/Windows:
Some implementations leave the leading slash off before the drive
letter when authority is blank, e.g. "file://c:/..."

DOS/Windows:
Some implementations replace ":" with "|", and others leave it off
completely. e.g. "file:///c|/..." or "file:///c/..."

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3.2.  Translating UNC String to file URI

A UNC filespace selector string can be directly translated to an
Internationalized Resource Identifier (IRI) [RFC3987], which can then
be translated to a URI as per Section 3.1 of [RFC3987]; see:
Section 4.

1.  Initialise the URI with the "file:" scheme identifier.

2.  Append the authority:

1.  Append the "//" authority sigil to the URI.

2.  Append the hostname field of the UNC string to the URI.

3.  Append the sharename:

1.  Transform the sharename to a path segment ([RFC3986],
Section 3.3) as per Section 2 of [RFC3986].

2.  Append a delimiting slash character "/" and the transformed
segment to the URI.

4.  For each objectname:

1.  Transform the objectname to a path segment ([RFC3986],
Section 3.3) as per Section 2 of [RFC3986].

2.  Append a delimiting slash character "/" and the transformed
segment to the URI.

Example:

UNC String:   \\host.example.com\Share\path\to\file.txt
URI:          file://host.example.com/Share/path/to/file.txt

Exceptions

Many implementations accept the full UNC string in the URI path (with
all backslashes "\" converted to slashes "/").  Additionally, because
[RFC1738] said that the first "/" after "file://[authority]" wasn't
part of the path, some implementations (including Firefox) require an
additional slash before the UNC string.

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For example:

file:////hostname/share/object/names
\_____/\__________________________ /
Scheme     Transformed UNC string

Firefox:
file://///hostname/share/object/names
\_____/|\__________________________ /
Scheme |    Transformed UNC string
Extra slash

3.3.  Translating Non-local File Path to file URI

Translating a non-local file path other than a UNC string to a file
URI follows the same basic algorithm as for local files, above,
except that the authority MUST refer to the network-accesible node
that hosts the file.

For example, in a clustered OpenVMS Files-11 system the authority
would contain the node name.  Where the original node reference
includes a username and password in an access control string, they
MAY be transcribed into the userinfo field of the authority
([RFC3986], Section 3.2.1), security considerations (Section 5)
notwithstanding.

3.4.  Incompatible File Paths

Some conventional file path formats are known to be incompatible with
the file URI scheme.

3.4.1.  Namespaces

The Microsoft Windows API defines Win32 Namespaces [Win32-Namespaces]
for interacting with files and devices using Windows API functions.
These namespaced paths are prefixed by "\\?\" for Win32 File
Namespaces and "\\.\" for Win32 Device Namespaces.  There is also a
special case for UNC file paths in Win32 File Namespaces, referred to
as "Long UNC", using the prefix "\\?\UNC\".

This specification does not define a mechanism for translating
namespaced paths to or from file URIs.

4.  Encoding

The encoding of a file URI depends on the file system.  If the file
system uses a known non-Unicode character encoding, the path SHOULD

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be converted to a sequence of characters from the Universal Character
Set [ISO10646] normalized according to Normalization Form C (NFC)
[UTR15], before being translated to a file URI, and conversely a file
URI SHOULD be converted back to the file system's native encoding
when translating to a file path.

Note that many modern file systems encode directory and file names
as arbitrary sequences of octets.  In those cases, the
representation as an encoded string often depends on the user's
localization settings, or defaults to UTF-8 [STD63].

When the file system's encoding is not known the file URI SHOULD be
transported as an Internationalized Resource Identifier (IRI)
[RFC3987].

Example: file IRI:

| Bytes of file IRI in a UTF-8 document:
|    66 69 6c 65 3a 43 3a 2f 72 65 c3 a7 75 2e 74 78 74
|    f  i  l  e  :  c  :  /  r  e  ( c ) u  .  t  x  t
|
| Interpretation:
|    A file named "recu.txt" with a cedilla on the "c", in the
|    directory "C:\" of a DOS or Windows file system.
|
| Character value sequences of file paths, for various file system
| encodings:
|
|  o UTF-16 (e.g. NTFS):
|       0043 003a 005c 0072 0065 00e7 0075 002e 0074 0078 0074
|
|  o Codepage 437 (e.g. MS-DOS):
|       43   3a   5c   72   65   87   75   2e   74   78   74

Counter-example: ambiguous file URI:

| File URI, in any ASCII-compatible document:
|    "file:///%E3%81%A1"
|
| Possible interpretations of the file name, depending on the
| (unknown) encoding of the file system:
|
|  o UTF-8:
|       <HIRAGANA LETTER TI (U+3061)>
|

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|
|  o Codepage 437:
|       <GREEK SMALL LETTER PI (U+03C0)> +
|       <LATIN SMALL LETTER U WITH DIAERESIS (U+00FC)> +
|       <LATIN SMALL LETTER I WITH ACUTE (U+00ED)>
|
|  o EBCDIC:
|       "Ta~"
|
|  o US-ASCII:
|       "%E3%81%A1"
|
| etc.

