Status of this Memo

This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.

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This Internet-Draft will expire on October 30, 2002.

Copyright Notice

Copyright (C) The Internet Society (2002). All Rights Reserved.

Abstract

This draft proposes a SIP event package to carry message waiting status and message summaries from a messaging system to an interested User Agent.

Table of Contents

1.  Conventions
2.  Background and Appropriateness
3.  Event Package Formal Definition
3.1  Event Package Name
3.2  Event Package Parameters
3.3  SUBSCRIBE Bodies
3.4  Subscription Duration
3.5  NOTIFY Bodies
3.6  Subscriber generation of SUBSCRIBE requests
3.7  Notifier processing of SUBSCRIBE requests
3.8  Notifier generation of NOTIFY requests
3.9  Subscriber processing of NOTIFY requests
3.10  Handling of Forked Requests
3.11  Rate of notifications
3.12  State Agents
3.13  Behavior of a Proxy Server
4.  Examples of Usage
4.1  Example Message Flow
4.2  Example Usage with Caller Preferences
5.  Formal Syntax
5.1  New event-package definition
5.2  Body Format Syntax
6.  IANA Considerations
6.1  SIP Event Package Registration for message-summary
6.2  MIME Registration for application/simple-message-summary
7.  Security Considerations
8.  Revision history
8.1  Open Issues
8.2  Changes from draft-mahy-sipping-mwi-00
8.3  Changes from draft-mahy-sip-mwi-01
8.4  Changes from draft-mahy-sip-mwi-00
9.  Contributors
10.  Acknowledgments
§  Normative References
§  Informational References
§  Author's Address
§  Full Copyright Statement

1. Conventions

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[3].

2. Background and Appropriateness

Messaging Waiting Indication is a common feature of telephone networks. It typically involves playing a special dial tone (called message-waiting dial tone), lighting a light or indicator on the phone, or both. Message-waiting dial tone is similar but distinct from stutter dial tone. Both are defined in GR-506[10].

The methods in the SIP[1] base specification were only designed to solve the problem of session initiation for multimedia sessions, and rendezvous. Since Message Waiting Indication is really status information orthogonal to a session, it was not clear how an IP telephone acting as a SIP User Agent would implement comparable functionality. Members of the telephony community viewed this as a shortcoming of SIP.

Users want the useful parts of the functionality they had using traditional analog and PBX telephones. It is also desirable to provide comparable functionality in a flexible way that allows for more customization and new features.

SIP Specific Event Notification SIP Events[2] is an appropriate mechanism to use in this environment, as it preserves the user mobility and rendezvous features which SIP provides.

Using SIP-Specific Event Notification, A Subscriber User Agent (typically an IP phone or SIP software User Agent) subscribes to the status of their messages. A SIP User Agent acting on behalf of the user's messaging system then notifies the Subscriber whenever the account's messages have changed. The Notifier sends this message summary information in the body of the NOTIFY, encoded in a new MIME type defined later in this draft. A User Agent can also explicitly fetch the current status.

A SIP User Agent MAY subscribe to multiple accounts (distinguished by the Request URI). Multiple SIP User Agents MAY subscribe to the same account.

Before any subscriptions or notifications are sent, each interested User Agent must be made aware of its messaging server(s). This MAY be manually configured on interested User Agents, manually configured on an appropriate SIP Proxy, or dynamically discovered using caller preferences[4]. (For more information on usage with caller preferences, see Example Usage with Caller Preferences)

3. Event Package Formal Definition

3.1 Event Package Name

This document defines a SIP Event Package as defined in SIP Events[2]. The event-package token name for this package is:

3.2 Event Package Parameters

This package does not define any event package parameters.

3.3 SUBSCRIBE Bodies

This package does not define any SUBSCRIBE bodies.

3.4 Subscription Duration

Subscriptions to this event package MAY range from minutes to weeks. Subscriptions in hours or days are more typical and are RECOMMENDED.

3.5 NOTIFY Bodies

A simple text-based format is proposed to prevent an undue burden on low-end user agents, for example, inexpensive IP phones with no display. Although this format is text-based, it is intended for machine consumption only.

A future extension MAY define other NOTIFY bodies. If no "Accept" header is present in the SUBSCRIBE, the body type defined in this document MUST be assumed.

The format specified in this proposal attempts to separate orthogonal attributes of messages as much as possible. Messages are separated by media type (audio, text, image, and video); by message status (new and old); and by urgent and non-urgent type.

The text format begins with a simple status line, and optionally a summary line per media type. Valid media types are Voicemail (audio), Email (text), Fax (image), and Video (video). For each media type the total number of new and old messages is reported in the new and old fields.

In some cases, detailed message summaries are not available. The status line allows messaging systems or messaging gateways to provide the traditional boolean message waiting notification.

In the example that follows, more functionality is available to the User Agent. There are one new and three old voice messages.

