Internet Engineering Task Force (IETF) C. Daboo

Internet Engineering Task Force (IETF) C. Daboo
Request for Comments: 6868 Apple
Updates: 5545, 6321, 6350, 6351 February 2013
Category: Standards Track
ISSN: 2070-1721

Parameter Value Encoding in iCalendar and vCard

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 is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6868.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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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.

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RFC 6868 Parameter Encoding February 2013

Table of Contents

1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. Parameter Value Encoding Scheme .................................3
3.1. iCalendar Example ..........................................4
3.2. vCard Example ..............................................4
4. Security Considerations .........................................4
5. Acknowledgments .................................................4
6. Normative References ............................................5
Appendix A. Choice of Quoting Mechanism ............................6

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" that 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 characters 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 minimize 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].

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RFC 6868 Parameter Encoding February 2013

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
apply:

o formatted text line breaks are encoded into ^n (U+005E, U+006E)

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
apply:

o the character sequence ^n (U+005E, U+006E) is decoded into an
appropriate formatted line break according to the type of system
being used

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 character other 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.

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3.1. iCalendar Example

The following example is 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:[email protected]

The unescaped parameter value is

George Herman "Babe" Ruth

3.2. vCard Example

The following example is a "GEO" property with an "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
sburgh, PA 15212":geo:40.446816,-80.00566

The unescaped parameter value (where each line is terminated by a
line break character sequence) is

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. Acknowledgments

Thanks to Michael Angstadt, Tim Bray, Mike Douglass, Barry Leiba,
Simon Perreault, and Pete Resnick for feedback on this specification.

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6. 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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RFC 6868 Parameter Encoding February 2013

Appendix A. Choice of Quoting Mechanism

Having recognized the need for escaping parameter 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:[email protected]

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, and 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 implementation 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.

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RFC 6868 Parameter Encoding February 2013

Author's Address

Cyrus Daboo
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA

EMail: [email protected]
URI: http://www.apple.com/

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