iCalendar Message-Based Interoperability Protocol (iMIP)

Internet Engineering Task Force (IETF)                  A. Melnikov, Ed.
Request for Comments: 6047                                     Isode Ltd
Obsoletes: <a href="./rfc2447">2447</a>                                            December 2010
Category: Standards Track
ISSN: 2070-1721


        <span class="h1">iCalendar Message-Based Interoperability Protocol (iMIP)</span>

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 <a href="./rfc5322">RFC 5322</a> and MIME (<a href="./rfc2045">RFC 2045</a>, <a href="./rfc2046">RFC</a>
   <a href="./rfc2046">2046</a>, <a href="./rfc2047">RFC 2047</a>, and <a href="./rfc2049">RFC 2049</a>), 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 <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="https://www.rfc-editor.org/info/rfc6047">http://www.rfc-editor.org/info/rfc6047</a>.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.





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   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

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
      <a href="#section-1.1">1.1</a>. Related Memos ..............................................<a href="#page-3">3</a>
      <a href="#section-1.2">1.2</a>. Formatting Conventions .....................................<a href="#page-3">3</a>
      <a href="#section-1.3">1.3</a>. Terminology ................................................<a href="#page-4">4</a>
   <a href="#section-2">2</a>. MIME Message Format Binding .....................................<a href="#page-4">4</a>
      <a href="#section-2.1">2.1</a>. MIME Media Type ............................................<a href="#page-4">4</a>
      <a href="#section-2.2">2.2</a>. Security ...................................................<a href="#page-5">5</a>
           <a href="#section-2.2.1">2.2.1</a>. Authorization .......................................<a href="#page-5">5</a>
           <a href="#section-2.2.2">2.2.2</a>. Authentication ......................................<a href="#page-5">5</a>
           <a href="#section-2.2.3">2.2.3</a>. Confidentiality .....................................<a href="#page-5">5</a>
      <a href="#section-2.3">2.3</a>. Email Addresses ............................................<a href="#page-6">6</a>
      <a href="#section-2.4">2.4</a>. Content-Type Header Field ..................................<a href="#page-6">6</a>
      <a href="#section-2.5">2.5</a>. Content-Transfer-Encoding Header Field .....................<a href="#page-7">7</a>
      <a href="#section-2.6">2.6</a>. Content-Disposition Header Field ...........................<a href="#page-8">8</a>
   <a href="#section-3">3</a>. Security Considerations .........................................<a href="#page-8">8</a>
   <a href="#section-4">4</a>. Examples .......................................................<a href="#page-11">11</a>
      <a href="#section-4.1">4.1</a>. Single Component with an ATTACH Property ..................<a href="#page-11">11</a>
      <a href="#section-4.2">4.2</a>. Using multipart/alternative for Low-Fidelity Clients ......<a href="#page-11">11</a>
      4.3. Single Component with an ATTACH Property and
           Inline Attachment .........................................<a href="#page-12">12</a>
      <a href="#section-4.4">4.4</a>. Multiple Similar Components ...............................<a href="#page-14">14</a>
      <a href="#section-4.5">4.5</a>. Multiple Mixed Components .................................<a href="#page-15">15</a>
      <a href="#section-4.6">4.6</a>. Detailed Components with an ATTACH Property ...............<a href="#page-16">16</a>
   <a href="#section-5">5</a>. Recommended Practices ..........................................<a href="#page-18">18</a>
      <a href="#section-5.1">5.1</a>. Use of Content and Message IDs ............................<a href="#page-18">18</a>
   <a href="#section-6">6</a>. IANA Considerations ............................................<a href="#page-18">18</a>
   <a href="#section-7">7</a>. References .....................................................<a href="#page-19">19</a>
      <a href="#section-7.1">7.1</a>. Normative References ......................................<a href="#page-19">19</a>
      <a href="#section-7.2">7.2</a>. Informative References ....................................<a href="#page-20">20</a>
   <a href="#appendix-A">Appendix A</a>. Changes since <a href="./rfc2447">RFC 2447</a> ................................<a href="#page-21">21</a>
   <a href="#appendix-B">Appendix B</a>. Acknowledgements ......................................<a href="#page-22">22</a>






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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This document provides the transport-specific information ("binding")
   necessary to convey iCalendar Transport-independent Interoperability
   Protocol (iTIP) [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>] over Internet email (using MIME) as defined in
   [<a href="./rfc5322" title=""Internet Message Format"">RFC5322</a>] and [<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>].  Therefore, this document defines the
   iCalendar Message-Based Interoperability Protocol (iMIP).

