This extension allows XMPP servers to dispatch incoming and outgonig messages to all connected resources of an XMPP entity. This allows all devices of a user to be aware of chats which are happening or happened on other devices of that user.
The server returns a stream header to the client along with stream features, where the features include an <mr2/> element qualified by the 'urn:xmpp:mr2:0' namespace (see Namespace Versioning regarding the possibility of incrementing the version number).
To enable use of message routing 2.0, the client sends an <enable/> nonza to the server.
Upon receiving the enable request, the server MUST reply with an <enabled/> nonza or a <failed/> nonza qualified by the 'urn:xmpp:mr2:0' namespace. The <failed/> element indicates that there was a problem creating a message routing 2.0 enabled stream. The <enabled/> nonza indicates successful creation of a message routing 2.0 enabled stream.
TODO: Now the server MUST use MR2.
The client MUST NOT attempt to negotiate message routing 2.0 until it is authenticated; i.e., it MUST NOT send an <enable/> element until after authentication (such as SASL, Non-SASL Authentication (XEP-0078)  or Server Dialback (XEP-0220) ) has been completed successfully. the client MUST enable message routing 2.0 before it has completed Resource Binding.
The server SHALL enforce this order and return a <failed/> element in response if the order is violated (see Error Handling).
When the server receives a <message/> that is not of type "groupchat" or "error" to a bare JID (localpart@domainpart), it delivers a copy to each resource using a Message Routing 2.0 enabled XMPP stream for the bare JID – in addition to delivering according to RFC 6121 § 8.5.2. This process is sometimes called "forking".
The receiving server MUST deliver a copy to every Message Routing 2.0-enabled resource, even if that resource normally would not receive <message/> stanzas addressed to the bare JID (e.g., resources which have broadcast <presence/> with a negative priority). A Message Routing 2.0-enabled resource MUST NOT receive more than one copy of the <message/>.
When the server receives a <message/> of type "chat" addressed to a full JID (localpart@domainpart/resourcepart), it delivers the <message/> according to RFC 6121 § 8.5.3, and delivers a forwarded copy to each Message Routing 2.0-enabled resource for the matching bare JID recipient.
Each forwarded copy is wrapped using Stanza Forwarding (XEP-0297) . The wrapping message SHOULD maintain the same 'type' attribute value; the 'from' attribute MUST be the Message Routing 2.0-enabled user's bare JID (e.g., "localpart@domainpart"); and the 'to' attribute MUST be the full JID of the resource receiving the copy. The content of the wrapping message MUST contain a <received/> element qualified by the namespace "urn:xmpp:mr2:0", which itself contains a <forwarded/> element qualified by the namespace "urn:xmpp:forward:0" that contains the original <message/>.
The receiving server MUST NOT send a forwarded copy to the full JID the original <message/> stanza was addressed to, as that recipient receives the original <message/> stanza.
When a client sends a <message/> of type "chat", its sending server delivers the <message/> according to RFC 6120 and RFC 6121, and delivers a forwarded copy to each Message Routing 2.0-enabled resource for the matching bare JID sender.
Each forwarded copy is wrapped using Stanza Forwarding (XEP-0297) . The wrapping message SHOULD maintain the same 'type' attribute value; the 'from' attribute MUST be the Message Routing 2.0-enabled user's bare JID (e.g., "localpart@domainpart"); and the 'to' attribute SHOULD be the full JID of the resource receiving the copy. The content of the wrapping message MUST contain a <sent/> element qualified by the namespace "urn:xmpp:mr2:0", which itself contains a <forwarded/> qualified by the namespace "urn:xmpp:forward:0" that contains the original <message/> stanza.
The sending server SHOULD NOT send a forwarded copy to the sending full JID if it is a Message Routing 2.0-enabled resource.
The Message Routing 2.0 state is kept if the stream is resumed by means of Stream Management (XEP-0198) . That is, clients are not required to enable Message Routing 2.0 if the stream is resumed. They MUST however, enable Message Routing 2.0 if stream resumption failed and they wish to use Message Routing 2.0.
TODO should Client State Indication (XEP-0352)  delay certain forked messages, e.g. the onces caused by a message send to a full JID?
Additionally, there are other considerations for clients that implement MR2s and XEP-0085:
When a receiving server attempts to deliver a forked message, and that message bounces with an error for any reason, the receiving server MUST NOT forward that error back to the original sender. The receiving server SHOULD use the sent element in the bounce to determine that an error is from a forked message.
This rule is used to prevent some of the half-failure modes that have been an issue in other prototocols.
Clients that automatically respond to messages for any reason (e.g., when in the "dnd" presence show state) MUST take adequate care when enabling MR2s in order to prevent storms or loops. MR2 copies of messages MUST NOT be auto-replied to under any circumstances. Forked inbound messages MUST NOT be auto-replied to unless the client has some way of ensuring no more than one auto-reply is sent from all of its user's resources.
Enabling this protocol on mobile devices needs to be undertaken with care. This protocol can result in additional bandwidth and power usage, possibly decreasing battery lifetime and increasing monetary costs. Additional mechanisms for controlling the MR2-copying or forking of individual conversations might need to be added to deal with mobile clients in the future.
As the notifications defined in this document contain full <message/> stanzas, it is important for implementations to take care that the receiving entities of such notifications are allowed to see that information. This usually means they receiving entities share the same bare JID with the intended receiver. (TODO wording)
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
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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