RFC 3801
Voice Profile for Internet Mail - version 2 (VPIMv2)
Network Working Group G. Vaudreuil Request for Comments: 3801 Lucent Technologies Obsoletes: 2421 G. Parsons Category: Standards Track Nortel Networks June 2004 Voice Profile for Internet Mail - version 2 (VPIMv2) 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 (2004).Abstract
This document specifies a restricted profile of the Internet multimedia messaging protocols for use between voice processing server platforms. The profile is referred to as the Voice Profile for Internet Mail (VPIM) in this document. 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 conforming systems. This document obsoletes RFC 2421 and describes version 2 of the profile with greater precision. No protocol changes were made in this revision. A list of changes from RFC 2421 are noted in Appendix F. Appendix A summarizes the protocol profiles of this version of VPIM.
Table of Contents
1. Introduction...................................................3 1.1. Voice Messaging System Limitations.......................3 1.2. Design Goals.............................................4 1.3. Applicability for VPIM...................................5 2. Requirements Language..........................................5 3. Protocol Restrictions..........................................6 4. Voice Message Interchange Format...............................6 4.1. VPIM Message Addressing Formats..........................7 4.2. Message Header Fields....................................9 4.3. MIME Audio Content Descriptions.........................17 4.4. Voice Message Content Types.............................19 4.5. Other MIME Contents.....................................23 4.6. Delivery Status Notification (DSN)......................25 4.7. Message Disposition Notification (MDN)..................26 4.8. Forwarded Messages......................................26 4.9. Reply Messages..........................................27 5. Message Transport Protocol....................................27 5.1. Base SMTP Protocol......................................28 5.2. SMTP Service Extensions.................................28 5.3. ESMTP - SMTP Downgrading................................30 6. Directory Address Resolution..................................30 7. Management Protocols..........................................30 7.1. Network Management......................................31 8. Conformance Requirements......................................31 9. Security Considerations.......................................32 9.1. General Directive.......................................32 9.2. Threats and Problems....................................32 9.3. Security Techniques.....................................33 10. Normative References..........................................33 11. Acknowledgments...............................................36 12. Appendix A - VPIM Requirements Summary........................37 13. Appendix B - Example Voice Messages...........................43 14. Appendix C - Example Error Voice Processing Error Codes.......49 15. Appendix D - Example Voice Processing Disposition Types.......50 16. Appendix E - IANA Registrations...............................50 16.1. Voice Content-Disposition Parameter Definition.........51 16.2. Multipart/Voice-Message MIME Media Type Definition.....51 17. Appendix F - Change History: RFC 2421 (VPIM V2) To This Doc...53 18. Authors' Addresses............................................54 19. Full Copyright Statement......................................55
1. Introduction
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. Voice messaging evolved as telephone answering service into a full send, receive, and forward messaging paradigm with unique message features, semantics and usage patterns. Voice messaging was introduced on special purpose computers that interface to a telephone switch and provide call answering and voice messaging services. Traditionally, messages sent from one voice messaging system to another were transported using analog networking protocols based on DTMF signaling and analog voice playback. As the demand for networking increases, there was a need for a standard high-quality digital protocol to connect these machines. VPIM has successfully demonstrated its usefulness as this new standard. VPIM is widely implemented and is seeing deployment in customer networks. This document clarifies ambiguities found in the earlier specification and is consistent with implementation practice. The profile is referred to as Voice Profile for Internet Mail (VPIM) in this document. 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 conforming systems. This document obsoletes RFC 2421 and describes VPIM version 2 of with greater precision. No protocol changes were made in this revision. A list of changes from RFC 2421 are noted in Appendix F. Appendix A summarizes the protocol profiles of this version of VPIM.1.1. Voice Messaging System Limitations
The following are typical limitations of voice messaging platforms that 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.
2) Voice mail machines usually act as an integrated Message
Transfer Agent, Message Store and User Agent. There is typically
no relaying of messages. RFC822 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.
It should be noted that newer systems are based natively on SMTP/MIME
and do not suffer these limitations. In particular, some systems may
support media other than voice and fax.
1.2. 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 are
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 that do not understand MIME. 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 special configuration is required to receive VPIM conforming messages, some may be required to originate conforming structures. It is expected that a system administrator who can perform TCP/IP network configuration will manage a VPIM messaging system. 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.1.3. Applicability for VPIM
VPIM is intended for the exchange of voice messages between traditional voice messaging systems and for systems that need to interoperate with such systems. VPIM is intended connect voice- messaging systems into special-purpose voice messaging networks. VPIM may also be used between message store servers and VPIM-aware clients such as web servers, TUI, and GUI clients. VPIM is not intended or optimized for downloading to, or sending from commercial email clients. Internet Voice Messaging, the subject of a separate standards initiative, is intended to enable general-purpose email clients to send and receive voice content through general-purpose message stores in an interoperable way. IVM may also be a suitable format for downloading voice messages from a VPIM server to a commercial email client. It may also be a suitable format for submission of a voice message from a general-purpose client into a VPIM system.2. Requirements Language
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].
