feat(modem): Add support for multiple connection in AT based example

components/esp_modem/examples/modem_tcp_client/README.md

@@ -22,3 +22,18 @@ To enable this mode, please set `EXAMPLE_CUSTOM_TCP_TRANSPORT=y`
 This configuration could be used with any network library, which is connecting to a localhost endpoint instead of remote one. This example creates a localhost listener which basically mimics the remote endpoint by forwarding the traffic between the library and the TCP/socket layer of the modem (which is already secure if the TLS is used in the network library)
 
 ![with localhost listener](at_client_localhost.png)
+
+### Multi-connection support
+
+This example supports opening multiple TCP connections concurrently when the modem firmware allows it.
+
+- ESP-AT: Multi-connection mode is enabled via `AT+CIPMUX=1`. The example assigns a unique link ID per DCE instance and includes the link ID in `CIPSTART/CIPSEND/CIPRECVDATA` commands.
+- BG96/SIM7600: The example uses module-specific multi-connection syntax (for example `QIOPEN/CIPOPEN` with a connection ID) and tracks link IDs internally.
+
+How it works:
+- The `sock_dce` layer creates multiple DCE instances over a shared DTE. A lightweight mutex coordinates access to the UART so only one DCE issues AT commands at a time.
+- Asynchronous URCs (for example `+IPD`, `+QIURC`, `+CIPRXGET: 1,<cid>`) wake the corresponding DCE which then performs receive operations for its link.
+
+Usage:
+- `app_main` starts two DCE tasks to demonstrate concurrent connections. Adjust the number of DCE instances as needed.
+- For ESP-AT, ensure your firmware supports `CIPMUX=1` and passive receive (`CIPRECVTYPE`).

components/esp_modem/examples/modem_tcp_client/main/Kconfig.projbuild

@@ -44,6 +44,18 @@ menu "Example Configuration"
         help
             Set APN (Access Point Name), a logical name to choose data network
 
+    config EXAMPLE_USE_TLS
+        bool "Use TLS for MQTT broker"
+        default n
+        help
+            Enable TLS for connection to the MQTT broker.
+
+    config EXAMPLE_BROKER_HOST
+        string "MQTT broker host"
+        default "test.mosquitto.org"
+        help
+            Hostname or IP address of the MQTT broker.
+
     menu "UART Configuration"
         config EXAMPLE_MODEM_UART_TX_PIN
             int "TXD Pin Number"

components/esp_modem/examples/modem_tcp_client/main/command/sock_dce.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#include <algorithm>
 #include <charconv>
 #include <sys/socket.h>
 #include "esp_vfs.h"
@@ -14,6 +15,29 @@
 namespace sock_dce {
 
 constexpr auto const *TAG = "sock_dce";
+constexpr auto WAIT_TO_IDLE_TIMEOUT = 5000;
+
+// Definition of the static member variables
+std::vector<DCE *> DCE::dce_list{};
+bool DCE::network_init = false;
+int Responder::s_link_id = 0;
+SemaphoreHandle_t Responder::s_dte_mutex{};
+
+// Constructor - add this DCE instance to the static list
+DCE::DCE(std::shared_ptr<esp_modem::DTE> dte_arg, const esp_modem_dce_config *config)
+    : Module(std::move(dte_arg), config)
+{
+    dce_list.push_back(this);
+}
+
+// Destructor - remove this DCE instance from the static list
+DCE::~DCE()
+{
+    auto it = std::find(dce_list.begin(), dce_list.end(), this);
+    if (it != dce_list.end()) {
+        dce_list.erase(it);
+    }
+}
 
 
 bool DCE::perform_sock()
@@ -61,13 +85,26 @@ bool DCE::perform_sock()
 
 void DCE::perform_at(uint8_t *data, size_t len)
 {
-    ESP_LOG_BUFFER_HEXDUMP(TAG, data, len, ESP_LOG_VERBOSE);
+    if (state != status::RECEIVING) {
+        std::string_view resp_sv((char *)data, len);
+        at.check_urc(state, resp_sv);
+        if (state == status::IDLE) {
+            return;
+        }
+    }
+    // Trace incoming AT bytes when handling a response; use DEBUG level
+    ESP_LOG_BUFFER_HEXDUMP(TAG, data, len, ESP_LOG_DEBUG);
     switch (at.process_data(state, data, len)) {
     case Responder::ret::OK:
+        // Release DTE access for this link after processing data
+        ESP_LOGD(TAG, "GIVE data %d", at.link_id);
+        xSemaphoreGive(at.s_dte_mutex);
         state = status::IDLE;
         signal.set(IDLE);
         return;
     case Responder::ret::FAIL:
+        ESP_LOGD(TAG, "GIVE data %d", at.link_id);
+        xSemaphoreGive(at.s_dte_mutex);
         state = status::FAILED;
         signal.set(IDLE);
         return;
@@ -82,10 +119,14 @@ void DCE::perform_at(uint8_t *data, size_t len)
     std::string_view response((char *)data, len);
     switch (at.check_async_replies(state, response)) {
     case Responder::ret::OK:
+        ESP_LOGD(TAG, "GIVE command %d", at.link_id);
+        xSemaphoreGive(at.s_dte_mutex);
         state = status::IDLE;
         signal.set(IDLE);
         return;
     case Responder::ret::FAIL:
+        ESP_LOGD(TAG, "GIVE command %d", at.link_id);
+        xSemaphoreGive(at.s_dte_mutex);
         state = status::FAILED;
         signal.set(IDLE);
         return;
@@ -121,7 +162,7 @@ bool DCE::at_to_sock()
     uint64_t data;
     read(data_ready_fd, &data, sizeof(data));
     ESP_LOGD(TAG, "select read: modem data available %" PRIu64, data);
-    if (!signal.wait(IDLE, 1000)) {
+    if (!signal.wait(IDLE, WAIT_TO_IDLE_TIMEOUT)) {
         ESP_LOGE(TAG, "Failed to get idle");
         close_sock();
         return false;
@@ -131,6 +172,10 @@ bool DCE::at_to_sock()
         close_sock();
         return false;
     }
+    // Take DTE mutex before issuing receive on this link
+    ESP_LOGD(TAG, "TAKE RECV %d", at.link_id);
+    xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
+    ESP_LOGD(TAG, "TAKEN RECV %d", at.link_id);
     state = status::RECEIVING;
     at.start_receiving(at.get_buf_len());
     return true;
@@ -139,7 +184,7 @@ bool DCE::at_to_sock()
 bool DCE::sock_to_at()
 {
     ESP_LOGD(TAG,  "socket read: data available");
-    if (!signal.wait(IDLE, 1000)) {
+    if (!signal.wait(IDLE, WAIT_TO_IDLE_TIMEOUT)) {
         ESP_LOGE(TAG,  "Failed to get idle");
         close_sock();
         return false;
@@ -149,6 +194,10 @@ bool DCE::sock_to_at()
         close_sock();
         return false;
     }
+    // Take DTE mutex before issuing send on this link
+    ESP_LOGD(TAG, "TAKE SEND %d", at.link_id);
+    xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
+    ESP_LOGD(TAG, "TAKEN SEND %d", at.link_id);
     state = status::SENDING;
     int len = ::recv(sock, at.get_buf(), at.get_buf_len(), 0);
     if (len < 0) {
@@ -201,7 +250,7 @@ void DCE::start_listening(int port)
     }
     int opt = 1;
     setsockopt(listen_sock, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
-    ESP_LOGI(TAG, "Socket created");
+    ESP_LOGD(TAG, "Socket created");
     struct sockaddr_in addr = { };
     addr.sin_family = AF_INET;
     addr.sin_port = htons(port);
@@ -213,7 +262,7 @@ void DCE::start_listening(int port)
         ESP_LOGE(TAG, "Socket unable to bind: errno %d", errno);
         return;
     }
-    ESP_LOGI(TAG, "Socket bound, port %d", 1883);
+    ESP_LOGD(TAG, "Socket bound, port %d", 1883);
     err = listen(listen_sock, 1);
     if (err != 0) {
         ESP_LOGE(TAG, "Error occurred during listen: errno %d", errno);
@@ -224,12 +273,12 @@ void DCE::start_listening(int port)
 
