diff --git a/core/src/main/kotlin/com/r3corda/core/messaging/Messaging.kt b/core/src/main/kotlin/com/r3corda/core/messaging/Messaging.kt
index e89d7863af..1bb07d52d3 100644
--- a/core/src/main/kotlin/com/r3corda/core/messaging/Messaging.kt
+++ b/core/src/main/kotlin/com/r3corda/core/messaging/Messaging.kt
@@ -1,6 +1,7 @@
 package com.r3corda.core.messaging
 
 import com.google.common.util.concurrent.ListenableFuture
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.serialization.DeserializeAsKotlinObjectDef
 import com.r3corda.core.serialization.serialize
 import java.time.Instant
@@ -22,7 +23,7 @@ import javax.annotation.concurrent.ThreadSafe
 interface MessagingService {
     /**
      * The provided function will be invoked for each received message whose topic matches the given string, on the given
-     * executor. The topic can be the empty string to match all messages.
+     * executor.
      *
      * If no executor is received then the callback will run on threads provided by the messaging service, and the
      * callback is expected to be thread safe as a result.
@@ -30,8 +31,28 @@ interface MessagingService {
      * The returned object is an opaque handle that may be used to un-register handlers later with [removeMessageHandler].
      * The handle is passed to the callback as well, to avoid race conditions whereby the callback wants to unregister
      * itself and yet addMessageHandler hasn't returned the handle yet.
+     *
+     * @param topic identifier for the general subject of the message, for example "platform.network_map.fetch".
+     * The topic can be the empty string to match all messages (session ID must be [DEFAULT_SESSION_ID]).
+     * @param sessionID identifier for the session the message is part of. For services listening before
+     * a session is established, use [DEFAULT_SESSION_ID].
      */
-    fun addMessageHandler(topic: String = "", executor: Executor? = null, callback: (Message, MessageHandlerRegistration) -> Unit): MessageHandlerRegistration
+    fun addMessageHandler(topic: String = "", sessionID: Long = DEFAULT_SESSION_ID, executor: Executor? = null, callback: (Message, MessageHandlerRegistration) -> Unit): MessageHandlerRegistration
+
+    /**
+     * The provided function will be invoked for each received message whose topic and session matches, on the
+     * given executor.
+     *
+     * If no executor is received then the callback will run on threads provided by the messaging service, and the
+     * callback is expected to be thread safe as a result.
+     *
+     * The returned object is an opaque handle that may be used to un-register handlers later with [removeMessageHandler].
+     * The handle is passed to the callback as well, to avoid race conditions whereby the callback wants to unregister
+     * itself and yet addMessageHandler hasn't returned the handle yet.
+     *
+     * @param topicSession identifier for the topic and session to listen for messages arriving on.
+     */
+    fun addMessageHandler(topicSession: TopicSession, executor: Executor? = null, callback: (Message, MessageHandlerRegistration) -> Unit): MessageHandlerRegistration
 
     /**
      * Removes a handler given the object returned from [addMessageHandler]. The callback will no longer be invoked once
@@ -55,34 +76,81 @@ interface MessagingService {
 
     /**
      * Returns an initialised [Message] with the current time, etc, already filled in.
+     *
+     * @param topic identifier for the general subject of the message, for example "platform.network_map.fetch".
+     * Must not be blank.
+     * @param sessionID identifier for the session the message is part of. For messages sent to services before the
+     * construction of a session, use [DEFAULT_SESSION_ID].
      */
-    fun createMessage(topic: String, data: ByteArray): Message
+    fun createMessage(topic: String, sessionID: Long = DEFAULT_SESSION_ID, data: ByteArray): Message
+
+    /**
+     * Returns an initialised [Message] with the current time, etc, already filled in.
+     *
+     * @param topicSession identifier for the topic and session the message is sent to.
+     */
+    fun createMessage(topicSession: TopicSession, data: ByteArray): Message
 
     /** Returns an address that refers to this node. */
     val myAddress: SingleMessageRecipient
 }
 
 /**
- * Registers a handler for the given topic that runs the given callback with the message and then removes itself. This
- * is useful for one-shot handlers that aren't supposed to stick around permanently. Note that this callback doesn't
- * take the registration object, unlike the callback to [MessagingService.addMessageHandler].
+ * Registers a handler for the given topic and session ID that runs the given callback with the message and then removes
+ * itself. This is useful for one-shot handlers that aren't supposed to stick around permanently. Note that this callback
+ * doesn't take the registration object, unlike the callback to [MessagingService.addMessageHandler], as the handler is
+ * automatically deregistered before the callback runs.
+ *
+ * @param topic identifier for the general subject of the message, for example "platform.network_map.fetch".
+ * The topic can be the empty string to match all messages (session ID must be [DEFAULT_SESSION_ID]).
+ * @param sessionID identifier for the session the message is part of. For services listening before
+ * a session is established, use [DEFAULT_SESSION_ID].
  */
-fun MessagingService.runOnNextMessage(topic: String = "", executor: Executor? = null, callback: (Message) -> Unit) {
+fun MessagingService.runOnNextMessage(topic: String, sessionID: Long, executor: Executor? = null, callback: (Message) -> Unit)
+    = runOnNextMessage(TopicSession(topic, sessionID), executor, callback)
+
+/**
+ * Registers a handler for the given topic and session that runs the given callback with the message and then removes
+ * itself. This is useful for one-shot handlers that aren't supposed to stick around permanently. Note that this callback
+ * doesn't take the registration object, unlike the callback to [MessagingService.addMessageHandler].
+ *
+ * @param topicSession identifier for the topic and session to listen for messages arriving on.
+ */
+fun MessagingService.runOnNextMessage(topicSession: TopicSession, executor: Executor? = null, callback: (Message) -> Unit) {
     val consumed = AtomicBoolean()
-    addMessageHandler(topic, executor) { msg, reg ->
+    addMessageHandler(topicSession, executor) { msg, reg ->
         removeMessageHandler(reg)
         check(!consumed.getAndSet(true)) { "Called more than once" }
-        check(msg.topic == topic) { "Topic mismatch: ${msg.topic} vs $topic" }
+        check(msg.topicSession == topicSession) { "Topic/session mismatch: ${msg.topicSession} vs $topicSession" }
         callback(msg)
     }
 }
 
