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<div class="section" id="networking-and-messaging">
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<h1>Networking and messaging<a class="headerlink" href="#networking-and-messaging" title="Permalink to this headline">¶</a></h1>
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<p>Corda uses AMQP/1.0 over TLS between nodes which is currently implemented using Apache Artemis, an embeddable message
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queue broker. Building on established MQ protocols gives us features like persistence to disk, automatic delivery
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retries with backoff and dead-letter routing, security, large message streaming and so on.</p>
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<p>Artemis is hidden behind a thin interface that also has an in-memory only implementation suitable for use in
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unit tests and visualisation tools.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">A future version of Corda will allow the MQ broker to be split out of the main node and run as a
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separate server. We may also support non-Artemis implementations via JMS, allowing the broker to be swapped
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out for alternative implementations.</p>
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</div>
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<p>There are multiple ways of interacting with the network. When writing an application you typically won’t use the
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messaging subsystem directly. Instead you will build on top of the <a class="reference internal" href="flow-state-machines.html"><span class="doc">flow framework</span></a>,
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which adds a layer on top of raw messaging to manage multi-step flows and let you think in terms of identities
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rather than specific network endpoints.</p>
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<div class="section" id="network-map-service">
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<span id="id1"></span><h2>Network Map Service<a class="headerlink" href="#network-map-service" title="Permalink to this headline">¶</a></h2>
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<p>Supporting the messaging layer is a network map service, which is responsible for tracking public nodes on the network.</p>
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<p>Nodes have an internal component, the network map cache, which contains a copy of the network map (which is just a
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document). When a node starts up its cache fetches a copy of the full network map, and requests to be notified of
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changes. The node then registers itself with the network map service, and the service notifies subscribers that a new
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node has joined the network. Nodes do not automatically deregister themselves, so (for example) nodes going offline
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briefly for maintenance are retained in the network map, and messages for them will be queued, minimising disruption.</p>
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<p>Nodes submit signed changes to the map service, which then forwards update notifications on to nodes which have
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requested to be notified of changes.</p>
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<p>The network map currently supports:</p>
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<ul class="simple">
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<li>Looking up nodes by service</li>
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<li>Looking up node for a party</li>
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<li>Suggesting a node providing a specific service, based on suitability for a contract and parties, for example suggesting
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an appropriate interest rates oracle for an interest rate swap contract. Currently no recommendation logic is in place.</li>
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</ul>
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</div>
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<div class="section" id="message-queues">
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<h2>Message queues<a class="headerlink" href="#message-queues" title="Permalink to this headline">¶</a></h2>
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<p>The node makes use of various queues for its operation. The more important ones are described below. Others are used
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for maintenance and other minor purposes.</p>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field-odd field"><th class="field-name"><code class="docutils literal"><span class="pre">p2p.inbound</span></code>:</th><td class="field-body">The node listens for messages sent from other peer nodes on this queue. Only clients who are authenticated to be
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nodes on the same network are given permission to send. Messages which are routed internally are also sent to this
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queue (e.g. two flows on the same node communicating with each other).</td>
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</tr>
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<tr class="field-even field"><th class="field-name" colspan="2"><code class="docutils literal"><span class="pre">internal.peers.$identity</span></code>:</th></tr>
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<tr class="field-even field"><td> </td><td class="field-body">These are a set of private queues only available to the node which it uses to route messages destined to other peers.