 bool DCE::connect(std::string host, int port)
 {
-    dte->on_read(nullptr);
+    data_ready_fd = eventfd(0, EFD_SUPPORT_ISR);
-    tcp_close();
+    assert(data_ready_fd > 0);
-    dte->on_read([this](uint8_t *data, size_t len) {
+    // Take DTE mutex before starting connect for this link
-        this->perform_at(data, len);
+    ESP_LOGD(TAG, "TAKE CONNECT %d", at.link_id);
-        return esp_modem::command_result::TIMEOUT;
+    xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
-    });
+    ESP_LOGD(TAG, "TAKEN CONNECT %d", at.link_id);
     if (!at.start_connecting(host, port)) {
         ESP_LOGE(TAG, "Unable to start connecting");
         dte->on_read(nullptr);
@@ -241,12 +290,15 @@ bool DCE::connect(std::string host, int port)
 
 bool DCE::init()
 {
+    if (network_init) {
+        return true;
+    }
+    network_init = true;
+    Responder::s_dte_mutex = xSemaphoreCreateBinary();
+    xSemaphoreGive(at.s_dte_mutex);
     esp_vfs_eventfd_config_t config = ESP_VFS_EVENTD_CONFIG_DEFAULT();
     esp_vfs_eventfd_register(&config);
 
-    data_ready_fd = eventfd(0, EFD_SUPPORT_ISR);
-    assert(data_ready_fd > 0);
-
     dte->on_read(nullptr);
     const int retries = 5;
     int i = 0;
@@ -287,6 +339,10 @@ bool DCE::init()
         esp_modem::Task::Delay(5000);
     }
     ESP_LOGI(TAG, "Got IP %s", ip_addr.c_str());
+    dte->on_read([](uint8_t *data, size_t len) {
+        read_callback(data, len);
+        return esp_modem::command_result::TIMEOUT;
+    });
     return true;
 }
 

components/esp_modem/examples/modem_tcp_client/main/command/sock_dce.hpp

@@ -34,6 +34,7 @@ public:
         sock(s), data_ready_fd(ready_fd), dte(dte_arg) {}
     ret process_data(status state, uint8_t *data, size_t len);
     ret check_async_replies(status state, std::string_view &response);
+    ret check_urc(status state, std::string_view &response);
 
     void start_sending(size_t len);
     void start_receiving(size_t len);
@@ -63,13 +64,19 @@ public:
         return total_len;
     }
 
+    // Unique link identifier used to target multi-connection AT commands
+    int link_id{s_link_id++};
+    // Shared mutex guarding DTE access across concurrent DCE instances
+    static SemaphoreHandle_t s_dte_mutex;
 private:
+    static int s_link_id;
     static constexpr size_t buffer_size = 512;
 
     bool on_read(char *data, size_t len)
     {
 #ifndef CONFIG_EXAMPLE_CUSTOM_TCP_TRANSPORT
         ::send(sock, data, len, 0);
+        printf("sending %d\n", len);
 #else
         ::memcpy(&buffer[actual_read], data, len);
         actual_read += len;
@@ -101,6 +108,8 @@ private:
 class DCE : public Module {
     using Module::Module;
 public:
+    DCE(std::shared_ptr<esp_modem::DTE> dte_arg, const esp_modem_dce_config *config);
+    ~DCE();
 
     /**
      * @brief Opens network in AT command mode
@@ -163,6 +172,10 @@ public:
             return 0;
         }
         at.clear_offsets();
+        // Take DTE mutex before issuing receive on this link
+        ESP_LOGD("TAG", "TAKE RECV %d", at.link_id);
+        xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
+        ESP_LOGD("TAG", "TAKEN RECV %d", at.link_id);
         state = status::RECEIVING;
         uint64_t data;
         read(data_ready_fd, &data, sizeof(data));
@@ -184,6 +197,10 @@ public:
         if (!wait_to_idle(timeout_ms)) {
             return -1;
         }
+        // Take DTE mutex before issuing send on this link
+        ESP_LOGD("TAG", "TAKE SEND %d", at.link_id);
+        xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
+        ESP_LOGD("TAG", "TAKEN SEND %d", at.link_id);
         state = status::SENDING;
         memcpy(at.get_buf(), buffer, len_to_send);
         ESP_LOG_BUFFER_HEXDUMP("dce", at.get_buf(), len, ESP_LOG_VERBOSE);
@@ -224,6 +241,14 @@ public:
         }
         return -1;
     }
+    static std::vector<DCE *> dce_list;
+    static bool network_init;
+    static void read_callback(uint8_t *data, size_t len)
+    {
+        for (auto dce : dce_list) {
+            dce->perform_at(data, len);
+        }
+    }
 private:
     esp_modem::SignalGroup signal;
     void close_sock();

components/esp_modem/examples/modem_tcp_client/main/generate/sock_dce.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#include <algorithm>
 #include <charconv>
 #include <sys/socket.h>
 #include "esp_vfs.h"
@@ -14,6 +15,29 @@
 namespace sock_dce {
 
 constexpr auto const *TAG = "sock_dce";
+constexpr auto WAIT_TO_IDLE_TIMEOUT = 5000;
+
+// Definition of the static member variables
+std::vector<DCE*> DCE::dce_list{};
+bool DCE::network_init = false;
+int Responder::s_link_id = 0;
+SemaphoreHandle_t Responder::s_dte_mutex{};
+
+// Constructor - add this DCE instance to the static list
+DCE::DCE(std::shared_ptr<esp_modem::DTE> dte_arg, const esp_modem_dce_config *config)
+    : Module(std::move(dte_arg), config)
+{
+    dce_list.push_back(this);
+}
+
+// Destructor - remove this DCE instance from the static list
+DCE::~DCE()
+{
+    auto it = std::find(dce_list.begin(), dce_list.end(), this);
+    if (it != dce_list.end()) {
+        dce_list.erase(it);
+    }
+}
 
 
 bool DCE::perform_sock()
@@ -61,13 +85,26 @@ bool DCE::perform_sock()
 
