iCalendar in XML Format (xCal-Basic)

The Extended Markup Language (XML) as defined in [XML] is gaining widespread attention as a "web friendly" syntax for representing and exchanging documents and data on the Internet. This interest includes requests for and discussion of possible document type definitions (DTD) and name-space for IETF standard formats such as that defined by [iCAL]. This memo defines how XML can be used to represent iCalendar objects and does not specify a DTD as iCalendar can be extended. This format was selected to ease its translation back to the iCalendar format using an XSLT transform. (See project iCalendar on SourceForge.com - http://sourceforge.net/projects/icalendar/ ) NOTE: The [iCAL] is the definitive reference for the definition of iCalendar semantics. This memo only provides an alternative, XML representation for the standard syntax defined in [iCAL]. This memo does not introduce any semantics not already defined by [RFC 2445]. An attempt has been made to leverage the standard features of the XML syntax in order to represent the component iCalendar semantics. For example, strong data typing is specified using the XML notation declaration. The element type attributes are used to represent many of the calendar properties that provide a "global attribute" capability in an iCalendar object. Binary content in the ATTACH component property may either be specified through an external entity reference to a non-XML binary content or may be included in the XML document's content information, after first being encoding using the BASE64 scheme of [RFC 2146]. The publication of XML version 1.0 was followed by the publication of a World-wide Web Consortium (W3C) recommendation on "Namespaces in XML". A XML name-space is a collection of names, identified by a URI. In anticipation of the broader use of XML namespaces. Within this memo the term "xCal" will mean the XML namespace usage as described in this memo.

XML is a simplified version of the text markup syntax defined by ISO 8879, Standard Generalized Markup Language (SGML). XML was published as a proposed recommendation [XML] by the World-wide Web Consortium (W3C) on February 10, 1998. In iCalendar names can be in upper case, lower case, or mixed case. In xCal the predefined iCaledars names will be represented in lower case only as XML element and attribute names are case sensitive.. Values to properties and parameters that are user specified may be in upper, lower, or mixed case. All iCalendar component names will be represented in xCal as XML element names in lower case. The "BEGIN" iCalendar component will be represented in xCal as: <begin>. All iCalendar property names will be represented in xCal as XML element names in lower case. All iCalendar parameter names will be represented in xCal as XML attribute names in lower case. All iCalendar predefined names that are used as values will be represented in xCal in lower case. This example:

BEGIN:VCALENDAR VERSION:2.0 PRODID:-//hacksw/handcal//NONSGML v1.0//EN BEGIN:VEVENT DTSTART:19970714T170000Z DTEND:19970715T035959Z SUMMARY;LANGUAGE="en_US":Bastille Day Party END:VEVENT END:VCALENDAR Is represented in xCal as:

2.0 -//hacksw/handcal//NONSGML v1.0//EN 19970714T170000Z 19970715T035959Z

Bastille Day Party

]]>

This memo specifies the XML representation for the standard iCalendar format defined by [iCAL]. There are no XML dependencies other than the [XML] and the [XMLNS] recommendations.

This memo defines an alternative, XML representation for the standard iCalendar format defined in [iCAL]. This alternative representation provides the same semantics as that defined in the standard format. It is the goal of this memo to allow all [iCAL] extensions and modifications to be translated into and from this XML format.

The standard format can be converted to and from this XML format without loss of any calendaring information. When the XML representation was defined, every attempt was made to use existing component, property and parameter naming conventions. This greatly facilitates transformations between the two representations.

As previously indicated, conversion between the standard and XML representations of iCalendar is a straightforward process. In addition, mixed use of both representations is also possible using MIME objects. With the use of the MIME multipart content-types, compound MIME entities containing a mix of the standard and XML representations can be specified. This capability is useful in applications where both representations might be encountered. In addition, this capability demonstrates the isomeric nature of the two representations. XML applications conforming to this specification MUST be able to properly parse and process a MIME multipart entity containing the MIME type associated with this iCalendar XML document type. Internet applications conforming to this memo MUST only send the iCalendar XML document in a "multipart/alternative" MIME entity that also contains an equivalent iCalendar object in the standard format defined by [RFC2445]. This restriction will guarantee that the iCalendar object can also be processed by Internet applications that only support the standard iCalendar representation.

