diff --git a/src/Blockcore/P2P/Protocol/Payloads/NotFoundPayload.cs b/src/Blockcore/P2P/Protocol/Payloads/NotFoundPayload.cs
index 82ab15eaf..09d613e5c 100644
--- a/src/Blockcore/P2P/Protocol/Payloads/NotFoundPayload.cs
+++ b/src/Blockcore/P2P/Protocol/Payloads/NotFoundPayload.cs
@@ -9,6 +9,7 @@ namespace Blockcore.P2P.Protocol.Payloads
     /// <summary>
     /// A getdata message for an asked hash is not found by the remote peer.
     /// </summary>
+    [Payload("notfound")]
     public class NotFoundPayload : Payload, IEnumerable<InventoryVector>
     {
         private List<InventoryVector> inventory = new List<InventoryVector>();
diff --git a/src/Blockcore/Utilities/LruHashSet.cs b/src/Blockcore/Utilities/LruHashSet.cs
new file mode 100644
index 000000000..7b8ff4dd3
--- /dev/null
+++ b/src/Blockcore/Utilities/LruHashSet.cs
@@ -0,0 +1,84 @@
+using System.Collections.Generic;
+
+namespace Blockcore.Utilities
+{
+    public class LruHashSet<T>
+    {
+        private readonly LinkedList<T> lru;
+
+        private HashSet<T> items;
+
+        private long maxSize;
+
+        private long itemCount;
+
+        private readonly object lockObject = new object();
+
+        public LruHashSet(long maxSize = long.MaxValue)
+        {
+            this.lru = new LinkedList<T>();
+            this.items = new HashSet<T>();
+
+            this.maxSize = maxSize;
+            this.itemCount = 0;
+        }
+
+        public void AddOrUpdate(T item)
+        {
+            lock (this.lockObject)
+            {
+                // First check if we are performing the 'Update' case. No change to item count.
+                if (this.items.Contains(item))
+                {
+                    this.lru.Remove(item);
+                    this.lru.AddLast(item);
+
+                    return;
+                }
+
+                // Otherwise it's 'Add'.
+                // First perform the size test.
+                if ((this.itemCount + 1) > this.maxSize)
+                {
+                    LinkedListNode<T> tempItem = this.lru.First;
+                    this.lru.RemoveFirst();
+                    this.items.Remove(tempItem.Value);
+                    this.itemCount--;
+                }
+
+                this.lru.AddLast(item);
+                this.items.Add(item);
+                this.itemCount++;
+            }
+        }
+
+        public void Clear()
+        {
+            lock (this.lockObject)
+            {
+                this.lru.Clear();
+                this.items.Clear();
+                this.itemCount = 0;
+            }
+        }
+
+        public bool Contains(T item)
+        {
+            lock (this.lockObject)
+            {
+                // Fastest to check the hashmap.
+                return this.items.Contains(item);
+            }
+        }
+
+        public void Remove(T item)
+        {
+            lock (this.lockObject)
+            {
+                this.lru.Remove(item);
+                this.items.Remove(item);
+                this.itemCount--;
+            }
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Features/Blockcore.Features.BlockStore/BlockStoreBehavior.cs b/src/Features/Blockcore.Features.BlockStore/BlockStoreBehavior.cs
index c29b5c8f0..b6264e24b 100644
--- a/src/Features/Blockcore.Features.BlockStore/BlockStoreBehavior.cs
+++ b/src/Features/Blockcore.Features.BlockStore/BlockStoreBehavior.cs
@@ -281,18 +281,24 @@ private async Task ProcessGetDataAsync(INetworkPeer peer, GetDataPayload getData
             // TODO: bring logic from core
             foreach (InventoryVector item in getDataPayload.Inventory.Where(inv => inv.Type.HasFlag(InventoryType.MSG_BLOCK)))
             {
+                if (!peer.IsConnected)
+                    continue;
+
                 ChainedHeaderBlock chainedHeaderBlock = this.consensusManager.GetBlockData(item.Hash);
 
                 if (chainedHeaderBlock?.Block != null)
                 {
                     this.logger.LogDebug("Sending block '{0}' to peer '{1}'.", chainedHeaderBlock.ChainedHeader, peer.RemoteSocketEndpoint);
 
