Internet Engineering Task Force (IETF) J. Rosenberg Request for Comments: 6914 jdrosen.net Category: Informational April 2013 ISSN: 2070-1721
SIMPLE Made Simple: An Overview of the IETF Specifications for Instant Messaging and Presence Using the Session Initiation Protocol (SIP)
The IETF has produced many specifications related to Presence and Instant Messaging with the Session Initiation Protocol (SIP). Collectively, these specifications are known as SIP for Instant Messaging and Presence Leveraging Extensions (SIMPLE). This document serves as a guide to the SIMPLE suite of specifications. It categorizes the specifications, explains what each is for, and how they relate to each other.
Status of This Memo
This document is not an Internet Standards Track specification; it is published for informational purposes.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.
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The IETF has produced many specifications related to Presence and Instant Messaging with the Session Initiation Protocol (SIP) [RFC3261]. Collectively, these specifications are known as SIP for Instant Messaging and Presence Leveraging Extensions (SIMPLE). These specifications cover topics ranging from protocols for subscription and publication to presence document formats to protocols for managing privacy preferences. The large number of specifications can make it hard to figure out exactly what SIMPLE is, what specifications cover it, what functionality it provides, and how these specifications relate to each other.
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This document serves to address these problems. It provides an enumeration of the protocols that make up the SIMPLE suite of specifications from IETF. It categorizes them into related areas of functionality, briefly explains the purpose of each, and how the specifications relate to each other. Each specification also includes a letter that designates its category [RFC2026]. These values are:
SIMPLE provides for both presence and instant messaging (IM) capabilities. Though both of these fit underneath the broad SIMPLE umbrella, they are well separated from each other and are supported by different sets of specifications. That is a key part of the SIMPLE story; presence is much broader than just IM, and it enables communications using voice and video along with IM.
The SIMPLE presence specifications can be broken up into:
o The core protocol machinery, which provides the actual SIP extensions for subscriptions, notifications, and publications
o Presence documents, which are XML documents that provide for rich presence and are carried by the core protocol machinery
o Privacy and policy, which are documents for expressing privacy preferences about how those presence documents are to be shown (or not shown) to other users
o Provisioning, which describes how users manage their privacy policies, buddy lists, and other pieces of information required for SIMPLE presence to work
o Optimizations, which are improvements in the core protocol machinery that were defined to improve the performance of SIMPLE, particularly on wireless links
RFC 6665, SIP-Specific Event Notification (S): [RFC6665] defines the SUBSCRIBE and NOTIFY methods for SIP, forming the core of the SIP event notification framework. To actually use the framework, extensions need to be defined for specific event packages. Presence is defined as an event package [RFC3856] within this framework. Packages exist for other, non-presence related functions, such as message waiting indicators and dialog state changes.
RFC 3856, A Presence Event Package for the Session Initiation Protocol (SIP) (S): [RFC3856] defines an event package for indicating user presence through SIP. Through this package, a SIP user agent (UA) can ask to be notified of the presence state of a presentity (presence entity). The contents of the NOTIFY messages in this package are presence documents discussed in Section 2.2.
RFC 4662, A Session Initiation Protocol (SIP) Event Notification Extension for Resource Lists (S): [RFC4662] defines an extension to [RFC3265] (which has now been obsoleted by RFC 6665) that allows a client to subscribe to a list of resources using a single subscription. The server, called a Resource List Server (RLS), will "expand" the subscription and subscribe to each individual member of the list. Its primary usage with presence is to allow subscriptions to "buddy lists". Without RFC 4662, a UA would need to subscribe to each presentity individually. With RFC 4662, they can have a single subscription to all buddies. A user can manage the entries in their buddy list using the provisioning mechanisms in Section 2.4.
RFC 5367, Subscriptions to Request-Contained Resource Lists in the Session Initiation Protocol (SIP) (S): [RFC5367] is very similar to RFC 4662. It allows a client to subscribe to a list of resources using a single subscription. However, with this mechanism, the list is included within the body of the SUBSCRIBE request. In RFC 4662, it is provisioned ahead of time on the server.