-fun MessagingService.send(topic: String, payload: Any, to: MessageRecipients) {
-    send(createMessage(topic, payload.serialize().bits), to)
-}
+fun MessagingService.send(topic: String, sessionID: Long, payload: Any, to: MessageRecipients)
+    = send(TopicSession(topic, sessionID), payload, to)
+
+fun MessagingService.send(topicSession: TopicSession, payload: Any, to: MessageRecipients)
+    = send(createMessage(topicSession, payload.serialize().bits), to)
 
 interface MessageHandlerRegistration
 
+/**
+ * An identifier for the endpoint [MessagingService] message handlers listen at.
+ *
+ * @param topic identifier for the general subject of the message, for example "platform.network_map.fetch".
+ * The topic can be the empty string to match all messages (session ID must be [DEFAULT_SESSION_ID]).
+ * @param sessionID identifier for the session the message is part of. For services listening before
+ * a session is established, use [DEFAULT_SESSION_ID].
+ */
+data class TopicSession(val topic: String, val sessionID: Long = DEFAULT_SESSION_ID) {
+    companion object {
+        val Blank = TopicSession("", DEFAULT_SESSION_ID)
+    }
+    fun isBlank() = topic.isBlank() && sessionID == DEFAULT_SESSION_ID
+
+    override fun toString(): String = "${topic}.${sessionID}"
+}
+
 /**
  * A message is defined, at this level, to be a (topic, timestamp, byte arrays) triple, where the topic is a string in
  * Java-style reverse dns form, with "platform." being a prefix reserved by the platform for its own use. Vendor
@@ -94,7 +162,7 @@ interface MessageHandlerRegistration
  * the timestamp field they probably will be, even if an implementation just uses a hash prefix as the message id.
  */
 interface Message {
-    val topic: String
+    val topicSession: TopicSession
     val data: ByteArray
     val debugTimestamp: Instant
     val debugMessageID: String
diff --git a/core/src/main/kotlin/com/r3corda/core/node/services/Services.kt b/core/src/main/kotlin/com/r3corda/core/node/services/Services.kt
index 5c22ec20f7..5a21444757 100644
--- a/core/src/main/kotlin/com/r3corda/core/node/services/Services.kt
+++ b/core/src/main/kotlin/com/r3corda/core/node/services/Services.kt
@@ -10,9 +10,10 @@ import java.security.PrivateKey
 import java.security.PublicKey
 
 /**
- * Postfix for base topics when sending a request to a service.
+ * Session ID to use for services listening for the first message in a session (before a
+ * specific session ID has been established).
  */
-val TOPIC_DEFAULT_POSTFIX = ".0"
+val DEFAULT_SESSION_ID = 0L
 
 /**
  * This file defines various 'services' which are not currently fleshed out. A service is a module that provides
diff --git a/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt b/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt
index e55e28b6c9..727421be9e 100644
--- a/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt
+++ b/core/src/main/kotlin/com/r3corda/protocols/TwoPartyDealProtocol.kt
@@ -7,6 +7,7 @@ import com.r3corda.core.crypto.DigitalSignature
 import com.r3corda.core.crypto.Party
 import com.r3corda.core.crypto.signWithECDSA
 import com.r3corda.core.node.NodeInfo
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.protocols.ProtocolLogic
 import com.r3corda.core.random63BitValue
 import com.r3corda.core.seconds
@@ -157,7 +158,7 @@ object TwoPartyDealProtocol {
                 // Copy the transaction to every regulator in the network. This is obviously completely bogus, it's
                 // just for demo purposes.
                 for (regulator in regulators) {
-                    send(regulator.identity, 0, fullySigned)
+                    send(regulator.identity, DEFAULT_SESSION_ID, fullySigned)
                 }
             }
 
@@ -461,7 +462,7 @@ object TwoPartyDealProtocol {
                 val initation = FixingSessionInitiation(sessionID, sortedParties[0], serviceHub.storageService.myLegalIdentity, timeout)
 
                 // Send initiation to other side to launch one side of the fixing protocol (the Fixer).
-                send(sortedParties[1], 0, initation)
+                send(sortedParties[1], DEFAULT_SESSION_ID, initation)
 
                 // Then start the other side of the fixing protocol.
                 val protocol = Floater(ref, sessionID)
diff --git a/node/src/main/kotlin/com/r3corda/node/internal/AbstractNode.kt b/node/src/main/kotlin/com/r3corda/node/internal/AbstractNode.kt
index ef9f6c1c56..a9ec16792c 100644
--- a/node/src/main/kotlin/com/r3corda/node/internal/AbstractNode.kt
+++ b/node/src/main/kotlin/com/r3corda/node/internal/AbstractNode.kt
@@ -273,11 +273,10 @@ abstract class AbstractNode(val dir: Path, val configuration: NodeConfiguration,
         val reg = NodeRegistration(info, networkMapSeq++, type, expires)
         val sessionID = random63BitValue()
         val request = NetworkMapService.RegistrationRequest(reg.toWire(storage.myLegalIdentityKey.private), net.myAddress, sessionID)
-        val message = net.createMessage("$REGISTER_PROTOCOL_TOPIC.0", request.serialize().bits)
+        val message = net.createMessage(REGISTER_PROTOCOL_TOPIC, DEFAULT_SESSION_ID, request.serialize().bits)
         val future = SettableFuture.create<NetworkMapService.RegistrationResponse>()
-        val topic = "$REGISTER_PROTOCOL_TOPIC.$sessionID"
 
-        net.runOnNextMessage(topic, RunOnCallerThread) { message ->
+        net.runOnNextMessage(REGISTER_PROTOCOL_TOPIC, sessionID, RunOnCallerThread) { message ->
             future.set(message.data.deserialize())
         }
         net.send(message, serviceInfo.address)
diff --git a/node/src/main/kotlin/com/r3corda/node/services/api/AbstractNodeService.kt b/node/src/main/kotlin/com/r3corda/node/services/api/AbstractNodeService.kt
index dc354efbc0..94eced249f 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/api/AbstractNodeService.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/api/AbstractNodeService.kt
@@ -2,8 +2,9 @@ package com.r3corda.node.services.api
 
 import com.r3corda.core.messaging.Message
 import com.r3corda.core.messaging.MessagingService
+import com.r3corda.core.messaging.TopicSession
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.node.services.NetworkMapCache
-import com.r3corda.core.node.services.TOPIC_DEFAULT_POSTFIX
 import com.r3corda.core.serialization.SingletonSerializeAsToken
 import com.r3corda.core.serialization.deserialize
 import com.r3corda.core.serialization.serialize
@@ -29,13 +30,13 @@ abstract class AbstractNodeService(val net: MessagingService, val networkMapCach
             addMessageHandler(topic: String,
                               crossinline handler: (Q) -> R,
                               crossinline exceptionConsumer: (Message, Exception) -> Unit) {
-        net.addMessageHandler(topic + TOPIC_DEFAULT_POSTFIX, null) { message, r ->
+        net.addMessageHandler(topic, DEFAULT_SESSION_ID, null) { message, r ->
             try {
                 val request = message.data.deserialize<Q>()
                 val response = handler(request)
                 // If the return type R is Unit, then do not send a response
                 if (response.javaClass != Unit.javaClass) {
-                    val msg = net.createMessage("$topic.${request.sessionID}", response.serialize().bits)
+                    val msg = net.createMessage(topic, request.sessionID, response.serialize().bits)
                     net.send(msg, request.getReplyTo(networkMapCache))
                 }
             } catch(e: Exception) {
diff --git a/node/src/main/kotlin/com/r3corda/node/services/clientapi/FixingSessionInitiation.kt b/node/src/main/kotlin/com/r3corda/node/services/clientapi/FixingSessionInitiation.kt
index fff47b7735..6f7706176c 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/clientapi/FixingSessionInitiation.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/clientapi/FixingSessionInitiation.kt
@@ -1,6 +1,7 @@
 package com.r3corda.node.services.clientapi
 
