Many &xep0060; and &xep0163; 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 &xep0256; 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:

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 &xep0059; 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).

If a pubsub or PEP service supports the protocol defined herein, it MUST report that by including a &xep0030; 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 &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 &rfc1305; and &xep0202;. The five-minute grace period is merely a suggestion for developers; implementation and deployment experience might indicate that other values are more prudent.