XEP-0207: XMPP Eventing via Pubsub

1. Introduction¶

Personal Eventing Protocol (XEP-0163) [1] (PEP) introduced the concept of "eventing" into the Extensible Messaging and Presence Protocol (see XMPP Core [2]). But PEP merely scratched the surface of eventing technologies based on the XMPP Publish-Subscribe (XEP-0060) [3] extension. This document extends the eventing concept to its ultimate conclusion: the ability to communicate all XMPP semantics via pubsub.

2. Concepts and Approach¶

Jabber technologies as invented by Jeremie Miller started out as a relatively lightweight XML messaging transport, but they have become unnecessarily -- even ridiculously -- bloated over the years. Formalization of the core Jabber protocols as the Extensible Messaging and Presence Protocol (XMPP) within the IETF seemed like a good idea at the time, but the extensible nature of the core protocols has tempted the developer community to extend XMPP six ways from Sunday. The result has been a plethora of different conceptual models for various extensions, such as Multi-User Chat (XEP-0045) [4] for multi-user communication and Ad-Hoc Commands (XEP-0050) [5] for structured entity-to-entity request/response semantics. These different models are inelegant and unnecessary. Indeed, even the inclusion of three different basic packet types (presence, message, and IQ) in the core protocol is overkill.

We can do better. In fact, we can reduce all the communication types and styles that are currently defined within the XMPP ecosystem to one model: publish-subscribe as specified in XEP-0060.

Consider:

• It has often been observed that presence (see XMPP IM [6]) is a form of publish-subscribe. [7]
• The primitive XMPP "roster" can be easily implemented using the pubsub permissions model.
• Multi-user chat too is a kind of publish-subscribe, since a single "publish" to the room results in multiple notifications to the room occupants, who are really subscribers to an information node.
• Even one-to-one messaging is just another example of publish-subscribe (in fact it is a special case of multi-user chat).

The remainder of this document will prove beyond a doubt that the older, multiple approaches are obsolete, and that there is indeed one model that serves all our needs: pubsub.

3. Presence¶

Presence simply is pubsub, since it follows the classic "observer" pattern: multiple subscribers receive notifications whenever the publisher (typically an end user) generates an event related to network availability. Currently in XMPP this is done with the <presence/> stanza, which serves as a kind of pubsub primitive (though only for availability information). For example, Juliet may log into the capulet.lit server and send presence:

<presence from='juliet@capulet.lit/balcony'>
  <status>Moping</status>
</presence>

The capulet.lit server will then send notifications to all of the users who have subscribed to Juliet's presence:

<presence from='juliet@capulet.lit/balcony' to='romeo@montague.lit/mobile'>
  <status>Moping</status>
</presence>

<presence from='juliet@capulet.lit/balcony' to='nurse@capulet.lit/chamber'>
  <status>Moping</status>
</presence>

<presence from='juliet@capulet.lit/balcony' to='benvolio@montague.lit/pda'>
  <status>Moping</status>
</presence>

[etc.]

But the same functionality can be implemented more elegantly using pubsub:

<iq from='juliet@capulet.lit/balcony' type='set' id='pres1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='presence'>
      <item>
        <presence from='juliet@capulet.lit/balcony' xmlns='jabber:client'>
          <status>Moping</status>
        </presence>
      </item>
    </publish>
  </pubsub>
</iq>

<iq to='juliet@capulet.lit/balcony' type='result' id='pres1'/>

The server (here implementing PEP) then sends notifications to the subscribers:

<message from='juliet@capulet.lit'
         to='romeo@montague.lit/mobile'
         id='presfoo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='presence'>
      <item>
        <presence from='juliet@capulet.lit/balcony' xmlns='jabber:client'>
          <status>Moping</status>
        </presence>
      </item>
    </items>
  </event>
</message>

<message from='juliet@capulet.lit'
         to='nurse@capulet.lit/chamber'
         id='presfoo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='presence'>
      <item>
        <presence from='juliet@capulet.lit/balcony' xmlns='jabber:client'>
          <status>Moping</status>
        </presence>
      </item>
    </items>
  </event>
</message>

<message from='juliet@capulet.lit'
         to='benvolio@montague.lit/pda'
         id='presfoo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='presence'>
      <item>
        <presence from='juliet@capulet.lit/balcony' xmlns='jabber:client'>
          <status>Moping</status>
        </presence>
      </item>
    </items>
  </event>
</message>

It is true that in this case the packets are significantly larger in the pubsub realization than in the old-fashioned presence realization. This is the price of elegance. Implementations SHOULD use native Transport Layer Security compression (see RFC 5246 [8]) or Stream Compression (XEP-0138) [9] at the application layer to conserve bandwidth.

