code: public static <T extends VersionedProtocol> ProtocolProxy<T> getProtocolProxy(
        Class<T> protocol,
        long clientVersion, InetSocketAddress addr, Configuration conf)
        throws IOException {
  
      return getProtocolProxy(protocol, clientVersion, addr, conf, NetUtils
          .getDefaultSocketFactory(conf));
    }
docs: Construct a client-side proxy object with the default SocketFactory
  
  @param protocol
  @param clientVersion
  @param addr
  @param conf
  @return a proxy instance
  @throws IOException