 void DCE::perform_at(uint8_t *data, size_t len)
 {
-    ESP_LOG_BUFFER_HEXDUMP(TAG, data, len, ESP_LOG_VERBOSE);
+    if (state != status::RECEIVING) {
+        std::string_view resp_sv((char *)data, len);
+        at.check_urc(state, resp_sv);
+        if (state == status::IDLE) {
+            return;
+        }
+    }
+    // Trace incoming AT bytes when handling a response; use DEBUG level
+    ESP_LOG_BUFFER_HEXDUMP(TAG, data, len, ESP_LOG_DEBUG);
     switch (at.process_data(state, data, len)) {
     case Responder::ret::OK:
+        // Release DTE access for this link after processing data
+        ESP_LOGD(TAG, "GIVE data %d", at.link_id);
+        xSemaphoreGive(at.s_dte_mutex);
         state = status::IDLE;
         signal.set(IDLE);
         return;
     case Responder::ret::FAIL:
+        ESP_LOGD(TAG, "GIVE data %d", at.link_id);
+        xSemaphoreGive(at.s_dte_mutex);
         state = status::FAILED;
         signal.set(IDLE);
         return;
@@ -82,10 +119,14 @@ void DCE::perform_at(uint8_t *data, size_t len)
     std::string_view response((char *)data, len);
     switch (at.check_async_replies(state, response)) {
     case Responder::ret::OK:
+        ESP_LOGD(TAG, "GIVE command %d", at.link_id);
+        xSemaphoreGive(at.s_dte_mutex);
         state = status::IDLE;
         signal.set(IDLE);
         return;
     case Responder::ret::FAIL:
+        ESP_LOGD(TAG, "GIVE command %d", at.link_id);
+        xSemaphoreGive(at.s_dte_mutex);
         state = status::FAILED;
         signal.set(IDLE);
         return;
@@ -121,7 +162,7 @@ bool DCE::at_to_sock()
     uint64_t data;
     read(data_ready_fd, &data, sizeof(data));
     ESP_LOGD(TAG, "select read: modem data available %" PRIu64, data);
-    if (!signal.wait(IDLE, 1000)) {
+    if (!signal.wait(IDLE, WAIT_TO_IDLE_TIMEOUT)) {
         ESP_LOGE(TAG, "Failed to get idle");
         close_sock();
         return false;
@@ -131,6 +172,10 @@ bool DCE::at_to_sock()
         close_sock();
         return false;
     }
+    // Take DTE mutex before issuing receive on this link
+    ESP_LOGD(TAG, "TAKE RECV %d", at.link_id);
+    xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
+    ESP_LOGD(TAG, "TAKEN RECV %d", at.link_id);
     state = status::RECEIVING;
     at.start_receiving(at.get_buf_len());
     return true;
@@ -139,7 +184,7 @@ bool DCE::at_to_sock()
 bool DCE::sock_to_at()
 {
     ESP_LOGD(TAG,  "socket read: data available");
-    if (!signal.wait(IDLE, 1000)) {
+    if (!signal.wait(IDLE, WAIT_TO_IDLE_TIMEOUT)) {
         ESP_LOGE(TAG,  "Failed to get idle");
         close_sock();
         return false;
@@ -149,6 +194,10 @@ bool DCE::sock_to_at()
         close_sock();
         return false;
     }
+    // Take DTE mutex before issuing send on this link
+    ESP_LOGD(TAG, "TAKE SEND %d", at.link_id);
+    xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
+    ESP_LOGD(TAG, "TAKEN SEND %d", at.link_id);
     state = status::SENDING;
     int len = ::recv(sock, at.get_buf(), at.get_buf_len(), 0);
     if (len < 0) {
@@ -201,7 +250,7 @@ void DCE::start_listening(int port)
     }
     int opt = 1;
     setsockopt(listen_sock, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
-    ESP_LOGI(TAG, "Socket created");
+    ESP_LOGD(TAG, "Socket created");
     struct sockaddr_in addr = { };
     addr.sin_family = AF_INET;
     addr.sin_port = htons(port);
@@ -213,7 +262,7 @@ void DCE::start_listening(int port)
         ESP_LOGE(TAG, "Socket unable to bind: errno %d", errno);
         return;
     }
-    ESP_LOGI(TAG, "Socket bound, port %d", 1883);
+    ESP_LOGD(TAG, "Socket bound, port %d", 1883);
     err = listen(listen_sock, 1);
     if (err != 0) {
         ESP_LOGE(TAG, "Error occurred during listen: errno %d", errno);
@@ -224,12 +273,12 @@ void DCE::start_listening(int port)
 
 bool DCE::connect(std::string host, int port)
 {
-    dte->on_read(nullptr);
+    data_ready_fd = eventfd(0, EFD_SUPPORT_ISR);
-    tcp_close();
+    assert(data_ready_fd > 0);
-    dte->on_read([this](uint8_t *data, size_t len) {
+    // Take DTE mutex before starting connect for this link
-        this->perform_at(data, len);
+    ESP_LOGD(TAG, "TAKE CONNECT %d", at.link_id);
-        return esp_modem::command_result::TIMEOUT;
+    xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
-    });
+    ESP_LOGD(TAG, "TAKEN CONNECT %d", at.link_id);
     if (!at.start_connecting(host, port)) {
         ESP_LOGE(TAG, "Unable to start connecting");
         dte->on_read(nullptr);
@@ -241,12 +290,15 @@ bool DCE::connect(std::string host, int port)
 
 bool DCE::init()
 {
+    if (network_init) {
+        return true;
+    }
+    network_init = true;
+    Responder::s_dte_mutex = xSemaphoreCreateBinary();
+    xSemaphoreGive(at.s_dte_mutex);
     esp_vfs_eventfd_config_t config = ESP_VFS_EVENTD_CONFIG_DEFAULT();
     esp_vfs_eventfd_register(&config);
 
-    data_ready_fd = eventfd(0, EFD_SUPPORT_ISR);
-    assert(data_ready_fd > 0);
-
     dte->on_read(nullptr);
     const int retries = 5;
     int i = 0;
@@ -287,6 +339,10 @@ bool DCE::init()
         esp_modem::Task::Delay(5000);
     }
     ESP_LOGI(TAG, "Got IP %s", ip_addr.c_str());
+    dte->on_read([](uint8_t *data, size_t len) {
+        read_callback(data, len);
+        return esp_modem::command_result::TIMEOUT;
+    });
     return true;
 }
 

components/esp_modem/examples/modem_tcp_client/main/generate/sock_dce.hpp

@@ -34,6 +34,7 @@ public:
         sock(s), data_ready_fd(ready_fd), dte(dte_arg) {}
     ret process_data(status state, uint8_t *data, size_t len);
     ret check_async_replies(status state, std::string_view &response);
+    ret check_urc(status state, std::string_view &response);
 
     void start_sending(size_t len);
     void start_receiving(size_t len);
@@ -63,13 +64,19 @@ public:
         return total_len;
     }
 
