Internet Engineering Task Force (IETF)
Request for Comments: 6047
Obsoletes: 2447
Category: Standards Track
ISSN: 2070-1721

A. Melnikov, Ed.
Isode Ltd
December 2010

iCalendar Message-Based Interoperability Protocol (iMIP)

Abstract

This document, "iCalendar Message-Based Interoperability Protocol (iMIP)", specifies a binding from the iCalendar Transport-independent Interoperability Protocol (iTIP) to Internet email-based transports. Calendaring entries defined by the iCalendar Object Model (iCalendar) are wrapped using constructs from RFC 5322 and MIME (RFC 2045, RFC 2046, RFC 2047, and RFC 2049), and then transported over SMTP.

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/rfc6047.

Copyright Notice

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 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.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

   1. Introduction ....................................................3
      1.1. Related Memos ..............................................3
      1.2. Formatting Conventions .....................................3
      1.3. Terminology ................................................4
   2. MIME Message Format Binding .....................................4
      2.1. MIME Media Type ............................................4
      2.2. Security ...................................................5
           2.2.1. Authorization .......................................5
           2.2.2. Authentication ......................................5
           2.2.3. Confidentiality .....................................5
      2.3. Email Addresses ............................................6
      2.4. Content-Type Header Field ..................................6
      2.5. Content-Transfer-Encoding Header Field .....................7
      2.6. Content-Disposition Header Field ...........................8
   3. Security Considerations .........................................8
   4. Examples .......................................................11
      4.1. Single Component with an ATTACH Property ..................11
      4.2. Using multipart/alternative for Low-Fidelity Clients ......11
      4.3. Single Component with an ATTACH Property and
           Inline Attachment .........................................12
      4.4. Multiple Similar Components ...............................14
      4.5. Multiple Mixed Components .................................15
      4.6. Detailed Components with an ATTACH Property ...............16
   5. Recommended Practices ..........................................18
      5.1. Use of Content and Message IDs ............................18
   6. IANA Considerations ............................................18
   7. References .....................................................19
      7.1. Normative References ......................................19
      7.2. Informative References ....................................20
   Appendix A. Changes since RFC 2447 ................................21
   Appendix B. Acknowledgements ......................................22

1. Introduction

This document provides the transport-specific information ("binding") necessary to convey iCalendar Transport-independent Interoperability Protocol (iTIP) [iTIP] over Internet email (using MIME) as defined in [RFC5322] and [RFC2045]. Therefore, this document defines the iCalendar Message-Based Interoperability Protocol (iMIP).

1.1. Related Memos

Implementers will need to be familiar with several other memos that, along with this memo, form a framework for Internet calendaring and scheduling standards.

This document specifies an Internet email binding for iTIP.

[iCAL] specifies a core specification of objects, data types, properties, and property parameters.

[iTIP] specifies an interoperability protocol for scheduling between different implementations.

This memo does not attempt to repeat the specification of concepts or definitions from these other memos. Where possible, references are made to the memo that provides for the specification of these concepts or definitions.

1.2. Formatting Conventions

The mechanisms defined in this memo are defined in prose. In order to refer to elements of the calendaring and scheduling model, core object, or interoperability protocol defined in [iCAL] and [iTIP], some formatting conventions have been used.

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 RFC 2119 [RFC2119].

Calendaring and scheduling roles are referred to in quoted strings of text with the first character of each word in uppercase. For example, "Organizer" refers to a role of a "Calendar User" within the scheduling protocol defined by [iTIP].

Calendar components defined by [iCAL] are referred to with capitalized, quoted strings of text. All calendar components start with the letter "V". For example, "VEVENT" refers to the event calendar component, "VTODO" refers to the to-do calendar component, and "VJOURNAL" refers to the daily journal calendar component.

Scheduling methods defined by [iTIP] are referred to with capitalized, quoted strings of text. For example, "REQUEST" refers to the method for requesting a scheduling calendar component be created or modified; "REPLY" refers to the method a recipient of a request uses to update their status with the "Organizer" of the calendar component.

