XEP-0312: PubSub Since

1. Introduction¶

Many Publish-Subscribe (XEP-0060) [1] and Personal Eventing Protocol (XEP-0163) [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 (XEP-0256) [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:0' namespace; the 'secs' attribute indicates the number of seconds since this resource was last online.

<presence from='juliet@capulet.com/balcony'>
  <ago xmlns='urn:xmpp:ago:0' 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 (although see the rule #2 and rule #5 below).
2. The service SHOULD NOT include items that were deleted from the node, and MAY purge the node if items are deleted (naturally, if the node is purged then the service would not be able to meet rule #1 above).
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 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).
5. The service MAY limit the number of notifications that it sends to avoid resource contraints.
6. The service MAY include Result Set Management (XEP-0059) [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 delivery toggles notifications "on" when receiving initial presence and toggles 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 (XEP-0030) [5] feature of "http://jabber.org/protocol/pubsub#since" in response to disco#info requests:

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

<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 (XEP-0202) [8]. The five-minute grace period is merely a suggestion for developers; 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-0312</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:0'
    xmlns='urn:xmpp:ago:0'
    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

Deferred

Type

Standards Track

Version

0.3

Last Updated

2012-05-29

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¶

Joe Hildebrand

Email

jhildebr@cisco.com

JabberID

hildjj@jabber.org

Peter Saint-Andre

Email

stpeter@stpeter.im

JabberID

stpeter@jabber.org

URI

https://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 <https://xmpp.org/community/> 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 https://xmpp.org/extensions/attic/

1. Version 0.3 (2012-05-29)¶

   Corrected namespace to use namespace versioning; tweaked some of the delivery rules.

   psa
2. Version 0.2 (2012-03-08)¶

   Replaced jabber:iq:last with a specialized namespace.

   psa
3. Version 0.1 (2012-02-29)¶

   Initial published version.

   psa
4. Version 0.0.2 (2012-02-29)¶

   Addressed some feedback from the XMPP Council.

   psa
5. Version 0.0.1 (2012-02-27)¶

   First draft.

   psa

Appendix I: Bib(La)TeX Entry

@report{hildebrand2012xep0312,
  title = {PubSub Since},
  author = {Hildebrand, Joe and Saint-Andre, Peter},
  type = {XEP},
  number = {0312},
  version = {0.3},
  institution = {XMPP Standards Foundation},
  url = {https://xmpp.org/extensions/xep-0312.html},
  date = {2012-02-27/2012-05-29},
}

END