public String pduTypeToString() {
       switch (pdu_type) {
        case SNMP_MSG_GET:
            return "SNMP-Get";
        case SNMP_MSG_GETNEXT:
            return "SNMP-GetNext";
        case SNMP_MSG_RESPONSE:
            return "SNMP-Response";
        case SNMP_MSG_SET:
            return "SNMP-Set";
        case SNMP_MSG_TRAP:
            return "SNMP-TrapV1";
        case SNMP_MSG_INFORM:
            return "SNMP-Inform";
        case SNMP_MSG_TRAP2:
            return "SNMP-TrapV2";
        case SNMP_MSG_REPORT:
            return "SNMP-Report";
        case SNMP_MSG_GETBULK:
            return "SNMP-GetBulk";
        }
        return "Unknown " + (pdu_type & 0xFF) + "'";
    }