4. Rosters and Presence Subscriptions¶

The original Jabber technologies included a kind of Buddy List ™ (called the "roster"). But the roster is simply a list of the entities that are subscribed to a user's presence. Occam's Razor would indicate that it is foolish to implement two concepts (presence subscription and roster) when one will solve the problem at hand. In XMPP Eventing via Pubsub, there is no need for a specialized "roster" since the same information is represented in the list of entities who are subscribed to the user's "presence" node in pubsub/PEP.

Using XMPP Eventing via Pubsub also cleans up the syntax for presence subscription management, which currently uses four specialized values of the <presence/> element's 'type' attribute: "subscribe", "subscribed", "unsubscribe", and "unsubscribed".

Thus for example a presence subscription request is currently made by sending the following presence stanza:

<presence from='bard@shakespeare.lit' to='juliet@capulet.lit' type='subscribe'/>

And that request is then delivered to the intended recipient:

<presence from='bard@shakespeare.lit' to='juliet@capulet.lit' type='subscribe'/>

In order to approve the subscription request, the user sends a presence stanza of type "subscribed":

<presence from='juliet@capulet.lit' to='bard@shakespeare.lit' type='subscribed'/>

At this point the user's server also creates an entry on the user's roster for the relevant contact and pushes that entry to the user:

<iq to='juliet@example.com/balcony'
    type='set'
    id='a78b4q6ha463'>
  <query xmlns='jabber:iq:roster'>
    <item jid='bard@shakespeare.lit'
          subscription='from'/>
  </query>
</iq>

Observe how much more elegant it is to use XMPP Eventing via Pubsub:

<iq type='set'
    from='bard@shakespeare.lit/globe'
    to='juliet@capulet.lit'
    id='sub1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <subscribe
        node='presence'
        jid='bard@shakespeare.lit'/>
  </pubsub>
</iq>

The pubsub service then sends an authorization request to the user:

<message to='juliet@capulet.lit' from='capulet.lit' id='approve1'>
  <x xmlns='jabber:x:data' type='form'>
    <title>PubSub subscriber request</title>
    <instructions>
      To approve this entity&apos;s subscription request,
      click the OK button. To deny the request, click the
      cancel button.
    </instructions>
    <field var='FORM_TYPE' type='hidden'>
      <value>http://jabber.org/protocol/pubsub#subscribe_authorization</value>
    </field>
    <field var='pubsub#node' type='text-single' label='Node ID'>
      <value>presence</value>
    </field>
    <field var='pusub#subscriber_jid' type='jid-single' label='Subscriber Address'>
      <value>bard@shakespeare.lit</value>
    </field>
    <field var='pubsub#allow' type='boolean'
           label='Allow this JID to subscribe to this pubsub node?'>
      <value>false</value>
    </field>
  </x>
</message>

In order to approve the request, the owner shall submit the form and set the "pubsub#allow" field to a value of "1" or "true"; for tracking purposes the message MUST reflect the 'id' attribute originally provided.

<message from='juliet@capulet.lit/balcony' to='montague.lit' id='approve1'>
  <x xmlns='jabber:x:data' type='submit'>
    <field var='FORM_TYPE' type='hidden'>
      <value>http://jabber.org/protocol/pubsub#subscribe_authorization</value>
    </field>
    <field var='pubsub#node'>
      <value>presence</value>
    </field>
    <field var='pubsub#subscriber_jid'>
      <value>bard@shakespeare.lit</value>
    </field>
    <field var='pubsub#allow'>
       <value>true</value>
    </field>
  </x>
</message>

Simple. Elegant. And no need for a roster! The pubsub approach is bit more verbose, but then again clients and servers should implement and deploy stream compression if they are really worred about bandwidth usage.

5. Multi-User Chat¶

The existing groupchat protocol for XMPP overloads the <presence/> stanza for temporary "subscriptions" to a virtual room and uses the <message/> stanza (with a special type of "groupchat") to communicate information to multiple room occupants. Sound familiar? It's just another form of pubsub!

