Network Working Group J. Reschke
Internet-Draft greenbytes
Intended status: Standards Track February 21, 2010
Expires: August 25, 2010
Application of RFC 2231 Encoding to
Hypertext Transfer Protocol (HTTP) Header Fields
draft-reschke-rfc2231-in-http-10
Abstract
By default, message header field parameters in Hypertext Transfer
Protocol (HTTP) messages can not carry characters outside the ISO-
8859-1 character set. RFC 2231 defines an escaping mechanism for use
in Multipurpose Internet Mail Extensions (MIME) headers. This
document specifies a profile of that encoding suitable for use in
HTTP header fields.
Editorial Note (To be removed by RFC Editor before publication)
There are multiple HTTP header fields that already use RFC 2231
encoding in practice (Content-Disposition) or might use it in the
future (Link). The purpose of this document is to provide a single
place where the generic aspects of RFC 2231 encoding in HTTP header
fields are defined.
Distribution of this document is unlimited. Although this is not a
work item of the HTTPbis Working Group, comments should be sent to
the Hypertext Transfer Protocol (HTTP) mailing list at
ietf-http-wg@w3.org [1], which may be joined by sending a message
with subject "subscribe" to ietf-http-wg-request@w3.org [2].
Discussions of the HTTPbis Working Group are archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>.
XML versions, latest edits and the issues list for this document are
available from
<http://greenbytes.de/tech/webdav/#draft-reschke-rfc2231-in-http>. A
collection of test cases is available at
<http://greenbytes.de/tech/tc2231/>.
Note: as of January 2010, there were at least three independent
implementations of the encoding defined in Section 3.2: Konqueror
(trunk), Mozilla Firefox, and Opera.
Status of this Memo
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Copyright Notice
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4
3. A Profile of RFC 2231 for Use in HTTP . . . . . . . . . . . . 4
3.1. Parameter Continuations . . . . . . . . . . . . . . . . . 5
3.2. Parameter Value Character Set and Language Information . . 5
3.2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Language specification in Encoded Words . . . . . . . . . 8
4. Guidelines for Usage in HTTP Header Field Definitions . . . . 8
4.1. When to Use the Extension . . . . . . . . . . . . . . . . 9
4.2. Error Handling . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Using Multiple Instances for Internationalization . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Document History and Future Plans (to be removed
by RFC Editor before publication) . . . . . . . . . . 12
Appendix B. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 12
B.1. Since draft-reschke-rfc2231-in-http-00 . . . . . . . . . . 12
B.2. Since draft-reschke-rfc2231-in-http-01 . . . . . . . . . . 12
B.3. Since draft-reschke-rfc2231-in-http-02 . . . . . . . . . . 13
B.4. Since draft-reschke-rfc2231-in-http-03 . . . . . . . . . . 13
B.5. Since draft-reschke-rfc2231-in-http-04 . . . . . . . . . . 13
B.6. Since draft-reschke-rfc2231-in-http-05 . . . . . . . . . . 13
B.7. Since draft-reschke-rfc2231-in-http-06 . . . . . . . . . . 13
B.8. Since draft-reschke-rfc2231-in-http-07 . . . . . . . . . . 13
B.9. Since draft-reschke-rfc2231-in-http-08 . . . . . . . . . . 13
B.10. Since draft-reschke-rfc2231-in-http-09 . . . . . . . . . . 13
Appendix C. Resolved issues (to be removed by RFC Editor
before publication) . . . . . . . . . . . . . . . . . 13
C.1. rfc2978-normative . . . . . . . . . . . . . . . . . . . . 13
C.2. rfc3986-normative . . . . . . . . . . . . . . . . . . . . 14
C.3. usascii-normative . . . . . . . . . . . . . . . . . . . . 14
Appendix D. Open issues (to be removed by RFC Editor prior to
publication) . . . . . . . . . . . . . . . . . . . . 14
D.1. edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
D.2. parameter-abnf . . . . . . . . . . . . . . . . . . . . . . 14
D.3. iso8859 . . . . . . . . . . . . . . . . . . . . . . . . . 15
D.4. when-ext-value . . . . . . . . . . . . . . . . . . . . . . 15
D.5. i18n-spoofing . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
By default, message header field parameters in HTTP ([RFC2616])
messages can not carry characters outside the ISO-8859-1 character
set ([ISO-8859-1]). RFC 2231 (Appendix of [RFC2231]) defines an
escaping mechanism for use in MIME headers. This document specifies
a profile of that encoding for use in HTTP header fields.