+    // Unique link identifier used to target multi-connection AT commands
+    int link_id{s_link_id++};
+    // Shared mutex guarding DTE access across concurrent DCE instances
+    static SemaphoreHandle_t s_dte_mutex;
 private:
+    static int s_link_id;
     static constexpr size_t buffer_size = 512;
 
     bool on_read(char *data, size_t len)
     {
 #ifndef CONFIG_EXAMPLE_CUSTOM_TCP_TRANSPORT
         ::send(sock, data, len, 0);
+        printf("sending %d\n", len);
 #else
         ::memcpy(&buffer[actual_read], data, len);
         actual_read += len;
@@ -101,6 +108,8 @@ private:
 class DCE : public Module {
     using Module::Module;
 public:
+    DCE(std::shared_ptr<esp_modem::DTE> dte_arg, const esp_modem_dce_config *config);
+    ~DCE();
 
 //  --- ESP-MODEM command module starts here ---
 #include "esp_modem_command_declare_helper.inc"
@@ -141,6 +150,10 @@ esp_modem::return_type name(ESP_MODEM_COMMAND_PARAMS(__VA_ARGS__));
             return 0;
         }
         at.clear_offsets();
+        // Take DTE mutex before issuing receive on this link
+        ESP_LOGD("TAG", "TAKE RECV %d", at.link_id);
+        xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
+        ESP_LOGD("TAG", "TAKEN RECV %d", at.link_id);
         state = status::RECEIVING;
         uint64_t data;
         read(data_ready_fd, &data, sizeof(data));
@@ -163,6 +176,10 @@ esp_modem::return_type name(ESP_MODEM_COMMAND_PARAMS(__VA_ARGS__));
         if (!wait_to_idle(timeout_ms)) {
             return -1;
         }
+        // Take DTE mutex before issuing send on this link
+        ESP_LOGD("TAG", "TAKE SEND %d", at.link_id);
+        xSemaphoreTake(at.s_dte_mutex, portMAX_DELAY);
+        ESP_LOGD("TAG", "TAKEN SEND %d", at.link_id);
         state = status::SENDING;
         memcpy(at.get_buf(), buffer, len_to_send);
         ESP_LOG_BUFFER_HEXDUMP("dce", at.get_buf(), len, ESP_LOG_VERBOSE);
@@ -204,6 +221,14 @@ esp_modem::return_type name(ESP_MODEM_COMMAND_PARAMS(__VA_ARGS__));
         }
         return -1;
     }
+    static std::vector<DCE*> dce_list;
+    static bool network_init;
+    static void read_callback(uint8_t *data, size_t len)
+    {
+        for (auto dce : dce_list) {
+            dce->perform_at(data, len);
+        }
+    }
 
 private:
     esp_modem::SignalGroup signal;

components/esp_modem/examples/modem_tcp_client/main/modem_client.cpp

@@ -22,15 +22,26 @@
 #include "esp_log.h"
 #include "tcp_transport_mbedtls.h"
 #include "tcp_transport_at.h"
+#include "sdkconfig.h"
 
-#define BROKER_URL "test.mosquitto.org"
+#define USE_TLS CONFIG_EXAMPLE_USE_TLS
+#define BROKER_HOST CONFIG_EXAMPLE_BROKER_HOST
+#if USE_TLS
+#define BROKER_SCHEME "mqtts"
+#define BROKER_PORT 8883
+#else
+#define BROKER_SCHEME "mqtt"
 #define BROKER_PORT 1883
+#endif
+#define BROKER_URL BROKER_SCHEME "://" BROKER_HOST
 
 
 static const char *TAG = "modem_client";
 static EventGroupHandle_t event_group = NULL;
 static const int CONNECT_BIT = BIT0;
 static const int GOT_DATA_BIT = BIT2;
+static const int DCE0_DONE = BIT3;
+static const int DCE1_DONE = BIT4;
 
 static void mqtt_event_handler(void *handler_args, esp_event_base_t base, int32_t event_id, void *event_data)
 {
@@ -73,13 +84,15 @@ static void mqtt_event_handler(void *handler_args, esp_event_base_t base, int32_
     }
 }
 
+static void perform(void* ctx);
 
 extern "C" void app_main(void)
 {
-
     /* Init and register system/core components */
     ESP_ERROR_CHECK(esp_netif_init());
     ESP_ERROR_CHECK(esp_event_loop_create_default());
+    // Default to INFO; individual modules use DEBUG for verbose tracing
+    esp_log_level_set("*", ESP_LOG_INFO);
 
     event_group = xEventGroupCreate();
 
@@ -104,30 +117,60 @@ extern "C" void app_main(void)
     esp_modem_dce_config_t dce_config = ESP_MODEM_DCE_DEFAULT_CONFIG(CONFIG_EXAMPLE_MODEM_APN);
 
     /* create the DCE and initialize network manually (using AT commands) */
-    auto dce = sock_dce::create(&dce_config, std::move(dte));
+    auto dce = sock_dce::create(&dce_config, dte);
     if (!dce->init()) {
         ESP_LOGE(TAG,  "Failed to setup network");
         return;
     }
 
-    esp_mqtt_client_config_t mqtt_config = {};
+    xTaskCreate(perform, "perform", 4096, dce.get(), 4, nullptr);
-    mqtt_config.broker.address.port = BROKER_PORT;
-    mqtt_config.session.message_retransmit_timeout = 10000;
-#ifndef CONFIG_EXAMPLE_CUSTOM_TCP_TRANSPORT
-    mqtt_config.broker.address.uri = "mqtt://127.0.0.1";
-    dce->start_listening(BROKER_PORT);
-#else
-    mqtt_config.broker.address.uri = "mqtt://" BROKER_URL;
-    esp_transport_handle_t at = esp_transport_at_init(dce.get());
-    esp_transport_handle_t ssl = esp_transport_tls_init(at);
 
