Network Working Group G. Vaudreuil
Request for Comments: 2421 Lucent Technologies
Obsoletes: 1911 G. Parsons
Category: Standards Track Northern Telecom
September 1998
Voice Profile for Internet Mail - version 2
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 (1998). All Rights Reserved.
Overview
This document profiles Internet mail for voice messaging. It
obsoletes RFC 1911 which describes version 1 of the profile. A list
of changes from that document are noted in Appendix F. As well,
Appendix A summarizes the protocol profiles of this version of VPIM.
Please send comments on this document to the EMA VPIM Work Group
mailing list: <vpim-l@ema.org>
Working Group Summary
This profile is not the product of an IETF working group, though
several have reviewed the document. It is instead the product of the
VPIM Work Group of the Electronic Messaging Association (EMA). This
work group, which has representatives from most major voice mail
vendors and several email vendors, has held several interoperability
demonstrations between voice messaging vendors and is currently
promoting VPIM trials and deployment.
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Table of Contents
1. ABSTRACT .........................................................3
2. SCOPE ............................................................3
2.1 Voice Messaging System Limitations ............................3
2.2 Design Goals ..................................................4
3. PROTOCOL RESTRICTIONS ............................................5
4. VOICE MESSAGE INTERCHANGE FORMAT .................................6
4.1 Message Addressing Formats ....................................6
4.2 Message Header Fields .........................................9
4.3 Voice Message Content Types ..................................15
4.4 Other Message Content Types ..................................21
4.5 Forwarded Messages ...........................................23
4.6 Reply Messages ...............................................23
4.7 Notification Messages ........................................24
5. MESSAGE TRANSPORT PROTOCOL ......................................24
5.1 ESMTP Commands ...............................................25
5.2 ESMTP Keywords ...............................................27
5.3 ESMTP Parameters - MAIL FROM .................................28
5.4 ESMTP Parameters - RCPT TO ...................................29
5.5 ESMTP - SMTP Downgrading .....................................29
6. DIRECTORY ADDRESS RESOLUTION ....................................30
7. IMAP ............................................................30
8. MANAGEMENT PROTOCOLS ............................................30
8.1 Network Management ...........................................31
9. CONFORMANCE REQUIREMENTS ........................................31
10. SECURITY CONSIDERATIONS ........................................32
10.1 General Directive ...........................................32
10.2 Threats and Problems ........................................32
10.3 Security Techniques .........................................33
11. REFERENCES .....................................................33
12. ACKNOWLEDGMENTS ................................................36
13. AUTHORS' ADDRESSES .............................................36
14. APPENDIX A - VPIM REQUIREMENTS SUMMARY .........................37
15. APPENDIX B - EXAMPLE VOICE MESSAGES ............................45
16. APPENDIX C - EXAMPLE ERROR VOICE PROCESSING ERROR CODES ........50
17. APPENDIX D - EXAMPLE VOICE PROCESSING DISPOSITION TYPES ........51
18. APPENDIX E - IANA REGISTRATIONS ................................52
18.1 vCard EMAIL Type Definition for VPIM ........................52
18.2 Voice Content-Disposition Parameter Definition ..............52
19. APPENDIX F - CHANGE HISTORY: RFC 1911 TO THIS DOCUMENT .........54
20. FULL COPYRIGHT NOTICE ..........................................56
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1. Abstract
A class of special-purpose computers has evolved to provide voice
messaging services. These machines generally interface to a
telephone switch and provide call answering and voice messaging
services. Traditionally, messages sent to a non-local machine are
transported using analog networking protocols based on DTMF signaling
and analog voice playback. As the demand for networking increases,
there is a need for a standard high-quality digital protocol to
connect these machines. The following document is a profile of the
Internet standard MIME and ESMTP protocols for use as a digital voice
messaging networking protocol. The profile is referred to as VPIM
(Voice Profile for Internet Mail) in this document.
This profile is based on earlier work in the Audio Message
Interchange Specification (AMIS) group that defined a voice messaging
protocol based on X.400 technology. This profile is intended to
satisfy the user requirements statement from that earlier work with
the industry standard ESMTP/MIME mail protocol infrastructures
already used within corporate intranets. This second version of VPIM
is based on implementation experience and obsoletes RFC 1911 which
describes version 1 of the profile.
2. Scope
MIME is the Internet multipurpose, multimedia messaging standard.
This document explicitly recognizes its capabilities and provides a
mechanism for the exchange of various messaging technologies,
primarily voice and facsimile.
This document specifies a restricted profile of the Internet
multimedia messaging protocols for use between voice processing
server platforms. These platforms have historically been special-
purpose computers and often do not have the same facilities normally
associated with a traditional Internet Email-capable computer. As a
result, VPIM also specifies additional functionality as it is needed.
This profile is intended to specify the minimum common set of
features to allow interworking between compliant systems.
2.1 Voice Messaging System Limitations
The following are typical limitations of voice messaging platform
which were considered in creating this baseline profile.
1) Text messages are not normally received and often cannot be
easily displayed or viewed. They can often be processed only via
text-to-speech or text-to-fax features not currently present in
many of these machines.
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2) Voice mail machines usually act as an integrated Message
Transfer Agent, Message Store and User Agent. There is no relaying
of messages, and RFC 822 header fields may have limited use in the
context of the limited messaging features currently deployed.
3) Voice mail message stores are generally not capable of
preserving the full semantics of an Internet message. As such, use
of a voice mail machine for gatewaying is not supported. In
particular, storage of recipient lists, "Received" lines, and
"Message-ID" may be limited.
