XEP-0312: PubSub Since

WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.

Table of Contents

1. Introduction

Many Publish-Subscribe [1] and Personal Eventing Protocol [2] services send notifications based on information about the presence of subscribers. This implies that subscribers might not receive notifications that were generated when they are offline at a particular resource. This specification defines how pubsub and PEP services can use the last logout information to send interim notifications to subscribers. (Although Last Activity in Presence [3] defines a way for clients to note their last logout time, this document defines a specialized namespace to reduce the possibility that non-initial presence notifications would trigger the synchronization behavior.)

2. How It Works

When sending initial presence, A subscriber (more precisely, a subscriber's specific full JID) can indicate how long ago it last logged out by including an <ago/> element qualifed by the 'urn:xmpp:ago' namespace; the 'secs' attribute indicates the number of seconds since this resource was last online.

Example 1. Last Logout Indication in Initial Presence

<presence from='juliet@capulet.com/balcony'>
  <ago xmlns='urn:xmpp:ago' secs='86511'/>
</presence>
  

Upon receiving such an indication, a pubsub or PEP service that supports presence-based notifications and the "pubsub-since" feature defined herein would behave as follows:

1. The service MUST send to the subscriber's full JID all of the notifications stored in the relevant node's history that were generated since the last logout time.
2. The service MAY adjust its definition of "interim notification" to account for reasonable clock skew (e.g., including notifications up to five minutes older than the subscriber's last logout time).
3. The service SHOULD include only the final version of each pubsub notification (e.g., in case of items that were updated during the period when the subscriber was offline).
4. The service SHOULD NOT include items that were removed from the node.
5. The service MAY limit the number of notifications that it sends to avoid resource contraints.
6. The service MAY include Result Set Management [4] data so that the subscriber can page through the set of interim notifications.

If the pubsub service receives subsequent available presence from that full JID (even a presence update that includes the last availability indication), it MUST behave according to the rules in XEP-0060 or XEP-0163 (typically this means it would do nothing, since presence-based notifications toggle notifications on when receiving initial presence and toggling notifications off when receiving unavailable presence).

3. Determining Support

If a pubsub or PEP service supports the protocol defined herein, it MUST report that by including a Service Discovery [5] feature of "http://jabber.org/protocol/pubsub#since" in response to disco#info requests:

Example 2. Service discovery information request

<iq from='bard@shakespeare.lit/globe'
    id='sb2t1d49'
    to='pubsub.shakespeare.lit'
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
  

Example 3. Service discovery information response

<iq from='pubsub.shakespeare.lit'
    id='sb2t1d49'
    to='bard@shakespeare.lit/globe'
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    <identity category='pubsub' type='service'/>
    <feature var='http://jabber.org/protocol/pubsub'/>
    <feature var='http://jabber.org/protocol/pubsub#auto-subscribe'/>
    <feature var='http://jabber.org/protocol/pubsub#presence-notifications'/>
    <feature var='http://jabber.org/protocol/pubsub#since'/>
  </query>
</iq>
  

4. Security Considerations

The number or cumulative size of the notifications published since the subscriber's last login time might be large, causing a significant load on the service. Implementations might consider truncating the interim notifications to avoid a denial of service.

5. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].

6. Implementation Notes

The Last Activity extension (XEP-0012) notates time in seconds before the moment of stanza generation. Although some commenters have suggested that it would be easier to implement last activity notations in terms of UTC timestamps, clients can mitigate some implementation problems by storing the last activity time in UTC instead of local time (in case the device is moved across time zones) and by using standard technologies for clock synchronization such as RFC 1305 [7] and Entity Time [8]. The five-minute grace period is merely an implementation suggestion; implementation and deployment experience might indicate that other values are more prudent.

7. XMPP Registrar Considerations

<var>
  <name>http://jabber.org/protocol/pubsub#since</name>
  <desc>
    The pubsub or PEP service sends interim notifications upon receiving 
    initial presence containing the subscriber's last logout time.
  </desc>
  <doc>XEP-xxxx</doc>
</var>
    

8. XML Schema

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:ago'
    xmlns='urn:xmpp:ago'
    elementFormDefault='qualified'>

  <xs:element name='ago'>
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base='empty'>
          <xs:attribute name='secs' type='xs:unsignedLong' use='required'/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>
    

Appendices

Appendix A: Document Information

Series: XEP
Number: 0312
Publisher: XMPP Standards Foundation
Status: Experimental
Type: Standards Track
Version: 0.2
Last Updated: 2012-03-08
Approving Body: XMPP Council
Dependencies: XMPP Core
Supersedes: None
Superseded By: None
Short Name: NOT_YET_ASSIGNED
Source Control: HTML
This document in other formats: XML  PDF

Appendix B: Author Information

Appendix C: Legal Notices

Appendix D: 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.

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/

END