diff --git a/node/src/main/kotlin/core/testing/IRSSimulation.kt b/node/src/main/kotlin/core/testing/IRSSimulation.kt
index fa1437517e..b741788d68 100644
--- a/node/src/main/kotlin/core/testing/IRSSimulation.kt
+++ b/node/src/main/kotlin/core/testing/IRSSimulation.kt
@@ -126,9 +126,9 @@ class IRSSimulation(runAsync: Boolean, latencyInjector: InMemoryMessagingNetwork
         }
     }
 
-    override fun iterate() {
+    override fun iterate(): InMemoryMessagingNetwork.MessageTransfer? {
         if (executeOnNextIteration.isNotEmpty())
             executeOnNextIteration.removeAt(0)()
-        super.iterate()
+        return super.iterate()
     }
 }
\ No newline at end of file
diff --git a/node/src/main/kotlin/core/testing/InMemoryMessagingNetwork.kt b/node/src/main/kotlin/core/testing/InMemoryMessagingNetwork.kt
index 2dcb728d48..ff4068d3c6 100644
--- a/node/src/main/kotlin/core/testing/InMemoryMessagingNetwork.kt
+++ b/node/src/main/kotlin/core/testing/InMemoryMessagingNetwork.kt
@@ -36,11 +36,16 @@ class InMemoryMessagingNetwork {
 
     private var counter = 0   // -1 means stopped.
     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'"
+    }
+
     // All messages are kept here until the messages are pumped off the queue by a caller to the node class.
     // Queues are created on-demand when a message is sent to an address: the receiving node doesn't have to have
     // been created yet. If the node identified by the given handle has gone away/been shut down then messages
     // stack up here waiting for it to come back. The intent of this is to simulate a reliable messaging network.
-    private val messageQueues = HashMap<Handle, LinkedBlockingQueue<Message>>()
+    private val messageQueues = HashMap<Handle, LinkedBlockingQueue<MessageTransfer>>()
 
     val endpoints: List<InMemoryMessaging> @Synchronized get() = handleEndpointMap.values.toList()
 
@@ -73,9 +78,9 @@ class InMemoryMessagingNetwork {
         return Builder(manuallyPumped, Handle(id, description ?: "In memory node $id"))
     }
 
-    private val _allMessages = PublishSubject.create<Triple<SingleMessageRecipient, Message, MessageRecipients>>()
+    private val _allMessages = PublishSubject.create<MessageTransfer>()
     /** A stream of (sender, message, recipients) triples */
-    val allMessages: Observable<Triple<SingleMessageRecipient, Message, MessageRecipients>> = _allMessages
+    val allMessages: Observable<MessageTransfer> = _allMessages
 
     interface LatencyCalculator {
         fun between(sender: SingleMessageRecipient, receiver: SingleMessageRecipient): Duration
@@ -87,28 +92,29 @@ class InMemoryMessagingNetwork {
 
     @Synchronized
     private fun msgSend(from: InMemoryMessaging, message: Message, recipients: MessageRecipients) {
-        log.trace { "${message.topic} from '${from.myAddress}' to '$recipients'" }
+        val transfer = MessageTransfer(from, message, recipients)
+        log.trace { transfer.toString() }
         val calc = latencyCalculator
         if (calc != null && recipients is SingleMessageRecipient) {
             // Inject some artificial latency.
             timer.schedule(calc.between(from.myAddress, recipients).toMillis()) {
-                msgSendInternal(message, recipients)
+                msgSendInternal(transfer)
             }
         } else {
-            msgSendInternal(message, recipients)
+            msgSendInternal(transfer)
         }
-        _allMessages.onNext(Triple(from.myAddress, message, recipients))
+        _allMessages.onNext(MessageTransfer(from, message, recipients))
     }
 
-    private fun msgSendInternal(message: Message, recipients: MessageRecipients) {
-        when (recipients) {
-            is Handle -> getQueueForHandle(recipients).add(message)
+    private fun msgSendInternal(transfer: MessageTransfer) {
+        when (transfer.recipients) {
+            is Handle -> getQueueForHandle(transfer.recipients).add(transfer)
 
             is AllPossibleRecipients -> {
                 // This means all possible recipients _that the network knows about at the time_, not literally everyone
                 // who joins into the indefinite future.
                 for (handle in handleEndpointMap.keys)
-                    getQueueForHandle(handle).add(message)
+                    getQueueForHandle(handle).add(transfer)
             }
             else -> throw IllegalArgumentException("Unknown type of recipient handle")
         }
@@ -170,7 +176,7 @@ class InMemoryMessagingNetwork {
 
         protected inner class InnerState {
             val handlers: MutableList<Handler> = ArrayList()
-            val pendingRedelivery = LinkedList<Message>()
+            val pendingRedelivery = LinkedList<MessageTransfer>()
         }
 
         protected val state = ThreadBox(InnerState())
@@ -197,7 +203,7 @@ class InMemoryMessagingNetwork {
                 Pair(handler, items)
             }
             for (it in items)
-                msgSend(this, it, handle)
+                msgSend(this, it.message, handle)
             return handler
         }
 