In groupchat, a user joins a room by sending presence to "room@service/nick":

<presence from='juliet@capulet.lit/balcony' to='characters@chat.shakespeare.lit/JC'/>

But on the pubsub model that is merely a temporary subscription, which can be handled quite elegantly as so:

<iq type='set'
    from='juliet@capulet.lit/balcony'
    to='chat.shakespeare.lit'
    id='sub2'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <subscribe
        node='characters'
        jid='juliet@capulet.lit/balcony'/>
    <options node='characters' jid='juliet@capulet.lit/balcony'>
      <x xmlns='jabber:x:data' type='submit'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#subscribe_options</value>
        </field>
        <field var='roomnick'><value>JC</value></field>
      </x>
    </options>
  </pubsub>
</iq>

In groupchat, room occupants can send messages to all other occupants via the <message/> stanza:

<message from='juliet@capulet.lit/balcony' to='characters@chat.shakespeare.lit' type='groupchat'>
  <body>hi</body>
</message>

The groupchat service then "reflects" that message to all the occupants:

<message from='characters@chat.shakespeare.lit/JC' to='bard@shakespeare.lit/globe' type='groupchat'>
  <body>hi</body>
</message>

<message from='characters@chat.shakespeare.lit/JC' to='romeo@montague.lit/mobile' type='groupchat'>
  <body>hi</body>
</message>

<message from='characters@chat.shakespeare.lit/JC' to='benvolio@montague.lit/pda' type='groupchat'>
  <body>hi</body>
</message>

But on the pubsub model that is merely a publish resulting in multiple notifications, which can be handled quite elegantly as so:

<iq from='juliet@capulet.lit/balcony' to='chat.shakespeare.lit' type='set' id='gc1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='characters'>
      <item>
        <message xmlns='jabber:client'>
          <body>hi</body>
        </message>
      </item>
    </publish>
  </pubsub>
</iq>

<iq from='chat.shakespeare.lit' to='juliet@capulet.lit/balcony' type='result' id='gc1'/>

The service then sends notifications to all the node subscribers:

<message from='chat.shakespeare.lit'
         to='bard@shakespeare.lit/globe'
         id='mucfoo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='characters'>
      <item>
        <message xmlns='jabber:client'>
          <body>hi</body>
        </message>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.lit/balcony'/>
  </addresses>
</message>

<message from='chat.shakespeare.lit'
         to='romeo@montague.lit/mobile'
         id='mucfoo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='characters'>
      <item>
        <message xmlns='jabber:client'>
          <body>hi</body>
        </message>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.lit/balcony'/>
  </addresses>
</message>

<message from='chat.shakespeare.lit'
         to='benvolio@montague.lit/pda'
         id='mucfoo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='characters'>
      <item>
        <message xmlns='jabber:client'>
          <body>hi</body>
        </message>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.lit/balcony'/>
  </addresses>
</message>

Here again the pubsub approach is slightly more verbose, but that's what stream compression is for.

6. One-to-One Messaging¶

It's really rather silly that XMPP includes two different models for messaging, one for groupchat and the other for one-to-one messages. Pubsub solves that problem by using one model for everything. In order to exchange messages, one of the parties simply creates a pubsub node with a whitelist model and adds the other person as a publisher (it may also be necessary to add the other party to the whitelist):

<iq type='set'
    from='juliet@capulet.lit/balcony'
    to='pubsub.shakespeare.lit'
    id='create1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <create node='me_and_romeo'/>
    <configure>
      <x xmlns='jabber:x:data' type='submit'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#node_config</value>
        </field>
        <field var='pubsub#access_model'><value>whitelist</value></field>
        <field var='pubsub#publisher'><value>romeo@montague.lit</value></field>
      </x>
    </configure>
  </pubsub>
</iq>

<iq type='result' from='pubsub.shakespeare.lit' to='juliet@capulet.lit/balcony' id='create1'/>

<iq type='set'
    from='juliet@capulet.lit/balcony'
    id='manage1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub#owner'>
    <subscriptions node='me_and_romeo'/>
      <subscription jid='romeo@capulet.lit' subscription='subscribed'/>
    </subscriptions>
  </pubsub>
</iq>

<iq type='result'
    to='juliet@capulet.lit/balcony'
    id='manage1'/>

Now Juliet can send a message to the node and it will be delivered to both parties (it's always nice to receive a bcc to one's sending address, the client can simply ignore it, or treat it as an ack):