Note: this profile does not apply to message payloads transmitted
over HTTP, such as when using the media type "multipart/form-data"
([RFC2388]).
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].
This specification uses the ABNF (Augmented Backus-Naur Form)
notation defined in [RFC5234]. The following core rules are included
by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters),
DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f) and LWSP
(linear white space).
Note that this specification uses the term "character set" for
consistency with other IETF specifications such as RFC 2277 (see
[RFC2277], Section 3). A more accurate term would be "character
encoding" (a mapping of code points to octet sequences).
3. A Profile of RFC 2231 for Use in HTTP
RFC 2231 defines several extensions to MIME. The sections below
discuss if and how they apply to HTTP.
In short:
o Parameter Continuations aren't needed (Section 3.1),
o Character Set and Language Information are useful, therefore a
simple subset is specified (Section 3.2), and
o Language Specifications in Encoded Words aren't needed
(Section 3.3).
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3.1. Parameter Continuations
Section 3 of [RFC2231] defines a mechanism that deals with the length
limitations that apply to MIME headers. These limitations do not
apply to HTTP ([RFC2616], Section 19.4.7).
Thus in HTTP, senders MUST NOT use parameter continuations, and
therefore recipients do not need to support them.
3.2. Parameter Value Character Set and Language Information
Section 4 of [RFC2231] specifies how to embed language information
into parameter values, and also how to encode non-ASCII characters,
dealing with restrictions both in MIME and HTTP header parameters.
However, RFC 2231 does not specify a mandatory-to-implement character
set, making it hard for senders to decide which character set to use.
Thus, recipients implementing this specification MUST support the
character sets "ISO-8859-1" [ISO-8859-1] and "UTF-8" [RFC3629].
Furthermore, RFC 2231 allows leaving out the character set
information. The profile defined by this specification does not
allow that.
The syntax for parameters is defined in Section 3.6 of [RFC2616]
(with RFC 2616 implied LWS translated to RFC 5234 LWSP):
parameter = attribute LWSP "=" LWSP value
attribute = token
value = token / quoted-string
quoted-string = <quoted-string, defined in [RFC2616], Section 2.2>
token = <token, defined in [RFC2616], Section 2.2>
This specification extends the grammar to:
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parameter = reg-parameter / ext-parameter
reg-parameter = attribute LWSP "=" LWSP value
ext-parameter = attribute "*" LWSP "=" LWSP ext-value
ext-value = charset "'" [ language ] "'" value-chars
; extended-initial-value,
; defined in [RFC2231], Section 7
charset = "UTF-8" / "ISO-8859-1" / mime-charset
mime-charset = 1*mime-charsetc
mime-charsetc = ALPHA / DIGIT
/ "!" / "#" / "$" / "%" / "&"
/ "+" / "-" / "^" / "_" / "`"
/ "{" / "}" / "~"
; as <mime-charset> in Section 2.3 of [RFC2978]
; except that the single quote is not included
language = <Language-Tag, defined in [RFC5646], Section 2.1>
value-chars = *( pct-encoded / attr-char )
pct-encoded = "%" HEXDIG HEXDIG
; see [RFC3986], Section 2.1
attr-char = ALPHA / DIGIT
/ "!" / "#" / "$" / "&" / "+" / "-" / "."
/ "^" / "_" / "`" / "|" / "~"
; token except ( "*" / "'" / "%" )
Thus, a parameter is either regular parameter (reg-parameter), as
previously defined in Section 3.6 of [RFC2616], or an extended
parameter (ext-parameter).
Extended parameters are those where the left hand side of the
assignment ends with an asterisk character.