-                    //TODO strip block of witness if node does not support
                     await peer.SendMessageAsync(new BlockPayload(chainedHeaderBlock.Block.WithOptions(this.ChainIndexer.Network.Consensus.ConsensusFactory, peer.SupportedTransactionOptions))).ConfigureAwait(false);
                 }
                 else
                 {
                     this.logger.LogDebug("Block with hash '{0}' requested from peer '{1}' was not found in store.", item.Hash, peer.RemoteSocketEndpoint);
+
+                    // https://btcinformation.org/en/developer-reference#notfound
+                    // https://github.com/bitcoin/bitcoin/pull/2192
+                    await peer.SendMessageAsync(new NotFoundPayload(InventoryType.MSG_BLOCK, item.Hash)).ConfigureAwait(false);
                 }
 
                 // If the peer is syncing using "getblocks" message we are supposed to send
diff --git a/src/Features/Blockcore.Features.MemoryPool/MempoolBehavior.cs b/src/Features/Blockcore.Features.MemoryPool/MempoolBehavior.cs
index a7c26b6c2..5b89408bc 100644
--- a/src/Features/Blockcore.Features.MemoryPool/MempoolBehavior.cs
+++ b/src/Features/Blockcore.Features.MemoryPool/MempoolBehavior.cs
@@ -38,6 +38,14 @@ public class MempoolBehavior : NetworkPeerBehavior
         /// </summary>
         private const int InventoryBroadcastMax = 7 * InventoryBroadcastInterval;
 
+        /// <summary>
+        /// Prevents a single peer from sending us huge quantities of fake `inv` messages with nonexistent hashes.
+        /// <remarks>This value is somewhat arbitrarily chosen, it is far greater than the maximum number of transactions that can be stored in any single block.
+        /// The number of unmined transactions in the mempool should ideally hover around the number in a single block or below, otherwise the mempool will eventually
+        /// fill completely. However, on some networks it is not uncommon for there to be surges in transaction volume, so we leave some leeway.</remarks>
+        /// </summary>
+        private const int MaximumInventoryToTrackCount = 50000;
+
         /// <summary>Memory pool validator for validating transactions.</summary>
         private readonly IMempoolValidator validator;
 
@@ -75,7 +83,7 @@ public class MempoolBehavior : NetworkPeerBehavior
         /// Filter for inventory known.
         /// State that is local to the behavior.
         /// </summary>
-        private readonly HashSet<uint256> filterInventoryKnown;
+        private readonly LruHashSet<uint256> filterInventoryKnown;
 
         /// <summary>
         /// Locking object for memory pool behaviour.
@@ -120,7 +128,7 @@ public MempoolBehavior(
 
             this.lockObject = new object();
             this.inventoryTxToSend = new HashSet<uint256>();
-            this.filterInventoryKnown = new HashSet<uint256>();
+            this.filterInventoryKnown = new LruHashSet<uint256>(MaximumInventoryToTrackCount);
             this.isPeerWhitelistedForRelay = false;
             this.isBlocksOnlyMode = false;
 
@@ -267,14 +275,14 @@ private async Task SendMempoolPayloadAsync(INetworkPeer peer, MempoolPayload mes
                         }
                     }
 
-                    this.filterInventoryKnown.Add(hash);
+                    this.filterInventoryKnown.AddOrUpdate(hash);
                     transactionsToSend.Add(hash);
                     this.logger.LogDebug("Added transaction ID '{0}' to inventory list.", hash);
                 }
             }
 
             this.logger.LogDebug("Sending transaction inventory to peer '{0}'.", peer.RemoteSocketEndpoint);
-            await this.SendAsTxInventoryAsync(peer, transactionsToSend);
+            await this.SendAsTxInventoryAsync(peer, transactionsToSend).ConfigureAwait(false);
             this.LastMempoolReq = this.mempoolManager.DateTimeProvider.GetTime();
         }
 
