Publish-Subscribe (XEP-0060)  in conjuction with Entity Capabilities (XEP-0115)  defines a way for a client to get rid of unwanted pubsub notifications and hence save significant amount of traffic. The protocol is quite flexible and reliable and is able to work even with servers that don't support filtering. Unfortunately, this reliability has its costs and the protocol has the following disadvantages:
The most actual example of the problems that are beget with these disadvantages is the Microblogging Over XMPP (XEP-0277) : you won't be able to subscribe to a user's blog if he doesn't want to read your blog as well. This will be a big problem for popular bloggers that can't read all the feeds of their readers.
Another one is a bot which want to gather information from a lot of users. For instance, we can want to write a bot which gather geo-location from a lot of devices, then it will subscribe to these devices but they may not want to do so. The new protocol will allow such bots to operate this way.
Also, some clients may want to receive retract items events to appropriately update their feed data storage but some other clients may not want to store feeds at all and just show the recent events and corollary don't need in these events. This standard allows the first one to receive those events and the last one to get rid of them saving traffic, regardless of node settings which can be configured solely by the node's owner.
The new protocol is needed to solve the stated problems that will allow to move the notifications filtering process from the sender side to the recipient side, that will allow us to filter them in a more flexible way. Also, we must care of:
This document should cover the following use cases:
Since we require the implementations to be backwards compatible, we need a mechanism for a sender to know if the recipient's server supports the modern protocol or not.
To know this, sender's server should check recipient's capabilities as described at Entity Capabilities (XEP-0115)  in the "Stream Feature" section: if the server supports the "urn:xmpp:rsf:0" feature then the server SHOULD use the protocol that's described here, if the server lacks support of this feature or doesn't provide its capabilities at all then the sender MUST NOT use this protocol.
When using this protocol to deliver notifications, the server MUST follow these rules:
Recipient server is responsible now to filter out and broadcast that messages with events that were request by each user's resource.
As described at Publish-Subscribe (XEP-0060) 's "Filtered notifications" section, client should add payload's NodeID and "+notify" postfix to tell which types of content it can and want to process.
This document extends this filtering feature to allow clients also select which event types it wants to receive.
To advertise user's preferences it should add a feature in its capabilities that MUST consist of payload's NodeID with appended "+events-" postfix and then append a list of necessary payload types. Here are the rules of generating this list:
For example, the following string will mean that user wants to receive new items, node configuration changes and new subscriptions geolocation events, it should add the following features in its capabilities:
The recipient's server MUST drop the notifications that are not in the list. New items notifications are implied by the "+notify" filter. If user doesn't specify the "+events" filter then the server SHOULD treat it as absence of any filtering at all.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA).
This protocol doesn't define any XML stanzas.
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