The value part of an extended parameter (ext-value) is a token that
consists of three parts: the REQUIRED character set name (charset),
the OPTIONAL language information (language), and a character
sequence representing the actual value (value-chars), separated by
single quote characters. Note that both character set names and
language tags are restricted to the US-ASCII character set, and are
matched case-insensitively (see [RFC2978], Section 2.3 and [RFC5646],
Section 2.1.1).
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Inside the value part, characters not contained in attr-char are
encoded into an octet sequence using the specified character set.
That octet sequence then is percent-encoded as specified in Section
2.1 of [RFC3986].
Producers MUST NOT use character sets other than "UTF-8" ([RFC3629])
or "ISO-8859-1" ([ISO-8859-1]). Extension character sets (ext-
charset) are reserved for future use.
Note: recipients should be prepared to handle encoding errors,
such as malformed or incomplete percent escape sequences, or non-
decodable octet sequences, in a robust manner. This specification
does not mandate any specific behavior, for instance the following
strategies are all acceptable:
* ignoring the parameter,
* stripping a non-decodable octet sequence,
* substituting a non-decodable octet sequence by a replacement
character, such as the Unicode character U+FFFD (Replacement
Character).
Note: the RFC 2616 token production ([RFC2616], Section 2.2)
differs from the production used in RFC 2231 (imported from
Section 5.1 of [RFC2045]) in that curly braces ("{" and "}") are
excluded. Thus, these two characters are excluded from the attr-
char production as well.
Note: the <mime-charset> ABNF defined here differs from the one in
Section 2.3 of [RFC2978] in that it does not allow the single
quote character (see also RFC Editor Errata ID 1912 [3]). In
practice, no character set names using that character have been
registered at the time of this writing.
3.2.1. Examples
Non-extended notation, using "token":
foo: bar; title=Economy
Non-extended notation, using "quoted-string":
foo: bar; title="US-$ rates"
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Extended notation, using the unicode character U+00A3 (POUND SIGN):
foo: bar; title*=iso-8859-1'en'%A3%20rates
Note: the Unicode pound sign character U+00A3 was encoded using ISO-
8859-1 into the single octet A3, then percent-encoded. Also note
that the space character was encoded as %20, as it is not contained
in attr-char.
Extended notation, using the unicode characters U+00A3 (POUND SIGN)
and U+20AC (EURO SIGN):
foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
Note: the unicode pound sign character U+00A3 was encoded using UTF-8
into the octet sequence C2 A3, then percent-encoded. Likewise, the
unicode euro sign character U+20AC was encoded into the octet
sequence E2 82 AC, then percent-encoded. Also note that HEXDIG
allows both lower-case and upper-case character, so recipients must
understand both, and that the language information is optional, while
the character set is not.
3.3. Language specification in Encoded Words
Section 5 of [RFC2231] extends the encoding defined in [RFC2047] to
also support language specification in encoded words. Although the
HTTP/1.1 specification does refer to RFC 2047 ([RFC2616], Section
2.2), it's not clear to which header field exactly it applies, and
whether it is implemented in practice (see
<http://tools.ietf.org/wg/httpbis/trac/ticket/111> for details).
Thus, the RFC 2231 profile defined by this specification does not
include this feature.
4. Guidelines for Usage in HTTP Header Field Definitions
Specifications of HTTP header fields that use the extensions defined
in Section 3.2 should clearly state that. A simple way to achieve
this is to normatively reference this specification, and to include
the ext-value production into the ABNF for that header field.
For instance:
foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
title-param = "title" LWSP "=" LWSP value
/ "title*" LWSP "=" LWSP ext-value
ext-value = <see RFCxxxx, Section 3.2>
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[[rfcno: Note to RFC Editor: in the figure above, please replace
"xxxx" by the RFC number assigned to this specification.]]
4.1. When to Use the Extension
Section 4.2 of [RFC2277] requires that protocol elements containing
text are able to carry language information. Thus, the ext-value
production should always be used when the parameter value is of
textual nature.
Furthermore, the extension should also be used whenever the parameter
value needs to carry characters not present in the US-ASCII
([USASCII]) character set (note that it would be unacceptable to
define a new parameter that would be restricted to a subset of the
Unicode character set).