@@ -237,19 +243,21 @@ class InMemoryMessagingNetwork {
          * Delivers a single message from the internal queue. If there are no messages waiting to be delivered and block
          * is true, waits until one has been provided on a different thread via send. If block is false, the return
          * result indicates whether a message was delivered or not.
+         *
+         * @return the message that was processed, if any in this round.
          */
-        fun pump(block: Boolean): Boolean {
+        fun pump(block: Boolean): MessageTransfer? {
             check(manuallyPumped)
             check(running)
             return pumpInternal(block)
         }
 
-        private fun pumpInternal(block: Boolean): Boolean {
+        private fun pumpInternal(block: Boolean): MessageTransfer? {
             val q = getQueueForHandle(handle)
-            val message = (if (block) q.take() else q.poll()) ?: return false
+            val transfer = (if (block) q.take() else q.poll()) ?: return null
 
             val deliverTo = state.locked {
-                val h = handlers.filter { if (it.topic.isBlank()) true else message.topic == it.topic }
+                val h = handlers.filter { if (it.topic.isBlank()) true else transfer.message.topic == it.topic }
 
                 if (h.isEmpty()) {
                     // Got no handlers for this message yet. Keep the message around and attempt redelivery after a new
@@ -257,8 +265,8 @@ class InMemoryMessagingNetwork {
                     // 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.
-                    pendingRedelivery.add(message)
-                    return false
+                    pendingRedelivery.add(transfer)
+                    return null
                 }
 
                 h
@@ -268,14 +276,14 @@ class InMemoryMessagingNetwork {
                 // Now deliver via the requested executor, or on this thread if no executor was provided at registration time.
                 (handler.executor ?: MoreExecutors.directExecutor()).execute {
                     try {
-                        handler.callback(message, handler)
+                        handler.callback(transfer.message, handler)
                     } catch(e: Exception) {
                         loggerFor<InMemoryMessagingNetwork>().error("Caught exception in handler for $this/${handler.topic}", e)
                     }
                 }
             }
 
-            return true
+            return transfer
         }
     }
 }
diff --git a/node/src/main/kotlin/core/testing/MockNode.kt b/node/src/main/kotlin/core/testing/MockNode.kt
index f66593be35..e4ed552616 100644
--- a/node/src/main/kotlin/core/testing/MockNode.kt
+++ b/node/src/main/kotlin/core/testing/MockNode.kt
@@ -131,8 +131,9 @@ class MockNetwork(private val threadPerNode: Boolean = false,
      */
     fun runNetwork(rounds: Int = -1) {
         fun pumpAll() = messagingNetwork.endpoints.map { it.pump(false) }
+
         if (rounds == -1)
-            while (pumpAll().any { it }) {
+            while (pumpAll().any { it != null }) {
             }
         else
             repeat(rounds) { pumpAll() }
diff --git a/node/src/main/kotlin/core/testing/Simulation.kt b/node/src/main/kotlin/core/testing/Simulation.kt
index edfe3c6cbb..f58b95eeda 100644
--- a/node/src/main/kotlin/core/testing/Simulation.kt
+++ b/node/src/main/kotlin/core/testing/Simulation.kt
@@ -175,18 +175,22 @@ abstract class Simulation(val runAsync: Boolean,
      * will carry on from where this one stopped. In an environment where you want to take actions between anything
      * interesting happening, or control the precise speed at which things operate (beyond the latency injector), this
      * is a useful way to do things.
+     *
+     * @return the message that was processed, or null if no node accepted a message in this round.
      */
-    open fun iterate() {
+    open fun iterate(): InMemoryMessagingNetwork.MessageTransfer? {
         // Keep going until one of the nodes has something to do, or we have checked every node.
         val endpoints = network.messagingNetwork.endpoints
         var countDown = endpoints.size
         while (countDown > 0) {
             val handledMessage = endpoints[pumpCursor].pump(false)
-            if (handledMessage) break
+            if (handledMessage != null)
+                return handledMessage
             // If this node had nothing to do, advance the cursor with wraparound and try again.
             pumpCursor = (pumpCursor + 1) % endpoints.size
             countDown--
         }
+        return null
     }
 
     protected fun linkProtocolProgress(node: SimulatedNode, protocol: ProtocolLogic<*>) {
diff --git a/node/src/test/kotlin/core/messaging/TwoPartyTradeProtocolTests.kt b/node/src/test/kotlin/core/messaging/TwoPartyTradeProtocolTests.kt
index a9fd7feeb3..45253076d1 100644
--- a/node/src/test/kotlin/core/messaging/TwoPartyTradeProtocolTests.kt
+++ b/node/src/test/kotlin/core/messaging/TwoPartyTradeProtocolTests.kt
@@ -36,6 +36,7 @@ import java.util.jar.JarOutputStream
 import java.util.zip.ZipEntry
 import kotlin.test.assertEquals
 import kotlin.test.assertFailsWith
+import kotlin.test.assertNotNull
 import kotlin.test.assertTrue
 
 /**
@@ -171,7 +172,7 @@ class TwoPartyTradeProtocolTests {
 
             // Alice doesn't know that and carries on: she wants to know about the cash transactions he's trying to use.
             // She will wait around until Bob comes back.
-            assertTrue(pumpAlice())
+            assertNotNull(pumpAlice())
 
             // ... bring the node back up ... the act of constructing the SMM will re-register the message handlers
             // that Bob was waiting on before the reboot occurred.