Rayo (XEP-0327)  documents the behaviour of an individual Rayo server and its interaction with a client. Two of the goals of Rayo, however, are to support clustering of servers and multi-tenancy. This specification outlines how that is to be achieved.
This specification is required to provide a framework for implementation of the following goals:
In examples, the following JIDs are used:
All communication between the Rayo Gateways and Clients, and the Gateways and Nodes MUST be conformant with Rayo (XEP-0327)  or any extensions defined in this specification. A conformant Rayo Gateway MUST NOT derive any critical functionality through proprietary knowledge of the Nodes it is managing.
The Rayo Gateway communicates both with Rayo Nodes and third-party XMPP servers through S2S XMPP connections. To differentiate the messages that come from Rayo Nodes and from client applications, the Rayo Gateway MUST present two separate domains, an internal (for communication with cluster nodes) and an external (for communication with Rayo clients) interface. In cases of multiple gateways, the internal and external domains MUST be the same, and DNS SHOULD be used for load-balancing.
Inbound call flow is:
Outbound call flow is:
The Gateway(s) in a Cluster are responsible for managing the routing of calls between relevant nodes and clients, and SHOULD retain knowledge of the presence of each for this purpose. Nodes and Clients SHOULD NOT be aware of each others identity of presence, and SHOULD only communicate with the Gateway(s).
The Gateway(s) in a Cluster MUST attempt to evenly balance outbound calls among Nodes; at a minimum they MUST implement round-robin dispatch of dial commands. Gateways MAY attempt load-based distribution by monitoring the number of active sessions (inbound and outbound) per Node and distributing accordingly.
The rules by which the PCPs for an inbound call are determined is implementation specific. In cases where a server permits registration of multiple JIDs as PCPs, it MAY opt to load-balance offers between them by an unspecified algorithm, though it may not assume any knowledge of the clients outside of this specification or Rayo (XEP-0327) .
In order for a Rayo Node to be considered available for processing dial requests, it MUST first notify the Gateway that it is available for such by sending directed presence to the Gateway internal interface with a <show/> element containing 'chat' as in the example:
Conversely, when a Rayo Node wishes not to process dial requests, it SHOULD send directed presence to the Gateway with a <show/> element containing 'dnd' as in the example:
A Rayo Gateway MAY transparently retry failed operations like dial requests, or MAY automatically remove from rotation the Rayo Nodes that fail to satisfy such requests repeatedly.
The Rayo Gateway MUST validate permissions on incoming Rayo commands from Clients (check that they are one of the call's DCP/PCP as appropriate to the rules defined in Rayo (XEP-0327) ). The Rayo Gateway MUST enforce its own rules on Node membership of the Cluster, ensuring communication via its internal interface with only trusted Nodes. The rules by which inbound calls are permitted are implementation specific. When configured as members of a cluster, Rayo Nodes SHOULD accept communication *only* with the gateway.
Rayo gateways MUST advertise support for "urn:xmpp:rayo:1" on their external interface, and "urn:xmpp:rayo:gateway:1" on their internal interface. Rayo nodes MUST advertise support for "urn:xmpp:rayo:node:1", indicating that they may be used as part of a cluster, and additionally "urn:xmpp:rayo:1" if they may also be used separately from the cluster.
Rayo sessions can be resource-intensive. Therefore, it is possible to launch a denial-of-service attack against an entity by burdening it with too many Rayo sessions. Care must be taken to accept sessions only from known entities and only if the entity's device is able to process such sessions.
Rayo communications can be enabled through gateways to non-XMPP networks, whose security characteristics can be quite different from those of XMPP networks. For example, on some SIP networks authentication is optional and "from" addresses can be easily forged. Care must be taken in communicating through such gateways.
Mere negotiation of a Rayo session can expose sensitive information about the parties (e.g. IP addresses). Care must be taken in communicating such information, and end-to-end encryption should be used if the parties do not trust the intermediate servers or gateways.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This specification defines the following XML namespaces:
If the protocol defined in this specification undergoes a major revision that is not fully backward-compatible with an older version, or that contains significant new features, 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.
The authors would like to acknowledge the input of teams at Tropo, Mojo Lingo and Grasshopper in the development of the specification.
Specific individuals who have contributed to the specification or to software significant to its completion include:
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