4.2. Error Handling
Header specifications that include parameters should also specify
whether same-named parameters can occur multiple times. If
repetitions are not allowed (and this is believed to be the common
case), the specification should state whether regular or the extended
syntax takes precedence. In the latter case, this could be used by
producers to use both formats without breaking recipients that do not
understand the syntax.
Example:
foo: bar; title="EURO exchange rates";
title*=utf-8''%e2%82%ac%20exchange%20rates
In this case, the sender provides an ASCII version of the title for
legacy recipients, but also includes an internationalized version for
recipients understanding this specification -- the latter obviously
should prefer the new syntax over the old one.
Note: at the time of this writing, many implementations failed to
ignore the form they do not understand, or prioritize the ASCII
form although the extended syntax was present.
4.3. Using Multiple Instances for Internationalization
It is expected that in many cases, internationalization of parameters
in response headers is implemented using server driven content
negotiation ([RFC2616], Section 12.1) using the Accept-Language
header ([RFC2616], Section 14.4). However, the format described in
this specification also allows using multiple instances providing
multiple languages in a single header. Specifications that want to
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take advantage of this should clearly specify the expected processing
by the recipient.
Example:
foo: bar; title*=utf-8'en'Document%20Title;
title*=utf-8'de'Titel%20des%20Dokuments
5. Security Considerations
This document does not discuss security issues and is not believed to
raise any security issues not already endemic in HTTP.
6. IANA Considerations
There are no IANA Considerations related to this specification.
7. Acknowledgements
Thanks to Martin Duerst and Frank Ellermann for help figuring out
ABNF details, to Graham Klyne and Alexey Melnikov for general review,
and to Benjamin Carlyle and Roar Lauritzsen for implementer's
feedback.
8. References
8.1. Normative References
[ISO-8859-1]
International Organization for Standardization,
"Information technology -- 8-bit single-byte coded graphic
character sets -- Part 1: Latin alphabet No. 1", ISO/
IEC 8859-1:1998, 1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, October 2000.
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[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", RFC 3986,
STD 66, January 2005.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, September 2009.
[USASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
8.2. Informative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, November 1996.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions:
Character Sets, Languages, and Continuations", RFC 2231,
November 1997.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC2388] Masinter, L., "Returning Values from Forms: multipart/
form-data", RFC 2388, August 1998.
URIs
[1] <mailto:ietf-http-wg@w3.org>
[2] <mailto:ietf-http-wg-request@w3.org?subject=subscribe>
[3] <http://www.rfc-editor.org/errata_search.php?eid=1912>
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Appendix A. Document History and Future Plans (to be removed by RFC
Editor before publication)
Problems with the internationalization of the HTTP Content-
Disposition header field have been known for many years (see test
cases at <http://greenbytes.de/tech/tc2231/>).
During IETF 72
(<http://tools.ietf.org/wg/httpbis/minutes?item=minutes72.html>), the
HTTPbis Working Group shortly discussed how to deal with the
underspecification of (1) Content-Disposition, and its (2)
internationalization aspects. Back then, there was rough consensus
in the room to move the definition into a separate draft.
This specification addresses problem (2), by defining a simple subset
of the encoding format defined in RFC 2231. A separate
specification, draft-reschke-rfc2183-in-http, is planned to address
problem (1). Note that this approach was chosen because Content-
Disposition is just an example for an HTTP header field using this
kind of encoding. Another example is the currently proposed Link
header field (draft-nottingham-http-link-header).
This document is planned to be published on the IETF Standards Track,
so that other standards-track level documents can depend on it, such
as the new specification of Content-Disposition, or potentially
future revisions of the HTTP Link Header specification.
Also note that this document specifies a proper subset of the
extensions defined in RFC 2231, but does not normatively refer to it.
Thus, RFC 2231 can be revised separately, should the email community
decide to.
Appendix B. Change Log (to be removed by RFC Editor before publication)
B.1. Since draft-reschke-rfc2231-in-http-00
Use RFC5234-style ABNF, closer to the one used in RFC 2231.
Make RFC 2231 dependency informative, so this specification can
evolve independently.
Explain the ABNF in prose.
