Added HEIGHT_V3 message with the following additions:

- latest block's reference
- latest block's online accounts count
- latest blocks transaction count

These are then retained locally in PeerChainTipData (as optional values, which will be null if peer doesn't support HEIGHT_V3 yet).

src/main/java/org/qortal/controller/Controller.java

@@ -1197,6 +1197,10 @@ public class Controller extends Thread {
 				onNetworkHeightV2Message(peer, message);
 				break;
 
+			case HEIGHT_V3:
+				onNetworkHeightV3Message(peer, message);
+				break;
+
 			case GET_TRANSACTION:
 				TransactionImporter.getInstance().onNetworkGetTransactionMessage(peer, message);
 				break;
@@ -1537,6 +1541,27 @@ public class Controller extends Thread {
 		Synchronizer.getInstance().requestSync();
 	}
 
+	private void onNetworkHeightV3Message(Peer peer, Message message) {
+		HeightV3Message heightV3Message = (HeightV3Message) message;
+
+		if (!Settings.getInstance().isLite()) {
+			// If peer is inbound and we've not updated their height
+			// then this is probably their initial HEIGHT_V3 message
+			// so they need a corresponding HEIGHT_V3 message from us
+			if (!peer.isOutbound() && (peer.getChainTipData() == null || peer.getChainTipData().getLastHeight() == null))
+				peer.sendMessage(Network.getInstance().buildHeightMessage(peer, getChainTip()));
+		}
+
+		// Update peer chain tip data
+		PeerChainTipData newChainTipData = new PeerChainTipData(heightV3Message.getHeight(), heightV3Message.getSignature(),
+				heightV3Message.getReference(), heightV3Message.getTimestamp(), heightV3Message.getMinterPublicKey(),
+				heightV3Message.getOnlineAccountsCount(), heightV3Message.getTransactionCount());
+		peer.setChainTipData(newChainTipData);
+
+		// Potentially synchronize
+		Synchronizer.getInstance().requestSync();
+	}
+
 	private void onNetworkGetAccountMessage(Peer peer, Message message) {
 		GetAccountMessage getAccountMessage = (GetAccountMessage) message;
 		String address = getAccountMessage.getAddress();

src/main/java/org/qortal/data/network/PeerChainTipData.java

@@ -6,19 +6,34 @@ public class PeerChainTipData {
 	private Integer lastHeight;
 	/** Latest block signature as reported by peer. */
 	private byte[] lastBlockSignature;
+	/** Latest block reference as reported by peer. */
+	private byte[] lastBlockReference;
 	/** Latest block timestamp as reported by peer. */
 	private Long lastBlockTimestamp;
 	/** Latest block minter public key as reported by peer. */
 	private byte[] lastBlockMinter;
+	/** Latest block's online accounts count as reported by peer. */
+	private Integer lastBlockOnlineAccountsCount;
+	/** Latest block's transaction count as reported by peer. */
+	private Integer lastBlockTransactionCount;
 
-	public PeerChainTipData(Integer lastHeight, byte[] lastBlockSignature, Long lastBlockTimestamp, byte[] lastBlockMinter) {
+	public PeerChainTipData(Integer lastHeight, byte[] lastBlockSignature, byte[] lastBlockReference,
+							Long lastBlockTimestamp, byte[] lastBlockMinter, Integer lastBlockOnlineAccountsCount,
+							Integer lastBlockTransactionCount) {
 		this.lastHeight = lastHeight;
 		this.lastBlockSignature = lastBlockSignature;
+		this.lastBlockReference = lastBlockReference;
 		this.lastBlockTimestamp = lastBlockTimestamp;
 		this.lastBlockMinter = lastBlockMinter;
+		this.lastBlockOnlineAccountsCount = lastBlockOnlineAccountsCount;
+		this.lastBlockTransactionCount = lastBlockTransactionCount;
 	}
 
-	public Integer getLastHeight() {
+	public PeerChainTipData(Integer lastHeight, byte[] lastBlockSignature, Long lastBlockTimestamp, byte[] lastBlockMinter) {
+		this(lastHeight, lastBlockSignature, null, lastBlockTimestamp, lastBlockMinter, null, null);
+	}
+
+		public Integer getLastHeight() {
 		return this.lastHeight;
 	}
 
@@ -26,6 +41,10 @@ public class PeerChainTipData {
 		return this.lastBlockSignature;
 	}
 
+	public byte[] getLastBlockReference() {
+		return this.lastBlockReference;
+	}
+
 	public Long getLastBlockTimestamp() {
 		return this.lastBlockTimestamp;
 	}
@@ -34,4 +53,12 @@ public class PeerChainTipData {
 		return this.lastBlockMinter;
 	}
 
