diff --git a/hydra-node/src/Hydra/Network/Reliability.hs b/hydra-node/src/Hydra/Network/Reliability.hs
index 805311e763e..1c34dab17d3 100644
--- a/hydra-node/src/Hydra/Network/Reliability.hs
+++ b/hydra-node/src/Hydra/Network/Reliability.hs
@@ -66,15 +66,12 @@ import qualified Data.Map.Strict as Map
 import Data.Vector (
   Vector,
   elemIndex,
-  empty,
   fromList,
   generate,
   length,
   replicate,
-  snoc,
   zipWith,
   (!),
-  (!?),
  )
 import Hydra.Logging (traceWith)
 import Hydra.Network (Network (..), NetworkComponent)
@@ -152,7 +149,7 @@ withReliability ::
   NetworkComponent m (Authenticated (Heartbeat msg)) (Heartbeat msg) a
 withReliability tracer me otherParties withRawNetwork callback action = do
   ackCounter <- newTVarIO $ replicate (length allParties) 0
-  sentMessages <- newTVarIO empty
+  sentMessages <- newTVarIO Map.empty
   seenMessages <- newTVarIO $ Map.fromList $ (,0) <$> toList allParties
   resendQ <- newTQueueIO
   ourIndex <- findPartyIndex me
@@ -174,7 +171,7 @@ withReliability tracer me otherParties withRawNetwork callback action = do
                   acks <- readTVar ackCounter
                   let newAcks = constructAcks acks ourIndex
                   writeTVar ackCounter newAcks
-                  modifyTVar' sentMessages (`snoc` msg)
+                  modifyTVar' sentMessages (insertNewMsg msg)
                   readTVar ackCounter
 
                 traceWith tracer (BroadcastCounter ourIndex ackCounter')
@@ -241,14 +238,14 @@ withReliability tracer me otherParties withRawNetwork callback action = do
       let missing = fromList [messageAckForUs + 1 .. knownAckForUs]
       messages <- readTVarIO sentMessages
       forM_ missing $ \idx -> do
-        case messages !? (idx - 1) of
+        case messages Map.!? (idx - 1) of
           Nothing ->
             throwIO $
               ReliabilityFailedToFindMsg $
                 "FIXME: this should never happen, there's no sent message at index "
                   <> show idx
                   <> ", messages length = "
-                  <> show (length messages)
+                  <> show (Map.size messages)
                   <> ", latest message ack: "
                   <> show knownAckForUs
                   <> ", acked: "
@@ -270,6 +267,11 @@ withReliability tracer me otherParties withRawNetwork callback action = do
     -- modifyTVar' sentMessages (Map.delete (const $ Just messageAckForUs) party)
     traceWith tracer (ClearedMessageQueue queueLength deleted)
 
+  insertNewMsg msg m =
+    case Map.lookupMax m of
+      Nothing -> Map.insert 1 msg m
+      Just (k, _) -> Map.insert (k + 1) msg m
+
   -- find the index of a party in the list of parties or fail with 'ReliabilityMissingPartyIndex'
   findPartyIndex party =
     maybe (throwIO $ ReliabilityMissingPartyIndex party) pure $ elemIndex party allParties