Properties defined by [iCAL] are referred to with capitalized, quoted strings of text, followed by the word "property". For example, "ATTENDEE" property refers to the iCalendar property used to convey the calendar address of a "Calendar User".

Property parameters defined by [iCAL] are referred to with lowercase, quoted strings of text, followed by the word "parameter". For example, "value" parameter refers to the iCalendar property parameter used to override the default data type for a property value.

1.3. Terminology

The email terms used in this memo are defined in [RFC5322] and [RFC2045]. The calendaring and scheduling terms used in this memo are defined in [iCAL] and [iTIP].

2. MIME Message Format Binding

This section defines the message binding to the MIME electronic mail transport.

The sections below refer to the "originator" and the "recipient" of an iMIP message. In the case of a "request" method, the originator is the "Organizer" and the recipient is an "Attendee" of the event. In the case of a "response" method, the originator is an "Attendee" and the recipient is the "Organizer" of the event.

The [RFC5322] "Reply-To" header field typically contains the email address of the originator of the scheduling message. However, this cannot be guaranteed because the sender of the iMIP message might not be the originator of the scheduling message and the sender's "Mail User Agent" (MUA) might not enforce iMIP semantics by translating the originator's address into the "Reply-To" email header field.

2.1. MIME Media Type

A MIME entity containing content information formatted according to this document will be referenced as a "text/calendar" content type [iCAL]. It is assumed that this content type will be transported through a MIME electronic mail transport.

2.2. Security

This section addresses several aspects of security including authentication, authorization, and confidentiality. Authentication and confidentiality can be achieved using Secure/MIME (S/MIME) [RFC5750] [RFC5751], which uses the Security Multiparts framework for MIME [RFC1847].

2.2.1. Authorization

In iTIP messages [iTIP], only the "Organizer" is authorized to modify or cancel calendar entries she organizes. That is, spoof@xyz.example.net is not allowed to modify or cancel a meeting that was organized by a@example.com. Furthermore, only the respondent has the authorization to indicate their status to the "Organizer". That is, the "Organizer" MUST ignore an iTIP message from spoof@xyz.example.net that declines a meeting invitation for b@example.com.

Implementations of iMIP SHOULD verify the authenticity of the creator of an iCalendar object before taking any action. Methods for doing this are presented later in this document.

[RFC1847] message flow in iTIP supports someone working on behalf of a "Calendar User" through use of the "sent-by" parameter that is associated with the "ATTENDEE" and "ORGANIZER" properties. However, there is no mechanism to verify whether or not a "Calendar User" has authorized someone to work on their behalf. It is left to implementations to provide mechanisms for the "Calendar Users" to make that decision.

2.2.2. Authentication

Authentication MUST be performed using S/MIME [RFC5750] [RFC5751]. Authentication is possible only on messages that have been signed. Unauthenticated messages (i.e., unsigned messages) may not be trusted.

2.2.3. Confidentiality

To ensure confidentiality using iMIP, implementations SHOULD utilize encryption specified in S/MIME [RFC5750] [RFC5751]. iMIP does not restrict a "Calendar User Agent" (CUA) from forwarding iCalendar objects to other users or agents.

2.3. Email Addresses

The calendar address specified within the "ORGANIZER" and "ATTENDEE" properties in an iCalendar object sent using iMIP MUST be a proper "mailto:" [MAILTO] URI specification for the corresponding "Organizer" or "Attendee" of the "VEVENT" or "VTODO".

Because [iTIP] does not preclude "Attendees" from forwarding "VEVENT"s or "VTODO"s to others, the [RFC5322] "Sender" value may not equal that of the "Organizer". Additionally, the "Organizer" or "Attendee" cannot be reliably inferred by the [RFC5322] "Sender" or "Reply-To" header field values of an iMIP message. The relevant address MUST be ascertained by opening the "text/calendar" MIME body part and examining the "ATTENDEE" and "ORGANIZER" properties.

