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WEBDAV Working Group                                          J. Reschke
Internet-Draft                                                greenbytes
Expires: May 17, 2006                                  November 13, 2005


   Web Distributed Authoring and Versioning (WebDAV) URL constraints
                draft-reschke-webdav-url-constraints-00

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   Both WebDAV servers and clients frequently map URI-escaped characters
   inside a path segment to non-ASCII characters.  These mappings can
   only be interoperable if there is a consensus about the appropriate
   character encoding.  This document specifies a default encoding that
   is compatible with both the recommendations for URIs in HTML content
   and the "Internationalized Resource Identifiers" (IRI) specification.

   Furthermore, servers that implement a mapping to locally constrained
   names frequently do not support specific names, or silently map



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   "similar" names to the same resource (for instance when content is
   stored in a filesystem that is case-preserving, but not case-
   sensitive).  For these cases, discovery and error signalling features
   are defined.

Editorial Note

   Distribution of this document is unlimited.  Please send comments to
   the Distributed Authoring and Versioning (WebDAV) working group at
   w3c-dist-auth@w3.org [1], which may be joined by sending a message
   with subject "subscribe" to w3c-dist-auth-request@w3.org [2].

   Discussions of the WEBDAV working group are archived at URL:
   <http://lists.w3.org/Archives/Public/w3c-dist-auth/>.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Name to URL segment mapping  . . . . . . . . . . . . . . . . .  3
   5.  Server Considerations  . . . . . . . . . . . . . . . . . . . .  4
     5.1   Overview of common mapping methods . . . . . . . . . . . .  4
       5.1.1   Mapping URL segments to byte sequences . . . . . . . .  4
       5.1.2   Mapping URL segments to character sequences  . . . . .  5
       5.1.3   Identity mapping . . . . . . . . . . . . . . . . . . .  5
     5.2   Caveats  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Client Considerations  . . . . . . . . . . . . . . . . . . . .  6
   7.  Additional Method Semantics  . . . . . . . . . . . . . . . . .  6
     7.1   Additional Preconditions . . . . . . . . . . . . . . . . .  6
       7.1.1   DAV:name-allowed precondition  . . . . . . . . . . . .  6
   8.  Compatibility Considerations . . . . . . . . . . . . . . . . .  6
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   10.   Internationalization Considerations  . . . . . . . . . . . .  6
   11.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  7
   12.   References . . . . . . . . . . . . . . . . . . . . . . . . .  7
     12.1  Normative References . . . . . . . . . . . . . . . . . . .  7
     12.2  Informative References . . . . . . . . . . . . . . . . . .  7
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  8
       Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 10










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1.  Introduction

   Both WebDAV servers and clients frequently map URI-escaped characters
   (see [RFC3986]) inside a path segment to non-ASCII characters.  These
   mappings can only be interoperable if there is a consensus about the
   appropriate character encoding.  This document specifies a default
   encoding that is compatible with both the recommendations for URIs in
   HTML content (see [HTML], Appendix B.2.1) and the IRI specification
   [RFC3987].

   Furthermore, servers that implement a mapping to locally constrained
   names frequently do not support specific names, or silently map
   "similar" names to the same resource (for instance when content is
   stored in a filesystem that is case-preserving, but not case-
   sensitive).  For these cases, discovery and error signalling features
   are defined.

2.  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].

3.  Terminology

   The terminology used here follows that in WebDAV [RFC2518], HTTP
   [RFC2616] and "Versioning Extensions to WebDAV" [RFC3253].
   Definitions of the terms resource, Uniform Resource Identifier (URI),
   and Uniform Resource Locator (URL) are provided in [RFC3986].

   This document uses the terms "precondition" and "postcondition" as
   defined in [RFC3253].  Servers SHOULD report pre-/postcondition
   failures as described in section 1.6 of this document.

4.  Name to URL segment mapping

   In proposing a common mapping, the following requirements were taken
   into account:

   R1 For URL characters inside the US-ASCII range (0..127), the mapping
      should be the identity mapping.

   R2 The mapping should provide support for all characters defined in
      the Unicode character set.

   The only widely-deployed character encoding fulfilling these
   requirements is the UTF-8 character decoding, defined in [RFC3629].
   Consequently, it's also the encoding recommended for URLs in HTML



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   content ([HTML], Appendix B.2.1) and for IRIs ([RFC3987]).

   Therefore, clients and servers SHOULD use the UTF-8 character
   encoding to map non-ASCII characters to/from character sequences in
   URL segments.

5.  Server Considerations

   When mapping HTTP URL segments (see [RFC3986], section 3.3) to local
   storage, the server's behaviour usually depends on the API used to
   access that storage.  In practice, two styles are widely deployed:
   binary and character-based.  The sections below discuss the
   implications of each and also describe an "identity" mapping.

5.1  Overview of common mapping methods

5.1.1  Mapping URL segments to byte sequences

   A typical scenario for this case is when the server does a direct
   mapping between URLs and objects in a filesystem, and the filesystem
   uses filenames based on byte sequences.  This is the case for typical
   Unix filesystem implementations.

   In this case, mapping between URL segments and local names is
   straightforward:

   o  To map from URL segments, just apply URL unescaping to obtain a
      byte sequence (see [RFC3986], section 2.1)

   o  To map to URL segments, just apply URL escaping to obtain a
      sequence of characters suitable for use in a URL segment

   The advantage of this simple mapping is that it faithfully stores
   whatever the original URL contained.  On the other hand, this is a
   binary encoding, and programs that display filenames usually have to
   map the byte sequence to a character sequence for display.  Unless
   both character encodings match, the results will be either inaccurate
   (incorrect characters) or the display function will break completely
   (for instance when an attempt is made to UTF-8-decode a byte stream
   that was originally encoded using an incompatible encoding such as
   ISO-8859-1).