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The queue name ends in the base 58 encoding of the peer’s identity key. There is at most one queue per peer. The broker
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creates a bridge from this queue to the peer’s <code class="docutils literal"><span class="pre">p2p.inbound</span></code> queue, using the network map service to lookup the
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peer’s network address.</td>
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</tr>
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<tr class="field-odd field"><th class="field-name" colspan="2"><code class="docutils literal"><span class="pre">internal.services.$identity</span></code>:</th></tr>
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<tr class="field-odd field"><td> </td><td class="field-body">These are private queues the node may use to route messages to services. The queue name ends in the base 58 encoding
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of the service’s owning identity key. There is at most one queue per service identity (but note that any one service
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may have several identities). The broker creates bridges to all nodes in the network advertising the service in
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question. When a session is initiated with a service counterparty the handshake is pushed onto this queue, and a
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corresponding bridge is used to forward the message to an advertising peer’s p2p queue. Once a peer is picked the
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session continues on as normal.</td>
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</tr>
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<tr class="field-even field"><th class="field-name" colspan="2"><code class="docutils literal"><span class="pre">internal.networkmap</span></code>:</th></tr>
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<tr class="field-even field"><td> </td><td class="field-body">This is another private queue just for the node which functions in a similar manner to the <code class="docutils literal"><span class="pre">internal.peers.*</span></code> queues
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except this is used to form a connection to the network map node. The node running the network map service is treated
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differently as it provides information about the rest of the network.</td>
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</tr>
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<tr class="field-odd field"><th class="field-name"><code class="docutils literal"><span class="pre">rpc.requests</span></code>:</th><td class="field-body">RPC clients send their requests here, and it’s only open for sending by clients authenticated as RPC users.</td>
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</tr>
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<tr class="field-even field"><th class="field-name" colspan="2"><code class="docutils literal"><span class="pre">clients.$user.rpc.$random</span></code>:</th></tr>
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<tr class="field-even field"><td> </td><td class="field-body">RPC clients are given permission to create a temporary queue incorporating their username (<code class="docutils literal"><span class="pre">$user</span></code>) and sole
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permission to receive messages from it. RPC requests are required to include a random number (<code class="docutils literal"><span class="pre">$random</span></code>) from
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which the node is able to construct the queue the user is listening on and send the response to that. This mechanism
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prevents other users from being able listen in on the responses.</td>
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</tr>
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</tbody>
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</table>
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</div>
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<div class="section" id="security">
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<h2>Security<a class="headerlink" href="#security" title="Permalink to this headline">¶</a></h2>
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<p>Clients attempting to connect to the node’s broker fall in one of four groups:</p>
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<ol class="arabic simple">
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<li>Anyone connecting with the username <code class="docutils literal"><span class="pre">SystemUsers/Node</span></code> is treated as the node hosting the broker, or a logical
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component of the node. The TLS certificate they provide must match the one broker has for the node. If that’s the case
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they are given full access to all valid queues, otherwise they are rejected.</li>
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<li>Anyone connecting with the username <code class="docutils literal"><span class="pre">SystemUsers/Peer</span></code> is treated as a peer on the same Corda network as the node. Their
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TLS root CA must be the same as the node’s root CA - the root CA is the doorman of the network and having the same root CA
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implies we’ve been let in by the same doorman. If they are part of the same network then they are only given permission
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to send to our <code class="docutils literal"><span class="pre">p2p.inbound</span></code> queue, otherwise they are rejected.</li>
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<li>Every other username is treated as a RPC user and authenticated against the node’s list of valid RPC users. If that
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is successful then they are only given sufficient permission to perform RPC, otherwise they are rejected.</li>
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<li>Clients connecting without a username and password are rejected.</li>
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</ol>
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<p>Artemis provides a feature of annotating each received message with the validated user. This allows the node’s messaging
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service to provide authenticated messages to the rest of the system. For the first two client types described above the
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validated user is the X.500 subject DN of the client TLS certificate and we assume the common name is the legal name of
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the peer. This allows the flow framework to authentically determine the <code class="docutils literal"><span class="pre">Party</span></code> initiating a new flow. For RPC clients
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the validated user is the username itself and the RPC framework uses this to determine what permissions the user has.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last"><code class="docutils literal"><span class="pre">Party</span></code> lookup is currently done by the legal name which isn’t guaranteed to be unique. A future version will
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use the full X.500 name as it can provide additional structures for uniqueness.</p>
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</div>
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<p>The broker also does host verification when connecting to another peer. It checks that the TLS certificate common name
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matches with the advertised legal name from the network map service.</p>
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</div>
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<div class="section" id="messaging-types">
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<h2>Messaging types<a class="headerlink" href="#messaging-types" title="Permalink to this headline">¶</a></h2>
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<p>Every <code class="docutils literal"><span class="pre">Message</span></code> object has an associated <em>topic</em> and may have a <em>session ID</em>. These are wrapped in a <code class="docutils literal"><span class="pre">TopicSession</span></code>.
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An implementation of <code class="docutils literal"><span class="pre">MessagingService</span></code> can be used to create messages and send them. You can get access to the
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messaging service via the <code class="docutils literal"><span class="pre">ServiceHub</span></code> object that is provided to your app. Endpoints on the network are
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identified at the lowest level using <code class="docutils literal"><span class="pre">SingleMessageRecipient</span></code> which may be e.g. an IP address, or in future
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versions perhaps a routing path through the network.</p>
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</div>
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