Strong "data typing" is an integral design principle to the iCalendar format. Strong data typing in iCalendar means that the format type for each property value is well known. Within [iCAL], the data type is called the "value type". The standard format defined by [RFC 2445] specifies a default value type for each calendar and component property. In addition, many of the property definitions allow for the specification of alternate value types. This XML representation continues this design principle. Explicit value/data typing in the XML representation is specified with the "value" attribute on each element type. XML documents conforming to this memo need only specify the "value" attribute on element types when the value needs to override the default value/data type defined in the iCalendar specifications. The formal public identifier for standard value types all have the common string format of:

Binary content can be included in an iCalendar object with the "ATTACH" component property. In the standard iCalendar format this content may either be specified through an external entity reference, using a URI value type, or maybe specified within the iCalendar object, after first BASE64 encoding the content. The XML representation for iCalendar also supports including binary content in an iCalendar object with the "attach" element type. It also supports either an external reference to the non-XML binary content or inclusion of the binary content after first encoding the binary information using the BASE64 encoding of [MIME].

http://host.com/bin/foo.exe ]]>

MIICajCC AdOgAwIBAgICBEUwDQEEBQAwdzELMAkGA1UEBhMCVVMxLDAqBgNVBAoTI05l dHNjYXBlIENvbW11bmljYXR5z...and so on...IENvcnBvc== ]]>

The iCalendar format has the capability for including multiple, individual iCalendar objects in a single data stream. The XML representation can support this also. Individual iCalendar objects are specified by the "vcalendar" element type. One or more "vcalendar" element types are permitted within the parent element type, called "iCalendar". For example:

2.0 2.0 > ]]>

The property parameters defined in the standard iCalendar format are represented in the XML representation as an attribute on element types. The following table specifies some of the attribute name corresponding to each property parameter. This is true for all iCalendar object parameters defined in any iCalendar specification. The property and paramater names will be all lower case. Here are some iCalendar parameter names and how they are mapped to their lower case xCal names. NOTE: that the iCalendar "language" parameter is converted to the "xml:lang" attribute as an exception to the one to one mapping.

Calendar properties defined in the standard iCalendar format provide information about an iCalendar object, as a whole. The calendar properties are represented in the XML representation as element types as shown in lower case. Here is a list of some iCalendar properties and them mapped to xCal lower case names:

The semantics for these are as specified for the corresponding calendar property in [iCAL].

All components in xCal are mapped to their component name without the BEGIN tag. This example show how many component names are mapped to xCal lower case names:

... | | VTODO | vtodo | ... | | VJOURNAL | vjournal | ... | | VTIMEZONE | vtimezone | ... | | STANDARD | standard | ... | | DAYLIGHT | daylight | ... | | X-... | x-... | ... | | ... | ... | ... | +----------------+-------------+------------------------------- ]]>

The [iCAL] specification specifies that the equivalent component properties to the "comment", "description", "location", "summary" and "contact" element types can contain formatted content, such as is specified by multiple lines of text. In such cases, the formatted text should be specified in as CDATA Section content. The CDATA section specifies arbitrary character data that is not meant to be interpretted. It is not scanned for markup by the XML parser. The CDATA Section in these element types MUST NOT contain markup or other such alternate representation of the property value. Values MUST NOT be mapped to lower case. iCalendar property values and iCalendar parameter values are used without lower case conversion. Vaues that have characters forbidden by XML MUST be encoded using standard XML escaping mechanisms. Values that containe XML tags like in this example:

element. ]]> Would be encoded using standard XML encoding as shown here:

How to map xml DESCRIPTION into an XML <description> element. ]]>

It is expected that iCalendar XML documents will need to be sent over SMTP/MIME email. The "text/xml" and "application/xml" content-types have been registered for XML documents. However, use of these content-type definitions present some problems for XML applications such as calendaring and scheduling. The "text/xml" and "application/xml" content-type definitions do not provide for any header field parameters to identify the type of XML document contained in the MIME entity. This means that a recipient mail user agent must (MUA) open up each "text/xml" or "application/xml" content in order to determine what object handler is needed to process the information. To a MUA, all XML documents look like just plain "text/xml" or "application/xml" content. Additionally, it is accepted practice for a MUA to provide iconic feedback to the user for individual content-types that are supported by the MUA. For example, not only would feedback be provided for a calendaring and scheduling content. Some further unique identification would also be provided for each different scheduling message; such as a meeting invitation, response to an invitation, reschedule notice, cancellation notice, etc. In such cases, acceptable performance by the MUA is dependent on the existence of header field information, such as it provided in the definition of the "text/calendar" content-type by [iCAL]. Internet application conforming to this memo MUST identify iCalendar XML documents with the experimental content-type "application/calendar+xml". The content-type header field SHOULD also contain a "component" and "method" parameter to clearly identify a comma-separated list of components and the singular method used in the iCalendar XML document. For example, an iCalendar XML document specifying a REQUEST for a VEVENT would be specified with the following content-type header field:

