Network Working Group S. McRae
Request for Comments: 4239 IBM
Category: Standards Track G. Parsons
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 (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
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
(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
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [KEYWORDS].
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.
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
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].
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]).
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
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
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.
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
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
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].
11.1. 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,
[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,
[DRUMSMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
[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,
[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
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
[G711] ITU-T Recommendation G.711 (1993), General Aspects of
Digital Transmission Systems, Terminal Equipment - Pulse
Code Modulation (PCM) of Voice Frequencies.
Stuart J. McRae
Lotus Park, The Causeway<
Staines, TW18 3AG
Phone: +44 1784 445 112
Fax: +44 1784 499 112
Glenn W. Parsons
3500 Carling Avenue
Ottawa, ON K2H 8E9
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