2.4. Content-Type Header Field

A MIME body part containing content information that conforms to this document MUST have an [RFC2045] "Content-Type" value of "text/calendar". The [RFC2045] "Content-Type" header field MUST also include the MIME parameter "method". The value MUST be the same (ignoring case) as the value of the "METHOD" property within the iCalendar object.

Note 1: A MIME message containing multiple iCalendar objects with different "method" values MUST be further encapsulated with a "multipart/mixed" MIME entity [RFC2046]. This will allow each of the iCalendar objects to be encapsulated within their own "text/calendar" MIME entity.

Note 2: A MIME body part with a "Content-Type" value of "text/calendar" that lacks the "method" parameter is not considered to be an iMIP body part and thus is not subject to the requirements specified in this document.

Note that according to [iCAL] the default character set for iCalendar objects is UTF-8 [UTF-8]. However, the default character set for a "text/*" MIME entity according to [RFC2046] is US-ASCII. Thus, a "charset" MIME parameter MUST be present if the iCalendar object contains characters that can't be represented in the US-ASCII character set and, as specified in [iCAL], it MUST have the value "UTF-8".

The optional "component" MIME parameter defines the iCalendar component type contained within the iCalendar object.

The following is an example of this header field with a value that indicates an event message.

        Content-Type: text/calendar; method=REQUEST; charset=UTF-8;
              component=vevent

The "text/calendar" content type allows for the scheduling message type to be included in a MIME message with other content information (i.e., "multipart/mixed") or included in a MIME message with a clear- text, human-readable form of the scheduling message (i.e., "multipart/alternative" [RFC2046]).

In order to permit the information in the scheduling message to be understood by MIME User Agents (UAs) that do not support the "text/calendar" content type, scheduling messages SHOULD be sent with an alternative, human-readable form of the information.

Note that "multipart/alternative" MUST NOT be used to represent two slightly different iCalendar objects, for example, two "VEVENT"s with alternative starting times.

CUAs can use other MIME parameters of the "Content-Type" header field, as well as a language specified in the Content-Language header field [RFC3282], to pick a "text/calendar" part for processing if a "multipart/alternative" MIME message contains more than one "text/calendar" part.

Any receiving UA compliant with this specification MUST be able to process "text/calendar" body parts enclosed within "multipart/*". Note that a "multipart/mixed" MIME message can include multiple "text/calendar" components. The receiving UA MUST be able to process all of them.

2.5. Content-Transfer-Encoding Header Field

Unless an iMIP message is transported over 8-bit clean transport (such as SMTP [8BITMIME]), a transfer encoding such as quoted- printable or base64 [RFC2045] MUST be used for iCalendar objects containing any characters that can't be represented in the US-ASCII character set. For example:

   From: user1@example.com
   To: user2@example.com
   Subject: Phone Conference
   Mime-Version: 1.0
   Date: Wed, 07 May 2008 21:30:25 +0400
   Message-ID: <4821E731.5040506@laptop1.example.com>
   Content-Type: text/calendar; method=REQUEST; charset=UTF-8
   Content-Transfer-Encoding: quoted-printable
   
   BEGIN:VCALENDAR
   PRODID:-//Example/ExampleCalendarClient//EN
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:mailto:user1@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:user1@example.com
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:user2@example.com
   DTSTAMP:20080507T170000Z
   DTSTART:20080701T160000Z
   DTEND:20080701T163000Z
   SUMMARY:Phone call to discuss your last visit
   DESCRIPTION:=D1=82=D1=8B =D0=BA=D0=B0=D0=BA - =D0=B4=D0=BE=D0=
    =B2=D0=BE=D0=BB=D0=B5=D0=BD =D0=BF=D0=BE=D0=B5=D0=B7=D0=B4=D0=BA=D0
    =BE=D0=B9?
   UID:calsvr.example.com-8739701987387998
   SEQUENCE:0
   STATUS:TENTATIVE
   END:VEVENT
   END:VCALENDAR

2.6. Content-Disposition Header Field

Implementations MAY include a "Content-Disposition" header field to define a file name for an iCalendar object. However, the handling of a MIME part MUST be based on its [RFC2045] "Content-Type" and not on the extension specified in the "Content-Disposition", as different email malware is known to trick User Agents into misinterpreting content of messages by specifying a file extension in the Content- Disposition header field that doesn't correspond to the value of the "Content-Type" header field.

