XEP-0222: Best Practices for Persistent Storage of Public Data via Publish-Subscribe

This specification defines best practices for using the XMPP publish-subscribe extension to persistently store semi-public data objects such as public keys and personal profiles.

NOTICE: This document is currently within Last Call or under consideration by the XMPP Council for advancement to the next stage in the XSF standards process.

Document Information

Author Information

Peter Saint-Andre

JabberID: stpeter@jabber.org
URI: https://stpeter.im/

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## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. In no event shall the XMPP Standards Foundation or the authors of this Specification be liable for any claim, damages, or other liability, whether in an action of contract, tort, or otherwise, arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification. ##

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Discussion Venue

Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) 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.

Conformance Terms

The following 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".

1. Introduction
1.1. Motivation
1.2. How It Works
2. Concepts and Approach
3. Publishing an Item
4. Composition
5. Determining Support
6. Security Considerations
7. IANA Considerations
8. XMPP Registrar Considerations
Notes
Revision History

1. Introduction

1.1 Motivation

Personal Eventing via Pubsub [1] introduced the idea of a virtual Publish-Subscribe [2] service associated with an IM user's bare JID <localpart@domain.tld>. However, the default node configuration options associated with PEP nodes are not optimized for the persistent storage of semi-public data objects such as public keys or user profiles. Therefore this document defines a set of best practices that enable IM users to persistently store semi-public data objects at their virtual pubsub service; in effect, we "sub-class" PEP by showing how a particular pubsub node can be configured for persisting objects.

1.2 How It Works

Imagine that you are a Shakespearean character named Juliet and that you want to persistently store information such as your public keys (see Public Key Publishing [3]) and user profile (see User Profile [4]).

We assume that you have three contacts with the following relationship to you:

benvolio@montague.lit, who has no subscription to your presence
nurse@capulet.lit, who has a bidirectional subscription to your presence and who is in your "Servants" roster group
romeo@montague.lit, who has a bidirectional subscription to your presence and who is in your "Friends" roster group

We also assume that your server (capulet.lit) supports PEP along with the "publish-options" feature, and that your client discovered that support when you logged in.

Because you want to keep your communications with Romeo confidential, you decide to start encrypting your messages. Therefore you reconfigure your client, which generates an RSA key that it publishes to the virtual pubsub service hosted at your bare JID <juliet@capulet.lit>.

Example 1. Publishing a key

<iq from='juliet@capulet.lit/balcony' type='set' id='pub1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
          <KeyName>julietRSAkey1hash</KeyName>
          <KeyValue>
            <RSAKeyValue>
              <Modulus>...</Modulus>
              <Exponent>AQAB</Exponent>
            </RSAKeyValue>
          </KeyValue>
        </KeyInfo>
      </item>
    </publish>
    <publish-options>
      <x xmlns='jabber:x:data' type='submit'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#publish-options</value>
        </field>
        <field var='pubsub#persist_items'>
          <value>true</value>
        </field>
        <field var='pubsub#send_last_published_item'>
          <value>never</value>
        </field>
        <field var='pubsub#access_model'>
          <value>roster</value>
        </field>
        <field var='pubsub#roster_groups_allowed'>
          <value>Friends</value>
        </field>
      </x>
    </publish-options>
  </pubsub>
</iq>

Your publish request is a standard pubsub request except that:

The item is persisted (pubsub#persist_items is true).
The last published item is never sent (pubsub#send_last_published_item is set to "never" so that items are pushed out only when modified).

(In this case, access is limited to people in your Friends roster group.)

If all goes well (see Publishing an Item), your key will be pushed out to all appropriate individuals (in this case only Romeo). In particular, Romeo receives your key because he has auto-subscribed to the virtual pubsub service at your bare JID via a presence subscription and because his Entity Capabilities [5] data indicated that he is interested in the "urn:xmpp:tmp:pubkey" payload type.

Example 2. Appropriate entities receive event notifications

<message from='juliet@capulet.lit'
         to='romeo@montague.lit/orchard'
         type='headline'
         id='keyfoo1'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
          <KeyName>julietRSAkey1hash</KeyName>
          <KeyValue>
            <RSAKeyValue>
              <Modulus>...</Modulus>
              <Exponent>AQAB</Exponent>
            </RSAKeyValue>
          </KeyValue>
        </KeyInfo>
      </item>
    </items>
  </event>
</message>

Because PEP services must send notifications to the account owner, you too receive the notification at each of your resources (here "balcony" and "chamber").

Example 3. Publisher receives event notification

<message from='juliet@capulet.lit'
         to='juliet@capulet.lit/balcony'
         type='headline'
         id='keyfoo2'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
          <KeyName>julietRSAkey1hash</KeyName>
          <KeyValue>
            <RSAKeyValue>
              <Modulus>...</Modulus>
              <Exponent>AQAB</Exponent>
            </RSAKeyValue>
          </KeyValue>
        </KeyInfo>
      </item>
    </items>
  </event>
</message>

<message from='juliet@capulet.lit'
         to='juliet@capulet.lit/chamber'
         type='headline'
         id='keyfoo3'>
  <event xmlns='http://jabber.org/protocol/pubsub#event'>
    <items node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
          <KeyName>julietRSAkey1hash</KeyName>
          <KeyValue>
            <RSAKeyValue>
              <Modulus>...</Modulus>
              <Exponent>AQAB</Exponent>
            </RSAKeyValue>
          </KeyValue>
        </KeyInfo>
      </item>
    </items>
  </event>
</message>

So that's the general idea.

