WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.
Some XMPP features must be offered by the server itself, or can't be available, that's the case of Personal Eventing Protocol (XEP-0163)  which is used in several places (e.g. bookmarks storage). But it can be desirable to use an external entity to manage some of these features, because it implements things that the server don't, or because it uses a special implementation useful in a particular case. Some people may also want to decentralize a feature on an entity under their control. This XEP try to solve these cases. Additionaly, a method to do generic treatments (independent of server) on stanza is also provided.
admin mode, where delegation is specified by the server administrator.
client mode, where it can be requested by any user.
In admin mode, the managing entity manages stanza of the delegated namespace for all users registered on the server. The namespace delegation MUST be totally transparent for the managed entities.
In client mode, a managing entity MUST have an explicit authorization for any namespace he wants to use. Client SHOULD be able to check and revoke granted permissions, and if it's not possible, permissions MUST be revoked after a disconnection.
Once the managing entity is authenticated and stream is started, the server send it a <message/> stanza with a <delegation/> elements which MUST have the 'urn:xmpp:delegation:2' namespace. This element contains <delegated/> elements which MUST contain a 'namespace' attribute indicating the delegated namespace. If there is additional attribute filtering, the <delegated/> can have children <attribute/> elements which MUST contain a 'name' attribute with the name of the filtering attribute.
Here pubsub.capulet.lit will received all stanzas of pubsub namespace sent to capulet.lit. It will also receive MAM stanzas, but only if the 'node' attribute is present in <query/>.
When a server receives a stanza for a delegated namespace which is either directed to him (no 'to' attribute, or 'to' attribute with its own JID), or directed to any bare jid (and only bare jid) that it manages (i.e. the domainpart is a domain handled by the server where delegation is activated), it MUST forwards it to the managing entity:
The server gets this stanza, sees that this namespace is delegated to pubsub.capulet.lit, so it forwards it.
To forward, an <iq/> stanza of type "set" is used which contain a <delegation/> element (with namespace 'urn:xmpp:delegation:2') which in turn contain a <forwarded/> element encapsulating the initial stanza, according to Stanza Forwarding (XEP-0297) :
The managing entity replies to the stanza by encapsulating its <iq/> result in the same way:
Then the server MUST decapsulate the <iq/> result, MUST insure that the 'to' and the 'from' attribute corresponds to respectively the 'from' and the 'to' attributes of the initial stanza, MUST insure that the 'id' attribute of the decapsulated stanza is the same as the initial 'id' attribute and that 'type' is "result". If everything is alright, it can send the decapsulated stanza to Juliet.
If the forwarded result from managing entity is bad (i.e. wrong 'to', 'from', 'id' or 'result' attributes), the server MUST send an <iq/> error with condition <service-unavailable/> to managed entity, and MAY close the connexion with managing entity.
The workflow is fully transparent for Juliet.
N.B.₁: If the server encounter a delegated namespace and the managing entity is not available, it MUST return an <iq/> stanza of type "error" with an error condition of <service-unavailable/>
N.B.₂: Similarly, if the managing entity return an <iq/> stanza of type "error", the server must return itself an <iq/> stanza of type "error" with an error condition of <service-unavailable/>
N.B.₃: If the server encounter a delegated namespace but the filtering attribute does not match, it MUST follow its normal behaviour, i.e. it must follow the same behaviour it would have had if the namespace was not delegated at all
If a stanza is sent by the managing entity on a managed namespace, the server MUST NOT forward it. This way, the managing entity can use privileged entity to do specific treatments, a kind of universal plugin (i.e. working with all servers implementing Namespace Delegation (XEP-0355)  and Privileged Entity (XEP-0356) ).
In the following examples, email@example.com has its "jabber:iq:roster" namespace delegated to filter.capulet.lit. filter.capulet.lit is a server agnostic component which filters allowed entities (which can be added to a roster), and sort them in enforced groups.
filter.capulet.lit accepts to add Romeo, but all JIDs with a montaigu.lit must be in a "Rivals" group, so it first returns a success result (Romeo is accepted).
