Many Publish-Subscribe (XEP-0060)  and Personal Eventing Protocol (XEP-0163)  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)  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.)
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.
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:
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).
If a pubsub or PEP service supports the protocol defined herein, it MUST report that by including a Service Discovery (XEP-0030)  feature of "http://jabber.org/protocol/pubsub#since" in response to disco#info requests:
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.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
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  and Entity Time (XEP-0202) . The five-minute grace period is merely a suggestion for developers; implementation and deployment experience might indicate that other values are more prudent.
Support for the feature defined in this document is advertised by the "http://jabber.org/protocol/pubsub#since" Service Discovery feature.
The XMPP Registrar shall add this feature to its registry at <https://xmpp.org/registrar/disco-features.html>. The registration is as follows.
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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.
The primary venue for discussion of XMPP Extension Protocols is the <email@example.com> 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 <firstname.lastname@example.org>.
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".
6. 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/>.
9. 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 <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Corrected namespace to use namespace versioning; tweaked some of the delivery rules.
Replaced jabber:iq:last with a specialized namespace.
Initial published version.
Addressed some feedback from the XMPP Council.