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Related Memos</span>

   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.

   [<a id="ref-iCAL">iCAL</a>] specifies a core specification of objects, data types,
   properties, and property parameters.

   [<a id="ref-iTIP">iTIP</a>] 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.

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Formatting Conventions</span>

   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 [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>] and [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>],
   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 <a href="./rfc2119">RFC 2119</a> [<a href="./rfc2119" title=""Key words for use in RFCs to Indicate Requirement Levels"">RFC2119</a>].

   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 [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>].

   Calendar components defined by [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>] 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.



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   Scheduling methods defined by [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>] 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 [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>] 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 [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>] 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.

<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>.  Terminology</span>

   The email terms used in this memo are defined in [<a href="./rfc5322" title=""Internet Message Format"">RFC5322</a>] and
   [<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>].  The calendaring and scheduling terms used in this memo
   are defined in [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>] and [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>].

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  MIME Message Format Binding</span>

   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 [<a href="./rfc5322" title=""Internet Message Format"">RFC5322</a>] "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.

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  MIME Media Type</span>

   A MIME entity containing content information formatted according to
   this document will be referenced as a "text/calendar" content type
   [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>].  It is assumed that this content type will be transported
   through a MIME electronic mail transport.




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<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  Security</span>

   This section addresses several aspects of security including
   authentication, authorization, and confidentiality.  Authentication
   and confidentiality can be achieved using Secure/MIME (S/MIME)
   [<a href="./rfc5750" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Certificate Handling"">RFC5750</a>] [<a href="./rfc5751" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification"">RFC5751</a>], which uses the Security Multiparts framework for
   MIME [<a href="./rfc1847" title=""Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted"">RFC1847</a>].

<span class="h4"><a class="selflink" id="section-2.2.1" href="#section-2.2.1">2.2.1</a>.  Authorization</span>

   In iTIP messages [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>], only the "Organizer" is authorized to modify
   or cancel calendar entries she organizes.  That is,
   [email protected] is not allowed to modify or cancel a meeting
   that was organized by [email protected].  Furthermore, only the
   respondent has the authorization to indicate their status to the
   "Organizer".  That is, the "Organizer" MUST ignore an iTIP message
   from [email protected] that declines a meeting invitation for
   [email protected].

   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.

   [<a id="ref-RFC1847">RFC1847</a>] 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.

<span class="h4"><a class="selflink" id="section-2.2.2" href="#section-2.2.2">2.2.2</a>.  Authentication</span>

   Authentication MUST be performed using S/MIME [<a href="./rfc5750" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Certificate Handling"">RFC5750</a>] [<a href="./rfc5751" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification"">RFC5751</a>].
   Authentication is possible only on messages that have been signed.
   Unauthenticated messages (i.e., unsigned messages) may not be
   trusted.

<span class="h4"><a class="selflink" id="section-2.2.3" href="#section-2.2.3">2.2.3</a>.  Confidentiality</span>

   To ensure confidentiality using iMIP, implementations SHOULD utilize
   encryption specified in S/MIME [<a href="./rfc5750" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Certificate Handling"">RFC5750</a>] [<a href="./rfc5751" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification"">RFC5751</a>].  iMIP does not
   restrict a "Calendar User Agent" (CUA) from forwarding iCalendar
   objects to other users or agents.







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<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>.  Email Addresses</span>

   The calendar address specified within the "ORGANIZER" and "ATTENDEE"
   properties in an iCalendar object sent using iMIP MUST be a proper
   "mailto:" [<a href="#ref-MAILTO" title=""The 'mailto' URI Scheme"">MAILTO</a>] URI specification for the corresponding
   "Organizer" or "Attendee" of the "VEVENT" or "VTODO".

   Because [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>] does not preclude "Attendees" from forwarding
   "VEVENT"s or "VTODO"s to others, the [<a href="./rfc5322" title=""Internet Message Format"">RFC5322</a>] "Sender" value may not
   equal that of the "Organizer".  Additionally, the "Organizer" or
   "Attendee" cannot be reliably inferred by the [<a href="./rfc5322" title=""Internet Message Format"">RFC5322</a>] "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.