   Things get even more complicated when there is no single character
   encoding being used on the server.  For instance, in a Unix system
   multiple users may use different character encodings for filenames.
   However, the filesystem does not preserve information about what
   character encoding the filename was encoded with; thus, depending on
   their "locale" settings, different users will see different names for



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   the same filesystem object.

5.1.2  Mapping URL segments to character sequences

   This scenario is similar to the one discussed in the previous section
   (5.1.1).  For instance it occurs when objects are stored locally in a
   way that allows Unicode characters in names, such as filenames in the
   Windows filesystem.

   However, in addition to the mapping to byte sequences, an additional
   mapping to a character sequence is required.  As discussed in
   Section 4, this mapping should use the UTF-8 character encoding
   ([RFC3629]).  Thus, here the mapping can be described as:

   o  To map from URL segments, apply URL unescaping to obtain a byte
      sequence (see [RFC3986], section 2.1), then UTF-8-decode to a
      sequence of characters.

   o  To map to URL segments, UTF-8-encode the character sequence to a
      sequence of bytes, then apply URL escaping to obtain a sequence of
      characters suitable for use in a URL segment


5.1.3  Identity mapping

   Finally, it's also possible to simply store the URL segments
   character by character, in which case no special mapping
   considerations apply.  Note that this approach may be inefficient in
   case the names contain many URL-escaped sequences (such as when asian
   characters have been encoded using UTF-8).

5.2  Caveats

   The non-trivial mappings have the common drawback that certain sets
   of legal HTTP URLs can not be mapped to local names (and therefore
   usually need to be rejected).  For the byte sequence mapping
   described in Section 5.1.1, this will usually be just the null
   character.

   However, when using the character mapping described in Section 5.1.2,
   whole Unicode character ranges may either be impossible to represent
   (such as when the underlying filesystem does only support a Unicode
   subset), or explicitly disallowed (such as non-normalized character
   sequences, see [CNORM], section 3.2).

   In cases like these, servers SHOULD reject operations that attempt to
   create those non-mappable URLs.  Appropriate precondition names are
   defined in Section 7.1.



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6.  Client Considerations

   In general, the mappings discussed in Section 5.1.2 apply to clients
   as well.  Whether a client maps segments to byte or character
   sequences usually depends on the platform it runs on, and what system
   layer it uses.  For instance, a filesystem driver for a Unix system
   usually will have to translate to byte sequences (because that's how
   many Unix system internally represent filenames).

   However, if the client needs to do any mapping it all, there may be
   sitations where parts of a URL segment can't be mapped to what the
   client needs internally.  In cases like these, it is recommended that
   the client signals the problem, and provides a way to repair the
   problem (such as renaming the resource).

7.  Additional Method Semantics

7.1  Additional Preconditions

7.1.1  DAV:name-allowed precondition

   The name specified by the HTTP request as path segment is available
   for use as a new binding name (see [draft-ietf-webdav-bind], section
   4 and 6).

8.  Compatibility Considerations

   Servers that use a non-identity mapping may not be able to create new
   resources with the URLs specified by the client (such as in an MKCOL
   or a PUT request).

   Clients that use a non-identity mapping may not be able to handle all
   URLs returned by a server (such as a result of a PROPFIND request).

9.  Security Considerations

   All of the security considerations of HTTP/1.1 and the WebDAV
   Distributed Authoring Protocol specification also apply to this
   protocol specification.

   _TBD: add notes about the inherent security risks when a backend
   storage maps multiple notations to the same physical object (file),
   think uppercase/lowercase, trailing blanks/dots, resolution of
   relative paths ("./", "../")._

10.  Internationalization Considerations

   All internationalization considerations mentioned in [RFC2518] also



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   apply to this document.

11.  IANA Considerations

   There are no IANA Considerations.

12.  References

12.1  Normative References

   [HTML]     Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", W3C REC REC-html401-19991224,
              December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

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

   [RFC2518]  Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
              Jensen, "HTTP Extensions for Distributed Authoring --
              WEBDAV", RFC 2518, February 1999.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3253]  Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J.
              Whitehead, "Versioning Extensions to WebDAV", RFC 3253,
              March 2002.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 3629, STD 63, November 2003.

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

12.2  Informative References

   [CNORM]    Duerst, M., Yergau, F., Ishida, R., Wolf, M., Texin, T.,
              and A. Phillips, "Character Model for the World Wide Web
              1.0: Normalization", W3C WD-charmod-norm-20040225,
              February 2004,
              <http://www.w3.org/TR/2004/WD-charmod-norm-20040225>.

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




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   [draft-ietf-webdav-bind]
              Clemm, G., Crawford, J., Reschke, J., and J. Whitehead,
              "Binding Extensions to Web Distributed Authoring and
              Versioning (WebDAV)", draft-ietf-webdav-bind-12 (work in
              progress), July 2005, <http://greenbytes.de/tech/webdav/
              draft-ietf-webdav-bind-12.html>.

URIs

   [1]  <mailto:w3c-dist-auth@w3.org>

   [2]  <mailto:w3c-dist-auth-request@w3.org?subject=subscribe>


Author's Address

   Julian F. Reschke
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   Phone: +49 251 2807760
   Fax:   +49 251 2807761
   Email: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/

























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Index

   C
      Condition Names
         DAV:name-allowed (pre)  6

   D
      DAV:name-allowed precondition  6











































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