After the summary, the format can optionally list a summary count of urgent messages. Of the one new and three old voice messages, none of the new messages are urgent, but one of the old messages is. All counters have a maximum value of 4,294,967,295 ((2^32) - 1). Notifiers MUST NOT generate a request with a larger value. Subscribers MUST ignore a larger value.

Optionally, after the summary counts, the messaging systems MAY append RFC 2822[9]-style message headers, which further describe newly added messages. A messaging system which includes message headers in a NOTIFY, MUST provide an administrator configurable mechanism for selecting which headers are sent. Likely headers for inclusion include To, From, Date, Subject, Message-ID, and Priority. Note that the syntax for these headers is more restrictive than for RFC 2822 and SIP. For example, line folding is prohibited.

Implementations which generate large notifications are reminded to follow the message size restrictions for unreliable transports articulated in Section 18.1.1 of SIP.

Mapping local message state to new/old message status and urgency is an implementation issue of the messaging server.

Messaging systems MAY use any algorithm for determining the approporiate media type for a message. Such algorithms are out of scope of this document. Systems which use Internet Mail SHOULD use the Message-Context header[6] if present as a hint to make a media type determination.

3.6 Subscriber generation of SUBSCRIBE requests

Subscriber User Agents will typically SUBSCRIBE to message summary information for a period of hours or days, and automatically attempt to re-SUBSCRIBE well before the subscription is completely expired. If re-subscription fails, the Subscriber SHOULD periodically retry again until a subscription is successful, taking care to backoff to avoid network congestion. If a subscription has expired, new re-subscriptions MUST use a new Call-ID.

The Subscriber SHOULD SUBSCRIBE to that user's message summaries whenever a new user becomes associated with the device (a new login). The Subscriber MAY also explicitly fetch the current status at any time. The subscriber SHOULD renew its subscription immediately after a reboot, or when the subscriber's network connectivity has just been re-established.

The Subscriber MUST be prepared to receive and process a NOTIFY with new state immediately after sending a new SUBSCRIBE, a SUBSCRIBE, renewal, an unSUBSCRIBE or a fetch; or at any time during the subscription.

When a user de-registers from a device (logoff, power down of a mobile device, etc.), subscribers SHOULD unsubscribe by sending a SUBSCRIBE message with an Expires header of zero.

3.7 Notifier processing of SUBSCRIBE requests

When a SIP Messaging System receives SUBSCRIBE messages with the message-summary event-type, it SHOULD authenticate the subscription request. If authentication is successful, the Notifier MAY limit the duration of the subscription to an administrator defined amount of time as described in SIP Events.

3.8 Notifier generation of NOTIFY requests

Immediately after a subscription is accepted, the Notifier MUST send a NOTIFY with the current message summary information. This allows the Subscriber to resynchronize its state. This initial synchronization NOTIFY MUST NOT include the optional message headers.

When the status of the messages changes sufficiently for a messaging account to change the number of new or old messages, the Notifier SHOULD send a NOTIFY message to all active subscribers to that account. NOTIFY messages sent to subscribers of a group or alias, MUST place the account name in the user part of the Contact header URL.

A Messaging System MAY send a NOTIFY with an "Expires" header of "0" and a "Subscription-State" header of "terminated" before a graceful shutdown.

3.9 Subscriber processing of NOTIFY requests

Upon receipt of a valid NOTIFY request, the subscriber SHOULD immediately render the message status and summary information to the end user in an implementation specific way.

The Subscriber MUST be prepared to receive NOTIFYs from different Contacts corresponding to the same SUBSCRIBE. (the SUBSCRIBE may have been forked).

3.10 Handling of Forked Requests

Forked requests are allowed for this event type and may install multiple subscriptions. The Subscriber MAY render multiple summaries to the user, or MAY merge them as described below.

If any of the "Messages-Waiting" status lines report "yes", then the merged state is "yes"; otherwise the merged state is "no".

The Subscriber MAY merge summary lines in an implementation-specific way if all notifications contain at least one summary line.

A Subscriber MAY use an "alias" or "group" in the To header and/or Request-URI if that name is significant to the messaging system.

3.11 Rate of notifications

A Notifier MAY choose to buffer NOTIFY responses for a short administrator-defined period (seconds or minutes) when the message status is changing rapidly. This buffering behavior is RECOMMENDED. Note that timely notification of a newly added message is probably more significant to the end user than notifications of newly deleted messages which do not affect the overall message waiting state.

Notifiers SHOULD NOT generate NOTIFY requests more frequently than once per second.

3.12 State Agents

Implementers MAY create a service which consolidates and summarizes NOTIFYs from many Contacts. This document does not preclude implementations from building state agents which support this event package.

3.13 Behavior of a Proxy Server

There are no additional requirements on a SIP Proxy, other than to transparently forward the SUBSCRIBE and NOTIFY methods as required in SIP. However, Proxies SHOULD allow non-SIP URLs. Proxies and Redirect servers SHOULD be able to direct the SUBSCRIBE request to an appropriate messaging server User Agent. Proxies are encouraged to support routing to Contacts based on the existence of methods="SUBSCRIBE" and feature="voice-mail" parameters in an Accept-Contact header (as specified in the caller preferences specification).