3. Security Considerations

The security threats that applications must address when implementing iTIP are detailed in [iTIP]. In particular, two spoofing threats are identified in Section 6.1 of [iTIP]: spoofing the "Organizer", and spoofing an "Attendee". To address these threats, the originator of an iCalendar object must be authenticated by a recipient. Once authenticated, a determination can be made as to whether or not the originator is authorized to perform the requested operation. Compliant applications MUST support signing and encrypting "text/calendar" body parts using a mechanism based on S/MIME [RFC5750] [RFC5751] in order to facilitate the authentication of the originator of the iCalendar object (see Sections 2.2.2 and 2.2.3). The steps for processing a signed iMIP message are described below:

Using S/MIME, determine who signed the "text/calendar" body part containing the iCalendar object. This is the "signer". (Note that the email address of the signer MUST be specified in the rfc822Name field of the "subject alternative name" extension of the signer certificate, as specified in [RFC5280], Section 4.1.2.6.) Note that the signer is not necessarily the person sending an e-mail message, since an e-mail message can be forwarded.

Correlate the signer to either an "ATTENDEE" property or to the "ORGANIZER" property in the iCalendar object, based on the method and the calendar component specified in the iCalendar object, as defined in Section 1.4 of [iTIP]. If the signer cannot be correlated to an "ATTENDEE"/"ORGANIZER" property, then actively warn the user controlling the "Calendar User Agent" that the iCalendar object is untrusted, and encourage the user to ignore the message, but give advanced users the option to (a) view the certificate of the signer and the entire certificate chain (if any) in order to help decide if the signer should be trusted to send the message, and then (b) allow the CUA to accept and process the iCalendar object.

Determine whether or not the "ATTENDEE"/"ORGANIZER" is authorized to perform the operation as defined by [iTIP]. If the conditions are not met, ignore the message.

If all the above conditions are met, the message can be processed.

S/MIME signing also protects against malicious changes to messages in transit.

If calendar confidentiality is required by the sender, signed iMIP messages SHOULD be encrypted by a mechanism based on S/MIME [RFC5750] [RFC5751]. If iMIP is used within a single ADministrative Management Domain (ADMD) [RFC5598], SMTP STARTTLS [SMTP-TLS] (together with STARTTLS in IMAP/POP [IMAP-POP-TLS]) MAY alternatively be used to provide calendar confidentiality.

Once a signed and/or encrypted iMIP message is received and successfully verified (as detailed above) by a CUA, the CUA SHOULD remember whether the sender of the message is using signing and/or encrypting. If an unsigned iMIP message is received from the same sender later on, the receiving CUA SHOULD warn the receiving user about a possible man-in-the-middle attack and SHOULD ignore the message, unless explicitly overridden by the user.

Implementations MAY provide means for users to disable signing and encrypting.

It is possible to receive iMIP messages sent by someone working on behalf of another "Calendar User". This is determined by examining the "sent-by" parameter in the relevant "ORGANIZER" or "ATTENDEE" property. [iCAL] and [iTIP] provide no mechanism to verify that a "Calendar User" has authorized someone else to work on their behalf. To address this security issue, implementations MUST provide mechanisms for the "Calendar Users" to make that decision before applying changes from someone working on behalf of a "Calendar User". One way to achieve this is to reject iMIP messages sent by users other than the "ORGANIZER" or the "ATTENDEE"s. Alternatively, the receiver could have a list of trusted <sent-by, organizer> proxies in its local security policy. And yet another way is to prompt the user for confirmation.

iMIP-based calendaring is frequently deployed within a single ADMD, with boundary filtering employed to restrict email calendaring flows to be inside the ADMD. This can help in minimizing malicious changes to calendaring messages in transit, as well as in making authorization decisions less risky.