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 [SIZE] to declare the maximum message size supported. The following sections describe the restrictions and additions to Internet mail protocols that are required to be conforming 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. The table in Appendix A summarizes the protocol details of this profile.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 [MIME1-5], the X.400 gateway specification [X.400], and the delivery status and message disposition notifications [REPORT][DSN][DRPT][STATUS][MDN]. MIME, introduced in [MIME1], is a general-purpose message body format that is extensible to carry a wide range of body parts. It provides for encoding binary data so that it can be transported over the 7-bit text-oriented SMTP protocol. This transport encoding (denoted by the "Content-Transfer-Encoding:" MIME field) is in addition to the audio encoding required to generate a binary object. MIME defines two transport-encoding mechanisms to transform binary data into a 7-bit representation, one designed for text-like data ("Quoted-Printable"), and one for arbitrary binary data ("Base64"). While Base64 is dramatically more efficient for audio data, either will work. Where binary transport is available, no transport encoding is needed, and the data can be labeled as "Binary".
4.1. VPIM Message Addressing Formats
VPIM addresses SHALL use the RFC 822 format 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 SHALL be a printable string containing the mailbox ID of the originator or recipient. While alpha characters and long mailbox identifiers MAY be permitted, short numeric local parts SHOULD be used as 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 unreplyable. 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 Name 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 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 private numbering plan (any number of digits) - e.g., 2722@lucent.com
2) mailbox number+extension
- for use as a private numbering plan with extensions
any number of digits, use of "+" as separator
- e.g., 2722+111@Lucent.com
3) +international number
- for international telephone numbers conforming to E.164
maximum of 15 digits
- e.g., +16137637582@vm.nortel.ca
4) +international number+extension
- for international telephone numbers conforming to E.164
maximum of 15 digits, with an extension (e.g., behind a
PBX) that has a maximum of 15 digits.
- e.g., +17035245550+230@ema.org
Note that this address format is designed to be compatible with
current usage within the voice messaging industry. It is not
compatible with the addressing formats of RFCs 2303-2304. It is
expected that as telephony services become more widespread on the
Internet, these addressing formats will converge.
4.1.2. Special Addresses
Special addresses to represent the sender are provided for
compatibility with the conventions of Internet mail. These addresses
do not use numeric local addresses, both to conform to current
Internet practice and to avoid conflict with existing numeric
addressing plans. Two special addresses are RESERVED for use as
follows:
postmaster@domain
By convention, a special mailbox named "postmaster" MUST exist on all
systems. This address is used for diagnostics and should be checked
regularly by the system manager. This mailbox is particularly likely
to receive text messages, which is not normal on a voice-processing
platform. The specific handling of these messages is an individual
implementation choice.
non-mail-user@domain
If a reply to a message is not possible, such as a telephone-
answering message, then the special address "non-mail-user" SHOULD be
used as the originator's address. Any text name such as "Telephone
Answering", or the telephone number if it is available, is permitted.
This special address is used as a token to indicate an unreachable
originator. A conforming implementation MUST NOT permit a reply to an
address from "non-mail-user". For compatibility with the installed
base of mail user agents, implementations MUST reject the message
when a message addressed to "non-mail-user" is received. The status
code for such NDN's is 5.1.1 "Mailbox does not exist".
Example:
From: Telephone Answering <non-mail-user@mycompany.com>
4.1.3. Distribution Lists
There are many ways to handle distribution list (DL) expansions and
none are 'standard'. A VPIM implementation MAY support DLs. Using a
simple alias is a behavior closest to what many voice mail systems do
today and what is to be used with VPIM messages. A couple of
important features that need special care when DLs are used are:
Reply to the originator - (Address in the RFC822 "Reply-To:" or
"From" field)
Errors to the submitter - (Address in the MAIL FROM field of the
ESMTP exchange or the "Return-Path:"
RFC822 field)
Some proprietary voice messaging protocols include only the recipient
of the particular copy in the envelope and include no "header fields"
except date and per-message features. Most voice messaging systems
do not provide for "Header Information" in their messaging queues and
only include delivery information. As a result, recipient
information MAY be in either the "To:" or "Cc:" header fields. If all
recipients cannot be presented then the recipient header fields
SHOULD be omitted to indicate that an accurate list of recipients
(e.g., for use with a reply-all capability) is not known.
