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. While &xep0256; defines a way for clients to include their last logout time, this specification closes the loop by defining how pubsub and PEP services can use the last logout information to send interim notifications to subscribers.

As described in XEP-0256, a subscriber (i.e., a subscriber's specific full JID) can indicate its last logout time when sending initial 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 &xep0059; data so that the subscriber can page through the set of interim notifications, as described in Section 6.5.3 of XEP-0060.

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).

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 an implementation suggestion; implementation and deployment experience might indicate that other values are more prudent.