RFC 3903, Session Initiation Protocol (SIP) Extension for Event State Publication (S): [RFC3903] defines the PUBLISH method. With this method, a UA can publish its current state for any event package, including the presence event package. Once an agent publishes its presence state, the presence server would send notifications of this state change using RFC 3856.
Once a user has generated a subscription to presence using the core protocol machinery, they will receive notifications (SIP NOTIFY requests) that contain presence information. That presence information is in the form of an XML presence document. Several specifications have been defined to describe this document format, focusing on rich, multimedia presence.
RFC 3863, Presence Information Data Format (PIDF) (S): [RFC3863] defines the baseline XML format for a presence document. It defines the concept of a tuple as representing a basic communication modality and defines a simple status for it (open or closed).
RFC 4479, A Data Model for Presence (S): [RFC4479] extends the basic model in RFC 3863. It introduces the concepts of device and person status and explains how these relate to each other. It describes how presence documents are used to represent communications systems states in a consistent fashion. More than RFC 3863, it defines what a presence document is and what it means.
RFC 4480, RPID: Rich Presence Extensions to the Presence Information Data Format (PIDF) (S): [RFC4480] adds many more attributes to the presence document schema, building upon the model in RFC 4479. It allows for indications of activities, moods, places and place types, icons, and indications of whether or not a user is idle.
RFC 4481, Timed Presence Extensions to the Presence Information Data Format (PIDF) to Indicate Status Information for Past and Future Time Intervals (S): [RFC4481] adds attributes to the presence document schema, again building upon the model in RFC 4479. It allows documents to indicate status for the future or the past. For example, a user can indicate that they will be unavailable for voice communications from 2 p.m. to 3 p.m. due to a meeting.
RFC 4482, CIPID: Contact Information for the Presence Information Data Format (S): [RFC4482] adds attributes to the presence document schema for contact information, such as a vCard, display name, homepage, icon, or sound (such as the pronunciation of their name).
RFC 5196, Session Initiation Protocol (SIP) User Agent Capability Extension to Presence Information Data Format (PIDF) (S): [RFC5196] adds even more attributes to the presence document schema, this time to allow indication of capabilities for the user
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agent. For example, the extensions can indicate whether a UA supports audio and video, what SIP methods it supports, and so on.
The rich presence capabilities defined by the specifications in Section 2.2 introduces a strong need for privacy preferences. Users must be able to approve or deny subscriptions to their presence and indicate what information such watchers can see. In SIMPLE, this is accomplished through policy documents uploaded to the presence server using the provisioning mechanisms in Section 2.4.
RFC 4745, Common Policy: A Document Format for Expressing Privacy Preferences (S): [RFC4745] defines a general XML framework for expressing privacy preferences for both geolocation information and presence information. It introduces the concepts of conditions, actions, and transformations that are applied to privacy-sensitive data. The common policy framework provides privacy safety, a property by which network error or version incompatibilities can never cause more information to be revealed to a watcher than the user would otherwise desire.
RFC 5025, Presence Authorization Rules (S): [RFC5025] uses the framework of RFC 4745 to define a policy document format for describing presence-privacy policies. Besides basic yes/no approvals, this format allows a user to control what kind of information a watcher is allowed to see.
RFC 3857, A Watcher Information Event Template-Package for the Session Initiation Protocol (SIP) (S): [RFC3857], also known as watcherinfo, provides a mechanism for a user agent to find out what subscriptions are in place for a particular event package. Though it was defined to be used for any event package, it has particular applicability for presence. It is used to provide reactive authorization. With reactive authorization, a user gets alerted if someone tries to subscribe to their presence, so that they may provide an authorization decision. Watcherinfo is used to provide the alert that someone has subscribed to a user's presence.
RFC 3858, An Extensible Markup Language (XML) Based Format for Watcher Information (S): [RFC3858] is the companion to RFC 3857. It specifies the XML format of watcherinfo that is carried in notifications for the event template package in RFC 3857.