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>.  Content-Type Header Field</span>

   A MIME body part containing content information that conforms to this
   document MUST have an [<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>] "Content-Type" value of
   "text/calendar".  The [<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>] "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 [<a href="./rfc2046" title=""Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types"">RFC2046</a>].  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 [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>] the default character set for iCalendar
   objects is UTF-8 [<a href="#ref-UTF-8" title=""UTF-8, a transformation format of ISO 10646"">UTF-8</a>].  However, the default character set for a
   "text/*" MIME entity according to [<a href="./rfc2046" title=""Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types"">RFC2046</a>] 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 [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>], it MUST have the value
   "UTF-8".

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






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   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" [<a href="./rfc2046" title=""Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types"">RFC2046</a>]).

   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 [<a href="./rfc3282" title=""Content Language Headers"">RFC3282</a>], 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.

<span class="h3"><a class="selflink" id="section-2.5" href="#section-2.5">2.5</a>.  Content-Transfer-Encoding Header Field</span>

   Unless an iMIP message is transported over 8-bit clean transport
   (such as SMTP [<a href="#ref-8BITMIME" title=""SMTP Service Extension for 8bit-MIMEtransport"">8BITMIME</a>]), a transfer encoding such as quoted-
   printable or base64 [<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>] MUST be used for iCalendar objects
   containing any characters that can't be represented in the US-ASCII
   character set.  For example:











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   From: [email protected]
   To: [email protected]
   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:[email protected]
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:[email protected]
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:[email protected]
   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

<span class="h3"><a class="selflink" id="section-2.6" href="#section-2.6">2.6</a>.  Content-Disposition Header Field</span>

   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 [<a href="./rfc2045" title=""Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies"">RFC2045</a>] "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.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Security Considerations</span>

   The security threats that applications must address when implementing
   iTIP are detailed in [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>].  In particular, two spoofing threats are
   identified in Section 6.1 of [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>]: spoofing the "Organizer", and
   spoofing an "Attendee".  To address these threats, the originator of
   an iCalendar object must be authenticated by a recipient.  Once



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   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
   [<a href="./rfc5750" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Certificate Handling"">RFC5750</a>] [<a href="./rfc5751" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification"">RFC5751</a>] in order to facilitate the authentication of the
   originator of the iCalendar object (see Sections <a href="#section-2.2.2">2.2.2</a> and <a href="#section-2.2.3">2.2.3</a>).
   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 <a href="./rfc5280#section-4.1.2.6">[RFC5280],
      Section 4.1.2.6</a>.)  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 [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>].  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 [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>].  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 [<a href="./rfc5750" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Certificate Handling"">RFC5750</a>]
   [<a href="./rfc5751" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification"">RFC5751</a>].  If iMIP is used within a single ADministrative Management
   Domain (ADMD) [<a href="./rfc5598" title=""Internet Mail Architecture"">RFC5598</a>], SMTP STARTTLS [<a href="#ref-SMTP-TLS" title=""SMTP Service Extension for Secure SMTP over Transport Layer Security"">SMTP-TLS</a>] (together with
   STARTTLS in IMAP/POP [<a href="#ref-IMAP-POP-TLS">IMAP-POP-TLS</a>]) MAY alternatively be used to
   provide calendar confidentiality.






<span class="grey">Melnikov                     Standards Track                    [Page 9]</span>

<span id="page-10" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


   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.  [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>] and [<a href="#ref-iTIP" title=""iCalendar Transport-Independent Interoperability Protocol (iTIP)"">iTIP</a>] 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 <a href="#section-2.6">Section 2.6</a>.

   Use of S/MIME makes the security considerations discussed in
   [<a href="./rfc5750" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Certificate Handling"">RFC5750</a>] [<a href="./rfc5751" title=""Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.2 Message Specification"">RFC5751</a>] relevant to this document.  For additional
   security considerations regarding certificate and Certificate
   Revocation List (CRL) verification, please see [<a href="./rfc5280" title=""Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile"">RFC5280</a>].