4.2. Message Header Fields
Internet messages contain a header information block. This header
block contains information required to identify the sender, the list
of recipients, the message send time, and other information intended
for user presentation. Except for specialized gateway and mailing
list cases, header fields do not indicate delivery options for the
transport of messages.
Distribution list processors are noted for modifying or adding to the
header fields of messages that pass through them. VPIM systems MUST
be able to accept and ignore header fields that are not defined here.
The following header lines are permitted for use with VPIM messages:
4.2.1. From
SEND RULES The originator's fully qualified domain address (a mailbox address followed by the fully qualified domain name) MUST be present. Systems conforming with this profile SHOULD provide the text personal name of the voice message originator in a quoted phrase, if the name is available. Text names of corporate or positional mailboxes MAY be provided as a simple string. From: [RFC822] Example: From: "Joe S. User" <12145551212@mycompany.com> From: Technical Support <611@serviceprovider.com> From: Non-mail-user@myserver.mycompany.com Voice mail machines may not be able to support separate attributes for the "From:" header fields and the SMTP MAIL FROM, VPIM-conforming systems SHOULD set these values to the same address. Use of addresses different than those present in the "From:" header field address may result in unanticipated behavior. RECEIVE RULES The user listed in the "From:" field MUST be presented in the voice message envelope of the voice messaging system as the originator of the message, though the exact presentation is an implementation decision (e.g., the mailbox ID or the text name MAY be presented). The "From:" address SHOULD be used for replies (see 4.9).4.2.2. To
The "To:" field contains the recipient's fully-qualified domain address. Example: To: +12145551213@mycompany.com SEND RULES There MAY be one or more "To:" fields in any message. Systems SHOULD provide a list of recipients only if all recipients are available.
Systems, such as gateways from protocols or legacy platforms that do not indicate the complete list of recipients, MAY provide a "To:" line. Because these systems cannot accurately enumerate all recipients in the "To:" headers, recipients SHOULD NOT be enumerated. RECEIVE RULES Systems conforming to this profile MAY discard the addresses in the "To:" fields if they are unable to store the information. This would, of course, make a reply-to-all capability impossible. If present, the addresses in the "To:" field MAY be used for a reply message to all recipients.4.2.3. Cc
The "Cc:" field contains additional recipients' fully qualified domain addresses. Many voice mail systems maintain only sufficient envelope information for message delivery and are not capable of storing or providing a complete list of additional recipients. SEND RULES Conforming implementations MAY send "Cc:" lists if all recipients are known at the time of origination. If not, systems SHOULD omit the "Cc:" fields to indicate that the full list of recipients is unknown or otherwise unavailable. The list of disclosed recipients MUST NOT include undisclosed recipients (i.e., those sent via a blind copy). Example: Cc: +12145551213@mycompany.com RECEIVE RULES Systems conforming to this profile MAY add all the addresses in the "Cc:" field to the "To:" field, others MAY discard the addresses in the "Cc:" fields. If a list of "Cc:" addresses is present, these addresses MAY be used for a reply message to all recipients.4.2.4. Date
The "Date:" field contains the date and time the message was sent by the originator. SEND RULES The sending system MUST report the time the message was sent. The time zone MUST be present and SHOULD be represented in a four-digit
time zone offset, such as -0500 for North American Eastern Standard
Time. This MAY be supplemented by a time zone name in parentheses,
e.g., "-0700 (PDT)".
Example:
Date: Wed, 28 Jul 96 10:08:49 -0800 (PST)
If the VPIM sender is relaying a message from a system that does not
provide a time stamp, the time of arrival at the gateway system
SHOULD be used as the date.
RECEIVE RULES
Conforming implementations SHOULD be able to convert [RFC822] date
and time stamps into local time
4.2.5. Sender
The "Sender:" field contains the actual address of the originator if
an agent on behalf of the author indicated in the "From:" field sends
the message.
SEND RULES
This header field MAY be sent by VPIM-conforming systems.
RECEIVE RULES
If the address in the "Sender:" field cannot be preserved in the
recipient's message queues or in the next-hop protocol from a
gateway, the field MAY be silently discarded.
4.2.6. Return-Path
The "Return-path:" field is added by the final delivering SMTP
server. If present, it contains the address from the MAIL FROM
parameter of the ESMTP exchange (see [RFC822]). Any error messages
resulting from the delivery failure MUST be sent to this address.
Note that if the "Return-path:" is null ("<>") (e.g., a call answer
message would have no return path) delivery status notifications MUST
NOT be sent.