@@ -303,7 +311,13 @@ private async Task ProcessInvAsync(INetworkPeer peer, InvPayload invPayload)
             {
                 foreach (var inv in inventoryTxs)
                 {
-                    this.filterInventoryKnown.Add(inv.Hash);
+                    // It is unlikely that the transaction will be in the to-send hashmap, but there is no harm attempting to remove it anyway.
+                    this.inventoryTxToSend.Remove(inv.Hash);
+
+                    // At this point we have no idea whether the proffered hashes exist or are spurious. So we have to rely on the bounded LRU hashmap implementation
+                    // to control the maximum number of extant hashes that we track for this peer. It isn't really worth trying to evict mined transactions from this
+                    // hashmap.
+                    this.filterInventoryKnown.AddOrUpdate(inv.Hash);
                 }
             }
 
@@ -344,18 +358,22 @@ private async Task ProcessGetDataAsync(INetworkPeer peer, GetDataPayload getData
 
             foreach (InventoryVector item in getDataPayload.Inventory.Where(inv => inv.Type.HasFlag(InventoryType.MSG_TX)))
             {
-                // TODO: check if we need to add support for "not found"
+                if (!peer.IsConnected)
+                    continue;
 
                 TxMempoolInfo trxInfo = await this.mempoolManager.InfoAsync(item.Hash).ConfigureAwait(false);
 
-                if (trxInfo != null)
+                if (trxInfo == null)
                 {
-                    if (peer.IsConnected)
-                    {
-                        this.logger.LogDebug("Sending transaction '{0}' to peer '{1}'.", item.Hash, peer.RemoteSocketEndpoint);
-                        await peer.SendMessageAsync(new TxPayload(trxInfo.Trx.WithOptions(peer.SupportedTransactionOptions, this.network.Consensus.ConsensusFactory))).ConfigureAwait(false);
-                    }
+                    // https://btcinformation.org/en/developer-reference#notfound
+                    // https://github.com/bitcoin/bitcoin/pull/2192
+                    await peer.SendMessageAsync(new NotFoundPayload(InventoryType.MSG_TX, item.Hash)).ConfigureAwait(false);
+
+                    continue;
                 }
+
+                this.logger.LogDebug("Sending transaction '{0}' to peer '{1}'.", item.Hash, peer.RemoteSocketEndpoint);
+                await peer.SendMessageAsync(new TxPayload(trxInfo.Trx.WithOptions(peer.SupportedTransactionOptions, this.network.Consensus.ConsensusFactory))).ConfigureAwait(false);
             }
         }
 
@@ -381,14 +399,14 @@ private async Task ProcessTxPayloadAsync(INetworkPeer peer, TxPayload transactio
             // add to local filter
             lock (this.lockObject)
             {
-                this.filterInventoryKnown.Add(trxHash);
+                this.filterInventoryKnown.AddOrUpdate(trxHash);
             }
             this.logger.LogDebug("Added transaction ID '{0}' to known inventory filter.", trxHash);
 
             var state = new MempoolValidationState(true);
-            if (!await this.orphans.AlreadyHaveAsync(trxHash) && await this.validator.AcceptToMemoryPool(state, trx))
+            if (!await this.orphans.AlreadyHaveAsync(trxHash).ConfigureAwait(false) && await this.validator.AcceptToMemoryPool(state, trx).ConfigureAwait(false))
             {
-                await this.validator.SanityCheck();
+                await this.validator.SanityCheck().ConfigureAwait(false);
                 this.RelayTransaction(trxHash);
 
                 this.signals.Publish(new TransactionReceived(trx));
@@ -398,7 +416,7 @@ private async Task ProcessTxPayloadAsync(INetworkPeer peer, TxPayload transactio
 
                 this.logger.LogDebug("Transaction ID '{0}' accepted to memory pool from peer '{1}' (poolsz {2} txn, {3} kb).", trxHash, peer.RemoteSocketEndpoint, mmsize, memdyn / 1000);
 
