code: private boolean readFlow(ReadStream is, int length, int streamId)
      throws IOException
    {
      if (length != 4) {
        error("Invalid window update length {0}", length);
        return false;
      }
      
      int credit = BitsUtil.readInt(is);
      
      if (streamId == 0) {
        _conn.channelZero().addSendCredit(credit);
        return true;
      }
      
      ChannelHttp2 channel = _conn.getChannel(streamId);
      
      if (channel == null) {
        return true;
      }
      
      channel.getOutChannel().addSendCredit(_conn.outHttp(), credit);
      
      return true;
    }
docs: window-update