SEND RULES
The originating system MUST NOT add this header.
RECEIVE RULES If the receiving system is incapable of storing the return path (or MAIL FROM) to be used for subsequent delivery errors (i.e., it is a gateway to a legacy system or protocol), the receiving system must otherwise ensure that further delivery errors don't happen. Systems that do not support the return path MUST ensure that at the time the message is acknowledged (i.e., when a DSN would be sent), the message is delivered to the recipient's ultimate mailbox. Non-Delivery notifications SHOULD NOT be sent after that final delivery.4.2.7. Message-id
The "Message-Id:" field contains a globally unique per-message identifier. SEND RULES A globally unique message-id MUST be generated for each message sent from a VPIM-conforming implementation. Example: Message-Id: <12345678@mycompany.com> RECEIVE RULES When provided in the original message, it MUST be used when sending a MDN. This identifier MAY be used for tracking and auditing. From [RFC822]4.2.8. Reply-To
If present, the "Reply-To:" header provides a preferred address to which reply messages should be sent (see 4.9). Typically, voice mail systems can only support one originator of a message so it is likely that this field will be ignored by the receiving system. From: [RFC822] SEND RULES A conforming system SHOULD NOT send a "Reply-To:" header. RECEIVE RULES If a "Reply-To:" field is present, a reply-to-sender message MAY be sent to the address specified (that is, in lieu of the address in the "From:" field). If the receiving system (e.g., multi-protocol
gateway) only supports one address for the originator, then the address in the "From:" field MUST be used and the "Reply-To:" field MAY be silently discarded.4.2.9. Received
The "Received:" field contains trace information added to the beginning of a RFC822 message by MTAs. This is the only field that may be added by an MTA. Information in this header is useful for debugging when using an US-ASCII message reader or a header-parsing tool. From: [RFC822] SEND RULES A VPIM-conforming system MUST add a "Received:" field. When acting as a gateway, information about the system from which the message was received SHOULD be included. RECEIVE RULES A VPIM-conforming system MUST NOT remove any "Received:" fields when relaying messages to other MTAs or gateways. These header fields MAY be ignored or deleted when the message is received at the final destination.4.2.10. MIME Version
The "MIME-Version:" field MUST be present to indicate that the message conforms to [MIME1-5]. Systems conforming with this specification SHOULD include a comment with the words "(Voice 2.0)". [VPIM1] defines an earlier version of this profile and uses the token (Voice 1.0). Example: MIME-Version: 1.0 (Voice 2.0) This identifier is intended for information only and SHOULD NOT be used to semantically identify the message as being a VPIM message. Instead, the presence of the multipart/voice-message content type defined in section 18.2 SHOULD be used if identification is necessary.4.2.11. Content-Type
The "Content-Type:" header MUST be present to declare the type of content enclosed in the message. The typical top-level content in a VPIM Message SHOULD be Multipart/Voice-Message. The allowable contents are detailed starting in section 4.4 of this document. From: [MIME2]
4.2.12. Content-Transfer-Encoding
Because Internet mail was initially specified to carry only 7-bit US-ASCII text, it may be necessary to encode voice and fax data into a representation suitable for that environment. The "Content- Transfer-Encoding:" header describes this transformation if it is needed. SEND RULES An implementation in conformance with this profile SHOULD send audio and/or facsimile data in "Binary" form when binary message transport is available (see section 5). When binary transport is not available, implementations MUST encode the audio and/or facsimile data as "Base64". RECEIVE RULES Conforming implementations MUST recognize and decode the standard encodings, "Binary" (when binary support is available), "7bit, "8bit", "Base64" and "Quoted-Printable" per [MIME1]. The detection and decoding of "Quoted-Printable", "7bit", and "8bit" MUST be supported in order to meet MIME requirements and to preserve interoperability with the fullest range of possible devices.4.2.13. Sensitivity
The "Sensitivity:" field, if present, indicates the requested privacy level. If no privacy is requested, this field is omitted. The header definition is as follows: Sensitivity := "Sensitivity" ":" Sensitivity-value Sensitivity-value := "Personal" / "Private" / "Company-Confidential" SEND RULES A VPIM-conforming implementation MAY include this header to indicate the sensitivity of a message. If a user marks a message "Private", a conforming implementation MUST send only the "Private" sensitivity level. There are no VPIM-specific semantics defined for the values "Personal" or "Company-Confidential". A conforming implementation SHOULD NOT send the values "Personal" or "Company-Confidential". If the message is of "Normal" sensitivity, this field SHOULD be omitted. From: [X.400]
RECEIVE RULES If a "Sensitivity:" field with a value of "Private" is present in the message, a conforming system MUST prohibit the recipient from forwarding this message to any other user. A conforming system, however, SHOULD allow the responder to reply to a sensitive message, but SHOULD NOT include the original message content. The responder MAY set the sensitivity of the reply message. A receiving system MAY ignore sensitivity values of "Personal" and "Company Confidential". If the receiving system does not support privacy and the sensitivity is "Private", a negative delivery status notification MUST be sent to the originator with the appropriate status code (5.6.0) "Other or undefined protocol status" indicating that privacy could not be assured. The message contents SHOULD be returned to the sender to allow for a voice context with the notification. A non-delivery notification to a private message SHOULD NOT be tagged private since it will be sent to the originator. From: [X.400] A message with no privacy explicitly noted (i.e., no header) or with "Normal" sensitivity has no special treatment.4.2.14. Importance
Indicates the requested importance to be given by the receiving system. If no special importance is requested, this header MAY be omitted and the value of the absent header assumed to be "normal". From: [X.400] Importance := "Importance" ":" importance-value Importance-value := "low" / "normal" / "high" SEND RULES Conforming implementations MAY include this header to indicate the importance of a message. RECEIVE RULES If the receiving system does not support "Importance:", the attribute MAY be silently dropped.