+	public Integer getLastBlockOnlineAccountsCount() {
+		return this.lastBlockOnlineAccountsCount;
+	}
+
+	public Integer getLastBlockTransactionCount() {
+		return this.lastBlockTransactionCount;
+	}
+
 }

src/main/java/org/qortal/network/Network.java

@@ -1163,11 +1163,27 @@ public class Network {
     }
 
     public Message buildHeightMessage(Peer peer, BlockData blockData) {
+        final long HEIGHT_V3_PEER_VERSION = 0x0300030003L;
+
+        if (peer.getPeersVersion() >= HEIGHT_V3_PEER_VERSION) {
+            return Network.getInstance().buildHeightV3Message(peer, blockData);
+        } else {
+            return Network.getInstance().buildHeightV2Message(peer, blockData);
+        }
+    }
+    private Message buildHeightV2Message(Peer peer, BlockData blockData) {
         // HEIGHT_V2 contains way more useful info
         return new HeightV2Message(blockData.getHeight(), blockData.getSignature(),
                 blockData.getTimestamp(), blockData.getMinterPublicKey());
     }
 
+    private Message buildHeightV3Message(Peer peer, BlockData blockData) {
+        // HEIGHT_V3 contains even more useful info
+        return new HeightV3Message(blockData.getHeight(), blockData.getSignature(), blockData.getReference(),
+                blockData.getTimestamp(), blockData.getMinterPublicKey(), blockData.getOnlineAccountsCount(),
+                blockData.getTransactionCount());
+    }
+
     public Message buildNewTransactionMessage(Peer peer, TransactionData transactionData) {
         // In V2 we send out transaction signature only and peers can decide whether to request the full transaction
         return new TransactionSignaturesMessage(Collections.singletonList(transactionData.getSignature()));

src/main/java/org/qortal/network/message/HeightV3Message.java

@@ -0,0 +1,113 @@
+package org.qortal.network.message;
+
+import com.google.common.primitives.Ints;
+import com.google.common.primitives.Longs;
+import org.qortal.transform.Transformer;
+import org.qortal.transform.block.BlockTransformer;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+
+public class HeightV3Message extends Message {
+
+	private int height;
+	private byte[] signature;
+	private byte[] reference;
+	private long timestamp;
+	private byte[] minterPublicKey;
+	private int onlineAccountsCount;
+	private int transactionCount;
+
+	public HeightV3Message(int height, byte[] signature, byte[] reference, long timestamp, byte[] minterPublicKey,
+						   int onlineAccountsCount, int transactionCount) {
+		super(MessageType.HEIGHT_V3);
+
+		ByteArrayOutputStream bytes = new ByteArrayOutputStream();
+
+		try {
+			bytes.write(Ints.toByteArray(height));
+
+			bytes.write(signature);
+
+			bytes.write(reference);
+
+			bytes.write(Longs.toByteArray(timestamp));
+
+			bytes.write(minterPublicKey);
+
+			bytes.write(Ints.toByteArray(onlineAccountsCount));
+
+			bytes.write(Ints.toByteArray(transactionCount));
+		} catch (IOException e) {
+			throw new AssertionError("IOException shouldn't occur with ByteArrayOutputStream");
+		}
+
+		this.dataBytes = bytes.toByteArray();
+		this.checksumBytes = Message.generateChecksum(this.dataBytes);
+	}
+
+	private HeightV3Message(int id, int height, byte[] signature, byte[] reference, long timestamp, byte[] minterPublicKey,
+							int onlineAccountsCount, int transactionCount) {
+		super(id, MessageType.HEIGHT_V3);
+
+		this.height = height;
+		this.signature = signature;
+		this.reference = reference;
+		this.timestamp = timestamp;
+		this.minterPublicKey = minterPublicKey;
+		this.onlineAccountsCount = onlineAccountsCount;
+		this.transactionCount = transactionCount;
+	}
+
+	public int getHeight() {
+		return this.height;
+	}
+
+	public byte[] getSignature() {
+		return this.signature;
+	}
+
+	public byte[] getReference() {
+		return this.reference;
+	}
+
+	public long getTimestamp() {
+		return this.timestamp;
+	}
+
+	public byte[] getMinterPublicKey() {
+		return this.minterPublicKey;
+	}
+
+	public int getOnlineAccountsCount() {
+		return this.onlineAccountsCount;
+	}
+
+	public int getTransactionCount() {
+		return this.transactionCount;
+	}
+
+	public static Message fromByteBuffer(int id, ByteBuffer bytes) {
+		int height = bytes.getInt();
+
+		byte[] signature = new byte[BlockTransformer.BLOCK_SIGNATURE_LENGTH];
+		bytes.get(signature);
+
+		byte[] reference = new byte[BlockTransformer.BLOCK_SIGNATURE_LENGTH];
+		bytes.get(reference);
+
+		long timestamp = bytes.getLong();
+
+		byte[] minterPublicKey = new byte[Transformer.PUBLIC_KEY_LENGTH];
+		bytes.get(minterPublicKey);
+
+		int onlineAccountsCount = bytes.getInt();
+
+		int transactionCount = bytes.getInt();
+
+		return new HeightV3Message(id, height, signature, reference, timestamp, minterPublicKey, onlineAccountsCount,
+				transactionCount);
+	}
+
+}

src/main/java/org/qortal/network/message/MessageType.java

@@ -18,6 +18,7 @@ public enum MessageType {
     HEIGHT_V2(10, HeightV2Message::fromByteBuffer),
     PING(11, PingMessage::fromByteBuffer),
     PONG(12, PongMessage::fromByteBuffer),
+    HEIGHT_V3(13, HeightV3Message::fromByteBuffer),
 
     // Requesting data
     PEERS_V2(20, PeersV2Message::fromByteBuffer),