At this stade, the entity is accepted, but not added to the roster. filter.capulet.lit is also a privileged entity which can manage "jabber:iq:roster", so it uses this ability to add Romeo in the enforced group:
The namespace is delegated, but as the stanza is from the managing entity, the server manages it normally. The entity is also privileged, so it can change the stanza of Juliet, the server accepts:
The server will then send the roster pushes (with the enforced group) normally.
In client mode, the managing entity is not certified by the server administrator, so the delegation MUST be explicitly allowed by the managed entity. This is initiated by the managing entity (it can be after an interaction with a managed entity, like a subscription).
To request delegation for a particular entity, the managing entity MUST have an <iq/> stanza with 'urn:xmpp:delegation:2' namespace. The <query/> element MUST have a 'to' attribute which specify the entity it wants to manage.
Namespace delegations are asked with a <delegate/> element, which MUST contain a 'namespace' attribute set to the requested namespace.
If an entity want to manage PEP service for Juliet, it can ask the delegation like this:
Once received the delegation request, the server ask to the client if it grants access to the requested namespace using Data Forms (XEP-0004) . The server use a challenge which it MUST have generated itself.
The server SHOULD include a warning message, SHOULD translate the namespace to human friendly names (and MAY keep the original namespace in addition) and MUST set the default value to '0' (permission refused). The server MUST use namespaces as field var, so a client can use them to have a customized display.
The client can then answer to the form:
Here Juliet allows pubsub.montaigu.lit to manage the PubSub (and then PEP) service.
Finaly, the server notifies the entity of the granted delegation. It do this in the same way as for admin mode, except that the <delegation/> element has an attitional 'to' attribute set to the managed entity bare jid:
The managing entity can now manage the namespace in a similar way as in admin mode but with different rules:
if the stanza is directed to the delegating server (no 'to' attribute, or 'to' attribute with the server own JID), and the 'from' attribute belong to a managed entity (e.g. firstname.lastname@example.org/balcony if email@example.com is a managed entity) and the namespace of the stanza has been delegated by the managed entity, then the delegating server MUST forward it to the managing entity accepted by the managed entity, in the same way as in admin mode.
if the stanza is directed to the bare jid (and only the bare jid) of the managed entity (e.g. the 'to' attribute is set to firstname.lastname@example.org and the namespace has been delegated by the managed entity, then the server MUST forward it to the managing entity accepted by the managed entity, in the same way as in admin mode, for any 'from' attribute but the jid of the managing entity.
in all other cases, the server MUST NOT forward the stanza
Server SHOULD provide a way for clients to check already delegated namespaces, and revoke them by using Ad-Hoc Commands (XEP-0050)  on the well-defined command node 'urn:xmpp:delegation:2#configure'.
If present, the configuration commands MUST allow at least to check delegations granted to a managing entity, and to revoke them. A server MAY offer an option to keep delegations from one session to an other (see business rules).
If a server or an entity supports the namespace delegation protocol, it MUST report that fact by including a service discovery feature of "urn:xmpp:delegation:2" in response to a Service Discovery (XEP-0030)  information request:
When a server delegates a namespace to a managing entity, the later can have particular features which must be advertised by the former with disco protocol.
This is done by using a disco node, which is built in the following way: if pubsub.capulet.int manages pubsub namespace, it MUST report that fact in discovery feature, and have a 'urn:xmpp:delegation:2::http://jabber.org/protocol/pubsub' node which reports the managed features.
The node name is obtained by concatenating this XEP namespace (urn:xmpp:delegation:2), a '::' separator, and the delegated namespace (here http://jabber.org/protocol/pubsub). The server MUST advertise the result in its own discovery answer, and MUST ignore features of its internal component (here internal PubSub service).
In the following example, the capulet.int server delegates its internal PEP component to pubsub.capulet.int. capulet.int only supports REQUIRED PubSub features and auto-create, while pubsub.capulet.int supports REQUIRED PubSub features and publish-options, but not auto-create. email@example.com asks its server what it is capable of, she is specially interested in PubSub capabilities.