A security consideration associated with the use of the Content- Disposition header field is described in Section 2.6.

Use of S/MIME makes the security considerations discussed in [RFC5750] [RFC5751] relevant to this document. For additional security considerations regarding certificate and Certificate Revocation List (CRL) verification, please see [RFC5280].

4. Examples

4.1. Single Component with an ATTACH Property

This minimal message shows how an iCalendar object references an attachment. The attachment is accessible via its URL.

   From: sman@netscape.example.com
   To: stevesil@microsoft.example.com
   Subject: Phone Conference
   Mime-Version: 1.0
   Content-Type: text/calendar; method=REQUEST; charset=US-ASCII
   Content-Transfer-Encoding: 7bit

   BEGIN:VCALENDAR
   PRODID:-//Example/ExampleCalendarClient//EN
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:mailto:man@netscape.example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:man@netscape.example.com
   ATTENDEE;RSVP=YES:mailto:stevesil@microsoft.example.com
   DTSTAMP:19970611T190000Z
   DTSTART:19970701T210000Z
   DTEND:19970701T230000Z
   SUMMARY:Phone Conference
   DESCRIPTION:Please review the attached document.
   UID:calsvr.example.com-873970198738777
   ATTACH:ftp://ftp.bar.example.com/pub/docs/foo.doc
   STATUS:CONFIRMED
   END:VEVENT
   END:VCALENDAR

4.2. Using multipart/alternative for Low-Fidelity Clients

This example shows how a client can emit a multipart message that includes both a plain text version and the full iCalendar object. Clients that do not support "text/calendar" will still be capable of rendering the plain text representation.

   From: foo1@example.com
   To: foo2@example.com
   Subject: Phone Conference
   Mime-Version: 1.0
   Content-Type: multipart/alternative; boundary="01BD3665.3AF0D360"
   
   --01BD3665.3AF0D360
   Content-Type: text/plain; charset=us-ascii
   Content-Transfer-Encoding: 7bit

This is an alternative representation of a "text/calendar" MIME object.

   When: 7/1/1997 10:00AM PDT - 7/1/97 10:30AM PDT
   Where:
   Organizer: foo1@example.com
   Summary: Phone Conference
   
   --01BD3665.3AF0D360
   Content-Type: text/calendar; method=REQUEST; charset=US-ASCII
   Content-Transfer-Encoding: 7bit
   
   BEGIN:VCALENDAR
   PRODID:-//Example/ExampleCalendarClient//EN
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:mailto:foo1@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:foo1@example.com
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:foo2@example.com
   DTSTAMP:19970611T190000Z
   DTSTART:19970701T170000Z
   DTEND:19970701T173000Z
   SUMMARY:Phone Conference
   UID:calsvr.example.com-8739701987387771
   SEQUENCE:0
   STATUS:CONFIRMED
   END:VEVENT
   END:VCALENDAR
   
   --01BD3665.3AF0D360

4.3. Single Component with an ATTACH Property and Inline Attachment

This example shows how a message containing an iCalendar object references an attached document. The reference is made using a Content-ID (CID). Thus, the iCalendar object and the document are packaged in a "multipart/related" encapsulation.

   From: foo1@example.com
   To: foo2@example.com
   Subject: Phone Conference
   Mime-Version: 1.0
   Content-Type: multipart/related; boundary="boundary-example-1"
   
   --boundary-example-1
   
   Content-Type: text/calendar; method=REQUEST; charset=US-ASCII
   Content-Transfer-Encoding: 7bit
   Content-Disposition: attachment; filename="event.ics"
   
   BEGIN:VCALENDAR
   PRODID:-//Example/ExampleCalendarClient//EN
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:mailto:foo1@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:foo1@example.com
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:foo2@example.com
   DTSTAMP:19970611T190000Z
   DTSTART:19970701T180000Z
   DTEND:19970701T183000Z
   SUMMARY:Phone Conference
   UID:calsvr.example.com-8739701987387771
   ATTACH:cid:123456789@example.com
   SEQUENCE:0
   STATUS:CONFIRMED
   END:VEVENT
   END:VCALENDAR
   
