RFC 4239

Network Working Group                                           S. McRae
Request for Comments: 4239                                           IBM
Category: Standards Track                                     G. Parsons
                                                         Nortel Networks
                                                           November 2005

                     Internet Voice Messaging (IVM)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document describes the carriage of voicemail messages over
   Internet mail as part of a unified messaging infrastructure.

   The Internet Voice Messaging (IVM) concept described in this document
   is not a successor format to VPIM v2 (Voice Profile for Internet Mail
   Version 2), but rather an alternative specification for a different

1.  Introduction

   For some forms of communication, people prefer to communicate using
   their voices rather than typing.  By permitting voicemail to be
   implemented in an interoperable way on top of Internet Mail, voice
   messaging and electronic mail no longer need to remain in separate,
   isolated worlds, and users will be able to choose the most
   appropriate form of communication.  This will also enable new types
   of devices, without keyboards, to be used to participate in
   electronic messaging when mobile, in a hostile environment, or in
   spite of disabilities.

   There exist unified messaging systems that will transmit voicemail
   messages over the Internet using SMTP/MIME, but these systems suffer
   from a lack of interoperability because various aspects of such a
   message have not hitherto been standardized.  In addition, voicemail
   systems can now conform to the Voice Profile for Internet Messaging

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   (VPIM v2 as defined in RFC 2421 [VPIMV2] and revised in RFC 3801,
   Draft Standard [VPIMV2R2]) when forwarding messages to remote
   voicemail systems.  VPIM v2 was designed to allow two voicemail
   systems to exchange messages, not to allow a voicemail system to
   interoperate with a desktop e-mail client.  It is often not
   reasonable to expect a VPIM v2 message to be usable by an e-mail
   recipient.  The result is messages that cannot be processed by the
   recipient (e.g., because of the encoding used), or look ugly to the

   This document therefore proposes a standard mechanism for
   representing a voicemail message within SMTP/MIME, and a standard
   encoding for the audio content, which unified messaging systems and
   mail clients MUST implement to ensure interoperability.  By using a
   standard SMTP/MIME representation and a widely implemented audio
   encoding, this will also permit most users of e-mail clients not
   specifically implementing the standard to still access the voicemail
   messages.  In addition, this document describes features an e-mail
   client SHOULD implement to allow recipients to display voicemail
   messages in a more friendly, context-sensitive way to the user, and
   intelligently provide some of the additional functionality typically
   found in voicemail systems (such as responding with a voice message
   instead of e-mail).  Finally, how a client MAY provide a level of
   interoperability with VPIM v2 is explained.

   It is desirable that unified messaging mail clients also be able to
   fully interoperate with voicemail servers.  This is possible today,
   providing the client implements VPIM v2 [VPIMV2R2], in addition to
   this specification, and uses it to construct messages to be sent to a
   voicemail server.

   The definition in this document is based on the IVM Requirements
   document [GOALS].  It references separate work on critical content
   [CRITICAL] and message context [HINT].  Addressing and directory
   issues are discussed in related documents [ADDRESS], [VPIMENUM],

   Further information on VPIM and related activities can be found at
   http://www.vpim.org or http://www.ema.org/vpim.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119 [KEYWORDS].

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3.  Message Format

   Voice messages may be created explicitly by a user (e.g., recording a
   voicemail message in their mail client) or implicitly by a unified
   messaging system (when it records a telephone message).

   All messages MUST conform with the Internet Mail format, as updated
   by the DRUMS working group [DRUMSIMF], and the MIME format [MIME1].

   When creating a voice message from a client supporting IVM, the
   message header MUST indicate a message context of "voice-message"
   (see [HINT]).  However, to support interoperability with clients not
   explicitly supporting IVM, a recipient MUST NOT require its presence
   in order to correctly process voice messages.

   The receiving agent MUST be able to support multipart messages
   [MIME5].  If the receiving user agent identifies the message as a
   voice message (from the message context), it SHOULD present it to the
   user as a voice message rather than as an electronic mail message
   with a voice attachment (see [BEHAVIOUR]).

   Any content type is permitted in a message, but the top level content
   type on a new, forwarded or reply voice message SHOULD be
   multipart/mixed.  If the recipient is known to be VPIM v2 compliant,
   then multipart/voice-message MAY be used instead (in which case, all
   the provisions of [VPIMV2R2] MUST be implemented in constructing the

   If the message was created as a voice message, and so is not useful
   if the audio content is omitted, then the appropriate audio body part
   MUST be indicated as critical content, via a Criticality parameter of
   CRITICAL on the Content-Disposition (see [CRITICAL]).  Additional
   important body parts (such as the original audio message if a
   voicemail is being forwarded) MAY also be indicated via a Criticality
   of CRITICAL.  Contents that are not essential to communicating the
   meaning of the message (e.g., an associated vCard for the originator)
   MAY be indicated via a Criticality of IGNORE.