-                await this.orphans.ProcessesOrphansAsync(this, trx);
+                await this.orphans.ProcessesOrphansAsync(this, trx).ConfigureAwait(false);
             }
             else if (state.MissingInputs)
             {
@@ -470,7 +488,10 @@ private void AddTransactionToSend(uint256 hash)
             lock (this.lockObject)
             {
                 if (!this.filterInventoryKnown.Contains(hash))
-                {
+                {                    
+                    // We do not need to bound this in the same way as the 'known' map,
+                    // because transactions are only sent/relayed when they are considered valid,
+                    // plus this map is constantly shrinking as txes are sent.
                     this.inventoryTxToSend.Add(hash);
                 }
             }
@@ -569,7 +590,7 @@ public async Task SendTrickleAsync()
             foreach (uint256 hash in findInMempool)
             {
                 // Not in the mempool anymore? don't bother sending it.
-                TxMempoolInfo txInfo = await this.mempoolManager.InfoAsync(hash);
+                TxMempoolInfo txInfo = await this.mempoolManager.InfoAsync(hash).ConfigureAwait(false);
                 if (txInfo == null)
                 {
                     this.logger.LogDebug("Transaction ID '{0}' not added to inventory list, no longer in mempool.", hash);
diff --git a/src/Tests/Blockcore.Tests/Utilities/LruHashSetTest.cs b/src/Tests/Blockcore.Tests/Utilities/LruHashSetTest.cs
new file mode 100644
index 000000000..81afd546d
--- /dev/null
+++ b/src/Tests/Blockcore.Tests/Utilities/LruHashSetTest.cs
@@ -0,0 +1,110 @@
+using Blockcore.Utilities;
+using Xunit;
+
+namespace Blockcore.Tests.Utilities
+{
+    public class LruHashSetTests
+    {
+        [Fact]
+        public void AddOrUpdate_AddsItemToSet()
+        {
+            // Arrange
+            var set = new LruHashSet<int>(maxSize: 2);
+
+            // Act
+            set.AddOrUpdate(1);
+
+            // Assert
+            Assert.True(set.Contains(1));
+        }
+
+        [Fact]
+        public void AddOrUpdate_UpdatesExistingItem()
+        {
+            // Arrange
+            var set = new LruHashSet<int>(maxSize: 2);
+            set.AddOrUpdate(1);
+
+            // Act
+            set.AddOrUpdate(1);
+
+            // Assert
+            Assert.True(set.Contains(1));
+        }
+
+        [Fact]
+        public void AddOrUpdate_RemovesOldestItemWhenMaxSizeExceeded()
+        {
+            // Arrange
+            var set = new LruHashSet<int>(maxSize: 2);
+            set.AddOrUpdate(1);
+            set.AddOrUpdate(2);
+
+            // Act
+            set.AddOrUpdate(3);
+
+            // Assert
+            Assert.False(set.Contains(1));
+            Assert.True(set.Contains(2));
+            Assert.True(set.Contains(3));
+        }
+
+        [Fact]
+        public void Clear_RemovesAllItemsFromSet()
+        {
+            // Arrange
+            var set = new LruHashSet<int>(maxSize: 2);
+            set.AddOrUpdate(1);
+            set.AddOrUpdate(2);
+
+            // Act
+            set.Clear();
+
+            // Assert
+            Assert.False(set.Contains(1));
+            Assert.False(set.Contains(2));
+        }
+
+        [Fact]
+        public void Contains_ReturnsTrueIfItemInSet()
+        {
+            // Arrange
+            var set = new LruHashSet<int>(maxSize: 2);
+            set.AddOrUpdate(1);
+
+            // Act
+            var contains = set.Contains(1);
+
+            // Assert
+            Assert.True(contains);
+        }
+
+        [Fact]
+        public void Contains_ReturnsFalseIfItemNotInSet()
+        {
+            // Arrange
+            var set = new LruHashSet<int>(maxSize: 2);
+            set.AddOrUpdate(1);
+
+            // Act
+            var contains = set.Contains(2);
+
+            // Assert
+            Assert.False(contains);
+        }
+
+        [Fact]
+        public void Remove_RemovesItemFromSet()
+        {
+            // Arrange
+            var set = new LruHashSet<int>(maxSize: 2);
+            set.AddOrUpdate(1);
+
+            // Act
+            set.Remove(1);
+
+            // Assert
+            Assert.False(set.Contains(1));
+        }
+    }
+}