4.2.15. Subject
The "Subject:" field is often provided by email systems but is not widely supported on voice mail platforms. From: [RFC822] SEND RULES For compatibility with text-based mailbox interfaces, a text subject field SHOULD be generated by a conforming implementation. It is RECOMMENDED that voice-messaging systems that do not support any text user interfaces (e.g., access only by a telephone) insert a generic subject header of "VPIM Message" or "Voice Message" for the benefit of GUI-enabled recipients. RECEIVE RULES It is anticipated that many voice-only systems will be incapable of storing the subject line. The subject MAY be discarded by a receiving system.4.3. MIME Audio Content Descriptions
4.3.1. Content-Description
This field MAY be present to facilitate the text identification of these body parts in simple email readers. Any values may be used. Example: Content-Description: Big Telco Voice Message SEND RULES This field MAY be added to a voice body part to offer a freeform description of the voice content. It is useful to incorporate the values for Content-Disposition with additional descriptions. For example, this can be used to indicate product name or transcoding records. RECEIVE RULES This field MAY be displayed to the recipient. However, since it is only informative it MAY be ignored.
4.3.2. Content-Disposition
This field MUST be present to allow the parsable identification of body parts within a VPIM voice message. This is especially useful if, as is typical, more than one Audio/* body occurs within a single level (e.g., Multipart/Voice-Message). Since a VPIM voice message is intended to be automatically played in the order in which the audio contents occur, the audio contents MUST always be of disposition inline. However, it is still useful to include a filename value, so this SHOULD be present if this information is available. From: [DISP] SEND RULES In order to distinguish between the various types of audio contents in a VPIM voice message a new disposition parameter "voice" is defined with IANA (see section 18.1) with the parameter values below to be used as appropriate: Audio-Type := "voice" "=" Audio-type-value Audio-type-value := "Voice-Message" / "Voice-Message-Notification" / "Originator-Spoken-Name" /"Recipient-Spoken-Name" /"Spoken-Subject" Voice-Message - the primary voice message, Voice-Message-Notification - a spoken delivery notification or spoken disposition notification, Originator-Spoken-Name - the spoken name of the originator, Recipient-Spoken-Name - the spoken name of the recipient(s) if available to the originator Spoken-Subject- the spoken subject of the message, typically spoken by the originator Note that there SHOULD only be one instance of each of these types of audio contents per message level. Additional instances of a given type (i.e., parameter value) MAY occur within an attached forwarded or reply voice message. If there are multiple recipients for a given message, recipient-spoken-name MUST NOT be used. RECEIVE RULES Implementations SHOULD use this header. However, those that do not understand the "voice" parameter (or the "Content-Disposition:" header) can safely ignore it, and will present the audio body parts in order (but will not be able to distinguish between them). If more than one instance of the "voice" parameter type value is encountered at one level (e.g., multiple 'Voice-Message' tagged contents) then they SHOULD be presented together.
