diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -133,15 +133,16 @@ class Server(NetNode):
 		if not self.isLocked(): self._lock()
 		log.info("received data block #{0}: {1}...{2}".format(jsonData["index"],binData[:5],binData[-5:]))
 
-		i=jsonData["index"]
-		if self._lastIndex+1!=i:
-			self._dataFile.seek(i*self.BLOCK_SIZE)
-		self._dataFile.write(binData)
-		self._lastIndex=i
-		if self._treeFile:
-			self._newLeaves[i+self._tree.leafStart]=hashBlock(binData)
+		indices=jsonData["index"]
+		for (i,k) in enumerate(indices):
+			if self._lastIndex+1!=k:
+				self._dataFile.seek(k*self.BLOCK_SIZE)
+			self._dataFile.write(binData[i*self.BLOCK_SIZE:(i+1)*self.BLOCK_SIZE])
+			self._lastIndex=k
+			if self._treeFile:
+				self._newLeaves[k+self._tree.leafStart]=hashBlock(binData)
 
-		return ({"command": "ack", "index": i},)
+		return ({"command": "ack", "index": indices},)
 
 	def _finalize(self):
 		log.info("closing session...")