4) Internet-style distribution/exploder mailing lists are not
typically supported. Voice mail machines often implement only
local alias lists, with error-to-sender and reply-to-sender
behavior. Reply-all capabilities using a CC list are not generally
available.
5) Error reports must be machine-parsable so that helpful responses
can be voiced to users whose only access mechanism is a telephone.
6) The voice mail systems generally limit address entry to 16 or
fewer numeric characters, and normally do not support alphanumeric
mailbox names. Alpha characters are not generally used for mailbox
identification as they cannot be easily entered from a telephone
terminal.
2.2 Design Goals
It is a goal of this profile to make as few restrictions and
additions to the existing Internet mail protocols as possible while
satisfying the requirements for interoperability with current
generation voice messaging systems. This goal is motivated by the
desire to increase the accessibility to digital messaging by enabling
the use of proven existing networking software for rapid development.
This specification is intended for use on a TCP/IP network; however,
it is possible to use the SMTP protocol suite over other transport
protocols. The necessary protocol parameters for such use is outside
the scope of this document.
This profile is intended to be robust enough to be used in an
environment, such as the global Internet with installed-base gateways
which do not understand MIME, though typical use is expected to be
within corporate intranets. Full functionality, such as reliable
error messages and binary transport, will require careful selection
of gateways (e.g., via MX records) to be used as VPIM forwarding
agents. Nothing in this document precludes use of general purpose
MIME email packages to read and compose VPIM messages. While no
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special configuration is required to receive VPIM compliant messages,
some may be required to originate compliant structures.
It is expected that a VPIM messaging system will be managed by a
system administrator who can perform TCP/IP network configuration.
When using facsimile or multiple voice encodings, it is suggested
that the system administrator maintain a list of the capabilities of
the networked mail machines to reduce the sending of undeliverable
messages due to lack of feature support. Configuration,
implementation and management of these directory listing capabilities
are local matters.
3. Protocol Restrictions
This protocol does not limit the number of recipients per message.
Where possible, server implementations should not restrict the number
of recipients in a single message. It is recognized that no
implementation supports unlimited recipients, and that the number of
supported recipients may be quite low.
This protocol does not limit the maximum message length.
Implementers should understand that some machines will be unable to
accept excessively long messages. A mechanism is defined in the RFC
1425 SMTP service extensions to declare the maximum message size
supported.
The message size indicated in the ESMTP SIZE parameter is in bytes,
not minutes or seconds. The number of bytes varies by voice encoding
format and includes the MIME wrapper overhead. If the length must be
known before sending, an approximate translation into minutes or
seconds can be performed if the voice encoding is known.
The following sections describe the restrictions and additions to
Internet mail protocols that are required to be compliant with this
VPIM v2 profile. Though various SMTP, ESMTP and MIME features are
described here, the implementer is referred to the relevant RFCs for
complete details. It is also advisable to check for IETF drafts of
various Internet Mail specifications that are later than the most
recent RFCs since, for example, MIME has yet to be published as a
full IETF Standard. The table in Appendix A summarizes the protocol
details of this profile.
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 [REQ].
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4. Voice Message Interchange Format
The voice message interchange format is a profile of the Internet
Mail Protocol Suite. Any Internet Mail message containing the format
defined in this section is referred to as a VPIM Message in this
document. As a result, this document assumes an understanding of the
Internet Mail specifications. Specifically, VPIM references
components from the message format standard for Internet messages
[RFC822], the Multipurpose Internet Message Extensions [MIME], the
X.400 gateway specification [X.400], delivery status and message
disposition notifications [REPORT][DSN][DRPT][STATUS][MDN], and the
electronic business card [MIMEDIR][VCARD].
4.1 Message Addressing Formats
RFC 822 addresses are based on the domain name system. This naming
system has two components: the local part, used for username or
mailbox identification; and the host part, used for global machine
identification.
4.1.1 VPIM Addresses
The local part of the address shall be a US-ASCII string uniquely
identifying a mailbox on a destination system. For voice messaging,
the local part is a printable string containing the mailbox ID of the
originator or recipient. While alpha characters and long mailbox
identifiers are permitted, most voice mail networks rely on numeric
mailbox identifiers to retain compatibility with the limited 10 digit
telephone keypad. As a result, some voice messaging systems may only
be able to handle a numeric local part. The reception of
alphanumeric local parts on these systems may result in the address
being mapped to some locally unique (but confusing to the recipient)
number or, in the worst case the address could be deleted making the
message un-replyable. Additionally, it may be difficult to create
messages on these systems with an alphanumeric local part without
complex key sequences or some form of directory lookup (see 6).
The use of the domain naming system should be transparent to the
user. It is the responsibility of the voice mail machine to lookup
the fully-qualified domain name (FQDN) based on the address entered
by the user (see 6).
In the absence of a global directory, specification of the local part
is expected to conform to international or private telephone
numbering plans. It is likely that private numbering plans will
prevail and these are left for local definition. However, it is
RECOMMENDED that public telephone numbers be noted according to the
international numbering plan described in [E.164]. The indication
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that the local part is a public telephone number is given by a
preceding `+' (the `+' would not be entered from a telephone keypad,
it is added by the system as a flag). Since the primary information
in the numeric scheme is contained by the digits, other character
separators (e.g. `-') may be ignored (i.e. to allow parsing of the
numeric local mailbox) or may be used to recognize distinct portions
of the telephone number (e.g. country code). The specification of
the local part of a VPIM address can be split into the four groups
described below:
1) mailbox number
- for use as a