4. Examples of Usage

4.1 Example Message Flow

The examples shown below are for informational purposes only. For a normative description of the event package, please see sections 4 and 6 of this document.

In the example call flow below, Rohan's IP phone subscribes to the status of Rohan's messages. Via headers are omitted for clarity.

4.2 Example Usage with Caller Preferences

The use of caller preferences is optional but encouraged. If caller preferences is used, a messaging server MAY REGISTER a Contact with a "SUBSCRIBE" methods tag. If SUBSCRIBE is used by other services, the messaging server MAY also REGISTER as a Contact with the feature="voice-mail" tag. An example of this kind of registration follows below.

The following SUBSCRIBE message would find the Contact which registered in the example above.

5. Formal Syntax

The following syntax specification uses the augmented Backus-Naur Form (BNF) as described in RFC-2234[5].

5.1 New event-package definition

This document defines a new event-package with the package name:

5.2 Body Format Syntax

The formal syntax for application/simple-message-summary is below:

6. IANA Considerations

6.1 SIP Event Package Registration for message-summary

6.2 MIME Registration for application/simple-message-summary

Applications which use this media: The simple-message-summary application subtype supports the exchange of message waiting and message summary information in SIP networks.

7. Security Considerations

Message Summaries and optional message bodies contain information which is typically very privacy sensitive. At minimum, subscriptions to this event package SHOULD be authenticated; and notifications SHOULD be encrypted and integrity protected using either end-to-end mechanisms, or the hop-by-hop protection afforded messages sent to SIPS URIs.

Additional security considerations are covered in SIP[1] and SIP Events[2].

8. Revision history

8.1 Open Issues

1. Should the "media types" concept be replaced entirely with message contexts? This would be a non-backwards compatible change, but appears to be a better semantic match than what is in the draft now. This would also add extensibility and change control in a single document. The list of valid message-context-classes are voice-message, fax-message, pager-message, multimedia-message, text-message, and none.
2. Should the syntax follow that of SIP instead of the restrictive syntax now in place? A SIP syntax would add line folding, for example. The optional message-headers would borrow the "extension-header" syntax from SIP and the per-media (or per-message-context) summary lines would use the explicit whitespace separators defined in SIP (ex: HCOLON, SLASH).
3. If forking or state agents are supported, and the subscriber subscribes to a group alias, how is the specific messaging account specified in the corresponding notifies.

8.2 Changes from draft-mahy-sipping-mwi-00

1. Updated references and split into normative and informational
2. Removed normative behavior now specified in Events
3. Updated to address the event package sections now specified in Events.
4. Added the Subscription-State header field to the examples and removed the Event header field from responses.
5. Removed redundant BNF
6. Simplified text on how to choose the media type. For Internet Mail, this now references the Message-Context header.

8.3 Changes from draft-mahy-sip-mwi-01

1. This document is now formatted as a SIP Event Package as defined in Section 4 of SIP Events[2].
2. The event-package name is now "message-summary", to allow for other bodies to extend the package.
3. The "urgent" token was missing from the BNF.

8.4 Changes from draft-mahy-sip-mwi-00

This draft greatly simplifies and shortens the -00 version.

1. The generic behavior of SUBSCRIBE/NOTIFY is now greatly clarified in SIP Events[2] and made consistent with PINT and SIP for presence. This message waiting draft is now consistent with SIP Events[2].
2. The XML format has been removed due to lack of immediate interest. At a future date, similar functionality may be added as another body definition with an appropriate MIME type.
3. An IANA Considerations section was added to register the new "application/simple-message-summary" MIME type and the "simple-message-summary" SIP event package.
4. The "flag-list" was removed due to lack of interest and to encourage simplicity.
5. Due to synchronization issues, and the recommendation of the VPIM Working Group, support for message count "deltas" was removed.
6. The Messages-Waiting line in the body is now mandatory.
7. This version of the draft clarifies the role of caller preferences as optional but encouraged.
8. A set of SMTP-like headers from the triggering messages may now optionally follow the message summaries, provided that the resulting NOTIFY on UDP fits in a single datagram.

9. Contributors

Ilya Slain came up with the initial format of the text body contained in this document. He was previously listed as a co-author, however, he is no longer reachable.

10. Acknowledgments

Thanks to Dave Oran, Bill Foster, Steve Levy, Denise Caballero-McCann, Jeff Michel, Priti Patil, Satyender Khatter, Bich Nguyen, Manoj Bhatia, David Williams, and Bryan Byerly of Cisco; and Jonathan Rosenberg and Adam Roach of Dynamicsoft.

Normative References

Informational References

Author's Address

Full Copyright Statement

Copyright (C) The Internet Society (2002). All Rights Reserved.

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Acknowledgement

Funding for the RFC Editor function is currently provided by the Internet Society.