The "Server Rules for Processing XML Stanzas" section in RFC 6121  defines only a "one receives" or "all receive" algorithm for message routing if the receiving entity of a message stanza has multiple connected resources and the message is addressed to a bare JID. Also RFC 6121 does not define a method to check or change the currently used message routing algorithm. Furthermore, none of the described routing algorithms is a good match when it comes down to achieve load-balancing between the connected resources, which is sometimes desirable.
This extensions solves those issues by allowing clients to configure their desired message routing algorithm. By exploiting the freedom provided by RFC 6121 which allows servers to implement their own algorithm for determining the "most available" resource.
PubSub Queueing (XEP-0254)  defines a XEP to queue PubSub items, which could be used as alternative approach, instead of the herein defined message routing algorithms ('urn:xmpp:cmr:roundrobin' and 'urn:xmpp:cmr:weighted'). But this solution requires more and complexer code on the server and client side, while the guarantees provided by XEP-0254 are not always required.
The authors have designed the customizable message routing protocol with the following requirements in mind:
The protocol MUST NOT violate the server rules for processing XML Stanzas as defined in RFC 6121 § 8
The protocol should be easy to use by clients and easy to adopt by server implementations
The protocol must by extensible by further message routing algorithms
The following terms are used throughout this document
Non-balancing message algorithms consists of the traditional algorithms that are widely used by XMPP implementations as defined in RFC 6121 § 184.108.40.206.1 and RFC 3921 . They usually try to send a message either to the all resources or try to determine the best resource based on some heuristic. They where designed with human-to-human interaction in mind.
Examples for non-balancing algorithms include:
Deliver to all ('urn:xmpp:cmr:all')
Deliver to most active resource ('urn:xmpp:cmr:mostactive')
Balanced message routing algorithms try to distribute messages between the available resources of the receiver. They are ideal for use with the Internet of Things (IoT) and especially machine to machine (M2M) technologies.
Consider a large deployment of sensor nodes send their data to a cluster for further processing. Every cluster node establishes a connection using the same bare JID but a different resource for receiving the sensor data. Furthermore, after a cluster connection is authenticated, it queries the XMPP service for the CMR status, and enables a balancing message routing algorithm.
The sensor nodes can now send their data as payload of a message stanza to the cluster. The server will evenly distributed the data between the cluster nodes by using a round-robin scheme.
Since CMR balances message stanzas of type 'normal' or 'chat', PubSub notifications will also be evenly distributed between the connected resources of an connection where CMR is active.
Examples for balancing algorithms include:
Load balance using round robin ('urn:xmpp:cmr:roundrobin')
Weighted load balancing ('urn:xmpp:cmr:weighted')
CMR is not suitable for scenarios where the routing behavior should also consider resources with negative priority. This would violate RFC 6121 § 220.127.116.11.1. A suitable specification for such use cases may already exists. For example a proper solution that involves forking of messages even to resources with negatives priorities is explained in Section 6 of Message Carbons (XEP-0280) .
An entity advertises support for this protocol by including the 'urn:xmpp:cmr:0' feature in its service discovery information features as specified in Service Discovery (XEP-0030) or section 6.3 of Entity Capabilities (XEP-0015).
Clients are able to query the server for the currently active and available message routing algorithms.
The result IQ, must include exactly one <active/> element and at least one <available/> element. Both elements must posses a 'algorithm' attribute, which contains the name of the available or active algorithm.
To change the active message routing algorithm, the client sends an <iq/> of type 'set' containing a 'cmr' child element which contains the desired algorithm as value of the 'algorithm' attribute.
The server will respond with an result <iq/> if the routing algorithm was successfully changed.
If the server is unable to change the message routing algorithm, then an error <iq/> is returned to the client.
If allowed and supported by the server, clients are able to annotate message stanza with a routing hint, that SHOULD affect the used message routing algorithm for the annotated stanza.
Clients are allowed to change the CMR state at any time. CMR MUST only affect the routing of message stanzas of type 'normal' or 'chat', which are addressed to a bare JID and where the receiving entity has more then one available resource with a non-negative presence priority. That is, it affects the routing decision as described in RFC 6121 § 18.104.22.168.1 for messages of type 'normal' and 'chat'.
The CMR state, ie. the used routing algorithm, is identical for every session of an XMPP client. Therefore implementations MUST NOT allow different CMR states between the resources of the same bare JID.
If an entity advertises support for CMR as described in 3. it MUST support at least one message routing algorithm.
Algorithm Namespace: 'urn:xmpp:cmr:all'
Deliver to all non-negative resources with share the same maximum priority. And if message type is 'chat', only to those that have opted in to receive chat messages.
Algorithm Namespace: 'urn:xmpp:cmr:mostactive'
Deliver the message to the "most available" resource or resources, depending on the server's implementation.
Algorithm Namespace: 'urn:xmpp:cmr:roundrobin'
Deliver the message to the next resource selected by a round-robin algorithm.
Algorithm Namespace: 'urn:xmpp:cmr:weighted'
Deliver the message to a resource selected by a weighted round-robin algorithm. The weight of a resource is determined by its priority.
Servers implementing CMR MUST at least implement one message routing algorithm, and offer at least one of of 'urn:xmpp:cmr:all' and 'urn:xmpp:cmr:mostactive'. Technically this is a constraint derived from Section 22.214.171.124.1. of RFC 6121.
This specification introduces no known security considerations.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This specification defines the following XML namespace:
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar  shall add the foregoing namespace to the registry located at <https://xmpp.org/registrar/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) .
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.
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