<abstract>This specification describes an extension to the Rayo protocol to support clustering of Rayo servers and their presentation as a unified service.</abstract>
<legal>
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<p>&xep0327; 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.</p>
</section1>
<section1topic='Requirements'anchor='reqs'>
<p>This specification is required to provide a framework for implementation of the following goals:</p>
<ul>
<li>Routing of Rayo messages between clients and server nodes in a cluster.</li>
<li>Load balancing of calls between multiple clients and server nodes in a cluster.</li>
<li>Failover of calls between clients and server nodes, and management of the cluster.</li>
<li>Enforcement of security and multi-tenancy concerns.</li>
</ul>
</section1>
<section1topic='Terminology'anchor='terms'>
<section2topic='Glossary'anchor='terms-glossary'>
<dl>
<di>
<dt>Cluster</dt>
<dd>A collection of one or more Nodes which are coordinated by one or more Gateways.</dd>
</di>
<di>
<dt>Gateway</dt>
<dd>
The Rayo Gateway is the main element that differentiates a Rayo Cluster from a stand-alone Rayo deployment. The Rayo Gateway is an intermediate component that is in charge of coordinating the communication from clients to Rayo Nodes and from Rayo Nodes to clients. When a cluster contains multiple gateways, the protocol for coordination of multiple gateways is considered implementation specific.
</dd>
</di>
<di>
<dt>Node</dt>
<dd>The Rayo Node is an individual instance of a Rayo server from &xep0327;, which presents call control capability via Rayo on top of a signaling protocol such as SIP or Jingle. It presents as a member of the cluster under the management of the Gateway(s).</dd>
</di>
<di>
<dt>Third-party XMPP server</dt>
<dd>An arbitrary third-party XMPP server used for registration of Rayo clients. MAY form part of the cluster.</dd>
<li><strong>shakespeare.lit</strong> - The root domain of the Rayo service, presented as the external interface of the Gateway(s).</li>
<li><strong>gateway.shakespeare.lit</strong> - The internal domain of the Gateway(s).</li>
<li><strong>node1.shakespeare.lit</strong> - The domain of a Node in the Cluster.</li>
</ul>
</section2>
</section1>
<section1topic='Session Flow'anchor='session'>
<p>All communication between the Rayo Gateways and Clients, and the Gateways and Nodes MUST be conformant with &xep0327; 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.</p>
<p>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.</p>
<p>Inbound call flow is:</p>
<ol>
<li>The Rayo Node generates events for the call as usual, beginning with an offer event.</li>
<li>The Rayo Node sends the event to the Rayo Gateway using the internal interface.</li>
<li>The Rayo Gateway receives events on its internal interface, decides on the correct client to route the event to (the calls DCP or its set of PCPs).</li>
<li>The Rayo Gateway forwards the event to relevant clients via the external interface, replacing the presence "to" and "from" attributes. The "to" attribute is the client's jid and the "from" is based on the Rayo Gateway's external interface.</li>
<li>The client(s) receive the event, responding to the Gateway. The Gateway correspondingly rewrites commands received from the client to the relevant Node for each call, and responses to those commands in reverse.</li>
<li>A client sends a dial command to the Rayo Cluster, arriving on the external interface of a Gateway.</li>
<li>The Rayo Gateway finds an available Rayo Node and forwards the command to it, replacing the "from" attribute with its own internal interface and the "to" attribute based on the Node's domain.</li>
<li>The Rayo Node receives the dial command, processes the outbound call and returns responses/events to the Gateway for rewriting to the client.</li>
<p>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).</p>
<p>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.</p>
<p>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 &xep0327;.</p>
<p>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:</p>
<examplecaption="Node presents itself as available to the Rayo Gateway"><![CDATA[
<presencefrom='node1.shakespeare.lit'
to='gateway.shakespeare.lit'>
<cxmlns='http://jabber.org/protocol/caps'
hash='sha-1'
node='urn:xmpp:rayo:node:1'
ver='QgayPKawpkPSDYmwT/WM94uAlu0='/>
<show>chat</show>
</presence>
]]></example>
<p>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:</p>
<examplecaption="Node presents itself as unavailable to the Rayo Gateway"><