Serial PPPoS会话无法响应Ping请求的技术求助

Let's break down your issue and walk through targeted troubleshooting steps and fixes based on the details you shared.

Problem Background #

I'm working with an STM32F7 MCU connected to a Telit GL865 GSM module over UART (using STM32CubeMX + Keil for development). Here's what's working and what's not:

• ✅ Using AT commands via the GSM module's internal stack, I can successfully get an IP address and ping external hosts.
• ❌ After starting an LwIP PPPoS session, I can't ping the assigned PPP IP (5.26.61.173). The LwIP stack assigns a gateway of 192.168.202.0 and subnet mask 255.255.255.255 (a special value auto-assigned by LwIP).
• Additional observations:
  • The MCU and GSM module send periodic "~y " data to each other after the session is established.
  • Ping attempts trigger communication between the two devices, but no response is returned.
  • I'm using interrupt-driven UART reception: when no new data is received for 10ms, the buffer is passed to pppos_input_tcpip.

Relevant Code Snippet #

/* Includes ------------------------------------------------------------------*/ 
#include "FreeRTOS.h"
#include "task.h"
#include "cmsis_os.h"
#include <stdio.h>
#include <stdarg.h>
#include <time.h>
#include "string.h"
#include "main.h"
#include "stm32f7xx_hal.h"
#include "cmsis_os.h"
#include "lwip.h"
#include "usart.h"
#include "gpio.h"
#include <stdio.h>
#include "string.h"
#include <time.h>
#include "lwip/opt.h"
#include "lwip/sys.h"
#include "lwip/timeouts.h"
#include "lwip/debug.h"
#include "lwip/stats.h"
#include "lwip/init.h"
#include "lwip/tcpip.h"
#include "lwip/netif.h"
#include "lwip/api.h"
#include "lwip/tcp.h"
#include "lwip/udp.h"
#include "lwip/dns.h"
#include "lwip/dhcp.h"
#include "lwip/autoip.h"
#include "lwip/etharp.h"
#include "netif/ethernet.h"
#include "lwip/apps/netbiosns.h"
#include "lwip/apps/httpd.h"
#include "lwip/sio.h"
#include "pppapi.h"
#include <lwip/sockets.h>
#include "usart.h"
#include "test.h"

#define RX_SIZE 4096 

/* USER CODE END Includes */ 

/* Variables -----------------------------------------------------------------*/ 
osThreadId defaultTaskHandle; 

/* USER CODE BEGIN Variables */ 
uint8_t lcpBuffer[RX_SIZE];
uint8_t lcpBufferIndex, quantityOfPackages = 0;
uint8_t pppBuffer[4096];
uint32_t sysTickCounter = 0;
uint8_t sendData[50] = " \n";
uint8_t recData, recDataOlder, recDataOlder2,recDataOlder3,recDataOlder4,recDataOlder5,recDataOlder6;
uint8_t receivedValue,connected,called = 0;
uint32_t recIndex = 0;
uint8_t recBuffer[RX_SIZE];
uint8_t sent, send, connect, connectionStatus, endOfLcpNegotiation, NCPNegotiation = 0;
ppp_pcb *ppp;
struct netif ppp_netif;
u8_t sio_idx = 0;
sio_fd_t ppp_sio;
uint8_t ATconnected,lcpCame,length = 0;
uint8_t lcpStartIndex, lcpStopIndex, lcpStartOld, lcpStart, lcpStop, dataReceived, timeOut, fillThePPPBuffer, gsmInitEnd, sendLCP, sendNCP, socketOpened, socketBind, socketOpen = 0;
uint32_t pppBufferIndex = 0;
ip4_addr_t addr;
int socketStatus;
int socket_fd,accept_fd;
int addr_size,sent_data;
char data_buffer[80];
struct sockaddr_in sa,ra,isa;
uint32_t runTimeCnt = 0;

void StartDefaultTask(void const * argument);
extern void MX_LWIP_Init(void);
void MX_FREERTOS_Init(void);