 import com.r3corda.core.node.CordaPluginRegistry
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.serialization.deserialize
 import com.r3corda.node.internal.AbstractNode
 import com.r3corda.node.services.api.ServiceHubInternal
@@ -19,7 +20,7 @@ object FixingSessionInitiation {
 
     class Service(services: ServiceHubInternal) {
         init {
-            services.networkService.addMessageHandler("${TwoPartyDealProtocol.FIX_INITIATE_TOPIC}.0") { msg, registration ->
+            services.networkService.addMessageHandler(TwoPartyDealProtocol.FIX_INITIATE_TOPIC, DEFAULT_SESSION_ID) { msg, registration ->
                 val initiation = msg.data.deserialize<TwoPartyDealProtocol.FixingSessionInitiation>()
                 val protocol = TwoPartyDealProtocol.Fixer(initiation)
                 services.startProtocol("fixings", protocol)
diff --git a/node/src/main/kotlin/com/r3corda/node/services/messaging/ArtemisMessagingService.kt b/node/src/main/kotlin/com/r3corda/node/services/messaging/ArtemisMessagingService.kt
index b4f6e2cb08..c5da7a90ca 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/messaging/ArtemisMessagingService.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/messaging/ArtemisMessagingService.kt
@@ -81,6 +81,8 @@ class ArtemisMessagingService(val directory: Path,
         // confusion.
         val TOPIC_PROPERTY = "platform-topic"
 
+        val SESSION_ID_PROPERTY = "session-id"
+
         /** Temp helper until network map is established. */
         fun makeRecipient(hostAndPort: HostAndPort): SingleMessageRecipient = Address(hostAndPort)
         fun makeRecipient(hostname: String) = makeRecipient(toHostAndPort(hostname))
@@ -101,7 +103,7 @@ class ArtemisMessagingService(val directory: Path,
 
     /** A registration to handle messages of different types */
     inner class Handler(val executor: Executor?,
-                        val topic: String,
+                        val topicSession: TopicSession,
                         val callback: (Message, MessageHandlerRegistration) -> Unit) : MessageHandlerRegistration
 
     private val handlers = CopyOnWriteArrayList<Handler>()
@@ -180,12 +182,17 @@ class ArtemisMessagingService(val directory: Path,
                     log.warn("Received message without a $TOPIC_PROPERTY property, ignoring")
                     return@setMessageHandler
                 }
+                if (!message.containsProperty(SESSION_ID_PROPERTY)) {
+                    log.warn("Received message without a $SESSION_ID_PROPERTY property, ignoring")
+                    return@setMessageHandler
+                }
                 val topic = message.getStringProperty(TOPIC_PROPERTY)
+                val sessionID = message.getLongProperty(SESSION_ID_PROPERTY)
 
                 val body = ByteArray(message.bodySize).apply { message.bodyBuffer.readBytes(this) }
 
                 val msg = object : Message {
-                    override val topic = topic
+                    override val topicSession = TopicSession(topic, sessionID)
                     override val data: ByteArray = body
                     override val debugTimestamp: Instant = Instant.ofEpochMilli(message.timestamp)
                     override val debugMessageID: String = message.messageID.toString()
@@ -208,12 +215,12 @@ class ArtemisMessagingService(val directory: Path,
     private fun deliverMessage(msg: Message): Boolean {
         // Because handlers is a COW list, the loop inside filter will operate on a snapshot. Handlers being added
         // or removed whilst the filter is executing will not affect anything.
-        val deliverTo = handlers.filter { it.topic.isBlank() || it.topic == msg.topic }
+        val deliverTo = handlers.filter { it.topicSession.isBlank() || it.topicSession == msg.topicSession }
 
         if (deliverTo.isEmpty()) {
             // This should probably be downgraded to a trace in future, so the protocol can evolve with new topics
             // without causing log spam.
-            log.warn("Received message for ${msg.topic} that doesn't have any registered handlers yet")
+            log.warn("Received message for ${msg.topicSession} that doesn't have any registered handlers yet")
 
             // This is a hack; transient messages held in memory isn't crash resistant.
             // TODO: Use Artemis API more effectively so we don't pop messages off a queue that we aren't ready to use.
@@ -227,7 +234,7 @@ class ArtemisMessagingService(val directory: Path,
                 try {
                     handler.callback(msg, handler)
                 } catch(e: Exception) {
-                    log.error("Caught exception whilst executing message handler for ${msg.topic}", e)
+                    log.error("Caught exception whilst executing message handler for ${msg.topicSession}", e)
                 }
             }
         }
@@ -259,13 +266,24 @@ class ArtemisMessagingService(val directory: Path,
     override fun send(message: Message, target: MessageRecipients) {
         if (target !is Address)
             TODO("Only simple sends to single recipients are currently implemented")
-        val artemisMessage = session!!.createMessage(true).putStringProperty("platform-topic", message.topic).writeBodyBufferBytes(message.data)
+        val artemisMessage = session!!.createMessage(true).apply {
+            val sessionID = message.topicSession.sessionID
+            putStringProperty(TOPIC_PROPERTY, message.topicSession.topic)
+            putLongProperty(SESSION_ID_PROPERTY, sessionID)
+            writeBodyBufferBytes(message.data)
+        }
         getSendClient(target).send(artemisMessage)
     }
 
-    override fun addMessageHandler(topic: String, executor: Executor?,
+    override fun addMessageHandler(topic: String, sessionID: Long, executor: Executor?,
+                                   callback: (Message, MessageHandlerRegistration) -> Unit): MessageHandlerRegistration
+        = addMessageHandler(TopicSession(topic, sessionID), executor, callback)
+
+    override fun addMessageHandler(topicSession: TopicSession,
+                                   executor: Executor?,
                                    callback: (Message, MessageHandlerRegistration) -> Unit): MessageHandlerRegistration {
-        val handler = Handler(executor, topic, callback)
+        require(!topicSession.isBlank()) { "Topic must not be blank, as the empty topic is a special case." }
+        val handler = Handler(executor, topicSession, callback)
         handlers.add(handler)
         undeliveredMessages.removeIf { deliverMessage(it) }
         return handler
@@ -275,18 +293,21 @@ class ArtemisMessagingService(val directory: Path,
         handlers.remove(registration)
     }
 