   --boundary-example-1
   Content-Type: application/msword; name="FieldReport.doc"
   Content-Transfer-Encoding: base64
   Content-Disposition: inline; filename="FieldReport.doc"
   Content-ID: <123456789@example.com>
   
   0M8R4KGxGuEAAAAAAAAAAAAAAAAAAAAAPgADAP7/CQAGAAAAAAAAAAABAAAARAAAAAAA
   AAAAEAAAQAAAAAEAAAD+////AAAAAEUAAAD/////////////////////////////////
    ...
   
   --boundary-example-1--

4.4. Multiple Similar Components

Multiple iCalendar components of the same type can be included in the iCalendar object when the "METHOD" is the same for each component.

   From: foo1@example.com
   To: foo2@example.com
   Subject: Summer Company Holidays
   Mime-Version: 1.0
   Content-Type: text/calendar; method=PUBLISH; charset=US-ASCII
   Content-Transfer-Encoding: 7bit
   Content-Disposition: attachment; filename="event.ics"
   
   BEGIN:VCALENDAR
   PRODID:-//Example/ExampleCalendarClient//EN
   METHOD:PUBLISH
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:mailto:foo1@example.com
   DTSTAMP:19970611T150000Z
   DTSTART:19970701T150000Z
   DTEND:19970701T230000Z
   SUMMARY:Company Picnic
   DESCRIPTION:Food and drink will be provided
   UID:calsvr.example.com-873970198738777-1
   SEQUENCE:0
   STATUS:CONFIRMED
   END:VEVENT
   BEGIN:VEVENT
   ORGANIZER:mailto:foo1@example.com
   DTSTAMP:19970611T190000Z
   DTSTART:19970715T150000Z
   DTEND:19970715T230000Z
   SUMMARY:Company Bowling Tournament
   DESCRIPTION:We have 10 lanes reserved
   UID:calsvr.example.com-873970198738777-2
   SEQUENCE:0
   STATUS:CONFIRMED
   END:VEVENT
   END:VCALENDAR

4.5. Multiple Mixed Components

Different component types must be encapsulated in separate iCalendar objects.

   From: foo1@example.com
   To: foo2@example.com
   Subject: Phone Conference
   Mime-Version: 1.0
   Content-Type: multipart/mixed;
                 boundary="--FEE3790DC7E35189CA67CE2C"

This is a multi-part message in MIME format.

   ----FEE3790DC7E35189CA67CE2C
   Content-Type: text/calendar; method=REQUEST; charset=US-ASCII
   Content-Transfer-Encoding: 7bit
   Content-Disposition: attachment; filename="event1.ics"
   
   BEGIN:VCALENDAR
   PRODID:-//Example/ExampleCalendarClient//EN
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:mailto:foo1@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:foo1@example.com
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:foo2@example.com
   DTSTAMP:19970611T190000Z
   DTSTART:19970701T210000Z
   DTEND:19970701T230000Z
   SUMMARY:Phone Conference
   DESCRIPTION:Discuss what happened at the last meeting
   UID:calsvr.example.com-8739701987387772
   SEQUENCE:0
   STATUS:CONFIRMED
   END:VEVENT
   END:VCALENDAR
   ----FEE3790DC7E35189CA67CE2C
   Content-Type: text/calendar; method=REQUEST; charset=US-ASCII
   Content-Transfer-Encoding: 7bit
   Content-Disposition: attachment; filename="todo1.ics"
   
   BEGIN:VCALENDAR
   PRODID:-//Example/ExampleCalendarClient//EN
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VTODO
   DUE:19970701T160000Z
   ORGANIZER:mailto:foo1@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:foo1@example.com
   ATTENDEE;RSVP=YES:mailto:foo2@example.com
   SUMMARY:Phone Conference
   DESCRIPTION:Discuss a new location for the company picnic
   UID:calsvr.example.com-td-8739701987387773
   SEQUENCE:0
   STATUS:NEEDS-ACTION
   END:VEVENT
   END:VCALENDAR
   
   ----FEE3790DC7E35189CA67CE2C

4.6. Detailed Components with an ATTACH Property

This example shows the format of a message containing a group meeting between three individuals. The "multipart/related" encapsulation is used because the iCalendar object contains an ATTACH property that uses a CID to reference the attachment.