-    mqtt_config.network.transport = ssl;
+    /* create another DCE to serve a new connection */
+    auto dce1 = sock_dce::create(&dce_config, dte);
+    if (!dce1->init()) {
+        ESP_LOGE(TAG,  "Failed to setup network");
+        return;
+    }
+    xTaskCreate(perform, "perform", 4096, dce1.get(), 4, nullptr);
+
+    xEventGroupWaitBits(event_group, DCE0_DONE | DCE1_DONE, pdFALSE, pdTRUE, portMAX_DELAY);
+#ifdef CONFIG_EXAMPLE_CUSTOM_TCP_TRANSPORT
+    // this example does never keeps both DCEs running and in tcp-transport option
+    // we don't need a task to run so we exit main and keep DCE's "running"
+    vTaskDelay(portMAX_DELAY);
 #endif
+}
+
+static void perform(void* ctx)
+{
+    auto dce = static_cast<sock_dce::DCE*>(ctx);
+    char mqtt_client_id[] = "MQTT_CLIENT_0";
+    static int counter = 0;
+    const int id = counter++;
+    mqtt_client_id[sizeof(mqtt_client_id) - 2] += id;    // assumes a different client id per each thread
+    esp_mqtt_client_config_t mqtt_config = {};
+    mqtt_config.session.message_retransmit_timeout = 10000;
+    mqtt_config.credentials.client_id = mqtt_client_id;
+#ifndef CONFIG_EXAMPLE_CUSTOM_TCP_TRANSPORT
+    mqtt_config.broker.address.port = BROKER_PORT + id;
+    mqtt_config.broker.address.uri = BROKER_SCHEME "://127.0.0.1";
+    dce->start_listening(BROKER_PORT + id);
+#else
+    mqtt_config.broker.address.port = BROKER_PORT;
+    mqtt_config.broker.address.uri = BROKER_URL;
+    esp_transport_handle_t at = esp_transport_at_init(dce);
+#if USE_TLS
+    esp_transport_handle_t ssl = esp_transport_tls_init(at);
+    mqtt_config.network.transport = ssl;
+#else
+    mqtt_config.network.transport = at;
+#endif // USE_TLS
+#endif // CONFIG_EXAMPLE_CUSTOM_TCP_TRANSPORT
     esp_mqtt_client_handle_t mqtt_client = esp_mqtt_client_init(&mqtt_config);
     esp_mqtt_client_register_event(mqtt_client, static_cast<esp_mqtt_event_id_t>(ESP_EVENT_ANY_ID), mqtt_event_handler, nullptr);
     esp_mqtt_client_start(mqtt_client);
 #ifndef CONFIG_EXAMPLE_CUSTOM_TCP_TRANSPORT
-    if (!dce->connect(BROKER_URL, BROKER_PORT)) {
+    if (!dce->connect(BROKER_HOST, BROKER_PORT)) {
         ESP_LOGE(TAG, "Failed to start DCE");
         return;
     }
@@ -135,18 +178,17 @@ extern "C" void app_main(void)
         while (dce->perform_sock()) {
             ESP_LOGV(TAG, "...performing");
         }
-        ESP_LOGE(TAG, "Loop exit.. retrying");
+        ESP_LOGD(TAG, "Loop exit.. retrying");
         // handle disconnections errors
         if (!dce->init()) {
             ESP_LOGE(TAG, "Failed to reinit network");
             return;
         }
-        if (!dce->connect(BROKER_URL, BROKER_PORT)) {
+        if (!dce->connect(BROKER_HOST, BROKER_PORT)) {
             ESP_LOGI(TAG, "Network reinitialized, retrying");
         }
     }
-#else
-    vTaskDelay(portMAX_DELAY);
 #endif
-
+    xEventGroupSetBits(event_group, id ? DCE0_DONE : DCE1_DONE);
+    vTaskDelete(nullptr);
 }

components/esp_modem/examples/modem_tcp_client/main/sock_commands_bg96.cpp

@@ -109,17 +109,17 @@ void Responder::start_sending(size_t len)
 {
     data_to_send = len;
     send_stat = 0;
-    send_cmd("AT+QISEND=0," + std::to_string(len) + "\r");
+    send_cmd("AT+QISEND=" + std::to_string(link_id) + "," + std::to_string(len) + "\r");
 }
 
 void Responder::start_receiving(size_t len)
 {
-    send_cmd("AT+QIRD=0," + std::to_string(len) + "\r");
+    send_cmd("AT+QIRD=" + std::to_string(link_id) + "," + std::to_string(len) + "\r");
 }
 
 bool Responder::start_connecting(std::string host, int port)
 {
-    send_cmd(R"(AT+QIOPEN=1,0,"TCP",")" + host + "\"," + std::to_string(port) + "\r");
+    send_cmd(std::string("AT+QIOPEN=1,") + std::to_string(link_id) + R"(,"TCP",")" + host + "\"," + std::to_string(port) + "\r");
     return true;
 }
 
@@ -130,16 +130,24 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
     auto *recv_data = (char *)data;
     if (data_to_recv == 0) {
         const std::string_view head = "+QIRD: ";
+again:
         const std::string_view recv_data_view = std::string_view(recv_data, len);
         auto head_pos_found = recv_data_view.find(head);
         if (head_pos_found == std::string_view::npos) {
-            return ret::FAIL;
+            return ret::IN_PROGRESS;
         }
 
         auto *head_pos = recv_data + head_pos_found;
         auto next_nl = (char *)memchr(head_pos + head.size(), '\n', MIN_MESSAGE);
 
         if (next_nl == nullptr) {
+            if (head_pos + head.size() + 1 < recv_data + len) {
+                // might be that we misinterpreted the URC +QIRD: <>,<>,<> (notification) with the +QIRD: <> (read data)
+                // so we try to find the next +QIRD:
+                recv_data = head_pos + head.size() + 1;
+                goto again;
+            }
+            ESP_LOGD(TAG, "no new line found");
             return ret::FAIL;
         }
 
@@ -151,7 +159,9 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
         ESP_LOGD(TAG, "Received: actual len=%d", actual_len);
         if (actual_len == 0) {
             ESP_LOGD(TAG, "no data received");
-            return ret::FAIL;
+            data_to_recv = 0;
+            // return OK here, as BG96 would keep unacked data and notifies us with +QIRD: 0
+            return ret::OK;
         }
 
         if (actual_len > buffer_size) {
@@ -182,6 +192,7 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
         last_pos = (char *)memchr(recv_data + 1 + actual_len, 'O', MIN_MESSAGE);
         if (last_pos == nullptr || last_pos[1] != 'K') {
             data_to_recv = 0;
+            ESP_LOGD(TAG, "no OK after data");
             return ret::FAIL;
         }
     }
@@ -222,7 +233,7 @@ Responder::ret Responder::send(std::string_view response)
 {
     if (send_stat == 3) {
         if (response.find("SEND OK") != std::string::npos) {
-            send_cmd("AT+QISEND=0,0\r");
+            send_cmd(std::string("AT+QISEND=") + std::to_string(link_id) + ",0\r");
             send_stat++;
             return ret::IN_PROGRESS;
         } else if (response.find("SEND FAIL") != std::string::npos) {
@@ -267,7 +278,7 @@ Responder::ret Responder::send(std::string_view response)
             if (ack < total) {
                 ESP_LOGD(TAG, "all sending data are not ack (missing %d bytes acked)", (total - ack));
                 if (total - ack > 64) {
-                    ESP_LOGW(TAG, "Need a pause: missing %d bytes acked", (total - ack));
+                    ESP_LOGD(TAG, "Need a pause: missing %d bytes acked", (total - ack));
                     return ret::NEED_MORE_TIME;
                 }
             }
@@ -284,7 +295,8 @@ Responder::ret Responder::send(std::string_view response)
 
 Responder::ret Responder::connect(std::string_view response)
 {
-    if (response.find("+QIOPEN: 0,0") != std::string::npos) {
+    std::string open_response = "+QIOPEN: " + std::to_string(link_id) + ",0";
+    if (response.find(open_response) != std::string::npos) {
         ESP_LOGI(TAG, "Connected!");
         return ret::OK;
     }
@@ -295,10 +307,9 @@ Responder::ret Responder::connect(std::string_view response)
     return Responder::ret::IN_PROGRESS;
 }
 