-    override fun createMessage(topic: String, data: ByteArray): Message {
+    override fun createMessage(topicSession: TopicSession, data: ByteArray): Message {
         // TODO: We could write an object that proxies directly to an underlying MQ message here and avoid copying.
         return object : Message {
-            override val topic: String get() = topic
+            override val topicSession: TopicSession get() = topicSession
             override val data: ByteArray get() = data
             override val debugTimestamp: Instant = Instant.now()
             override fun serialise(): ByteArray = this.serialise()
             override val debugMessageID: String get() = Instant.now().toEpochMilli().toString()
-            override fun toString() = topic + "#" + String(data)
+            override fun toString() = topicSession.toString() + "#" + String(data)
         }
     }
 
+    override fun createMessage(topic: String, sessionID: Long, data: ByteArray): Message
+            = createMessage(TopicSession(topic, sessionID), data)
+
     override val myAddress: SingleMessageRecipient = Address(myHostPort)
 
     private enum class ConnectionDirection { INBOUND, OUTBOUND }
diff --git a/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryMessagingNetwork.kt b/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryMessagingNetwork.kt
index 5b54e81461..1940430fc7 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryMessagingNetwork.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryMessagingNetwork.kt
@@ -42,7 +42,7 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
     private val handleEndpointMap = HashMap<Handle, InMemoryMessaging>()
 
     data class MessageTransfer(val sender: InMemoryMessaging, val message: Message, val recipients: MessageRecipients) {
-        override fun toString() = "${message.topic} from '${sender.myAddress}' to '$recipients'"
+        override fun toString() = "${message.topicSession} from '${sender.myAddress}' to '$recipients'"
     }
 
     // All sent messages are kept here until pumpSend is called, or manuallyPumped is set to false
@@ -197,8 +197,7 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
      */
     @ThreadSafe
     inner class InMemoryMessaging(private val manuallyPumped: Boolean, private val handle: Handle) : SingletonSerializeAsToken(), MessagingServiceInternal {
-
-        inner class Handler(val executor: Executor?, val topic: String,
+        inner class Handler(val executor: Executor?, val topicSession: TopicSession,
                             val callback: (Message, MessageHandlerRegistration) -> Unit) : MessageHandlerRegistration
 
         @Volatile
@@ -226,10 +225,13 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
 
         override fun registerTrustedAddress(address: SingleMessageRecipient) {}
 
-        override fun addMessageHandler(topic: String, executor: Executor?, callback: (Message, MessageHandlerRegistration) -> Unit): MessageHandlerRegistration {
+        override fun addMessageHandler(topic: String, sessionID: Long, executor: Executor?, callback: (Message, MessageHandlerRegistration) -> Unit): MessageHandlerRegistration
+            = addMessageHandler(TopicSession(topic, sessionID), executor, callback)
+
+        override fun addMessageHandler(topicSession: TopicSession, executor: Executor?, callback: (Message, MessageHandlerRegistration) -> Unit): MessageHandlerRegistration {
             check(running)
             val (handler, items) = state.locked {
-                val handler = Handler(executor, topic, callback).apply { handlers.add(this) }
+                val handler = Handler(executor, topicSession, callback).apply { handlers.add(this) }
                 val items = ArrayList(pendingRedelivery)
                 pendingRedelivery.clear()
                 Pair(handler, items)
@@ -262,16 +264,20 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
             netNodeHasShutdown(handle)
         }
 
-        /** Returns the given (topic, data) pair as a newly created message object.*/
-        override fun createMessage(topic: String, data: ByteArray): Message {
+        /** Returns the given (topic & session, data) pair as a newly created message object. */
+        override fun createMessage(topic: String, sessionID: Long, data: ByteArray): Message
+                = createMessage(TopicSession(topic, sessionID), data)
+
+        /** Returns the given (topic & session, data) pair as a newly created message object. */
+        override fun createMessage(topicSession: TopicSession, data: ByteArray): Message {
             return object : Message {
-                override val topic: String get() = topic
+                override val topicSession: TopicSession get() = topicSession
                 override val data: ByteArray get() = data
                 override val debugTimestamp: Instant = Instant.now()
                 override fun serialise(): ByteArray = this.serialise()
                 override val debugMessageID: String get() = serialise().sha256().prefixChars()
 
-                override fun toString() = topic + "#" + String(data)
+                override fun toString() = topicSession.toString() + "#" + String(data)
             }
         }
 
@@ -300,7 +306,7 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
             while (deliverTo == null) {
                 val transfer = (if (block) q.take() else q.poll()) ?: return null
                 deliverTo = state.locked {
-                    val h = handlers.filter { if (it.topic.isBlank()) true else transfer.message.topic == it.topic }
+                    val h = handlers.filter { if (it.topicSession.isBlank()) true else transfer.message.topicSession == it.topicSession }
 