   From: foo1@example.com
   MIME-Version: 1.0
   To: foo2@example.com,foo3@example.com
   Subject: REQUEST - Phone Conference
   Content-Type: multipart/related;
                 boundary="--FEE3790DC7E35189CA67CE2C"
   
   ----FEE3790DC7E35189CA67CE2C
   Content-Type: multipart/alternative;
                 boundary="--00FEE3790DC7E35189CA67CE2C00"
   
   ----00FEE3790DC7E35189CA67CE2C00
   Content-Type: text/plain; charset=us-ascii
   Content-Transfer-Encoding: 7bit
   
   When: 7/1/1997 10:00PM PDT - 7/1/97 10:30 PM PDT
   Where:
   Organizer: foo1@example.com
   Summary: Let's discuss the attached document
   
   ----00FEE3790DC7E35189CA67CE2C00
   
   Content-Type: text/calendar; method=REQUEST; charset=US-ASCII;
                    Component=vevent
   Content-Transfer-Encoding: 7bit
   Content-Disposition: attachment; filename="event.ics"
   
   BEGIN:VCALENDAR
   PRODID:-//Example/ExampleCalendarClient//EN
   METHOD:REQUEST
   VERSION:2.0
   BEGIN:VEVENT
   ORGANIZER:foo1@example.com
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:foo1@example.com
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:foo2@example.com
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:foo3@example.com
   DTSTAMP:19970611T190000Z
   DTSTART:19970621T170000Z
   DTEND:199706211T173000Z
   SUMMARY:Let's discuss the attached document
   UID:calsvr.example.com-873970198738777-8aa
   ATTACH:cid:calsvr.example.com-12345aaa
   SEQUENCE:0
   STATUS:CONFIRMED
   END:VEVENT
   END:VCALENDAR
   
   ----00FEE3790DC7E35189CA67CE2C00
   ----FEE3790DC7E35189CA67CE2C
   Content-Type: application/msword; name="FieldReport.doc"
   Content-Transfer-Encoding: base64
   Content-Disposition: inline; filename="FieldReport.doc"
   Content-ID: <calsvr.example.com-12345aaa>
   
   R0lGODdhTAQZAJEAAFVVVd3d3e4AAP///ywAAAAATAQZAAAC/5yPOSLhD6OctNqLs94Xq
   AG4kiW5omm6sq27gvH8kzX9o1y+s73/g8MCofEovGITCoxKMbyCR16cNSq9YrNarfcrvd
   riIH5LL5jE6rxc3G+v2cguf0uv2Oz+v38L7/DxgoOKjURnjIIbe3yNjo+AgZWYVIWWl5i
   ZnJY6J
    ...
   
   ----FEE3790DC7E35189CA67CE2C

5. Recommended Practices

This section outlines a series of recommended practices when using a messaging transport to exchange iCalendar objects.

5.1. Use of Content and Message IDs

The [iCAL] specification makes frequent use of the URI for data types in properties such as "DESCRIPTION", "ATTACH", "CONTACT", and others. Two forms of URIs are the Message ID (MID) and the Content-ID (CID). These are defined in [RFC2392]. Although [RFC2392] allows referencing messages or MIME body parts in other MIME entities or stores, it is strongly RECOMMENDED that iMIP implementations include all referenced messages and body parts in a single MIME entity. Simply put, if an iCalendar object contains CID or MID references to other messages or body parts, implementations should ensure that these messages and/or body parts are transmitted with the iCalendar object. If they are not, there is no guarantee that the receiving CUA will have the access or the authorization to view those objects.