B.2. Since draft-reschke-rfc2231-in-http-01
Remove unneeded RFC5137 notation (code point vs character).
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B.3. Since draft-reschke-rfc2231-in-http-02
And and resolve issues "charset", "repeats" and "rfc4646".
B.4. Since draft-reschke-rfc2231-in-http-03
And and resolve issue "charsetmatch".
B.5. Since draft-reschke-rfc2231-in-http-04
Add and resolve issues "badseq" and "tokenquotcharset".
B.6. Since draft-reschke-rfc2231-in-http-05
Say "header field" instead of "header" in the context of HTTP.
B.7. Since draft-reschke-rfc2231-in-http-06
Add an appendix discussing document history and future plans, to be
removed before publication.
B.8. Since draft-reschke-rfc2231-in-http-07
Add and resolve issues "impl" and "rel-2388".
B.9. Since draft-reschke-rfc2231-in-http-08
Editorial improvements. Add and resolve issues "attrcharvstoken" and
"tokengrammar".
B.10. Since draft-reschke-rfc2231-in-http-09
Add issues "i18n-spoofing", "iso8859", "parameter-abnf", and "when-
ext-value". Add and resolve issues "rfc2978-normative", "rfc3986-
normative" and "usascii-normative".
Appendix C. Resolved issues (to be removed by RFC Editor before
publication)
Issues that were either rejected or resolved in this version of this
document.
C.1. rfc2978-normative
In Section 3.2:
Type: change
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julian.reschke@greenbytes.de (2010-02-20): The reference to RFC2978
needs to be normative (reported by Alexey Melnikov).
Resolution (2010-02-20): Done.
C.2. rfc3986-normative
In Section 3.2:
Type: change
julian.reschke@greenbytes.de (2010-02-20): The reference to percent-
encoding (RFC3986) needs to be normative (reported by Alexey
Melnikov).
Resolution (2010-02-20): Done.
C.3. usascii-normative
In Section 4.1:
Type: change
julian.reschke@greenbytes.de (2010-02-20): The reference to USASCII
needs to be normative.
Resolution (2010-02-20): Done.
Appendix D. Open issues (to be removed by RFC Editor prior to
publication)
D.1. edit
Type: edit
julian.reschke@greenbytes.de (2009-04-17): Umbrella issue for
editorial fixes/enhancements.
D.2. parameter-abnf
In Section 3.2:
Type: change
julian.reschke@greenbytes.de (2010-02-20): The ABNF for reg-parameter
and ext-parameter is ambiguous, as "*" is a valid token character;
furthermore, RFC 2616's "attribute" production allows "*" while RFC
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2231's does not. (reported by Alexey Melnikov).
julian.reschke@greenbytes.de (2010-02-21): Proposal: destrict the
allowable character set in parameter names to exclude "*" (amd maybe
even more non-name characters?). Also, consider extending the set of
value characters (for the right hand side) to allow more characters
that can be umabigouously parsed outside quoted strings, such as "/".
D.3. iso8859
In Section 3.2:
Type: change
julian.reschke@greenbytes.de (2010-02-20): The protocol could be
further simplified by mandating UTF-8 only (reported by Alexey
Melnikov). On the other hand and not surprinsingly, testing shows
that ISO-8859-1 support is widely implemented. The author is looking
for community feedback on this choice.
D.4. when-ext-value
In Section 4.1:
Type: change
julian.reschke@greenbytes.de (2010-02-18): There's no point in using
ext-value when the language is unknown and no "special" characters
are present.
D.5. i18n-spoofing
In Section 5:
Type: change
<http://www.ietf.org/mail-archive/web/apps-discuss/current/
msg01329.html>
GK@ninebynine.org (2010-02-20): I note that the security
considerations section says nothing about possible character
"spoofing" - i.e. making a displayed prompt or value appear to be
something other than it is. E.g. Non-ASCII characters have been
used to set up exploits involving dodgy URIs that may appear to a
user to be legitimate.
Reschke Expires August 25, 2010 [Page 15]
Internet-Draft RFC2231 Encoding in HTTP February 2010
Author's Address
Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
Reschke Expires August 25, 2010 [Page 16]