                     if (h.isEmpty()) {
                         // Got no handlers for this message yet. Keep the message around and attempt redelivery after a new
@@ -308,6 +314,7 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
                         // reliable, as a sender may attempt to send a message to a receiver that hasn't finished setting
                         // up a handler for yet. Most unit tests don't run threaded, but we want to test true parallelism at
                         // least sometimes.
+                        log.warn("Message to ${transfer.message.topicSession} could not be delivered")
                         pendingRedelivery.add(transfer)
                         null
                     } else {
@@ -335,7 +342,7 @@ class InMemoryMessagingNetwork(val sendManuallyPumped: Boolean) : SingletonSeria
                     try {
                         handler.callback(transfer.message, handler)
                     } catch(e: Exception) {
-                        loggerFor<InMemoryMessagingNetwork>().error("Caught exception in handler for $this/${handler.topic}", e)
+                        loggerFor<InMemoryMessagingNetwork>().error("Caught exception in handler for $this/${handler.topicSession}", e)
                     }
                 }
             }
diff --git a/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryNetworkMapCache.kt b/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryNetworkMapCache.kt
index 0f0c21ee54..07af6b3782 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryNetworkMapCache.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/network/InMemoryNetworkMapCache.kt
@@ -10,10 +10,10 @@ import com.r3corda.core.messaging.MessagingService
 import com.r3corda.core.messaging.runOnNextMessage
 import com.r3corda.core.messaging.send
 import com.r3corda.core.node.NodeInfo
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.node.services.NetworkCacheError
 import com.r3corda.core.node.services.NetworkMapCache
 import com.r3corda.core.node.services.ServiceType
-import com.r3corda.core.node.services.TOPIC_DEFAULT_POSTFIX
 import com.r3corda.core.random63BitValue
 import com.r3corda.core.serialization.SingletonSerializeAsToken
 import com.r3corda.core.serialization.deserialize
@@ -57,11 +57,11 @@ open class InMemoryNetworkMapCache(val netInternal: MessagingServiceInternal?) :
                                ifChangedSinceVer: Int?): ListenableFuture<Unit> {
         if (subscribe && !registeredForPush) {
             // Add handler to the network, for updates received from the remote network map service.
-            net.addMessageHandler(NetworkMapService.PUSH_PROTOCOL_TOPIC + ".0", null) { message, r ->
+            net.addMessageHandler(NetworkMapService.PUSH_PROTOCOL_TOPIC, DEFAULT_SESSION_ID, null) { message, r ->
                 try {
                     val req = message.data.deserialize<NetworkMapService.Update>()
                     val hash = SecureHash.sha256(req.wireReg.serialize().bits)
-                    val ackMessage = net.createMessage(NetworkMapService.PUSH_ACK_PROTOCOL_TOPIC + TOPIC_DEFAULT_POSTFIX,
+                    val ackMessage = net.createMessage(NetworkMapService.PUSH_ACK_PROTOCOL_TOPIC, DEFAULT_SESSION_ID,
                             NetworkMapService.UpdateAcknowledge(hash, net.myAddress).serialize().bits)
                     net.send(ackMessage, req.replyTo)
                     processUpdatePush(req)
@@ -81,13 +81,13 @@ open class InMemoryNetworkMapCache(val netInternal: MessagingServiceInternal?) :
         // Add a message handler for the response, and prepare a future to put the data into.
         // Note that the message handler will run on the network thread (not this one).
         val future = SettableFuture.create<Unit>()
-        net.runOnNextMessage("${NetworkMapService.FETCH_PROTOCOL_TOPIC}.$sessionID", MoreExecutors.directExecutor()) { message ->
+        net.runOnNextMessage(NetworkMapService.FETCH_PROTOCOL_TOPIC, sessionID, MoreExecutors.directExecutor()) { message ->
             val resp = message.data.deserialize<NetworkMapService.FetchMapResponse>()
             // We may not receive any nodes back, if the map hasn't changed since the version specified
             resp.nodes?.forEach { processRegistration(it) }
             future.set(Unit)
         }
-        net.send("${NetworkMapService.FETCH_PROTOCOL_TOPIC}.0", req, service.address)
+        net.send(NetworkMapService.FETCH_PROTOCOL_TOPIC, DEFAULT_SESSION_ID, req, service.address)
 
         return future
     }
@@ -114,7 +114,7 @@ open class InMemoryNetworkMapCache(val netInternal: MessagingServiceInternal?) :
         // Add a message handler for the response, and prepare a future to put the data into.
         // Note that the message handler will run on the network thread (not this one).
         val future = SettableFuture.create<Unit>()
-        net.runOnNextMessage("${NetworkMapService.SUBSCRIPTION_PROTOCOL_TOPIC}.$sessionID", MoreExecutors.directExecutor()) { message ->
+        net.runOnNextMessage(NetworkMapService.SUBSCRIPTION_PROTOCOL_TOPIC, sessionID, MoreExecutors.directExecutor()) { message ->
             val resp = message.data.deserialize<NetworkMapService.SubscribeResponse>()
             if (resp.confirmed) {
                 future.set(Unit)
@@ -122,7 +122,7 @@ open class InMemoryNetworkMapCache(val netInternal: MessagingServiceInternal?) :
                 future.setException(NetworkCacheError.DeregistrationFailed())
             }
         }
-        net.send("${NetworkMapService.SUBSCRIPTION_PROTOCOL_TOPIC}.0", req, service.address)
+        net.send(NetworkMapService.SUBSCRIPTION_PROTOCOL_TOPIC, DEFAULT_SESSION_ID, req, service.address)
 
         return future
     }
diff --git a/node/src/main/kotlin/com/r3corda/node/services/network/NetworkMapService.kt b/node/src/main/kotlin/com/r3corda/node/services/network/NetworkMapService.kt
index ccd6a3545c..0aaa53f39c 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/network/NetworkMapService.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/network/NetworkMapService.kt
@@ -7,9 +7,9 @@ import com.r3corda.core.messaging.MessageRecipients
 import com.r3corda.core.messaging.MessagingService
 import com.r3corda.core.messaging.SingleMessageRecipient
 import com.r3corda.core.node.NodeInfo
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.node.services.NetworkMapCache
 import com.r3corda.core.node.services.ServiceType
-import com.r3corda.core.node.services.TOPIC_DEFAULT_POSTFIX
 import com.r3corda.core.serialization.SerializedBytes
 import com.r3corda.core.serialization.deserialize
 import com.r3corda.core.serialization.serialize
@@ -114,7 +114,7 @@ class InMemoryNetworkMapService(net: MessagingService, home: NodeRegistration, v
         addMessageHandler(NetworkMapService.SUBSCRIPTION_PROTOCOL_TOPIC,
                 { req: NetworkMapService.SubscribeRequest -> processSubscriptionRequest(req) }
         )
-        net.addMessageHandler(NetworkMapService.PUSH_ACK_PROTOCOL_TOPIC + TOPIC_DEFAULT_POSTFIX, null) { message, r ->
+        net.addMessageHandler(NetworkMapService.PUSH_ACK_PROTOCOL_TOPIC, DEFAULT_SESSION_ID, null) { message, r ->
             val req = message.data.deserialize<NetworkMapService.UpdateAcknowledge>()
             processAcknowledge(req)
         }
@@ -144,8 +144,7 @@ class InMemoryNetworkMapService(net: MessagingService, home: NodeRegistration, v
         // to a MessageRecipientGroup that nodes join/leave, rather than the network map
         // service itself managing the group
         val update = NetworkMapService.Update(wireReg, net.myAddress).serialize().bits
-        val topic = NetworkMapService.PUSH_PROTOCOL_TOPIC + TOPIC_DEFAULT_POSTFIX
-        val message = net.createMessage(topic, update)
+        val message = net.createMessage(NetworkMapService.PUSH_PROTOCOL_TOPIC, DEFAULT_SESSION_ID, update)
 
