Skip to main content

Parameter Value Encoding in iCalendar and vCard

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6868.
Author Cyrus Daboo
Last updated 2013-01-10 (Latest revision 2013-01-08)
RFC stream Internet Engineering Task Force (IETF)
Stream WG state (None)
Document shepherd (None)
IESG IESG state IESG Evaluation::Revised I-D Needed
Consensus boilerplate Unknown
Telechat date (None)
Needs a YES. Needs 10 more YES or NO OBJECTION positions to pass.
Responsible AD Barry Leiba
Send notices to,,
Network Working Group                                           C. Daboo
Internet-Draft                                                     Apple
Updates: 5545,6321,6350,6351                             January 8, 2013
(if approved)
Intended status: Standards Track
Expires: July 12, 2013

            Parameter Value Encoding in iCalendar and vCard


   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.

   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

   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 July 12, 2013.

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
   ( 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.

Daboo                     Expires July 12, 2013                 [Page 1]
Internet-Draft             Parameter Encoding               January 2013

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  . . . . . . . . . . . . . . . . . . . . . . 5
   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)  . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 7

Daboo                     Expires July 12, 2013                 [Page 2]
Internet-Draft             Parameter Encoding               January 2013

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

   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",
   "OPTIONAL" in this document are to be interpreted as described in

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 used, the combination of ^ plus one other character is
   interpreted as a single character as per the table below:

Daboo                     Expires July 12, 2013                 [Page 3]
Internet-Draft             Parameter Encoding               January 2013

           | Escape Sequence     | Character                  |
           | ^' (U+005E, U+0027) | U+0022 (Quotation Mark)    |
           | ^n (U+005E, U+006E) | U+000A (Line Feed)         |
           | ^^ (U+005E, U+005E) | U+005E (Circumflex Accent) |

   When generating iCalendar or vCard parameter values, the characters
   in the second column above MUST be escaped using the sequence from
   the first column.  When parsing iCalendar or vCard parameter values,
   the two character sequence from the first column MUST be replaced by
   the single character from the second column.  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.

   In addition to the escaping procedure outlined above, text parameter
   values MUST also have line break character sequences canonicalized
   into a single line feed (U+000A) character when the parameter value
   is generated.  Specifically, any single carriage return character
   (U+000D) or the two character combination of carriage return followed
   by line feed (U+000D, U+000A) MUST be mapped to ^n in the generated
   parameter value.  When parsing a parameter value with ^n, clients can
   replace that with the appropriate line break character sequence for
   the system they are running on.

   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^'

   The unescaped parameter value is

   George Herman "Babe" Ruth

Daboo                     Expires July 12, 2013                 [Page 4]
Internet-Draft             Parameter Encoding               January 2013

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

   GEO;X-ADDRESS="Pittsburgh Pirates^n115 Federal St^nPitt
    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, and
   Simon Perreault 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,

Daboo                     Expires July 12, 2013                 [Page 5]
Internet-Draft             Parameter Encoding               January 2013

              August 2011.

   [RFC6351]  Perreault, S., "xCard: vCard XML Representation",
              RFC 6351, August 2011.

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"

   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

   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

Daboo                     Expires July 12, 2013                 [Page 6]
Internet-Draft             Parameter Encoding               January 2013

Appendix B.  Change History (To be removed by RFC Editor before

   Changes in -03:

   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

   Changes in -01:

   1.  Tweaked vCard example line-folding.

Author's Address

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


Daboo                     Expires July 12, 2013                 [Page 7]