5.  Security Considerations

There are many security considerations for URI schemes discussed in
[RFC3986].

File access and the granting of privileges for specific operations
are complex topics, and the use of file URIs can complicate the
security model in effect for file privileges.  Software using file
URIs MUST NOT grant greater access than would be available for other
file access methods.

Additionally, as discussed in the HP OpenVMS Systems Documentation
http://h71000.www7.hp.com/doc/84final/ba554_90015/ch03s09.html
"access control strings include sufficient information to allow
someone to break in to the remote account, [therefore] they create
serious security exposure."  In a similar vein, the presence of a
password in a "user:password" userinfo field is deprecated by
[RFC3986].  As such, the userinfo field of a file URI, if present,
MUST NOT contain a password.

6.  IANA Considerations

IANA maintains the registry of URI Schemes [BCP115] at
http://www.iana.org/assignments/uri-schemes/ .

This document defines the following URI scheme, so the "Permanent URI
Schemes" registry has been updated accordingly.

+------------+--------------------------+-----------+
| URI Scheme | Description              | Reference |
+------------+--------------------------+-----------+
| file       | Host-specific file names | RFC XXXX  |
+------------+--------------------------+-----------+

RFC Editor Note: Replace XXXX with this RFC's reference.

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7.  Acknowledgements

This specification is derived from [RFC1738], [RFC3986], and
[I-D.draft-hoffman-file-uri] (expired); the acknowledgements in those
documents still apply.

Additional thanks to Dave Risney, author of the informative IE Blog
article http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-
windows.aspx , and Dave Thaler for their comments and suggestions.

8.  References

8.1.  Normative References

[BCP115]   Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35, RFC
4395, February 2006.

[ISO10646]
International Organization for Standardization,
"Information Technology - Universal Multiple-Octet Coded
Character Set (UCS)", ISO/IEC 10646:2003, December 2003.

[MS-DTYP]  Microsoft Open Specifications, "Windows Data Types, 2.2.56
UNC", January 2013,
<http://msdn.microsoft.com/en-us/library/gg465305.aspx>.

[MS-NBTE]  Microsoft Open Specifications, "NetBIOS over TCP (NBT)
Extensions", May 2014,
<http://msdn.microsoft.com/en-us/library/dd891412.aspx>.

[RFC1035]  Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.

[RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.

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[RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.

[RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

[RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers", RFC 6874, February 2013.

[UTR15]    Davis, M. and K. Whistler, "Unicode Normalization Forms",
August 2012.

8.2.  Informative References

[I-D.draft-hoffman-file-uri]
Hoffman, P., "The file URI Scheme", draft-hoffman-file-
uri-03 (work in progress), January 2005.

[MS-SMB]   Microsoft Open Specifications, "Server Message Block (SMB)
Protocol", January 2013,
<http://msdn.microsoft.com/en-us/library/cc246231.aspx>.

[NOVELL]   Novell, "NetWare Core Protocols", 2013,
<http://www.novell.com/developer/ndk/
netware_core_protocols.html>.

[POSIX]    IEEE, "IEEE Std 1003.1, 2013 Edition", 2013.

[RFC1630]  Berners-Lee, T., "Universal Resource Identifiers in WWW: A
Unifying Syntax for the Expression of Names and Addresses
of Objects on the Network as used in the World-Wide Web",
RFC 1630, June 1994.

[RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.

[RFC3530]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
Beame, C., Eisler, M., and D. Noveck, "Network File System
(NFS) version 4 Protocol", RFC 3530, April 2003.

[RFC7231]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content", RFC 7231, June 2014.

[STD63]    Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

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Internet-Draft                 file-scheme                September 2014

[WHATWG-URL]
WHATWG, "URL Living Standard", May 2013,
<http://url.spec.whatwg.org/>.

[Win32-Namespaces]
Microsoft Developer Network, "Naming Files, Paths, and
Namespaces", June 2013.

Appendix A.  UNC Syntax

The UNC filespace selector string is a null-terminated sequence of
characters from the Universal Character Set [ISO10646].

The syntax of a UNC filespace selector string, as defined by
[MS-DTYP], is given here in Augmented Backus-Naur Form (ABNF)
[RFC5234] for convenience:

UNC = "\\" hostname "\" sharename *( "\" objectname )
hostname   = netbios-name / fqdn / ip-address
sharename  = <name of share or resource to be accessed>
objectname = <depends on resource being accessed>

o  "netbios-name" from [MS-NBTE], Section 2.2.1.

o  "fqdn" from [RFC1035] or [RFC1123]

o  "ip-address" from Section 2.1 of [RFC1123], or Section 2.2 of
[RFC4291].

The precise format of "sharename" depends on the protocol; see: SMB
[MS-SMB], NFS [RFC3530], NCP [NOVELL].

Matthew Kerwin
QUT

Email: matthew.kerwin@qut.edu.au

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