//PPP function
static void status_cb(ppp_pcb *pcb, int err_code, void *ctx) {
    struct netif *pppif = ppp_netif(pcb);
    called = 1;
    switch(err_code) {
        case PPPERR_NONE: {
            connectionStatus = 1;
            printf("status_cb: Connected\n");
            printf(" our_ipaddr = %s\r\n", ipaddr_ntoa(&pppif->ip_addr));
            printf(" gatewayaddr = %s\r\n", ipaddr_ntoa(&pppif->gw));
            printf(" netmask = %s\r\n", ipaddr_ntoa(&pppif->netmask));
            break;
        }
        case PPPERR_PARAM: {
            connectionStatus = 2;
            printf("status_cb: Invalid parameter\r\n");
            break;
        }
        case PPPERR_OPEN: {
            connectionStatus = 3;
            printf("status_cb: Unable to open PPP session\r\n");
            break;
        }
        case PPPERR_DEVICE: {
            connectionStatus = 4;
            printf("status_cb: Invalid I/O device for PPP\r\n");
            break;
        }
        case PPPERR_ALLOC: {
            connectionStatus = 5;
            printf("status_cb: Unable to allocate resources\r\n");
            break;
        }
        case PPPERR_USER: {
            connectionStatus = 6;
            ppp_free(pcb);
            printf("status_cb: User interrupt\r\n");
            break;
        }
        case PPPERR_CONNECT: {
            connectionStatus = 7;
            // Truncated in original code
            break;
        }
    }
}

Troubleshooting & Fixes #

1. Validate Subnet Mask & PPP Configuration #

The subnet mask 255.255.255.255 is normal for point-to-point PPP links, so that's not the issue. But double-check your lwipopts.h to ensure critical PPP flags are enabled:

#define PPP_SUPPORT                  1
#define PPPOS_SUPPORT                1
#define PPP_POINTTOPOINT             1
#define LWIP_ICMP                    1  // Required for ping
#define PPP_IPV4_SUPPORT             1
#define PPP_DEBUG                    LWIP_DBG_ON  // Enable debug logs to spot negotiation issues

2. Fix UART Data Handling #

Your 10ms timeout-based buffer submission can split or merge PPP frames, causing LwIP to fail parsing. PPP frames are wrapped in 0x7E flags—switch to frame-boundary detection instead:

void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart) {
    if (huart->Instance == USARTx) {
        static uint8_t in_ppp_frame = 0;
        static uint32_t frame_start_idx = 0;

        // Detect PPP frame boundaries
        if (recData == 0x7E) {
            if (in_ppp_frame) {
                // End of frame: pass complete buffer to LwIP
                uint32_t frame_len = recIndex - frame_start_idx + 1;
                if (frame_len > 2) {  // Skip empty frames
                    pppos_input_tcpip(ppp_sio, &recBuffer[frame_start_idx], frame_len);
                }
                in_ppp_frame = 0;
            } else {
                // Start of frame: reset buffer index
                in_ppp_frame = 1;
                frame_start_idx = recIndex;
            }
        }

        // Only store data if we're inside a valid PPP frame
        if (in_ppp_frame) {
            recBuffer[recIndex++] = recData;
            if (recIndex >= RX_SIZE) recIndex = 0;  // Prevent overflow
        }

        // Restart interrupt reception
        HAL_UART_Receive_IT(&huart, &recData, 1);
    }
}

Also, filter out the periodic "~y " data—it's not part of PPP frames and will corrupt LwIP's input.

3. Verify Network Interface Setup #

Ensure your PPP netif is properly registered as the default route:

// After creating the PPP PCB
netif_add(&ppp_netif, NULL, NULL, NULL, NULL, pppos_init, tcpip_input);
netif_set_default(&ppp_netif);
netif_set_up(&ppp_netif);

4. Enable Hardware Flow Control #

Telit GL865 often requires RTS/CTS flow control for PPP to prevent data loss. Enable it in your STM32 UART configuration and set the module's AT command:

AT&K3  // Enable RTS/CTS hardware flow control

5. Debug PPP Negotiation #

Turn on LwIP's PPP debug logs (via PPP_DEBUG) and monitor the UART output. Look for:

• Successful LCP and NCP negotiation steps
• No errors like "frame check sequence failure" or "invalid protocol"
• Confirm that the module is forwarding ICMP ping responses back to the MCU

内容的提问来源于stack exchange，提问作者Sarp Engin Daltaban