4.3.3. Content-Duration
The "Content-Duration:" header provides an indication of the audio length in seconds of the segment. Example: Content-Duration: 33 SEND RULES This field MAY be present to allow the specification of the length of the audio body part in seconds. RECEIVE RULES The use of this field on reception is a local implementation issue. From: [DUR]4.3.4. Content-Language:
This field MAY be present to allow the specification of the spoken language of the audio body part. The encoding is defined in [LANG]. Example for UK English: Content-Language: en-UK SEND RULES A sending system MAY add this field to indicate the language of the voice. The determination of this (e.g., automated or user-selected) is a local implementation issue. RECEIVE RULES The use of this field on reception is a local implementation issue. It MAY be used as a hint to the recipient (e.g., end-user or an automated translation process) as to the language of the voice message.4.4. Voice Message Content Types
The content types described in this section are identified for use within the Multipart/Voice-Message content. This content is referred to as a "VPIM message" in this document and is the fundamental part of a "VPIM message".
Only the contents profiled can be sent within a VPIM voice message construct (i.e., the Multipart/Voice-Message content type) to form a simple or a more complex structure (several examples are given in Appendix B). The presence of other contents within a VPIM voice message is not permitted. In the absence of a bilateral agreement, conforming implementations MUST NOT create a message containing prohibited contents. In the spirit of liberal acceptance, a conforming implementation MAY accept and render prohibited content. Systems unable to accept or render prohibited contents MAY discard the prohibited contents as necessary to deliver the acceptable content. When multiple contents are present within the Multipart/Voice-Message, they SHOULD be presented to the user in the order that they appear in the message. Some deployed implementations based on a common interpretation of the original VPIM v2 specification reject messages with prohibited content rather than discard the unsupported contents. For interoperability with these systems, it is especially important that prohibited contents not be sent within a Multipart/Voice-Message.4.4.1. Multipart/Voice-Message
This MIME multipart structure provides a mechanism for packaging a voice message into one container that is tagged as VPIM v2 conforming. The sub-type is identical in semantics and syntax to multipart/mixed, as defined in [MIME2]. As such, it may be safely interpreted as a multipart/mixed by systems that do not understand the sub-type (only the identification as a voice message would be lost). In addition to the MIME required boundary parameter, a version parameter is also required for this sub-type. This is to distinguish this refinement of the sub-type from the previous definition in [VPIM1]. The value of the version parameter is "2.0" if the content conforms to the requirements of this specification. Should there be further revisions of this content type, there MUST be backwards compatibility (i.e., systems implementing version n can read version 2, and systems implementing version 2 can read version 2 contents within a version n). SEND RULES The Multipart/Voice-Message content-type MUST only contain the profiled media and content types specified in this section (i.e., Audio/*, Image/*, and Message/RFC822). The most common will be: spoken name, spoken subject, the message itself, and an attached fax. Forwarded messages are created by simply using the Message/RFC822 construct.
Conformant implementations MUST use Multipart/Voice-Message in a VPIM message. In most cases, this Multipart/Voice-Message Content-Type will be the top level but may be included within a Message/RFC822 if the message is forwarded or within a multipart/mixed when more than one message is being forwarded. RECEIVE RULES Conformant implementations MUST recognize the Multipart/Voice-Message content (whether it is a top-level content or contained in a Multipart/Mixed) and MUST be able to separate the contents (e.g., spoken name or spoken subject). The semantic of Multipart/Voice-Message (defined in section 18.2) is identical to Multipart/Mixed and may be interpreted as that by systems that do not recognize this content-type.4.4.2. Message/RFC822
SEND RULES MIME requires support of the Message/RFC822 message encapsulation body part. This body part SHOULD be used within a Multipart/Voice- Message to forward complete messages (see 4.8) or to reply with original content (see 4.9). From: [MIME2] RECEIVE RULES The receiving system MUST accept this format and SHOULD treat this attachment as a forwarded message. The receiving system MAY flatten the forwarding structure (i.e., remove this construct to leave multiple voice contents or even concatenate the voice contents to fit in a recipient's mailbox), if necessary.4.4.3. Audio/32KADPCM
SEND RULES An implementation conforming to this profile MUST send Audio/32KADPCM by default for voice [ADPCM]. This encoding is a moderately- compressed encoding with a data rate of 32 kbits/second using moderate processing resources. Typically, this body contains several minutes of message content; however, if used for spoken name or subject the content is expected to be considerably shorter (i.e., about 5 and 10 seconds respectively).
