Bacnet MSTP帧头CRC校验失败问题排查求助

问题场景 #

将BACnet源码编译为动态库，编写客户端程序调用该库周期性发送请求读取服务器属性。程序初始运行正常，但数小时后开始出现间歇性属性读取失败。

调试捕获日志 #

开启Makefile调试选项后，捕获到如下错误日志：

MSTP: Rx Header: BadCRC [01]
MSTP: Rx Data: SilenceTimer 96ms > 95ms
MSTP: Rx Header: BadCRC [01]
MSTP: Rx Header: BadCRC [55]
MSTP: Rx Header: BadCRC [27]
MSTP: Rx Header: BadCRC [3B]
MSTP: Rx Header: BadCRC [3E]
MSTP: Rx Header: BadCRC [4B]
MSTP: Rx Header: BadCRC [4D]
MSTP: Rx Header: BadCRC [FF]
MSTP: Rx Header: BadCRC [5E]
MSTP: Rx Header: SilenceTimer 96 > 95
MSTP: Rx Header: BadCRC [6E]
MSTP: Rx Header: BadCRC [74]
MSTP: Rx Header: BadCRC [FF]
MSTP: Rx Header: BadCRC [FF]
MSTP: Rx Header: BadCRC [03]
MSTP: Rx Header: BadCRC [FF]
MSTP: Rx Header: BadCRC [55]
MSTP: Rx Header: BadCRC [55]
MSTP: Rx Header: BadCRC [55]
MSTP: Rx Header: BadCRC [FF]
MSTP: Rx Header: BadCRC [55]
MSTP: Rx Header: BadCRC [FF]
MSTP: Rx Header: BadCRC [00]
MSTP: Rx Header: BadCRC [45]
MSTP: Rx Header: BadCRC [FF]
MSTP: Rx Header: BadCRC [55]
MSTP: Rx Header: BadCRC [55]
MSTP: Rx Header: BadCRC [55]
MSTP: Rx Header: BadCRC [FF]
MSTP: Rx Header: BadCRC [00]
MSTP: Rx Header: BadCRC [55]
MSTP: Rx Header: BadCRC [FF]

源码定位与分析 #

报错为帧头CRC校验错误，触发后该帧会被标记为非法，程序切换到MSTP_RECEIVE_STATE_IDLE状态寻找下一帧起始码。理论上单帧校验错误不应持续影响后续帧，但实际结果与预期不符。

出错代码位于bacnet-stack-1.0.0/ports/linux/dlmstp.c的MSTP_Receive_Frame_FSM函数中，相关片段：

case MSTP_RECEIVE_STATE_HEADER:
            /* Timeout */
            if (mstp_port->SilenceTimer((void *)mstp_port) > Tframe_abort) {
                /* indicate that an error has occurred during the reception of a
                 * frame */
                mstp_port->ReceivedInvalidFrame = true;
                /* wait for the start of a frame. */
                mstp_port->receive_state = MSTP_RECEIVE_STATE_IDLE;
                debug_printf("MSTP: Rx Header: SilenceTimer %u > %d\n",
                    (unsigned)mstp_port->SilenceTimer((void *)mstp_port),
                    Tframe_abort);
            }

/* don't wait for next state - do it here */
                    if (mstp_port->HeaderCRC != 0x55) {
                        /* BadCRC */
                        /* indicate that an error has occurred during
                           the reception of a frame */
                        mstp_port->ReceivedInvalidFrame = true;
                        debug_printf("MSTP: Rx Header: BadCRC [%02X], Tframe_abort = %d\n",
                            mstp_port->DataRegister, Tframe_abort);
                        /* wait for the start of the next frame. */
                        mstp_port->receive_state = MSTP_RECEIVE_STATE_IDLE;
                    }

抓包验证 #

故障期间通过Wireshark抓包，发现报文正常且CRC校验无错误，推测问题出在程序接收报文后的自身处理环节。

诉求 #

寻求该问题的根因分析及解决思路。

内容的提问来源于stack exchange，提问作者PYHAO