   When forwarding IVM messages, clients MUST preserve the content type
   of all audio body parts in order to ensure that the new recipient is
   able to play the forwarded messages.

   The top level content type, on origination of a delivery notification
   message, MUST be a multipart/report.  This will allow automatic
   processing of the delivery notification, for example, so that text-
   to-speech processing can render a non-delivery notification in the
   appropriate language for the recipient.

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4.  Transport

   The message MUST be transmitted in accordance with the Simple Mail
   Transport Protocol, as updated by the DRUMS working group [DRUMSMTP].

   Delivery Status Notifications MAY be requested [DSN] if delivery of
   the message is important to the originator and a mechanism exists to
   return status indications to them (which may not be possible for
   voicemail originators).

5.  Addressing

   Any valid Internet Mail address may be used for a voice message.

   It is desirable to be able to use an onramp/offramp for delivery of a
   voicemail message to a user, which will result in specific addressing
   requirements, based on service selectors defined in [SELECTOR].
   Further discussion of addressing requirements for voice messages can
   be found in the VPIM Addressing document [ADDRESS].

   It is desirable to permit the use of a directory service to map
   between the E.164 phone number of the recipient and an SMTP mailbox
   address.  A discussion on how this may be achieved using the ENUM
   infrastructure is in [VPIMENUM].  A definition of the VPIM LDAP
   schema that a system would use is found in [SCHEMA].

   If a message is created and stored as a result of call answering, the
   caller's name and number MAY be stored in the message headers in its
   original format per [CLID].

6.  Notifications

   Delivery Status Notifications MUST be supported.  All non-delivery of
   messages MUST result in an NDN, if requested [DSN, DSN2, DSN3, DSN4].
   If the receiving system supports content criticality and is unable to
   process all of the critical media types within a voice message
   (indicated by the content criticality), then it MUST non-deliver the
   entire message per [CRITICAL].

   Message Disposition Notifications SHOULD be supported (but according
   to MDN rules, the user MUST be given the option of deciding whether
   MDNs are returned) per [MDN].

   If the recipient is unable to display all of the indicated critical
   content components indicated, then it SHOULD give the user the option
   of returning an appropriate MDN (see [CRITICAL]).

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7.  Voice Contents

   Voice messages may be contained at any location within a message and
   MUST always be contained in an audio/basic content-type, unless the
   originator is aware that the recipient can handle other content.
   Specifically, Audio/32kadpcm MAY be used when the recipient is known
   to support VPIM v2 [VPIMV2R2].

   The VOICE parameter on Content-Disposition from VPIM v2 [VPIMV2R2]
   SHOULD be used to identify any spoken names or spoken subjects (as
   distinct from voice message contents).  As well, the Content-Duration
   header [DUR] SHOULD be used to indicate the audio length.

   The originator's spoken name MAY be included with messages as
   separate audio contents, if known, and SHOULD be indicated by the
   Content-Disposition VOICE parameter as defined in VPIM v2 [VPIMV2R2].
   If there is a single recipient for the message, the spoken name MAY
   also be included (per VPIM v2).  A spoken subject MAY also be
   provided (per VPIM v2).

   A sending implementation MAY determine the recipient capabilities
   before sending a message and choose a codec accordingly (e.g., using
   some form of content negotiation).  In the absence of such recipient
   knowledge, sending implementations MUST use raw G.711 mu-law, which
   is indicated with a MIME content type of "audio/basic" (and SHOULD
   use a filename parameter that ends ".au") [G711], [MIME2].  A sending
   implementation MAY support interoperability with VPIM v2 [VPIMV2R2],
   in which case, it MUST be able to record G.726 (indicated as
   audio/32kadpcm) [G726], [ADPCM].

   Recipients MUST be able to play a raw G.711 mu-law message, and MAY
   be able to play G.726 (indicated as audio/32kadpcm) to provide
   interoperability with VPIM v2.  A receiving implementation MAY also
   be able to play messages encoded with other codecs (either natively
   or via transcoding).