The content-type can also include the "optinfo" parameter to specify any other optional iCalendar information. The semantics of these content-type parameters is as defined in [iCAL]. Internet applications conforming to this memo MUST only send the iCalendar XML document in a "multipart/alternative" MIME entity that also contains an equivalent iCalendar object in the standard format defined by [iCAL]. This restrict will guarantee that the iCalendar object can also be processed by internet applications that only support the standard iCalendar format. An XML application supporting the iCalendar XML document type MUST be able to receive and properly process the "application/calendar+xml" document contained within a "multipart" message content-type.

The iCalendar XML documents will be stored in file systems. The accepted practice for file extensions for XML documents is the text "XML". However, in order to uniquely identify iCalendar XML documents for file association with applications that can directly process this document type, it is RECOMMENDED that the file extension be the text "XCS".

The following sections provide various examples of iCalendar XML documents.

The following is a simple example of a iCalendar XML document. This document is both a well-formed and valid XML document. The iCalendar object specifies an appointment.

PUBLISH 2.0 -//HandGen//NONSGML vGen v1.0//EN 19981116T150000@cal10.host.com 19981116T145958Z

Project XYZ Review

Conference Room 23A 19981116T163000Z 19981116T190000Z

1998-ABC Corp-1234

Appointment,Work ]]>

The following is an example of a valid iCalendar XML document that also includes an external reference to an attachment. The iCalendar object specifies a meeting invitation with an attachment.

REQUEST 2.0 -//HandGen//NONSGML vGen v1.0//EN 19981211T133000@cal1.host.com 19981211T132928Z cap://host.com/jim 19981212T150000Z 19981212T160000Z

Department Meeting

Conference Room 23A jim@host.com MAILTO:joe@host.com MAILTO:steve@host.com http://host.com/pub/photos/holiday.jpg ]]> The following is an example of a well-formed and valid iCalendar XML document that includes an attachment as inline binary content. The iCalendar object specifies a meeting invitation with an attachment.

REQUEST 2.0 -//HandGen//NONSGML vGen v1.0//EN 19981211T133000@cal1.host.com 19981211T132928Z MAILTO:jim@host.com 19981212T150000Z 19981212T160000Z

Department Meeting

Conference Room 23A MAILTO:jim@host.com MAILTO:joe@host.com MAILTO:steve@host.com MIICajCCAdOgAwIBAgI CBEUwDQEEBQAwdzELMAkGA1UEBhMCVVMxLDAqBgNVBAoTI05ldHNjYXB lIEjYXRpb25z...and so on...IENvcnBvc== ]]>

The following is an example of a well-formed and valid iCalendar XML document that includes more than one iCalendar object.

PUBLISH 2.0 -//HandGen//NONSGML vGen v1.0//EN 2.0 -//HandGen//NONSGML vGen v1.0//EN PUBLISH 19981009T233010@cal1.host.com 19981009T233000Z 19981120T133000Z 19981122T183000Z

IT Conference

Downtowner Hotel ]]>

The following is an example of a snippet of a XML document that includes elements from the iCalendar name-space.

xmlns:pdi="http://pdi.org/schema"> 19981123T133000Z 19981123T203000Z 1234567 999.99 ]]>

The following is a snippet of an iCalendar XML document that publishes information about a meeting.

2.0 -//hacksw/handcal//NONSGML 1.0//EN PUBLISH 19970901T130000Z-123401@host.com 19970901T130000Z 19970903T163000Z 19970903T190000Z

Annual Employee Review

PRIVATE Business,Human Resources ]]>

The following is a snippet of an iCalendar XML document that publishes information about an annually repeating event that is transparent to busy time searches.

