code: public long getProtocolVersion(String protocol, long clientVersion
        ) throws IOException {
      if (protocol.equals(InterDatanodeProtocol.class.getName())) {
        return InterDatanodeProtocol.versionID;
      } else if (protocol.equals(ClientDatanodeProtocol.class.getName())) {
        checkVersion(protocol, clientVersion, ClientDatanodeProtocol.versionID);
        return ClientDatanodeProtocol.versionID;
      }
      throw new IOException("Unknown protocol to " + getClass().getSimpleName()
          + ": " + protocol);
    }
docs: {@inheritDoc}