   These requirements may be summarized as follows:

   Codec           No VPIM v2 Support      With VPIM v2 Support
                   Record    Playback      Record      Playback
   -------------   ------    --------      ------      --------

   G.711 mu-law     MUST      MUST          MUST        MUST
   G.726            *         MAY           MUST        MUST
   Other            *         MAY           *           MAY

      * = MUST NOT, but MAY only if recipient capabilities known

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8.  Fax Contents

   Fax contents SHOULD be carried according to RFC 2532 [IFAX].

9.  Interoperability with VPIM v2

   Interoperability between VPIM v2 systems and IVM systems can take a
   number of different forms.  While a thorough investigation of how
   full interoperability might be provided between IVM and VPIM v2
   systems is beyond the scope of this document; three key alternatives
   are discussed below.

9.1.  Handling VPIM v2 Messages in an IVM Client

   If an IVM-conformant client is able to process a content type of
   multipart/voice-message (by treating it as multipart/mixed) and play
   a G.726 encoded audio message within it (indicated by a content type
   of audio/32kadpcm), then a VPIM v2 message that gets routed to that
   desktop will be at least usable by the recipient.

   This delivers a level of partial interoperability that would ease the
   life of end users.  However, care should be taken to ensure that any
   attempt to reply to such a message does not result in an invalid VPIM
   v2 message being sent to a VPIM v2 system.  Note that replying to an
   e-mail user who has forwarded a VPIM v2 message to you is, however,

   A conformant IVM implementation MUST NOT send a non-VPIM v2 message
   to something it knows to be a VPIM v2 system, unless it also knows
   that the destination system can handle such a message (even though
   VPIM v2 systems are encouraged to handle non-VPIM v2 messages in a
   graceful manner).  In general, it must be assumed that if a system
   sends you a conformant VPIM v2 message, then it is a VPIM v2 system,
   and so you may only reply with a VPIM v2 compliant message (unless
   you know by some other means that the system supports IVM).

   In addition, it should be noted that an IVM client may not fully
   conform to VPIM v2, even if it supports playing a G.726 message
   (e.g., it may not respect the handling of the Sensitivity field
   required by VPIM v2).  This is one reason why VPIM v2 systems may
   choose not to route messages to any system they do not know to be
   VPIM v2 compliant.

9.2.  Dual Mode Systems and Clients

   A VPIM v2 system could be extended to also be able to support IVM
   compliant messages, and an IVM conformant client could be extended to
   implement VPIM v2 in full when corresponding with a VPIM v2 compliant

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   system.  This is simply a matter of implementing both specifications
   and selecting the appropriate one, depending on the received message
   content or the recipient's capabilities.  This delivers full
   interoperability for the relevant systems, providing the capabilities
   of the target users can be determined.

   Note that the mechanism for determining if a given recipient is using
   a VPIM v2 system or client is outside of the scope of this
   specification.  Various mechanisms for capabilities discovery exist
   that could be applied to this problem, but no standard solution has
   yet been defined.

9.3.  Gateways

   It would be possible to build a gateway linking a set of VPIM v2
   users with a set of IVM users.  This gateway would implement the
   semantics of the two worlds, and translate between them according to
   defined policies.

   For example, VPIM v2 messages with a Sensitivity of Private might be
   rejected instead of forwarded to an IVM recipient, because it might
   not implement the semantics of a Private message, while an IVM
   message containing content not supported in VPIM v2 (e.g., a PNG
   image), with a Criticality of CRITICAL, would be rejected in the

   Such a gateway MUST fully implement this specification and the VPIM
   v2 specification [VPIMV2R2], unless it knows somehow that the
   specific originators/recipients support capabilities beyond those
   required by these standards.

10.  Security Considerations

   This document presents an optional gateway between IVM and VPIM
   systems.  Gateways are inherently lossy systems and not all
   information can be accurately translated.  This applies to both the
   transcoding of the voice and the translation of features.  Two
   examples of feature translation are given in 9.3, but the risk
   remains that different gateways will handle the translation
   differently since it is undefined in this document.  This may lead to
   unexpected behavior through gateways.

   In addition, gateways present an additional point of attack for those
   interested in compromising a messaging system.  If a gateway is
   compromised, "monkey in the middle" attacks, conducted from the
   compromised gateway, may be difficult to detect or appear to be
   authorized transformations.

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RFC 4239                Internet Voice Messaging           November 2005

   Aside from the gateway issue, it is anticipated that there are no new
   additional security issues beyond those identified in VPIM v2
   [VPIMV2R2] and in the other RFCs referenced by this document --
   especially SMTP [DRUMSMTP], Internet Message Format [DRUMSIMF], MIME
   [MIME2], Critical Content [CRITICAL], and Message Context [HINT].