RECEIVE RULES Receivers MUST be able to accept and decode Audio/32KADPCM. If an implementation can only handle one voice body, then multiple voice bodies (if present) SHOULD be concatenated, and MUST NOT be discarded. If concatenated, the contents SHOULD be in the same order they appeared in the multipart.4.4.4. Image/TIFF
A common image encoding for facsimile, known as TIFF-F, is a derivative of the Tag Image File Format (TIFF) and is described in several documents. For the purposes of VPIM, the F Profile of TIFF for Facsimile (TIFF-F) is defined in [TIFF-F], and the Image/TIFF MIME content-type is defined in [TIFFREG]. While there are several formats of TIFF, only TIFF-F is profiled for use within Multipart/Voice-Message. Further, since the TIFF-F file format is used in a store-and-forward mode with VPIM, the image MUST be encoded so that there is only one image strip per facsimile page. SEND RULES All VPIM implementations that support facsimile MUST generate TIFF-F compatible facsimile contents in the Image/TIFF subtype using the application=faxbw encoding by default. If the VPIM message is a voice- annotated fax, the implementation SHOULD send this fax content in Multipart/Voice-Message. If the message is a simple fax, an implementation MAY send it without using the Multipart/Voice-Message to be more compatible with fax-only (RFC 2305) implementations. While any valid MIME body header MAY be used (e.g., Content- Disposition to indicate the filename), none are specified to have special semantics for VPIM and MAY be ignored. Note that the content-type parameter application=faxbw MUST be included in outbound messages. RECEIVE RULES Not all VPIM systems support fax, but all SHOULD accept it within the multipart/voice-message. Within a Multipart/Voice-Message, a receiving system that cannot render fax content SHOULD accept the voice content of a VPIM message and discard the fax content. Outside a Multipart/Voice-Message, a recipient system MAY reject (with appropriate NDN) the entire message if it cannot store or is not capable of rendering a message with fax attachments. VPIM conforming systems MAY support fax outside of (or without) the Multipart/Voice- Message.
Some deployed implementations based on a common interpretation of the original VPIM V2 specification reject messages with fax content within the Multipart/Voice-Message rather than discard the unsupported contents. These systems will return the message to the sender with an NDN indicating lack of support for fax.4.5. Other MIME Contents
The following MIME contents (with the exception of multipart/mixed in section 4.5.1) MAY be included within a multipart/voice message. Other contents MUST NOT be included. Their handling is a local implementation issue. Multipart/mixed is included to promote interoperability with a wider range of systems and also to allow the creation of more complex multimedia messages (with a VPIM message as one part).4.5.1. Multipart/Mixed
This common MIME content-type allows the enclosing of several body parts in a single message. SEND RULES A VPIM voice message (i.e., multipart/voice-message) MAY be included within a message with a Multipart/Mixed top-level content type. Typically, this would only be used when mixing non-voice and non-fax contents with a voice message. RECEIVE RULES Such a message is not itself a VPIM message and the handling of such a construct is outside the scope of the VPIM profile. However, an the spirit of liberal acceptance, a conforming implementation MUST accept and render a VPIM voice message contained in a Multipart/Mixed.4.5.2. Text/Directory
SEND RULES This content was profiled in the original specification of VPIM v2 as a means of transporting contact information from the sender to the recipient. This usage did not find widespread adoption and is no longer a feature of VPIM V2. Conforming implementations SHOULD NOT send the Text/Directory content type.
RECEIVE RULES For compatibility with an earlier specification of VPIM v2, the Text/Directory content type MUST be accepted by a conforming implementation, but need not be stored, processed, or rendered to the recipient.4.5.3. Proprietary Voice or Fax Formats
Use of any other encoding except the required codecs reduces interoperability in the absence of explicit knowledge about the capabilities of the recipient. A conforming implementation SHOULD NOT use any other encoding unless a unique identifier is registered with the IANA prior to use (see [MIME4]). The voice encodings SHOULD be registered as subtypes of Audio. The fax encodings SHOULD be registered as subtypes of Image. SEND RULES Proprietary voice encoding formats or other standard formats SHOULD NOT be sent under this profile unless the sender has a reasonable expectation that the recipient will accept the encoding. In practice, this requires explicit per-destination configuration information maintained either in a directory, personal address book, or gateway configuration tables. RECEIVE RULES Systems MAY accept other Audio/* or Image/* content types if they can decode them. Systems which receive Audio/* or Image/* content types which they are unable to deposit or unable to render MUST return the message (and SHOULD include the original content) to the originator with an NDN indicating media not supported.4.5.4. Text/Plain
MIME requires support of the basic Text/Plain content type (with the US-ASCII character set). This content type has limited applicability within the voice-messaging environment. However, because VPIM is a MIME profile, MIME requirements SHOULD be met. SEND RULES Conforming VPIM implementations SHOULD NOT send the Text/Plain content-type. Implementations MAY send the Text/Plain content-type outside the Multipart/Voice-Message.