         subscribers.locked {
             val toRemove = mutableListOf<SingleMessageRecipient>()
diff --git a/node/src/main/kotlin/com/r3corda/node/services/persistence/DataVendingService.kt b/node/src/main/kotlin/com/r3corda/node/services/persistence/DataVendingService.kt
index 515a7021fe..44a76dd1f1 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/persistence/DataVendingService.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/persistence/DataVendingService.kt
@@ -75,11 +75,11 @@ object DataVending {
                     .success {
                         services.recordTransactions(req.tx)
                         val resp = NotifyTxResponseMessage(true)
-                        val msg = net.createMessage(NOTIFY_TX_PROTOCOL_TOPIC + "." + req.sessionID, resp.serialize().bits)
+                        val msg = net.createMessage(NOTIFY_TX_PROTOCOL_TOPIC, req.sessionID, resp.serialize().bits)
                         net.send(msg, req.getReplyTo(services.networkMapCache))
                     }.failure {
                 val resp = NotifyTxResponseMessage(false)
-                val msg = net.createMessage(NOTIFY_TX_PROTOCOL_TOPIC + "." + req.sessionID, resp.serialize().bits)
+                val msg = net.createMessage(NOTIFY_TX_PROTOCOL_TOPIC, req.sessionID, resp.serialize().bits)
                 net.send(msg, req.getReplyTo(services.networkMapCache))
             }
         }
diff --git a/node/src/main/kotlin/com/r3corda/node/services/statemachine/FiberRequest.kt b/node/src/main/kotlin/com/r3corda/node/services/statemachine/FiberRequest.kt
index d088df8c74..d5714fee76 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/statemachine/FiberRequest.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/statemachine/FiberRequest.kt
@@ -1,6 +1,7 @@
 package com.r3corda.node.services.statemachine
 
 import com.r3corda.core.crypto.Party
+import com.r3corda.core.messaging.TopicSession
 
 // TODO: Clean this up
 sealed class FiberRequest(val topic: String,
@@ -13,8 +14,8 @@ sealed class FiberRequest(val topic: String,
     @Transient
     val stackTraceInCaseOfProblems: StackSnapshot? = StackSnapshot()
 
-    val receiveTopic: String
-        get() = topic + "." + sessionIDForReceive
+    val receiveTopicSession: TopicSession
+        get() = TopicSession(topic, sessionIDForReceive)
 
 
     override fun equals(other: Any?): Boolean
@@ -66,7 +67,7 @@ sealed class FiberRequest(val topic: String,
             false
 
         override fun toString(): String {
-            return "Expecting response via topic ${receiveTopic} of type ${responseTypeName}"
+            return "Expecting response via topic ${receiveTopicSession} of type ${responseTypeName}"
         }
 
         // We have to do an unchecked cast, but unless the serialized form is damaged, this was
diff --git a/node/src/main/kotlin/com/r3corda/node/services/statemachine/StateMachineManager.kt b/node/src/main/kotlin/com/r3corda/node/services/statemachine/StateMachineManager.kt
index 23df79646f..64c7f2c423 100644
--- a/node/src/main/kotlin/com/r3corda/node/services/statemachine/StateMachineManager.kt
+++ b/node/src/main/kotlin/com/r3corda/node/services/statemachine/StateMachineManager.kt
@@ -8,6 +8,7 @@ import com.esotericsoftware.kryo.Kryo
 import com.google.common.base.Throwables
 import com.google.common.util.concurrent.ListenableFuture
 import com.r3corda.core.abbreviate
+import com.r3corda.core.messaging.TopicSession
 import com.r3corda.core.messaging.runOnNextMessage
 import com.r3corda.core.messaging.send
 import com.r3corda.core.protocols.ProtocolLogic
@@ -124,7 +125,7 @@ class StateMachineManager(val serviceHub: ServiceHubInternal, tokenizableService
 
         when (checkpoint.request) {
             is FiberRequest.ExpectingResponse<*> -> {
-                val topic = checkpoint.request.receiveTopic
+                val topic = checkpoint.request.receiveTopicSession
                 val awaitingPayloadType = checkpoint.request.responseType
                 fiber.logger.info("Restored ${fiber.logic} - it was previously waiting for message of type ${awaitingPayloadType.name} on topic $topic")
                 iterateOnResponse(fiber, awaitingPayloadType, checkpoint.serialisedFiber, checkpoint.request) {
@@ -179,9 +180,9 @@ class StateMachineManager(val serviceHub: ServiceHubInternal, tokenizableService
         return createKryo(serializer.kryo)
     }
 
-    private fun logError(e: Throwable, payload: Any?, topic: String?, psm: ProtocolStateMachineImpl<*>) {
+    private fun logError(e: Throwable, payload: Any?, topicSession: TopicSession?, psm: ProtocolStateMachineImpl<*>) {
         psm.logger.error("Protocol state machine ${psm.javaClass.name} threw '${Throwables.getRootCause(e)}' " +
-                "when handling a message of type ${payload?.javaClass?.name} on topic $topic")
+                "when handling a message of type ${payload?.javaClass?.name} on queue $topicSession")
         if (psm.logger.isTraceEnabled) {
             val s = StringWriter()
             Throwables.getRootCause(e).printStackTrace(PrintWriter(s))
@@ -265,12 +266,12 @@ class StateMachineManager(val serviceHub: ServiceHubInternal, tokenizableService
             }
         }
         // If a non-null payload to send was provided, send it now.
+        val queueID = TopicSession(request.topic, request.sessionIDForSend)
         request.payload?.let {
-            val topic = "${request.topic}.${request.sessionIDForSend}"
-            psm.logger.trace { "Sending message of type ${it.javaClass.name} using topic $topic to ${request.destination} (${it.toString().abbreviate(50)})" }
+            psm.logger.trace { "Sending message of type ${it.javaClass.name} using queue $queueID to ${request.destination} (${it.toString().abbreviate(50)})" }
             val node = serviceHub.networkMapCache.getNodeByLegalName(request.destination!!.name)
             requireNotNull(node) { "Don't know about ${request.destination}" }
-            serviceHub.networkService.send(topic, it, node!!.address)
+            serviceHub.networkService.send(queueID, it, node!!.address)
         }
         if (request is FiberRequest.NotExpectingResponse) {
             // We sent a message, but don't expect a response, so re-enter the continuation to let it keep going.
@@ -278,7 +279,7 @@ class StateMachineManager(val serviceHub: ServiceHubInternal, tokenizableService
                 try {
                     Fiber.unpark(psm, QUASAR_UNBLOCKER)
                 } catch(e: Throwable) {
-                    logError(e, request.payload, request.topic, psm)
+                    logError(e, request.payload, queueID, psm)
                 }
             }
         }
@@ -286,15 +287,15 @@ class StateMachineManager(val serviceHub: ServiceHubInternal, tokenizableService
 
