To enable lightweight repeaters for Publish-Subscribe (XEP-0060) [1] notifications, we need the ability to subscribe one pubsub node to another pubsub node. This specification defines a method for doing so, using Ad-Hoc Commands (XEP-0050) [2].
The owner of a pubsub node can subscribe that "local" node to a "remote" node using the flow described below. In these examples, the node owner is admin@consumer.tld, the local node is weatherbot@consumer.tld/Chicagoland, and the remote node has a NodeID of "OHR" at the pubsub service notify.weather.tld.
<iq from='admin@consumer.tld/client'
id='chaining-1'
to='weatherbot@consumer.tld'
type='set'
xml:lang='en'>
<command xmlns='http://jabber.org/protocol/commands'
action='execute'
node='http://jabber.org/protocol/pubsub#chaining'/>
</iq>
Unless an error occurs, the service SHOULD return the appropriate form.
<iq from='weatherbot@consumer.tld'
id='chaining-1'
to='admin@consumer.tld/client'
type='result'
xml:lang='en'>
<command xmlns='http://jabber.org/protocol/commands'
node='http://jabber.org/protocol/pubsub#chaining'
sessionid='a73sjjvkla37jfea'
status='executing'>
<x xmlns='jabber:x:data' type='form'>
<title>Chaining Request</title>
<instructions>Fill out this form to complete your request.</instructions>
<field type='hidden' var='FORM_TYPE'>
<value>http://jabber.org/protocol/pubsub#chaining</value>
</field>
<field label='The Node ID of the local pubsub node'
type='text-single'
var='local-node'>
<required/>
</field>
<field label='The Jabber ID of the remote pubsub service'
type='jid-single'
var='remote-service'>
<required/>
</field>
<field label='The NodeID of the remote pubsub node'
type='text-single'
var='remote-node'>
<required/>
</field>
</x>
</command>
</iq>
<iq from='admin@consumer.tld/client'
id='chaining-2'
to='weatherbot@consumer.tld'
type='submit'
xml:lang='en'>
<command xmlns='http://jabber.org/protocol/commands'
node='http://jabber.org/protocol/pubsub#chaining'
sessionid='a73sjjvkla37jfea'
status='executing'>
<x xmlns='jabber:x:data' type='submit'>
<field type='hidden' var='FORM_TYPE'>
<value>http://jabber.org/protocol/pubsub#chaining</value>
</field>
<field var='local-node'>
<value>Chicagoland</value>
</field>
<field var='remote-service'>
<value>notify.weather.tld</value>
</field>
<field var='remote-node'>
<value>OHR</value>
</field>
</x>
</command>
</iq>
<iq from='weatherbot@consumer.tld'
id='chaining-2'
to='admin@consumer.tld/client'
type='result'
xml:lang='en'>
<command xmlns='http://jabber.org/protocol/commands'
node='http://jabber.org/protocol/pubsub#chaining'
sessionid='a73sjjvkla37jfea'
status='completed'/>
</iq>
At this point, the service itself will subscribe to the remote node.
<iq type='set'
from='weatherbot@consumer.tld/Chicagoland'
to='notify.weather.tld'
id='sub1'>
<pubsub xmlns='http://jabber.org/protocol/pubsub'>
<subscribe
node='OHR'
jid='weatherbot@consumer.tld/Chicagoland'/>
</pubsub>
</iq>
Now subscribers who are local to the consumer.tld XMPP service can subscribe directly to weatherbot@consumer.tld/Chicagoland instead of the remote pubsub node at notify.weather.tld. We therefore refer to weatherbot@consumer.tld/Chicagoland as a "chaining node" and the remote node as a "chained node".
When a chaining node delivers a notification to its subscribers, it SHOULD include an Extended Stanza Addressing (XEP-0033) [3] header of "ofrom" to specify the chained node or service that generated the notification (note: this header is not yet defined in XEP-0033).
<message from='weatherbot@consumer.tld/bot'
to='subscriber@consumer.tld'
id='foo'>
<event xmlns='http://jabber.org/protocol/pubsub#event'>
<items node='Chicagoland'>
<item id='ae890ac52d0df67ed7cfdf51b644e901'>
<example xmlns='urn:xmpp:example'>message</example>
</item>
</items>
</event>
<addresses xmlns='http://jabber.org/protocol/address'>
<address type='ofrom' jid='notify.weather.tld'/>
</addresses>
</message>
If a pubsub service supports Ad-Hoc Commands, it MUST advertise the commands it supports via Service Discovery (XEP-0030) [4] (as described in XEP-0050: Ad-Hoc Commands); such commands exist as well-defined discovery nodes associated with the service. In particular, if a pubsub service supports chaining it MUST advertise a command of "http://jabber.org/protocol/pubsub#chaining".
The ability to subscribe one node to another node introduces the possibility of exposing non-public information in a public way, since the access controls for the node that originates a notification might not be known or enforced by the downstream node. Therefore, the upstream node (or its owner) is advised to make a careful access decision before allowing a downstream node (or any other entity) to subscribe.
Note: The upstream node can discover the identity of the downstream node by sending a service discovery information ("disco#info") request to the downstream node, and MAY cancel or decline the downstream node's subscription if it determines that the node has an identity of "pubsub/leaf" or "pubsub/collection".
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [5].
The XMPP Registrar [6] shall include the following information in its registries.
The XMPP Registrar shall include 'http://jabber.org/protocol/pubsub#chaining' in its registry of protocol namespaces (see <https://xmpp.org/registrar/namespaces.html>).
Field Standardization for Data Forms (XEP-0068) [7] defines a process for standardizing the fields used within Data Forms (XEP-0004) [8] scoped by a particular namespace (see <https://xmpp.org/registrar/formtypes.html>). The reserved fields for the 'http://jabber.org/protocol/pubsub#chaining' namespace are specified below.
<form_type>
<name>http://jabber.org/protocol/pubsub#chaining</name>
<doc>XEP-0253</doc>
<desc>Forms used for chaining of pubsub nodes.</desc>
<field var='local-node'
type='text-single'
label='The Node ID of the local node'/>
<field var='remote-node'
type='text-single'
label='The NodeID of the remote node'/>
<field var='remote-service'
type='jid-single'
label='The Jabber ID of the remote pubsub service'/>
</form_type>
Thanks to Joe Hildebrand for his feedback.
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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".
1. XEP-0060: Publish-Subscribe <https://xmpp.org/extensions/xep-0060.html>.
2. XEP-0050: Ad-Hoc Commands <https://xmpp.org/extensions/xep-0050.html>.
3. XEP-0033: Extended Stanza Addressing <https://xmpp.org/extensions/xep-0033.html>.
4. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
5. 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/>.
6. 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/>.
7. XEP-0068: Field Data Standardization for Data Forms <https://xmpp.org/extensions/xep-0068.html>.
8. XEP-0004: Data Forms <https://xmpp.org/extensions/xep-0004.html>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Specifed protocol flow for the chained subscription.
Initial published version.
Added ofrom header to notifications.
First draft.
END