6. IANA Considerations

The "text/calendar" MIME media type was registered in [iCAL].

7. References

7.1. Normative References

   [iCAL]      Desruisseaux, B., Ed., "Internet Calendaring and
               Scheduling Core Object Specification (iCalendar)",
               RFC 5545, September 2009.
   
   [iTIP]      Daboo, C., Ed., "iCalendar Transport-Independent
               Interoperability Protocol (iTIP)", RFC 5546, December
               2009.
   
   [RFC5322]   Resnick, P., Ed., "Internet Message Format", RFC 5322,
               October 2008.
   
   [MAILTO]    Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
               URI Scheme", RFC 6068, October 2010.
   
   [RFC1847]   Galvin, J., Murphy, S., Crocker, S., and N. Freed,
               "Security Multiparts for MIME: Multipart/Signed and
               Multipart/Encrypted", RFC 1847, October 1995.
   
   [RFC2045]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part One: Format of Internet Message
               Bodies", RFC 2045, November 1996.
   
   [RFC2046]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part Two: Media Types", RFC 2046,
               November 1996.
   
   [RFC2392]   Levinson, E., "Content-ID and Message-ID Uniform Resource
               Locators", RFC 2392, August 1998.
   
   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [UTF-8]     Yergeau, F., "UTF-8, a transformation format of ISO
               10646", STD 63, RFC 3629, November 2003.
   
   [SMTP-TLS]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
               Transport Layer Security", RFC 3207, February 2002.

[IMAP-POP-TLS]

               Newman, C., "Using TLS with IMAP, POP3 and ACAP",
               RFC 2595, June 1999.
   
   [RFC5750]   Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
               Mail Extensions (S/MIME) Version 3.2 Certificate
               Handling", RFC 5750, January 2010.
   
   [RFC5751]   Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
               Mail Extensions (S/MIME) Version 3.2 Message
               Specification", RFC 5751, January 2010.
   
   [RFC5280]   Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
               Housley, R., and W. Polk, "Internet X.509 Public Key
               Infrastructure Certificate and Certificate Revocation
               List (CRL) Profile", RFC 5280, May 2008.

7.2. Informative References

   [8BITMIME]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
               Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
               RFC 1652, July 1994.
   
   [RFC5598]   Crocker, D., "Internet Mail Architecture", RFC 5598, July
               2009.
   
   [RFC3282]   Alvestrand, H., "Content Language Headers", RFC 3282, May
               2002.

Appendix A. Changes since RFC 2447

Updated references. Split them into Normative and Informative.

Updated examples to use example.com/example.net domains.

Corrected usage of RFC 2119 language.

Clarified that charset=UTF-8 is required, unless the calendar can be entirely represented in US-ASCII.

Clarified that 7-bit content transfer encodings should be used unless the calendar object is known to be transferred over 8-bit clean transport.

Clarified that file extension specified in the Content-Disposition header field is not to be used to override the "Content-Type" MIME type.

Disallowed use of "multipart/alternative" for slightly different representations of the same calendar.

Clarified handling of the "method" MIME parameter of the "Content- Type" header field.

Clarified that in an iMIP message an ORGANIZER/ATTENDEE property contains a mailto: URI.

Fixed examples with ATTENDEE property to use "CUTYPE=" instead of "TYPE=".

Clarified that message integrity/confidentiality should be achieved using S/MIME.

Provided additional examples.

Improved the Security Considerations section.

Made multiple editorial changes to different sections of the document.

Appendix B. Acknowledgements

The editor of this document wishes to thank Frank Dawson, Steve Mansour, and Steve Silverberg, the original authors of RFC 2447, as well as the following individuals who have participated in the drafting, review, and discussion of this memo:

Reinhold Kainhofer, Cyrus Daboo, Bernard Desruisseaux, Eliot Lear, and Peter Saint-Andre.

Author's Address

   Alexey Melnikov (editor)
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK
   
   EMail: Alexey.Melnikov@isode.com