     private fun checkpointOnExpectingResponse(psm: ProtocolStateMachineImpl<*>, request: FiberRequest.ExpectingResponse<*>) {
         executor.checkOnThread()
-        val topic = "${request.topic}.${request.sessionIDForReceive}"
+        val queueID = request.receiveTopicSession
         val serialisedFiber = serializeFiber(psm)
         updateCheckpoint(psm, serialisedFiber, request)
-        psm.logger.trace { "Preparing to receive message of type ${request.responseType.name} on topic $topic" }
+        psm.logger.trace { "Preparing to receive message of type ${request.responseType.name} on queue $queueID" }
         iterateOnResponse(psm, request.responseType, serialisedFiber, request) {
             try {
                 Fiber.unpark(psm, QUASAR_UNBLOCKER)
             } catch(e: Throwable) {
-                logError(e, it, topic, psm)
+                logError(e, it, queueID, psm)
             }
         }
     }
@@ -308,7 +309,7 @@ class StateMachineManager(val serviceHub: ServiceHubInternal, tokenizableService
                                   serialisedFiber: SerializedBytes<ProtocolStateMachineImpl<*>>,
                                   request: FiberRequest.ExpectingResponse<*>,
                                   resumeAction: (Any?) -> Unit) {
-        val topic = request.receiveTopic
+        val topic = request.receiveTopicSession
         serviceHub.networkService.runOnNextMessage(topic, executor) { netMsg ->
             // Assertion to ensure we don't execute on the wrong thread.
             executor.checkOnThread()
@@ -322,7 +323,7 @@ class StateMachineManager(val serviceHub: ServiceHubInternal, tokenizableService
             check(responseType.isInstance(payload)) { "Expected message of type ${responseType.name} but got ${payload.javaClass.name}" }
             // Update the fiber's checkpoint so that it's no longer waiting on a response, but rather has the received payload
             updateCheckpoint(psm, serialisedFiber, request)
-            psm.logger.trace { "Received message of type ${payload.javaClass.name} on topic $topic (${payload.toString().abbreviate(50)})" }
+            psm.logger.trace { "Received message of type ${payload.javaClass.name} on topic ${request.topic}.${request.sessionIDForReceive} (${payload.toString().abbreviate(50)})" }
             iterateStateMachine(psm, payload, resumeAction)
         }
     }
diff --git a/node/src/test/kotlin/com/r3corda/node/messaging/InMemoryMessagingTests.kt b/node/src/test/kotlin/com/r3corda/node/messaging/InMemoryMessagingTests.kt
index d939d93015..86a0e6444c 100644
--- a/node/src/test/kotlin/com/r3corda/node/messaging/InMemoryMessagingTests.kt
+++ b/node/src/test/kotlin/com/r3corda/node/messaging/InMemoryMessagingTests.kt
@@ -4,6 +4,7 @@ package com.r3corda.node.messaging
 
 import com.r3corda.core.messaging.Message
 import com.r3corda.core.messaging.TopicStringValidator
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.node.internal.testing.MockNetwork
 import org.junit.Before
 import org.junit.Test
@@ -61,7 +62,7 @@ class InMemoryMessagingTests {
         }
 
         // Node 1 sends a message and it should end up in finalDelivery, after we run the network
-        node1.net.send(node1.net.createMessage("test.topic", bits), node2.info.address)
+        node1.net.send(node1.net.createMessage("test.topic", DEFAULT_SESSION_ID, bits), node2.info.address)
 
         network.runNetwork(rounds = 1)
 
@@ -78,7 +79,7 @@ class InMemoryMessagingTests {
 
         var counter = 0
         listOf(node1, node2, node3).forEach { it.net.addMessageHandler { msg, registration -> counter++ } }
-        node1.net.send(node2.net.createMessage("test.topic", bits), network.messagingNetwork.everyoneOnline)
+        node1.net.send(node2.net.createMessage("test.topic", DEFAULT_SESSION_ID, bits), network.messagingNetwork.everyoneOnline)
         network.runNetwork(rounds = 1)
         assertEquals(3, counter)
     }
@@ -97,8 +98,8 @@ class InMemoryMessagingTests {
             received++
         }
 
-        val invalidMessage = node2.net.createMessage("invalid_message", ByteArray(0))
-        val validMessage = node2.net.createMessage("valid_message", ByteArray(0))
+        val invalidMessage = node2.net.createMessage("invalid_message", DEFAULT_SESSION_ID, ByteArray(0))
+        val validMessage = node2.net.createMessage("valid_message", DEFAULT_SESSION_ID, ByteArray(0))
         node2.net.send(invalidMessage, node1.net.myAddress)
         network.runNetwork()
         assertEquals(0, received)
diff --git a/node/src/test/kotlin/com/r3corda/node/services/ArtemisMessagingServiceTests.kt b/node/src/test/kotlin/com/r3corda/node/services/ArtemisMessagingServiceTests.kt
index e2cd96e023..a068ce61f9 100644
--- a/node/src/test/kotlin/com/r3corda/node/services/ArtemisMessagingServiceTests.kt
+++ b/node/src/test/kotlin/com/r3corda/node/services/ArtemisMessagingServiceTests.kt
@@ -1,10 +1,10 @@
 package com.r3corda.node.services
 
 import com.r3corda.core.messaging.Message
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.testing.freeLocalHostAndPort
 import com.r3corda.node.services.config.NodeConfiguration
 import com.r3corda.node.services.messaging.ArtemisMessagingService
-import org.assertj.core.api.Assertions.assertThat
 import org.assertj.core.api.Assertions.assertThatThrownBy
 import org.junit.After
 import org.junit.Rule
@@ -14,6 +14,9 @@ import java.net.ServerSocket
 import java.util.concurrent.LinkedBlockingQueue
 import java.util.concurrent.TimeUnit.MILLISECONDS
 import java.util.concurrent.TimeUnit.SECONDS
+import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
+import kotlin.test.assertNull
 
 class ArtemisMessagingServiceTests {
 
@@ -48,11 +51,13 @@ class ArtemisMessagingServiceTests {
             receivedMessages.add(message)
         }
 
-        val message = messagingNetwork.createMessage(topic, "first msg".toByteArray())
+        val message = messagingNetwork.createMessage(topic, DEFAULT_SESSION_ID, "first msg".toByteArray())
         messagingNetwork.send(message, messagingNetwork.myAddress)
 