-Responder::ret Responder::check_async_replies(status state, std::string_view &response)
+Responder::ret Responder::check_urc(status state, std::string_view &response)
 {
-    ESP_LOGD(TAG, "response %.*s", static_cast<int>(response.size()), response.data());
+    if (response.find(std::string("+QIURC: \"recv\",") + std::to_string(link_id)) != std::string::npos) {
-    if (response.find("+QIURC: \"recv\",0") != std::string::npos) {
         uint64_t data_ready = 1;
         write(data_ready_fd, &data_ready, sizeof(data_ready));
         ESP_LOGD(TAG, "Got data on modem!");
@@ -309,6 +320,9 @@ Responder::ret Responder::check_async_replies(status state, std::string_view &re
         response = response.substr(head_pos + head.size());
         int next_cr = response.find('\r');
         if (next_cr != std::string::npos) {
+            if (next_cr < 2) {
+                return ret::IN_PROGRESS;
+            }
             response = response.substr(next_cr - 2, next_cr);
             if (response.find(",0") != std::string::npos) {
                 ESP_LOGV(TAG, "Receiving done");
@@ -318,12 +332,21 @@ Responder::ret Responder::check_async_replies(status state, std::string_view &re
                 ESP_LOGD(TAG, "Got data on modem!");
             }
         }
-    } else if (response.find("+QIURC: \"closed\",0") != std::string::npos) {
+    } else if (response.find(std::string("+QIURC: \"closed\",") + std::to_string(link_id)) != std::string::npos) {
+        ESP_LOGE(TAG, "Connection closed");
         return ret::FAIL;
     }
+    return ret::IN_PROGRESS;
+}
+
+
+Responder::ret Responder::check_async_replies(status state, std::string_view &response)
+{
+    ESP_LOGD(TAG, "response %.*s", static_cast<int>(response.size()), response.data());
     if (state == status::SENDING) {
         return send(response);
-    } else if (state == status::CONNECTING) {
+    }
+    if (state == status::CONNECTING) {
         return connect(response);
     }
     return ret::IN_PROGRESS;
@@ -342,7 +365,7 @@ Responder::ret Responder::process_data(status state, uint8_t *data, size_t len)
 
 status Responder::pending()
 {
-    send_cmd("AT+QISEND=0,0\r");
+    send_cmd(std::string("AT+QISEND=") + std::to_string(link_id) + ",0\r");
     return status::SENDING;
 }
 

components/esp_modem/examples/modem_tcp_client/main/sock_commands_espat.cpp

@@ -58,12 +58,19 @@ command_result net_open(CommandableIf *t)
     }
     ESP_LOGI(TAG, "WiFi connected successfully");
 
-    // Set passive receive mode (1) for better control
+    // Enable multiple connections mode
-    ret = set_rx_mode(t, 1);
+    ret = dce_commands::generic_command(t, "AT+CIPMUX=1\r\n", "OK", "ERROR", 1000);
     if (ret != command_result::OK) {
-        ESP_LOGE(TAG, "Failed to set preferred Rx mode");
+        ESP_LOGE(TAG, "Failed to enable multiple connections mode");
         return ret;
     }
+    ESP_LOGD(TAG, "Multiple connections mode enabled");
+
+    // Set passive receive mode (1) for better control
+    for (int i = 0; i < 2; i++) {
+        std::string cmd = "AT+CIPRECVTYPE=" + std::to_string(i) + ",1\r\n";
+        dce_commands::generic_command(t, cmd, "OK", "ERROR", 1000);
+    }
     return command_result::OK;
 }
 
@@ -78,49 +85,20 @@ command_result net_close(CommandableIf *t)
     return command_result::OK;
 }
 
-command_result tcp_open(CommandableIf *t, const std::string &host, int port, int timeout)
-{
-    ESP_LOGV(TAG, "%s", __func__);
-
-    // Set single connection mode (just in case)
-    auto ret = dce_commands::generic_command(t, "AT+CIPMUX=0\r\n", "OK", "ERROR", 1000);
-    if (ret != command_result::OK) {
-        ESP_LOGW(TAG, "Failed to set single connection mode");
-    }
-
-    // Establish TCP connection
-    std::string tcp_cmd = "AT+CIPSTART=\"TCP\",\"" + host + "\"," + std::to_string(port) + "\r\n";
-    ret = dce_commands::generic_command(t, tcp_cmd, "CONNECT", "ERROR", timeout);
-    if (ret != command_result::OK) {
-        ESP_LOGE(TAG, "Failed to establish TCP connection to %s:%d", host.c_str(), port);
-        return ret;
-    }
-
-    ESP_LOGI(TAG, "TCP connection established to %s:%d", host.c_str(), port);
-    return command_result::OK;
-}
-
 command_result tcp_close(CommandableIf *t)
 {
+    return command_result::OK;
     ESP_LOGV(TAG, "%s", __func__);
-    return dce_commands::generic_command(t, "AT+CIPCLOSE\r\n", "CLOSED", "ERROR", 5000);
+    // Use link ID 0 for closing connection
+    const int link_id = 0;
+    std::string close_cmd = "AT+CIPCLOSE=" + std::to_string(link_id) + "\r\n";
+
+    // In multiple connections mode, response format is: <link ID>,CLOSED
+    std::string expected_response = std::to_string(link_id) + ",CLOSED";
+
+    return dce_commands::generic_command(t, close_cmd, expected_response, "ERROR", 5000);
 }
 
-command_result tcp_send(CommandableIf *t, uint8_t *data, size_t len)
-{
-    ESP_LOGV(TAG, "%s", __func__);
-    // This function is not used in the current implementation
-    // Data sending is handled by the DCE responder
-    return command_result::FAIL;
-}
-
-command_result tcp_recv(CommandableIf *t, uint8_t *data, size_t len, size_t &out_len)
-{
-    ESP_LOGV(TAG, "%s", __func__);
-    // This function is not used in the current implementation
-    // Data receiving is handled by the DCE responder
-    return command_result::FAIL;
-}
 
 command_result get_ip(CommandableIf *t, std::string &ip)
 {
@@ -150,9 +128,11 @@ command_result get_ip(CommandableIf *t, std::string &ip)
 
 command_result set_rx_mode(CommandableIf *t, int mode)
 {
-    ESP_LOGE(TAG, "%s", __func__);
+    ESP_LOGV(TAG, "%s", __func__);
+    // For multiple connections mode, set receive mode for link ID 0
+    const int link_id = 0;
     // Active mode (0) sends data automatically, Passive mode (1) notifies about data for reading
-    std::string cmd = "AT+CIPRECVTYPE=" + std::to_string(mode) + "\r\n";
+    std::string cmd = "AT+CIPRECVTYPE=" + std::to_string(link_id) + "," + std::to_string(mode) + "\r\n";
     return dce_commands::generic_command(t, cmd, "OK", "ERROR", 1000);
 }
 