2. Concepts and Approach

The best practices described herein re-use the concepts already defined in XEP-0060 and XEP-0163. In order to optimize for object persistence instead of transient event notifications, a node MUST be configured as follows:

3. Publishing an Item

An account owner publishes an item to a node by following the protocol specified in XEP-0060:

<iq from='juliet@capulet.lit/balcony' type='set' id='pub1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
          <KeyName>julietRSAkey1hash</KeyName>
          <KeyValue>
            <RSAKeyValue>
              <Modulus>...</Modulus>
              <Exponent>AQAB</Exponent>
            </RSAKeyValue>
          </KeyValue>
        </KeyInfo>
      </item>
    </publish>
  </pubsub>
</iq>

If the node does not already exist, the virtual pubsub service MUST create the node. As described in XEP-0163, this "auto-create" feature (defined in XEP-0060) MUST be supported by a PEP service. (Naturally, the account owner's client MAY follow the node creation use case specified in XEP-0060 before attempting to publish an item.)

In order for the client to reliably persist objects, the virtual pubsub service must also support the "publish-options" feature defined in XEP-0060. Typically, a client will publish with options so that the object is properly persisted.

<iq from='juliet@capulet.lit/balcony' type='set' id='pub1'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='urn:xmpp:tmp:pubkey'>
      <item id='julietRSAkey1hash'>
        <KeyInfo xmlns='http://www.w3.org/2000/09/xmldsig#'>
          <KeyName>julietRSAkey1hash</KeyName>
          <KeyValue>
            <RSAKeyValue>
              <Modulus>...</Modulus>
              <Exponent>AQAB</Exponent>
            </RSAKeyValue>
          </KeyValue>
        </KeyInfo>
      </item>
    </publish>
    <publish-options>
      <x xmlns='jabber:x:data' type='submit'>
        <field var='FORM_TYPE' type='hidden'>
          <value>http://jabber.org/protocol/pubsub#publish-options</value>
        </field>
        <field var='pubsub#persist_items'>
          <value>true</value>
        </field>
        <field var='pubsub#send_last_published_item'>
          <value>never</value>
        </field>
        <field var='pubsub#access_model'>
          <value>roster</value>
        </field>
        <field var='pubsub#roster_groups_allowed'>
          <value>Friends</value>
        </field>
      </x>
    </publish-options>
  </pubsub>
</iq>

If the publication logic dictates that event notifications shall be sent, the account owner's server generates notifications and sends them to all appropriate entities as described in the Receiving Event Notifications section of XEP-0163.

4. Composition

Each item published to the node is a logically separate instance of the data to be stored. It is the responsibility of the publishing and receiving entities to construct a complete view of all such items, if desired. For example, each bookmark published to a private data node is a separate piece of data, whereas the history of all items published to the node provides a complete list of the user's bookmarks. This history may include items that are republished with an existing ItemID (thus overwriting the previous version of that item).

5. Determining Support

Before an account owner attempts to complete the use cases defined herein, its client SHOULD verify that the account owner's server supports both PEP and the "publish-options" feature; to do so, it MUST send a Service Discovery [6] information request to the server (or cache Entity Capabilities information received from the server).

<iq from='juliet@capulet.lit/balcony'
    to='capulet.lit'
    id='disco1'
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>

The server MUST return an identity of "pubsub/pep" and include the "publish-options" feauture in the list of the namespaces and other features it supports:

<iq from='capulet.lit'
    to='juliet@capulet.lit/balcony'
    id='disco1'
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <identity category='server' type='im'/>
    <identity category='pubsub' type='pep'/>
    ...
    <feature var='http://jabber.org/protocol/pubsub#publish-options'/>
    ...
  </query>
</iq>

6. Security Considerations

This document introduces no security considerations above and beyond those specified in XEP-0060 and XEP-0163.

7. IANA Considerations

8. XMPP Registrar Considerations

Notes

1. XEP-0163: Personal Eventing via Pubsub <http://www.xmpp.org/extensions/xep-0163.html>.

2. XEP-0060: Publish-Subscribe <http://www.xmpp.org/extensions/xep-0060.html>.

3. XEP-0189: Public Key Publishing <http://www.xmpp.org/extensions/xep-0189.html>.

4. XEP-0154: User Profile <http://www.xmpp.org/extensions/xep-0154.html>.

5. XEP-0115: Entity Capabilities <http://www.xmpp.org/extensions/xep-0115.html>.

6. XEP-0030: Service Discovery <http://www.xmpp.org/extensions/xep-0030.html>.

7. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

8. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <http://www.xmpp.org/registrar/>.