Internet DRAFT - draft-daboo-ical-vcard-parameter-encoding
draft-daboo-ical-vcard-parameter-encoding
Network Working Group C. Daboo
Internet-Draft Apple
Updates: 5545,6321,6350,6351 January 10, 2013
(if approved)
Intended status: Standards Track
Expires: July 14, 2013
Parameter Value Encoding in iCalendar and vCard
draft-daboo-ical-vcard-parameter-encoding-04
Abstract
This specification updates the data formats for iCalendar (RFC 5545)
and vCard (RFC 6350) to allow parameter values to include certain
characters forbidden by the existing specifications.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 14, 2013.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. Parameter Value Encoding Scheme . . . . . . . . . . . . . . . . 3
3.1. iCalendar Example . . . . . . . . . . . . . . . . . . . . . 4
3.2. vCard Examples . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Choice of Quoting Mechanism . . . . . . . . . . . . . 6
Appendix B. Change History (To be removed by RFC Editor
before publication) . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
The iCalendar [RFC5545] specification defines a standard way to
describe calendar data. The vCard [RFC6350] specification defines a
standard way to describe contact data. Both of these use a similar
text-based data format. Each iCalendar and vCard data object can
include "properties" which have "parameters" and a "value". The
value of a "parameter" is typically a token or URI value, but a
"generic" text value is also allowed. However, the syntax rules for
both iCalendar and vCard prevent the use of a double-quote character
or control characters in such values, though double-quote and some
subset of control characters are allowed in the actual property
values.
As more and more extensions are being developed for these data
formats, there is a need to allow at least double-quotes and line
feeds to be included in parameter values. The \-escaping mechanism
used for property text values is not defined for use with parameter
values and cannot be easily used in a backwards compatible manner.
This specification defines a new character escaping mechanism,
compatible with existing parsers and chosen to minimise any impact on
existing data.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. Parameter Value Encoding Scheme
This specification defines the ^ character (U+005E - Circumflex
Accent) as an escape character in parameter values whose value type
is defined using the "param-value" syntax element (Section 3.1 of
iCalendar [RFC5545] and Section 3.3 of vCard [RFC6350]). The
^-escaping mechanism can be used when the value is either unquoted or
quoted (i.e., whether or not the value is surrounded by double-
quotes).
When generating iCalendar or vCard parameter values the following
applies:
o formatted text line breaks are encoded into ^n (U+005E, U+006E)
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o the ^ character (U+005E) is encoded into ^^ (U+005E, U+005E)
o the " character (U+0022) is encoded into ^' (U+005E, U+0027)
When parsing iCalendar or vCard parameter values the following
applies:
o the character sequence ^n (U+005E, U+006E) is decoded into an
appropriate formatted line break for the system being run on
o the character sequence ^^ (U+005E, U+005E) is decoded into the ^
character (U+005E)
o the character sequence ^' (U+005E, U+0027) is decoded into the "
character (U+0022)
o if a ^ (U+005E) character is followed by any other character than
the ones above, parsers MUST leave both the ^ and the following
character in place
When converting between iCalendar and vCard text-based data formats
and alternative data-format representations such as XML (as described
in [RFC6321] and [RFC6351] respectively), implementations MUST ensure
that parameter value escape sequences are generated correctly in the
text-based format and are decoded when the parameter values appear in
the alternate data formats.
3.1. iCalendar Example
An "ATTENDEE" property with a "CN" parameter whose value includes two
double-quote characters. The parameter value is not quoted as there
are no characters in the value that would trigger quoting as required
by iCalendar.
ATTENDEE;CN=George Herman ^'Babe^' Ruth:mailto:babe@example.com
The unescaped parameter value is
George Herman "Babe" Ruth
3.2. vCard Examples
A "GEO" property with a "X-ADDRESS" parameter whose value includes
several line feed characters. The parameter value is also quoted
since it contains a comma which triggers quoting as required by
vCard.
GEO;X-ADDRESS="Pittsburgh Pirates^n115 Federal St^nPitt
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sburgh, PA 15212":geo:40.446816,-80.00566
The unescaped parameter value is (where each line is terminated by a
line break character sequence)
Pittsburgh Pirates
115 Federal St
Pittsburgh, PA 15212
4. Security Considerations
There are no additional security issues beyond those of iCalendar
[RFC5545] and vCard [RFC6350].
5. IANA Considerations
There are none.
6. Acknowledgments
Thanks to Michael Angstadt, Tim Bray, Mike Douglass, Barry Leiba,
Simon Perreault, and Pete Resnick, for feedback on this
specification.
7. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5545] Desruisseaux, B., "Internet Calendaring and Scheduling
Core Object Specification (iCalendar)", RFC 5545,
September 2009.
[RFC6321] Daboo, C., Douglass, M., and S. Lees, "xCal: The XML
Format for iCalendar", RFC 6321, August 2011.
[RFC6350] Perreault, S., "vCard Format Specification", RFC 6350,
August 2011.
[RFC6351] Perreault, S., "xCard: vCard XML Representation",
RFC 6351, August 2011.
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Appendix A. Choice of Quoting Mechanism
Having recognized the need for escaping parameters values, the
question is what mechanism to use? One obvious choice would be to
adopt the \-escaping used for property values. However, that could
not be used as-is, because it escapes a double-quote as the sequence
of \ followed by double-quote. Consider what the example in
Section 3.1 might look like using \-escaping:
ATTENDEE;CN="George Herman \"Babe\" Ruth":mailto:babe@example.com
Existing iCalendar/vCard parsers know nothing about escape sequences
in parameters. So they would parse the parameter value as:
George Herman \
i.e. the text between the first and second occurrence of a double-
quote. However, the text after the second double-quote ought to be
either a : or a ; (to delimit the parameter value from the following
parameter or property) but is not - so the parser could legitimately
throw an error at that point because the data is syntactically
invalid. Thus for backwards compatibility reasons, a double-quote
cannot be escaped using a sequence that itself includes a double-
quote. Hence the choice of using a single-quote in this
specification.
Another option would be to use a form of \-escaping modified for use
in parameter values only. However, some incorrect, non-interoperable
use of \ in parameter values has been observed, and thus it is best
to steer clear of that to achieve guaranteed, reliable
interoperability. Also, given that double-quote gets changed to
single-quote in the escape sequence for a parameter, but not for a
value, it is better to not give the impression that the same escape
mechanism (and thus code) can be used for both (which could lead to
other issues such as an implementations incorrectly escaping a ; as
\; as opposed to quoting the parameter value).
The choice of ^ as the escape character was made based on the
requirement that an ASCII symbol (non-alphanumeric) character be
used, and it ought to be one least likely to be found in existing
data.
Appendix B. Change History (To be removed by RFC Editor before
publication)
Changes in -03:
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1. Tweak wording and change SHOULD to MUST for leaving ^+any other
character sequences unchanged.
2. Added paragraph on interaction with other data-format
representations such as XML.
Changes in -02:
1. Removed unnecessary HTAB escaping.
2. Added appendix to describe why ^ was chosen as the escape
mechanism.
Changes in -01:
1. Tweaked vCard example line-folding.
Author's Address
Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
Email: cyrus@daboo.name
URI: http://www.apple.com/
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