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rfc:rfc6047

Internet Engineering Task Force (IETF) A. Melnikov, Ed. Request for Comments: 6047 Isode Ltd Obsoletes: 2447 December 2010 Category: Standards Track ISSN: 2070-1721

      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

 Copyright (c) 2010 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
 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.

Melnikov Standards Track [Page 1] RFC 6047 iMIP December 2010

 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.

Table of Contents

 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

Melnikov Standards Track [Page 2] RFC 6047 iMIP December 2010

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.

Melnikov Standards Track [Page 3] RFC 6047 iMIP December 2010

 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.

Melnikov Standards Track [Page 4] RFC 6047 iMIP December 2010

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.

Melnikov Standards Track [Page 5] RFC 6047 iMIP December 2010

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.

Melnikov Standards Track [Page 6] RFC 6047 iMIP December 2010

 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:

Melnikov Standards Track [Page 7] RFC 6047 iMIP December 2010

 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

Melnikov Standards Track [Page 8] RFC 6047 iMIP December 2010

 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:
 1. 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.
 2. 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.
 3. 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.
 4. 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.

Melnikov Standards Track [Page 9] RFC 6047 iMIP December 2010

 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].

Melnikov Standards Track [Page 10] RFC 6047 iMIP December 2010

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.

Melnikov Standards Track [Page 11] RFC 6047 iMIP December 2010

 From: foo1@example.com
 To: foo2@example.com
 Subject: Phone Conference
 Mime-Version: 1.0
 Content-Type: multipart/alternative; boundary="01BD3665.3AF0D360"
  1. -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
  1. -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
  1. -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.

Melnikov Standards Track [Page 12] RFC 6047 iMIP December 2010

 From: foo1@example.com
 To: foo2@example.com
 Subject: Phone Conference
 Mime-Version: 1.0
 Content-Type: multipart/related; boundary="boundary-example-1"
  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
  1. -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/////////////////////////////////
  ...
  1. -boundary-example-1–

Melnikov Standards Track [Page 13] RFC 6047 iMIP December 2010

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

Melnikov Standards Track [Page 14] RFC 6047 iMIP December 2010

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.
  1. —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

Melnikov Standards Track [Page 15] RFC 6047 iMIP December 2010

  1. —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
  1. —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"
  1. —FEE3790DC7E35189CA67CE2C

Content-Type: multipart/alternative;

               boundary="--00FEE3790DC7E35189CA67CE2C00"

Melnikov Standards Track [Page 16] RFC 6047 iMIP December 2010

  1. —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
  1. —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
  1. —00FEE3790DC7E35189CA67CE2C00

Melnikov Standards Track [Page 17] RFC 6047 iMIP December 2010

  1. —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
  ...
  1. —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].

Melnikov Standards Track [Page 18] RFC 6047 iMIP December 2010

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.

Melnikov Standards Track [Page 19] RFC 6047 iMIP December 2010

 [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.

Melnikov Standards Track [Page 20] RFC 6047 iMIP December 2010

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.

Melnikov Standards Track [Page 21] RFC 6047 iMIP December 2010

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

Melnikov Standards Track [Page 22]

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