2.0

PUBLISH 19990101T125957Z-123403@host.com 19990101T130000Z 19991102

Our Blissful Anniversary

CONFIDENTIAL TRANSPARENT Anniversary,Personal,Special Occasion FREQ=YEARLY

]]>

The following is a snippet of an iCalendar XML document that specifies an invitation for a meeting. The meeting occurs on the first Monday of each year for five years.

REQUEST 2.0 -//hacksw/handcal//NONSGML 1.0//EN 19981220T130000Z-123403@host.com 19981220T130050Z MAILTO:corprel@host.com 19990104T140000Z 19990104T220000Z

Annual Stockholders Meeting

One Corporate Drive, Wilmington, DL MAILTO:mrbig@host.com CAP:host.com/stockholders Business,Meeting,Special Occasion FREQ=YEARLY;COUNT=5;BYDAY=1MO ]]>

The following is an iCalendar XML document that publishes busy time information. The default value for the "method" attribute is "PUBLISH" and does not need to be specified in this example.

2.0 -//hacksw/handcal//NONSGML 1.0//EN 19980313T133000@ical1.host.com 19990104T133010Z CAP:host.com/jsmith 19980313T141711Z 19980410T141711Z jsmith.ifb 19980314T233000Z/19980315T003000Z 19980316T153000Z/19980316T163000Z 19980318T030000Z/19980318T040000Z ]]>

The following is a snippet of an iCalendar XML document that requests a calendar user's busy time information.

REQUEST 2.0 -//hacksw/handcal//NONSGML 1.0//EN 19970901T083000@ical1.host.com 19970901T083000Z MAILTO:jane_doe@host1.com 19971015T050000Z 19971016T050000Z MAILTO:john_public@host2.com ]]>

The following is a snippet of an iCalendar XML document that issues a CAP command to delete a UID.

2.0 -//hacksw/handcal//NONSGML 1.0//EN relcalid-22 DELETE SELECT VEVENT FROM VAGENDA WHERE UID = 'abcd12345' ]]>

The following have participated in the drafting and discussion of this memo:

TODO - registration if application/calendar+xml

CDATA Sections - - A XML iCalendar document may contain CDATA sections to represent content for specific element types. The CDATA section specifies arbitrary character data that is not meant to be interpretted. It is not scanned by the XML parser for markup. While this memo restricts that any CDATA section MUST NOT contain markup or other such alternate representation for the property value, in general, CDATA section from a non-conformant implementation can contain content such as HTML markup. HTML text can be used to invoke programs. Implementors should be aware that this may leave an implementation open to malicious attack that might occur as a result of executing the markup in the CDATA section. PROCEDURAL ALARMS - - A XML iCalendar document can be created that contains a "VEVENT" calendar component with "VALARM" calendar components. The "VALARM" calendar component can be of type PROCEDURE and can have an attachment containing some sort of executable program. Implementations that incorporate these types of alarms are subject to any virus or malicious attack that might occur as a result of executing the attachment. ATTACHMENTS - - A XML iCalendar document can include references to Uniform Resource Locators that can be programmed resources. Implementers and users of this memo should be aware of the network security implications of accepting and parsing such information. In addition, the security considerations observed by implementations of electronic mail systems should be followed for this memo.

[BASIC] D. Royer, "Basic Internet Calendaring and Scheduling Core Object Specification", Internet Draft, http://www.internic.net/internet-drafts/draft-royer-ical-basic-03.txt, June 2005. [ISO9070] "Information Technology_SGML Support Facilities_ Registration Procedures for Public Text Owner Identifiers", ISO/IEC 9070, Second Edition, International Organization for Standardization, April 1991. [MIME] N. Freed, N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) - Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, http://www.ietf.org/rfc/rfc2119.txt, March 1997. [iCAL] F. Dawson and D. Stenerson, "Internet Calendaring and Scheduling Core Object Specification (iCalendar)", RFC 2445, http://www.ietf.org/rfc/rfc2445.txt, November 1998. [XML] "Extensible Markup Language (XML)", Worldwide Web Consortium, http://www.w3.org/TR/1998/REC-xml-19980210, February 1998. [XML] "Extensible Markup Language (XML)", Worldwide Web Consortium, http://www.w3.org/TR/1998/REC-xml-19980210, February 1998.