@@ -164,17 +144,20 @@ void Responder::start_sending(size_t len)
 {
     data_to_send = len;
     send_stat = 0;
-    send_cmd("AT+CIPSEND=" + std::to_string(len) + "\r\n");
+    // For multiple connections mode, include link ID
+    send_cmd("AT+CIPSEND=" + std::to_string(link_id) + "," + std::to_string(len) + "\r\n");
 }
 
 void Responder::start_receiving(size_t len)
 {
-    send_cmd("AT+CIPRECVDATA=" + std::to_string(len) + "\r\n");
+    // For multiple connections mode, include link ID
+    send_cmd("AT+CIPRECVDATA=" + std::to_string(link_id) + "," + std::to_string(len) + "\r\n");
 }
 
 bool Responder::start_connecting(std::string host, int port)
 {
-    std::string cmd = "AT+CIPSTART=\"TCP\",\"" + host + "\"," + std::to_string(port) + "\r\n";
+    // For multiple connections mode, include link ID
+    std::string cmd = "AT+CIPSTART=" + std::to_string(link_id) + ",\"TCP\",\"" + host + "\"," + std::to_string(port) + "\r\n";
     send_cmd(cmd);
     return true;
 }
@@ -187,16 +170,12 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
 
     if (data_to_recv == 0) {
         const std::string_view head = "+CIPRECVDATA:";
-
+        const std::string_view recv_data_view(recv_data, len);
-        // Find the response header
+        const auto head_pos_found = recv_data_view.find(head);
-        auto head_pos = std::search(recv_data, recv_data + len, head.data(), head.data() + head.size(), [](char a, char b) {
+        if (head_pos_found == std::string_view::npos) {
-            return a == b;
+            return ret::IN_PROGRESS;
-        });
-
-        if (head_pos == recv_data + len) {
-            return ret::FAIL;
         }
-
+        const auto *head_pos = recv_data + head_pos_found;
         // Find the end of the length field
         auto next_comma = (char *)memchr(head_pos + head.size(), ',', MIN_MESSAGE);
         if (next_comma == nullptr) {
@@ -245,12 +224,25 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
     char *ok_pos = nullptr;
     if (actual_len + 1 + 2 /* OK */ <= len) {
         ok_pos = (char *)memchr(recv_data + actual_len + 1, 'O', MIN_MESSAGE);
-        if (ok_pos == nullptr || ok_pos[1] != 'K') {
+        if (ok_pos == nullptr) { // || ok_pos[1] != 'K') {
             data_to_recv = 0;
+            ESP_LOGE(TAG, "Missed 'OK' marker");
+            return ret::OK;
+            return ret::FAIL;
+        }
+        if (ok_pos + 1 < recv_data + len && ok_pos[1] != 'K') {
+            // we ignore the condition when receiving 'O' as the last character in the last batch,
+            // don't wait for the 'K' in the next run, assume the data are valid and let higher layers deal with it.
+            data_to_recv = 0;
+            ESP_LOGE(TAG, "Missed 'OK' marker2");
             return ret::FAIL;
         }
     }
-
+    if (ok_pos != nullptr && (char *)data + len - ok_pos - 2 > MIN_MESSAGE) {
+        // check for async replies after the Recv header
+        std::string_view response((char *)ok_pos + 2 /* OK */, (char *)data + len - ok_pos);
+        check_urc(status::RECEIVING, response);
+    }
     // Reset and prepare for next receive
     data_to_recv = 0;
     return ret::OK;
@@ -299,7 +291,8 @@ Responder::ret Responder::send(std::string_view response)
 
 Responder::ret Responder::connect(std::string_view response)
 {
-    if (response.find("CONNECT") != std::string::npos) {
+    // In multiple connections mode, response format is: <link ID>,CONNECT
+    if (response.find(",CONNECT") != std::string::npos || response.find("CONNECT") != std::string::npos) {
         ESP_LOGI(TAG, "TCP connected!");
         return ret::OK;
     }
@@ -309,6 +302,17 @@ Responder::ret Responder::connect(std::string_view response)
     }
     return ret::IN_PROGRESS;
 }
+Responder::ret Responder::check_urc(status state, std::string_view &response)
+{
+    // Handle data notifications - in multiple connections mode, format is +IPD,<link ID>,<len>
+    std::string expected_urc = "+IPD," + std::to_string(link_id);
+    if (response.find(expected_urc) != std::string::npos) {
+        uint64_t data_ready = 1;
+        write(data_ready_fd, &data_ready, sizeof(data_ready));
+        ESP_LOGD(TAG, "Data available notification");
+    }
+    return ret::IN_PROGRESS;
+}
 
 Responder::ret Responder::check_async_replies(status state, std::string_view &response)
 {
@@ -318,24 +322,17 @@ Responder::ret Responder::check_async_replies(status state, std::string_view &re
     if (response.find("WIFI CONNECTED") != std::string::npos) {
         ESP_LOGI(TAG, "WiFi connected");
     } else if (response.find("WIFI DISCONNECTED") != std::string::npos) {
-        ESP_LOGW(TAG, "WiFi disconnected");
+        ESP_LOGD(TAG, "WiFi disconnected");
     }
 
-    // Handle TCP status messages
+    // Handle TCP status messages (multiple connections format: <link ID>,CONNECT or <link ID>,CLOSED)
     if (response.find("CONNECT") != std::string::npos && state == status::CONNECTING) {
         return connect(response);
     } else if (response.find("CLOSED") != std::string::npos) {
-        ESP_LOGW(TAG, "TCP connection closed");
+        ESP_LOGD(TAG, "TCP connection closed");
         return ret::FAIL;
     }
 
-    // Handle data notifications in active mode (if we switch to it later)
-    if (response.find("+IPD,") != std::string::npos) {
-        uint64_t data_ready = 1;
-        write(data_ready_fd, &data_ready, sizeof(data_ready));
-        ESP_LOGD(TAG, "Data available notification");
-    }
-
     if (state == status::SENDING) {
         return send(response);
     } else if (state == status::CONNECTING) {

components/esp_modem/examples/modem_tcp_client/main/sock_commands_sim7600.cpp

@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: 2022-2023 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2022-2025 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -18,13 +18,13 @@ using namespace esp_modem;
 
 command_result net_open(CommandableIf *term)
 {
-    ESP_LOGV(TAG, "%s", __func__ );
+    ESP_LOGV(TAG, "%s", __func__);
     std::string response;
     auto ret = dce_commands::generic_get_string(term, "AT+NETOPEN?\r", response, 1000);
     if (ret != command_result::OK) {
         return ret;
     }
-    ESP_LOGV(TAG, "%s", response.data() );
+    ESP_LOGV(TAG, "%s", response.data());
     if (response.find("+NETOPEN: 1") != std::string::npos) {
         ESP_LOGD(TAG, "Already there");
         ret = command_result::OK;
@@ -42,23 +42,23 @@ command_result net_open(CommandableIf *term)
 
 command_result net_close(CommandableIf *term)
 {
-    ESP_LOGV(TAG, "%s", __func__ );
+    ESP_LOGV(TAG, "%s", __func__);
     return dce_commands::generic_command(term, "AT+NETCLOSE\r", "+NETCLOSE:", "ERROR", 30000);
 }
 