<span class="grey">Melnikov                     Standards Track                   [Page 10]</span>

<span id="page-11" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Examples</span>

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Single Component with an ATTACH Property</span>

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

   From: [email protected]
   To: [email protected]
   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:[email protected]
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:[email protected]
   ATTENDEE;RSVP=YES:mailto:[email protected]
   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

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Using multipart/alternative for Low-Fidelity Clients</span>

   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.












<span class="grey">Melnikov                     Standards Track                   [Page 11]</span>

<span id="page-12" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


   From: [email protected]
   To: [email protected]
   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: [email protected]
   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:[email protected]
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:[email protected]
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:[email protected]
   DTSTAMP:19970611T190000Z
   DTSTART:19970701T170000Z
   DTEND:19970701T173000Z
   SUMMARY:Phone Conference
   UID:calsvr.example.com-8739701987387771
   SEQUENCE:0
   STATUS:CONFIRMED
   END:VEVENT
   END:VCALENDAR

   --01BD3665.3AF0D360

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Single Component with an ATTACH Property and Inline Attachment</span>

   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.



<span class="grey">Melnikov                     Standards Track                   [Page 12]</span>

<span id="page-13" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


   From: [email protected]
   To: [email protected]
   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:[email protected]
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:[email protected]
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:[email protected]
   DTSTAMP:19970611T190000Z
   DTSTART:19970701T180000Z
   DTEND:19970701T183000Z
   SUMMARY:Phone Conference
   UID:calsvr.example.com-8739701987387771
   ATTACH:cid:[email protected]
   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--









<span class="grey">Melnikov                     Standards Track                   [Page 13]</span>

<span id="page-14" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  Multiple Similar Components</span>

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

   From: [email protected]
   To: [email protected]
   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:[email protected]
   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:[email protected]
   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











<span class="grey">Melnikov                     Standards Track                   [Page 14]</span>

<span id="page-15" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


<span class="h3"><a class="selflink" id="section-4.5" href="#section-4.5">4.5</a>.  Multiple Mixed Components</span>

   Different component types must be encapsulated in separate iCalendar
   objects.

   From: [email protected]
   To: [email protected]
   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:[email protected]
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:[email protected]
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:[email protected]
   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














<span class="grey">Melnikov                     Standards Track                   [Page 15]</span>

<span id="page-16" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


   ----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:[email protected]
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:mailto:[email protected]
   ATTENDEE;RSVP=YES:mailto:[email protected]
   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

<span class="h3"><a class="selflink" id="section-4.6" href="#section-4.6">4.6</a>.  Detailed Components with an ATTACH Property</span>

   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: [email protected]
   MIME-Version: 1.0
   To: [email protected],[email protected]
   Subject: REQUEST - Phone Conference
   Content-Type: multipart/related;
                 boundary="--FEE3790DC7E35189CA67CE2C"

   ----FEE3790DC7E35189CA67CE2C
   Content-Type: multipart/alternative;
                 boundary="--00FEE3790DC7E35189CA67CE2C00"










<span class="grey">Melnikov                     Standards Track                   [Page 16]</span>

<span id="page-17" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


   ----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: [email protected]
   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:[email protected]
   ATTENDEE;ROLE=CHAIR;PARTSTAT=ACCEPTED:[email protected]
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:[email protected]
   ATTENDEE;RSVP=YES;CUTYPE=INDIVIDUAL:mailto:[email protected]
   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














<span class="grey">Melnikov                     Standards Track                   [Page 17]</span>

<span id="page-18" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


   ----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

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Recommended Practices</span>

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

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Use of Content and Message IDs</span>

   The [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>] 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 [<a href="./rfc2392" title=""Content-ID and Message-ID Uniform Resource Locators"">RFC2392</a>].  Although [<a href="./rfc2392" title=""Content-ID and Message-ID Uniform Resource Locators"">RFC2392</a>] 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.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  IANA Considerations</span>

   The "text/calendar" MIME media type was registered in [<a href="#ref-iCAL" title=""Internet Calendaring and Scheduling Core Object Specification (iCalendar)"">iCAL</a>].














<span class="grey">Melnikov                     Standards Track                   [Page 18]</span>

<span id="page-19" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  References</span>

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Normative References</span>

   [<a id="ref-iCAL">iCAL</a>]      Desruisseaux, B., Ed., "Internet Calendaring and
               Scheduling Core Object Specification (iCalendar)",
               <a href="./rfc5545">RFC 5545</a>, September 2009.