<iq from='juliet@capulet.lit/balcony' to='pubsub.shakespeare.lit' type='set' id='msg1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='me_and_romeo'>
      <item>
        <message xmlns='jabber:client'>
          <body>wherefore art thou?</body>
        </message>
      </item>
    </publish>
  </pubsub>
</iq>

<iq from='pubsub.shakespeare.lit' to='juliet@capulet.lit/balcony' type='result' id='msg1'/>

The service then sends notifications to both parties:

<message from='pubsub.shakespeare.lit'
         to='romeo@montague.lit/mobile'
         id='msgfoo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='me_and_romeo'>
      <item>
        <message xmlns='jabber:client'>
          <body>wherefore art thou?</body>
        </message>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.lit/balcony'/>
  </addresses>
</message>

<message from='pubsub.shakespeare.lit'
         to='juliet@capulet.lit/balcony'
         id='msgfoo'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='me_and_romeo'>
      <item>
        <message xmlns='jabber:client'>
          <body>wherefore art thou?</body>
        </message>
      </item>
    </items>
  </event>
  <addresses xmlns='http://jabber.org/protocol/address'>
    <address type='replyto' jid='juliet@capulet.lit/balcony'/>
  </addresses>
</message>

Beautiful, no?

7. Conclusion¶

Although this document shows how to do presence notifications, presence subscriptions, rosters, groupchat, and one-to-one messaging via pubsub, XMPP Eventing via Pubsub (XEP) is not limited to these functionality areas, which are provided only as examples. Indeed, XMPP Eventing via Pubsub (XEP) provides a generic mechanism for XMPP eventing that obviates the need for any future XMPP Extension Protocol (XEP) specifications other than payload formats for communication over the XMPP Eventing via Pubsub (XEP) transport. Truly, pubsub is the "one ring to bind them all" and the XEP XEP is the mother of all future XEPs. We have a clear path forward to a powerful, robust, payload-agnostic technology for the full range of eventing needs. Let us grasp the opportunity to rebuild XMPP the way it should have been built from the beginning: on top of a solid foundation of publish-subscribe. [10]

8. Security Considerations¶

In XMPP Eventing via Pubsub (XEP), access control is handled through a single permissions model, that of subscriptions to pubsub nodes. XEP nodes MUST have a default access model of "authorize" to prevent open data retrieval from potentially private data sources; this will result in a great deal of authorization requests and thus annoy typical end users to no end, but users will at least have the illusion of security, which is all they really want anyway.

End-to-end encryption is made more difficult in XMPP Eventing via Pubsub (XEP) since all information passes through the pubsub service, which is typically associated with or hosted by the user's server. The solution is to run your own XMPP server in a high-security fashion. In particular, universal deployment of personal XMPP servers, domain certificates (X.509 / PKI) for channel encryption (TLS) and server-to-server trust (SASL), IPv6, DNSSEC, and IPsec will solve all security problems.

If that is not enough, XMPP can utilize onion routing schemes such as Tor for added security. Typically this results in high latency. But the word "instant" in "instant messaging" has always made XMPP seem quite frivolous (especially back when we called it "Jabber", what a silly word that is!), whereas "secure messaging" sounds like a serious technology. Who cares if delivery takes forever? (Oh and while we're at it, we should add per-hop acknowledgements for every stanza and perhaps full transactional abilities; however, those initiatives are beyond the scope of this document.)

9. IANA Considerations¶

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [11].

10. XMPP Registrar Considerations¶

This document requires no interaction with the XMPP Registrar [12].

11. XML Schema¶

Because XMPP Eventing via Pubsub simply reuses the protocol specified in XEP-0060, a separate schema is not needed.

12. Acknowledgements¶

Thanks to Maciek Niedzielski for inspiration.

Appendices

Appendix A: Document Information¶

Series

XEP

Number

0207

Publisher

XMPP Standards Foundation

Status

Active

Type

Humorous

Version

1.0

Last Updated

2007-04-01

Approving Body

XMPP Council

Dependencies

XMPP Core, XEP-0060

Supersedes

None

Superseded By

None

Short Name

N/A

Source Control

HTML

This document in other formats: XML  PDF

Appendix B: Author Information¶

Peter Saint-Andre

Email

xsf@stpeter.im

JabberID

peter@jabber.org

URI

http://stpeter.im/

Appendix C: Legal Notices¶

Appendix D: Relation to XMPP¶

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue¶

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance¶

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes¶

Appendix H: Revision History¶

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

1. Version 1.0 (2007-04-01)¶

   April Fools!

   psa

END