 command_result tcp_open(CommandableIf *term, const std::string &host, int port, int timeout)
 {
-    ESP_LOGV(TAG, "%s", __func__ );
+    ESP_LOGV(TAG, "%s", __func__);
     auto ret = dce_commands::generic_command(term, "AT+CIPRXGET=1\r", "OK", "ERROR", 50000);
     if (ret != command_result::OK) {
         ESP_LOGE(TAG, "Setting Rx mode failed!");
         return ret;
     }
-    ESP_LOGV(TAG, "%s", __func__ );
+    ESP_LOGV(TAG, "%s", __func__);
     std::string ip_open = R"(AT+CIPOPEN=0,"TCP",")" + host + "\"," + std::to_string(port) + "\r";
     ret = dce_commands::generic_command(term, ip_open, "+CIPOPEN: 0,0", "ERROR", timeout);
     if (ret != command_result::OK) {
-        ESP_LOGE(TAG, "%s Failed", __func__ );
+        ESP_LOGE(TAG, "%s Failed", __func__);
         return ret;
     }
     return command_result::OK;
@@ -66,13 +66,13 @@ command_result tcp_open(CommandableIf *term, const std::string &host, int port,
 
 command_result tcp_close(CommandableIf *term)
 {
-    ESP_LOGV(TAG, "%s", __func__ );
+    ESP_LOGV(TAG, "%s", __func__);
     return dce_commands::generic_command(term, "AT+CIPCLOSE=0\r", "+CIPCLOSE:", "ERROR", 10000);
 }
 
 command_result tcp_send(CommandableIf *term, uint8_t *data, size_t len)
 {
-    ESP_LOGV(TAG, "%s", __func__ );
+    ESP_LOGV(TAG, "%s", __func__);
     std::string send = "AT+CIPSEND=0," + std::to_string(len) + "\r";
     auto ret = term->command(send, [&](uint8_t *data, size_t len) {
         std::string_view response((char *)data, len);
@@ -86,10 +86,10 @@ command_result tcp_send(CommandableIf *term, uint8_t *data, size_t len)
         return ret;
     }
     ret = command_result::TIMEOUT;
-    ESP_LOGW(TAG, "Before setting...");
+    ESP_LOGD(TAG, "Before setting...");
     term->on_read([&ret](uint8_t *cmd_data, size_t cmd_len) {
         std::string_view response((char *)cmd_data, cmd_len);
-        ESP_LOGW(TAG, "CIPSEND response %.*s", static_cast<int>(response.size()), response.data());
+        ESP_LOGD(TAG, "CIPSEND response %.*s", static_cast<int>(response.size()), response.data());
 
         if (response.find("+CIPSEND:") != std::string::npos) {
             ret = command_result::OK;
@@ -98,7 +98,7 @@ command_result tcp_send(CommandableIf *term, uint8_t *data, size_t len)
         }
         return ret;
     });
-    ESP_LOGW(TAG, "Before writing...");
+    ESP_LOGD(TAG, "Before writing...");
     auto written = term->write(data, len);
     if (written != len) {
         ESP_LOGE(TAG, "written %d (%d)...", written, len);
@@ -107,7 +107,7 @@ command_result tcp_send(CommandableIf *term, uint8_t *data, size_t len)
     uint8_t ctrl_z = '\x1A';
     term->write(&ctrl_z, 1);
     int count = 0;
-    while (ret == command_result::TIMEOUT && count++ < 1000 ) {
+    while (ret == command_result::TIMEOUT && count++ < 1000) {
         vTaskDelay(pdMS_TO_TICKS(1000));
     }
     term->on_read(nullptr);
@@ -116,7 +116,7 @@ command_result tcp_send(CommandableIf *term, uint8_t *data, size_t len)
 
 command_result tcp_recv(CommandableIf *term, uint8_t *data, size_t len, size_t &out_len)
 {
-    ESP_LOGV(TAG, "%s", __func__ );
+    ESP_LOGV(TAG, "%s", __func__);
     std::string out;
     auto ret = dce_commands::generic_get_string(term, "AT+CIPRXGET=4,0\r", out);
     if (ret != command_result::OK) {
@@ -195,17 +195,17 @@ void Responder::start_sending(size_t len)
 {
     data_to_send = len;
     send_stat = 0;
-    send_cmd("AT+CIPSEND=0," + std::to_string(len) + "\r");
+    send_cmd("AT+CIPSEND=" + std::to_string(link_id) + "," + std::to_string(len) + "\r");
 }
 
 void Responder::start_receiving(size_t len)
 {
-    send_cmd("AT+CIPRXGET=2,0," + std::to_string(len) + "\r");
+    send_cmd("AT+CIPRXGET=2," + std::to_string(link_id) + "," + std::to_string(len) + "\r");
 }
 
 bool Responder::start_connecting(std::string host, int port)
 {
-    send_cmd(R"(AT+CIPOPEN=0,"TCP",")" + host + "\"," + std::to_string(port) + "\r");
+    send_cmd(std::string("AT+CIPOPEN=") + std::to_string(link_id) + R"(,"TCP",")" + host + "\"," + std::to_string(port) + "\r");
     return true;
 }
 
@@ -215,7 +215,7 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
     size_t actual_len = 0;
     auto *recv_data = (char *)data;
     if (data_to_recv == 0) {
-        static constexpr std::string_view head = "+CIPRXGET: 2,0,";
+        const std::string head = std::string("+CIPRXGET: 2,") + std::to_string(link_id) + ",";
         auto head_pos = std::search(recv_data, recv_data + len, head.begin(), head.end());
         if (head_pos == recv_data + len) {
             return ret::FAIL;
@@ -329,7 +329,8 @@ Responder::ret Responder::send(std::string_view response)
 
 Responder::ret Responder::connect(std::string_view response)
 {
-    if (response.find("+CIPOPEN: 0,0") != std::string::npos) {
+    std::string open_response = "+CIPOPEN: " + std::to_string(link_id) + ",0";
+    if (response.find(open_response) != std::string::npos) {
         ESP_LOGI(TAG, "Connected!");
         return ret::OK;
     }
@@ -340,14 +341,22 @@ Responder::ret Responder::connect(std::string_view response)
     return Responder::ret::IN_PROGRESS;
 }
 
-Responder::ret Responder::check_async_replies(status state, std::string_view &response)
+Responder::ret Responder::check_urc(status state, std::string_view &response)
 {
-    ESP_LOGD(TAG, "response %.*s", static_cast<int>(response.size()), response.data());
+    // 1. When <mode> is set to 1 and the 2-4 mode will take effect.
-    if (response.find("+CIPRXGET: 1") != std::string::npos) {
+    // 2. If AT+CIPRXGET=1, it will report +CIPRXGET: 1,<cid>(multi client) when
+    const std::string expected = std::string("+CIPRXGET: 1,") + std::to_string(link_id);
+    if (response.find(expected) != std::string::npos) {
         uint64_t data_ready = 1;
         write(data_ready_fd, &data_ready, sizeof(data_ready));
         ESP_LOGD(TAG, "Got data on modem!");
     }
+    return ret::IN_PROGRESS;
+}
+
+Responder::ret Responder::check_async_replies(status state, std::string_view &response)
+{
+    ESP_LOGD(TAG, "response %.*s", static_cast<int>(response.size()), response.data());
     if (state == status::SENDING) {
         return send(response);
     } else if (state == status::CONNECTING) {