   [<a id="ref-iTIP">iTIP</a>]      Daboo, C., Ed., "iCalendar Transport-Independent
               Interoperability Protocol (iTIP)", <a href="./rfc5546">RFC 5546</a>, December
               2009.

   [<a id="ref-RFC5322">RFC5322</a>]   Resnick, P., Ed., "Internet Message Format", <a href="./rfc5322">RFC 5322</a>,
               October 2008.

   [<a id="ref-MAILTO">MAILTO</a>]    Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
               URI Scheme", <a href="./rfc6068">RFC 6068</a>, October 2010.

   [<a id="ref-RFC1847">RFC1847</a>]   Galvin, J., Murphy, S., Crocker, S., and N. Freed,
               "Security Multiparts for MIME: Multipart/Signed and
               Multipart/Encrypted", <a href="./rfc1847">RFC 1847</a>, October 1995.

   [<a id="ref-RFC2045">RFC2045</a>]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part One: Format of Internet Message
               Bodies", <a href="./rfc2045">RFC 2045</a>, November 1996.

   [<a id="ref-RFC2046">RFC2046</a>]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part Two: Media Types", <a href="./rfc2046">RFC 2046</a>,
               November 1996.

   [<a id="ref-RFC2392">RFC2392</a>]   Levinson, E., "Content-ID and Message-ID Uniform Resource
               Locators", <a href="./rfc2392">RFC 2392</a>, August 1998.

   [<a id="ref-RFC2119">RFC2119</a>]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-UTF-8">UTF-8</a>]     Yergeau, F., "UTF-8, a transformation format of ISO
               10646", STD 63, <a href="./rfc3629">RFC 3629</a>, November 2003.

   [<a id="ref-SMTP-TLS">SMTP-TLS</a>]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
               Transport Layer Security", <a href="./rfc3207">RFC 3207</a>, February 2002.

   [<a id="ref-IMAP-POP-TLS">IMAP-POP-TLS</a>]
               Newman, C., "Using TLS with IMAP, POP3 and ACAP",
               <a href="./rfc2595">RFC 2595</a>, June 1999.






<span class="grey">Melnikov                     Standards Track                   [Page 19]</span>

<span id="page-20" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


   [<a id="ref-RFC5750">RFC5750</a>]   Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
               Mail Extensions (S/MIME) Version 3.2 Certificate
               Handling", <a href="./rfc5750">RFC 5750</a>, January 2010.

   [<a id="ref-RFC5751">RFC5751</a>]   Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
               Mail Extensions (S/MIME) Version 3.2 Message
               Specification", <a href="./rfc5751">RFC 5751</a>, January 2010.

   [<a id="ref-RFC5280">RFC5280</a>]   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", <a href="./rfc5280">RFC 5280</a>, May 2008.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Informative References</span>

   [<a id="ref-8BITMIME">8BITMIME</a>]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and D.
               Crocker, "SMTP Service Extension for 8bit-MIMEtransport",
               <a href="./rfc1652">RFC 1652</a>, July 1994.

   [<a id="ref-RFC5598">RFC5598</a>]   Crocker, D., "Internet Mail Architecture", <a href="./rfc5598">RFC 5598</a>, July
               2009.

   [<a id="ref-RFC3282">RFC3282</a>]   Alvestrand, H., "Content Language Headers", <a href="./rfc3282">RFC 3282</a>, May
               2002.



























<span class="grey">Melnikov                     Standards Track                   [Page 20]</span>

<span id="page-21" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Changes since <a href="./rfc2447">RFC 2447</a></span>

   Updated references.  Split them into Normative and Informative.

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

   Corrected usage of <a href="./rfc2119">RFC 2119</a> 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.











<span class="grey">Melnikov                     Standards Track                   [Page 21]</span>

<span id="page-22" ></span>
<span class="grey"><a href="./rfc6047">RFC 6047</a>                          iMIP                     December 2010</span>


<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Acknowledgements</span>

   The editor of this document wishes to thank Frank Dawson, Steve
   Mansour, and Steve Silverberg, the original authors of <a href="./rfc2447">RFC 2447</a>, 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: [email protected]































Melnikov                     Standards Track                   [Page 22]