public String getFlagStr(){
        if (this.getFlag()==1) return "SYN";
        if (this.getFlag()==2) return "FIN";
        if (this.getFlag()==3) return "PSH";
        if (this.getFlag()==4) return "ACK";
        if (this.getFlag()==5) return "SYN_ACK";
        return "";
    }