11.  References

11.1.  Normative References

   [ADDRESS]    Parsons, G., "Voice Profile for Internet Mail (VPIM)
                Addressing", RFC 3804, June 2004.

   [ADPCM]      Vaudreuil, G. and G. Parsons, "Toll Quality Voice - 32
                kbit/s Adaptive Differential Pulse Code Modulation
                (ADPCM) MIME Sub-type Registration", RFC 3802, June

   [BEHAVIOUR]  Parsons, G. and J. Maruszak, "Voice Messaging Client
                Behaviour", RFC 4024, July 2005.

   [CLID]       Parsons, G. and J. Maruszak, "Calling Line
                Identification for Voice Mail Messages", RFC 3939,
                December 2004.

   [CRITICAL]   Burger, E., "Critical Content Multi-purpose Internet
                Mail Extensions (MIME) Parameter", RFC 3459, January

   [DSN]        Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
                Extension for Delivery Status Notifications (DSNs)", RFC
                3461, January 2003.

   [DSN2]       Vaudreuil, G., "The Multipart/Report Content Type for
                the Reporting of Mail System Administrative Messages",
                RFC 3462, January 2003.

   [DSN3]       Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
                3463, January 2003.

   [DSN4]       Moore, K. and G. Vaudreuil, "An Extensible Message
                Format for Delivery Status Notifications", RFC 3464,
                January 2003.

   [DRUMSMTP]   Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
                April 2001.

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RFC 4239                Internet Voice Messaging           November 2005

   [DRUMSIMF]   Resnick, P., "Internet Message Format", RFC 2822, April

   [DUR]        Vaudreuil, G. and G. Parsons, "Content Duration MIME
                Header Definition", RFC 3803, June 2004.

   [HINT]       Burger, E., Candell, E., Eliot, C., and G. Klyne,
                "Message Context for Internet Mail", RFC 3458, January

   [IFAX]       Masinter, L. and D. Wing, " Extended Facsimile Using
                Internet Mail", RFC 2532, March 1999.

   [KEYWORDS]   Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [MDN]        Hansen, T. and G. Vaudreuil, "Message Disposition
                Notification", RFC 3798, May 2004.

   [MIME1]      Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part One: Format of Internet Message
                Bodies", RFC 2045, November 1996.

   [MIME2]      Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part Two: Media Types", RFC 2046,
                November 1996.

   [MIME5]      Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part Five: Conformance Criteria and
                Examples", RFC 2049, November 1996.

   [SELECTOR]   Allocchio, C., "Minimal GSTN address format in Internet
                Mail", RFC 3191, October 2001.

   [SCHEMA]     Vaudreuil, G., "Voice Messaging Directory Service", RFC
                4237, October 2005.

   [VPIMENUM]   Vaudreuil, G., "Voice Message Routing Service", RFC
                4238, October 2005.

   [VPIMV2]     Vaudreuil, G. and G. Parsons, "Voice Profile for
                Internet Mail -  version 2", RFC 2421, September 1998.

   [VPIMV2R2]   Vaudreuil, G. and G. Parsons, "Voice Profile for
                Internet Mail - version 2 (VPIMv2)", RFC 3801, June

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RFC 4239                Internet Voice Messaging           November 2005

11.2.  Informative References

   [GOALS]      Candell, E., "High-Level Requirements for Internet Voice
                Mail", RFC 3773, June 2004.

   [G726]       CCITT Recommendation G.726 (1990), General Aspects of
                Digital Transmission Systems, Terminal Equipment - 40,
                32, 24, 16 kbit/s Adaptive Differential Pulse Code
                Modulation (ADPCM).

   [G711]       ITU-T Recommendation G.711 (1993), General Aspects of
                Digital Transmission Systems, Terminal Equipment - Pulse
                Code Modulation (PCM) of Voice Frequencies.

Authors' Addresses

   Stuart J. McRae
   Lotus Park, The Causeway<
   Staines, TW18 3AG
   United Kingdom

   Phone: +44 1784 445 112
   Fax: +44 1784 499 112
   EMail: stuart.mcrae@uk.ibm.com

   Glenn W. Parsons
   Nortel Networks
   3500 Carling Avenue
   Ottawa, ON K2H 8E9

   Phone: +1-613-763-7582
   Fax: +1-613-967-5060
   EMail: gparsons@nortel.com

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Full Copyright Statement

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