RECEIVE RULES Within a Multipart/Voice-Message, the Text/Plain content-type MAY be dropped from the message, if necessary, to deliver the audio/fax components. The recipient SHOULD NOT reject the entire message if the text component cannot be accepted or rendered. Outside a Multipart/Voice-Message, conforming implementations MUST accept Text/Plain; however, specific handling is left as an implementation decision. From: [MIME2] Some deployed implementations based on a common interpretation of the original VPIM V2 specification reject messages with any text content rather than discard the unsupported contents. These systems will return the message to the sender with an NDN indicating lack of support for text.4.6. Delivery Status Notification (DSN)
A DSN is a notification of delivery (positive DSN), non-delivery (negative DSN), or temporary delivery delay (delayed DSN). The top- level content-type of a DSN is Multipart/Report, which is defined in [REPORT]. The content-type which distinguishes DSN's from other types of notifications is Message/Delivery-Status, which is defined in [DSN]. SEND RULES A VPIM-compliant implementation MUST be able to send DSN's that conform to [REPORT] and [DSN]. Unless requested otherwise, a non- delivery DSN MUST be sent when any form of non-delivery of a message occurs. A VPIM-compliant implementation SHOULD provide a spoken delivery status in the "human-readable" body part of the DSN, but MAY provide a textual status. RECEIVE RULES A VPIM-compliant implementation MUST be able to receive DSN's that conform to [REPORT] and [DSN]. A VPIM-compliant implementation MUST be able to receive a DSN whose "human-readable" body part contains a spoken delivery status phrase or a textual description. Though subsequent use of the phrase or text is a local implementation issue, the intent of the DSN MUST be presented to the end user.
4.7. Message Disposition Notification (MDN)
An MDN is a notification indicating what happens to a message after it is deposited in the recipient's mailbox. An MDN can be positive (message was read/played/rendered/etc.) or negative (message was deleted before recipient could see it, etc.). The top-level content-type of a MDN is Multipart/Report, which is defined in [REPORT]. The content-type which distinguishes MDN's from other types of notifications is Message/Disposition-Notification, which is defined in [MDN]. SEND RULES A VPIM-compliant implementation SHOULD support the ability to request MDNs. This is done via the use of the "Disposition-Notification-To:" header field as defined in [MDN]. A VPIM-compliant implementation SHOULD support the ability to send MDNs, but these MDNs MUST conform to [REPORT] and [MDN]. When sending an MDN, a VPIM-compliant implementation SHOULD provide a spoken message disposition in the "human-readable" body part of the MDN, but MAY provide a textual status. RECEIVE RULES A VPIM-compliant implementation SHOULD respond to an MDN request with an MDN response. A VPIM-compliant implementation MUST be able to receive MDNs that conform to [REPORT] and [MDN], if it is capable of requesting MDNs. If a VPIM-compliant implementation is capable of receiving MDNs, it MUST be able to receive a MDN whose "human-readable" body part contains a spoken message disposition phrase or a textual disposition description. Though subsequent use of the phrase or text is a local implementation issue, the intent of the MDN MUST be presented to the end user.4.8. Forwarded Messages
VPIM v2 explicitly supports the forwarding of voice and fax content with voice or fax annotation. However, only the two constructs described below are acceptable in a VPIM message. Since only the first (i.e., Message/RFC822) can be recognized as a forwarded message (or even multiple forwarded messages), it is RECOMMENDED that this construct be used whenever possible.
Forwarded VPIM messages SHOULD be sent as a Multipart/Voice-Message with the entire original message enclosed in a Message/RFC822 content-type and the annotation as a separate Audio/* or Image/* body part. If the RFC822 header fields are not available for the forwarded content, simulated header fields with available information SHOULD be constructed to indicate the original sending timestamp, and the original sender as indicated in the "From:" field. Note that at least one of "From:", "Subject:", or "Date:" MUST be present. As well, the Message/RFC822 content MUST include at least the "MIME- Version:", and "Content-Type:" header fields. From: [MIME2] In the event that forwarding information is lost, the entire audio content MAY be sent as a single Audio/* segment without including any forwarding semantics. An example of this loss is an AMIS message being forwarded through an AMIS-to-VPIM gateway.4.9. Reply Messages
VPIM v2 explicitly supports replying to received messages. Support of multiple originator header fields in a reply message is often not possible on voice messaging systems, so it may be necessary to choose only one when gatewaying a VPIM message to another voice message system. However, implementers should note that this may make it impossible to send DSN's, MDN's, and replies to their proper destinations. In some cases, replying to a message is not possible, such as with a message created by telephone answering (i.e., classic voice mail). In this case, the From field SHOULD contain the special address non- mail-user@domain (see 4.1.2). The recipient's VPIM system SHOULD NOT offer the option to reply to this kind of message (unless an outcalling feature is offered - which is out of scope for VPIM).
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