Proper operation of a SIMPLE presence system requires that several pieces of data are correctly managed by the users and provisioned into the system. These include buddy lists (used by the resource list subscription mechanism in RFC 4662) and privacy policies (such as those described by the XML format in [RFC5025]).
RFC 4825, The Extensible Markup Language (XML) Configuration Access Protocol (XCAP) (S): [RFC4825] specifies XCAP, a usage of HTTP that allows a user agent to manipulate the contents of XML documents stored on a server. It can be used to manipulate any kind of XML, and the protocol itself is independent of the particular schema of the data it is modifying. XML schemas have been defined for buddy lists, privacy policies, and offline presence status, allowing all of those to be managed by a user with XCAP.
RFC 5874, An Extensible Markup Language (XML) Document Format for Indicating a Change in XML Configuration Access Protocol (XCAP) Resources (S): [RFC5874] defines an XML format for indicating changes in XCAP documents. It makes use of an XML diff format defined in [RFC5261]. It is used in conjunction with [RFC5875] to alert a user agent of changes made by someone else to their provisioned data.
RFC 4826, Extensible Markup Language (XML) Formats for Representing Resource Lists (S): [RFC4826] defines two XML document formats used to represent buddy lists. One is simply a list of users (or more generally, resources), and the other defines a buddy list whose membership is composed of a list of users or resources. These lists can be manipulated by XCAP, allowing a user to add or remove members from their buddy lists. The buddy list is also
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accessed by the resource list server specified in RFC 4662 for processing resource list subscriptions.
RFC 4827, An Extensible Markup Language (XML) Configuration Access Protocol (XCAP) Usage for Manipulating Presence Document Contents (S): [RFC4827] defines an XCAP usage that allows a user to store an "offline" presence document. This is a presence status that is used by a presence server when there are no presence documents published for that user by any user agents currently running.
Federation refers to the interconnection of different presence and instant messaging systems for the purposes of communications. Federation can be between domains or within a domain. A document has been developed that describes how presence and IM federation works.
RFC 5344, Presence and Instant Messaging Peering Use Cases (I): [RFC5344] describes a basic set of presence and instant messaging use cases for federating between providers.
When running over wireless links, presence can be a very expensive service. Notifications often get sent when the change is not really relevant to the watcher. Furthermore, when a notification is sent, it contains the full presence state of the watcher, rather than just an indication of what changed. Optimizations have been defined to address both of these cases.
RFC 4660, Functional Description of Event Notification Filtering (S): [RFC4660] defines a mechanism that allows a watcher to include filters in its subscription. These filters limit the cases in which notifications are sent. It is used in conjunction with RFC 4661, which specifies the XML format of the filters themselves. The mechanism, though targeted for presence, can be applied to any SIP event package.
RFC 4661, An Extensible Markup Language (XML)-Based Format for Event Notification Filtering (S): [RFC4661] defines an XML format used with the event notification filtering mechanism defined in RFC 4660 [RFC4660].
RFC 5262, Presence Information Data Format (PIDF) Extension for Partial Presence (S): [RFC5262] defines a new XML format for representing changes in presence documents, called a partial PIDF document. This format contains an XML patch operation [RFC5261] that, when applied to the previous presence document, yields the
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new presence document. The partial PIDF document is included in presence notifications when a watcher indicates that they support the format.
RFC 5263, Session Initiation Protocol (SIP) Extension for Partial Notification of Presence Information (S): [RFC5263] defines a mechanism for receiving notifications that contain partial presence documents.
RFC 5264, Publication of Partial Presence Information (S): [RFC5264] defines a mechanism for publishing presence status using a partial PIDF document.
RFC 5261, An Extensible Markup Language (XML) Patch Operations Framework Utilizing XML Path Language (XPath) Selectors (S): [RFC5261] defines an XML structure for representing changes in XML documents. It is a form of "diff" but specifically for XML documents. It is used by several of the optimization mechanisms defined for SIMPLE.