-        assertThat(String(receivedMessages.poll(2, SECONDS).data)).isEqualTo("first msg")
-        assertThat(receivedMessages.poll(200, MILLISECONDS)).isNull()
+        val actual = receivedMessages.poll(2, SECONDS)
+        assertNotNull(actual)
+        assertEquals("first msg", String(actual.data))
+        assertNull(receivedMessages.poll(200, MILLISECONDS))
     }
 
     private fun createMessagingService(): ArtemisMessagingService {
diff --git a/node/src/test/kotlin/com/r3corda/node/services/persistence/DataVendingServiceTests.kt b/node/src/test/kotlin/com/r3corda/node/services/persistence/DataVendingServiceTests.kt
index 4a2a24fb28..cc8968b67f 100644
--- a/node/src/test/kotlin/com/r3corda/node/services/persistence/DataVendingServiceTests.kt
+++ b/node/src/test/kotlin/com/r3corda/node/services/persistence/DataVendingServiceTests.kt
@@ -4,6 +4,7 @@ import co.paralleluniverse.fibers.Suspendable
 import com.r3corda.contracts.asset.Cash
 import com.r3corda.core.contracts.*
 import com.r3corda.core.node.NodeInfo
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.protocols.ProtocolLogic
 import com.r3corda.core.random63BitValue
 import com.r3corda.core.testing.DUMMY_NOTARY
@@ -39,7 +40,7 @@ class DataVendingServiceTests {
         override fun call(): Boolean {
             val sessionID = random63BitValue()
             val req = DataVending.Service.NotifyTxRequestMessage(tx, serviceHub.storageService.myLegalIdentity, sessionID)
-            return sendAndReceive<DataVending.Service.NotifyTxResponseMessage>(server.identity, 0, sessionID, req).validate { it.accepted }
+            return sendAndReceive<DataVending.Service.NotifyTxResponseMessage>(server.identity, DEFAULT_SESSION_ID, sessionID, req).validate { it.accepted }
         }
     }
 
diff --git a/src/main/kotlin/com/r3corda/demos/TraderDemo.kt b/src/main/kotlin/com/r3corda/demos/TraderDemo.kt
index ca2c357cfe..e3db2518b8 100644
--- a/src/main/kotlin/com/r3corda/demos/TraderDemo.kt
+++ b/src/main/kotlin/com/r3corda/demos/TraderDemo.kt
@@ -13,6 +13,7 @@ import com.r3corda.core.crypto.generateKeyPair
 import com.r3corda.core.days
 import com.r3corda.core.logElapsedTime
 import com.r3corda.core.node.NodeInfo
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.node.services.ServiceType
 import com.r3corda.core.protocols.ProtocolLogic
 import com.r3corda.core.random63BitValue
@@ -217,7 +218,7 @@ private fun runBuyer(node: Node, amount: Amount<Issued<Currency>>) {
     // next stage in our building site, we will just auto-generate fake trades to give our nodes something to do.
     //
     // As the seller initiates the two-party trade protocol, here, we will be the buyer.
-    node.services.networkService.addMessageHandler("$DEMO_TOPIC.0") { message, registration ->
+    node.services.networkService.addMessageHandler(DEMO_TOPIC, DEFAULT_SESSION_ID) { message, registration ->
         // We use a simple scenario-specific wrapper protocol to make things happen.
         val otherSide = message.data.deserialize<Party>()
         val buyer = TraderDemoProtocolBuyer(otherSide, attachmentsPath, amount)
diff --git a/src/main/kotlin/com/r3corda/demos/protocols/AutoOfferProtocol.kt b/src/main/kotlin/com/r3corda/demos/protocols/AutoOfferProtocol.kt
index 6b13d2b533..047e9a10da 100644
--- a/src/main/kotlin/com/r3corda/demos/protocols/AutoOfferProtocol.kt
+++ b/src/main/kotlin/com/r3corda/demos/protocols/AutoOfferProtocol.kt
@@ -7,6 +7,7 @@ import com.r3corda.core.contracts.DealState
 import com.r3corda.core.contracts.SignedTransaction
 import com.r3corda.core.crypto.Party
 import com.r3corda.core.node.CordaPluginRegistry
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.protocols.ProtocolLogic
 import com.r3corda.core.random63BitValue
 import com.r3corda.core.serialization.deserialize
@@ -53,7 +54,7 @@ object AutoOfferProtocol {
         }
 
         init {
-            services.networkService.addMessageHandler("$TOPIC.0") { msg, registration ->
+            services.networkService.addMessageHandler(TOPIC, DEFAULT_SESSION_ID) { msg, registration ->
                 val progressTracker = tracker()
                 progressTracker.currentStep = RECEIVED
                 val autoOfferMessage = msg.data.deserialize<AutoOfferMessage>()
diff --git a/src/main/kotlin/com/r3corda/demos/protocols/ExitServerProtocol.kt b/src/main/kotlin/com/r3corda/demos/protocols/ExitServerProtocol.kt
index a1cdc9ea11..d28b9a2910 100644
--- a/src/main/kotlin/com/r3corda/demos/protocols/ExitServerProtocol.kt
+++ b/src/main/kotlin/com/r3corda/demos/protocols/ExitServerProtocol.kt
@@ -4,6 +4,7 @@ import co.paralleluniverse.fibers.Suspendable
 import co.paralleluniverse.strands.Strand
 import com.r3corda.core.node.CordaPluginRegistry
 import com.r3corda.core.node.NodeInfo
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.protocols.ProtocolLogic
 import com.r3corda.core.serialization.deserialize
 import com.r3corda.node.services.api.ServiceHubInternal
@@ -27,7 +28,7 @@ object ExitServerProtocol {
     class Service(services: ServiceHubInternal) {
 
         init {
-            services.networkService.addMessageHandler("$TOPIC.0") { msg, registration ->
+            services.networkService.addMessageHandler(TOPIC, DEFAULT_SESSION_ID) { msg, registration ->
                 // Just to validate we got the message
                 if (enabled) {
                     val message = msg.data.deserialize<ExitMessage>()
diff --git a/src/main/kotlin/com/r3corda/demos/protocols/UpdateBusinessDayProtocol.kt b/src/main/kotlin/com/r3corda/demos/protocols/UpdateBusinessDayProtocol.kt
index 6692b946c4..4edad7436e 100644
--- a/src/main/kotlin/com/r3corda/demos/protocols/UpdateBusinessDayProtocol.kt
+++ b/src/main/kotlin/com/r3corda/demos/protocols/UpdateBusinessDayProtocol.kt
@@ -3,6 +3,7 @@ package com.r3corda.demos.protocols
 import co.paralleluniverse.fibers.Suspendable
 import com.r3corda.core.node.CordaPluginRegistry
 import com.r3corda.core.node.NodeInfo
+import com.r3corda.core.node.services.DEFAULT_SESSION_ID
 import com.r3corda.core.protocols.ProtocolLogic
 import com.r3corda.core.serialization.deserialize
 import com.r3corda.core.utilities.ProgressTracker
@@ -28,7 +29,7 @@ object UpdateBusinessDayProtocol {
     class Service(services: ServiceHubInternal) {
 
         init {
-            services.networkService.addMessageHandler("${TOPIC}.0") { msg, registration ->
+            services.networkService.addMessageHandler(TOPIC, DEFAULT_SESSION_ID) { msg, registration ->
                 val updateBusinessDayMessage = msg.data.deserialize<UpdateBusinessDayMessage>()
                 (services.clock as DemoClock).updateDate(updateBusinessDayMessage.date)
             }