public GossipDigestAckMessage deserialize(DataInputStream dis) throws IOException {
   List<GossipDigest> gDigestList = GossipDigestSerializationHelper.deserialize(dis);
   dis.readBoolean(); // 0.6 compatibility
   Map<InetAddress, EndpointState> epStateMap = EndpointStatesSerializationHelper.deserialize(dis);
   return new GossipDigestAckMessage(gDigestList, epStateMap);
 }