Server delegates its PubSub namespace to pubsub.capulet.lit, so it asks its available features for this namespace like this:
Note that in real situation, server has probably this information already in cache (see Implementation Notes). pubsub.capulet.lit returns its available features:
Server include the results in its own discovery info results:
Note that 'http://jabber.org/protocol/pubsub#auto-create' is not available.
N.B.: In the special case of attribute filtering, the server still display managing entity's features for the whole delegated namespace instead of its own internal ones.
Extensions of Service Discovery as specified in Service Discovery Extensions (XEP-0128)  follow the same rules: for the delegated namespace, internal extensions MUST be removed and extensions of managing entity MUST be included instead.
It may be necessary for a managing entity to advertise feature on a node which is unknown at the time of configuration. This is notably the case for a PEP component: features must be advertisable on disco nodes mapping pubsub nodes.
To allows that, the special namespace 'urn:xmpp:delegation:2:bare:disco#info:*' is used, and means "delegate information discovery requests made on bare jids for any discovery node which is not already managed by the server".
The node 'urn:xmpp:microblog:0' is not managed by the server. Normally, this would result as an <item-not-found> <iq/> error, but the namespace 'urn:xmpp:delegation:2:bare:disco#info:*' is delegated to pubsub.capulet.lit, thus the server delegate the stanza to the managing entity:
The managing entity answers with its own disco features for the requested node:
A managing entity must be able to answer items discovery requests on bare jids. For PEP components, this is used to show available nodes on the service. Furthermore, An information discovery request or an items discovery requests on a node corresponding to an item advertised by the managing entity must be forwarded to the managing entity too.
This is done by using the special namespace 'urn:xmpp:delegation:2:bare:disco#items:*' which means "delegate items discovery made on bare jids with no node specified and all items discovery requests made on bare jids with a node which is not already managed by the server".
Note: When no node is used, and unlike the disco#info case, the server SHOULD NOT nest its internal items with the ones from managing entity. The reason is that bare jids disco items are usually used for PEP nodes, and the server would then mix internal PEP nodes with the managing entity nodes, which would be confusing for XMPP clients and end-users. The server implementation MAY choose to nest items anyway, in which case the mechanism is the same as for disco#info.
A server SHOULD NOT cache result of forwarded disco#items requests, as this is highly dynamic.
The namespace 'urn:xmpp:delegation:2:bare:disco#items:*' is delegated to pubsub.capulet.lit, thus the server delegate the stanza to the managing entity:
The managing entity answers with its own disco items for Juliet's JID:
In client mode, server MAY keep delegations granted to an entity by a client from one session to an other, but if it does so, it MUST provide configuration like explained in the suitable section. If server offers this feature, it SHOULD add a field directly in configuration commands.
If a client can't check or revoke delegations (i.e. it doesn't support Ad-Hoc Commands (XEP-0050) ) when granting them, the server MUST NOT keep granted delegations from one session to an other, and delegations will be asked on each new session.
If delegations are kept between sessions in client mode, and the managing entity is not available or return an error, the server MUST return an <iq/> error with condition <service-unavailable/> when a delegated namespace is requested, like explained in "Server Forwards Delegated <iq/> Stanza" section.
The namespace of this XEP (urn:xmpp:delegation:2) MUST NOT be delegated. If an entity requests it, the server MUST return a <forbidden/> error.
As admin mode is far more easy to implement than client mode, and client mode may impact performances, a server MAY choose to only implement the former.
Because of the performance impact, a server SHOULD ask for disco features to nest to managing entity when delegation is accepted, and keep them in cache.
RFC 6120  section 10.1 require in-order processing of stanzas, which may be problematic for this extension: either the server blocks the traffic until the managing entity answers - which can lead to severe performance impact -, or the server doesn't block and may loose order. The recommended way is to not block the traffic while waiting for managing entity answer to avoid performance issues. A future version of this XEP may include an attribute to request traffic blocking. In admin mode the server implementation MAY chooses to have a blocking option (which SHOULD be per namespace, not global).
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
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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 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".
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