RFC 5112, The Presence-Specific Static Dictionary for Signaling Compression (Sigcomp) (S): [RFC5112] defines a dictionary for usage with Signaling Compression (Sigcomp) [RFC3320] to improve the compressibility of presence documents.
RFC 6446, Session Initiation Protocol (SIP) Event Notification Extension for Notification Rate Control (S): [RFC6446] specifies mechanisms for adjusting the rate of SIP event notifications. These mechanisms can be applied in subscriptions to all SIP event packages.
SIMPLE defines two modes of instant messaging. These are page mode and session mode. In page mode, instant messages are sent by sending a SIP request that contains the contents of the instant message. In session mode, IM is viewed as another media type -- along with audio and video -- and an INVITE request is used to set up a session that includes IM as a media type. While page mode is more efficient for one or two message conversations, session mode is more efficient for longer conversations since the messages are not sent through the SIP servers. Furthermore, by viewing IM as a media type, all of the features available in SIP signaling -- third party call control, forking, and so on, are available for IM.
RFC 3428, Session Initiation Protocol (SIP) Extension for Instant Messaging (S): [RFC3428] introduces the MESSAGE method, which can be used to send an instant message through SIP signaling.
RFC 5365, Multiple-Recipient MESSAGE Requests in the Session Initiation Protocol (SIP) (S): [RFC5365] defines a mechanism whereby a client can send a single SIP MESSAGE to multiple recipients. This is accomplished by including the list of recipients as an object in the body and having a network server send a copy to each recipient.
RFC 4975, The Message Session Relay Protocol (MSRP) (S): [RFC4975] defines a small text-based protocol for exchanging arbitrarily sized content of any kind between users. An MSRP session is set up by exchanging certain information, such as an MSRP URI, within SIP and Session Description Protocol (SDP) signaling.
RFC 3862, Common Presence and Instant Messaging (CPIM): Message Format (S): [RFC3862] defines a wrapper around instant message content providing metadata, such as the sender and recipient identity. The CPIM format is carried in MSRP.
RFC 4976, Relay Extensions for the Message Sessions Relay Protocol (MSRP) (S): [RFC4976] adds support for relays to MSRP. These relay servers receive MSRP messages and send them towards the destination. They provide support for firewall and NAT traversal and allow for features such as recording and inspection to be implemented.
RFC 6135, An Alternative Connection Model for the Message Session Relay Protocol (MSRP) (S): [RFC6135] allows clients to negotiate which endpoint in a session will establish the MSRP connection. Without this specification, the client generating the SDP offer would initiate the connection.
RFC 6714, Connection Establishment for Media Anchoring (CEMA) for the Message Session Relay Protocol (MSRP) (S): [RFC6714] allows middleboxes to anchor the MSRP connection, without the need for middleboxes to modify the MSRP messages; thus, it also enables a secure end-to-end MSRP communication in networks where such middleboxes are deployed.
In SIMPLE, IM multi-user chat (also known as chat-rooms) are provided using regular SIP conferencing mechanisms. The framework for SIP conferencing [RFC4353] and conference control [RFC5239] describe how all SIP-based conferencing works; including joining and leaving, persistent and temporary conferences, floor control and moderation, and learning of conference membership, amongst other functions. All that is necessary are extensions to provide features that are specific to IM.
Multi-party Chat Using the Message Session Relay Protocol (MSRP) (Work in Progress): [SIMPCHAT] defines how MSRP is used to provide support for nicknames and private chat within an IM conference.
Several specifications have been written to provide IM-specific features for SIMPLE. These include "is-typing" indications, allowing a user to know when their messaging peer is composing a response and allowing a user to know when their IM has been received via delivery notifications.
RFC 3994, Indication of Message Composition for Instant Messaging (S): [RFC3994] defines an XML format that can be sent in instant messages that indicates the status of message composition. This provides the familiar "is-typing" indication in IM systems, but also supports voice, video, and other message types.
RFC 5438, Instant Message Disposition Notification (IMDN) (S): [RFC5438] provides delivery notifications of IM receipt. This allows a user to know with certainty that a message has been received.