feat(modem): Support custom transport in AT TCP client example

components/esp_modem/examples/modem_tcp_client/README.md

@@ -5,6 +5,10 @@
 ## Overview
 This example demonstrates how to act as a MQTT client using modem's TCP commands (provided, the device supports "socket" related commands)
 
+This example could be used in two different configurations:
+1) Custom TCP transport: Implements a TCP transport in form of AT commands and uses it as custom transport for mqtt client.
+2) Localhost listener: Uses standard transports to connect and forwards socket layer data from the client to the modem using AT commands.
+
 ### Supported IDF versions
 
-This example is supported from IDF `v4.4`.
+This example is supported from IDF `v5.0`.

components/esp_modem/examples/modem_tcp_client/components/extra_tcp_transports/CMakeLists.txt

@@ -0,0 +1,4 @@
+idf_component_register(SRCS mbedtls_wrap.cpp
+                            tls_transport.cpp
+                       INCLUDE_DIRS include
+                       REQUIRES tcp_transport)

components/esp_modem/examples/modem_tcp_client/components/extra_tcp_transports/README.md

@@ -0,0 +1,9 @@
+# Custom transports
+
+This component is a placeholder of custom transports. It contains mbedTLS cxx wrapper which could be used to create a custom TLS layer on any kind of IO function.
+
+# List of Transports
+
+## TLS Transport
+
+TLS layer on top of any custom transport. Very similar to `ssl_transport` (standard IDF transport), but this is customizable and could work on any kind of transport, not only  the BSD socket based tcp transport (like `ssl_transport`).

components/esp_modem/examples/modem_tcp_client/components/extra_tcp_transports/include/mbedtls_wrap.hpp

@@ -0,0 +1,46 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#pragma once
+
+#include <utility>
+#include "mbedtls/ssl.h"
+#include "mbedtls/entropy.h"
+#include "mbedtls/ctr_drbg.h"
+#include "mbedtls/error.h"
+
+using  const_buf = std::pair<const unsigned char *, std::size_t>;
+using  buf = std::pair<unsigned char *, std::size_t>;
+
+class Tls {
+public:
+    Tls();
+    bool init(bool is_server, bool verify);
+    int handshake();
+    int write(const unsigned char *buf, size_t len);
+    int read(unsigned char *buf, size_t len);
+    bool set_own_cert(const_buf crt, const_buf key);
+    bool set_ca_cert(const_buf crt);
+    virtual int send(const unsigned char *buf, size_t len) = 0;
+    virtual int recv(unsigned char *buf, size_t len) = 0;
+    size_t get_available_bytes();
+
+private:
+    mbedtls_ssl_context ssl_{};
+    mbedtls_x509_crt public_cert_{};
+    mbedtls_pk_context pk_key_{};
+    mbedtls_x509_crt ca_cert_{};
+    mbedtls_ssl_config conf_{};
+    mbedtls_ctr_drbg_context ctr_drbg_{};
+    mbedtls_entropy_context entropy_{};
+
+    static void print_error(const char *function, int error_code);
+    static int bio_write(void *ctx, const unsigned char *buf, size_t len);
+    static int bio_read(void *ctx, unsigned char *buf, size_t len);
+    int mbedtls_pk_parse_key( mbedtls_pk_context *ctx,
+                              const unsigned char *key, size_t keylen,
+                              const unsigned char *pwd, size_t pwdlen);
+
+};

components/esp_modem/examples/modem_tcp_client/components/extra_tcp_transports/include/tcp_transport_mbedtls.h

@@ -0,0 +1,16 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#pragma once
+
+#include "esp_transport.h"
+
+/**
+ * @brief Initializes TLS transport based on mbetls wrapper
+ *
+ * @param parent Transport on top of which we run TLS layer
+ * @return Transport handle on success
+ */
+esp_transport_handle_t esp_transport_tls_init(esp_transport_handle_t parent);

components/esp_modem/examples/modem_tcp_client/components/extra_tcp_transports/mbedtls_wrap.cpp

@@ -0,0 +1,129 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#include "mbedtls/ctr_drbg.h"
+#include "mbedtls/ssl.h"
+#include "mbedtls_wrap.hpp"
+
+bool Tls::init(bool is_server, bool verify)
+{
+    const char pers[] = "mbedtls_wrapper";
+    mbedtls_entropy_init(&entropy_);
+    mbedtls_ctr_drbg_seed(&ctr_drbg_, mbedtls_entropy_func, &entropy_, (const unsigned char *)pers, sizeof(pers));
+    int ret = mbedtls_ssl_config_defaults(&conf_, is_server ? MBEDTLS_SSL_IS_SERVER : MBEDTLS_SSL_IS_CLIENT, MBEDTLS_SSL_TRANSPORT_STREAM, MBEDTLS_SSL_PRESET_DEFAULT);
+    if (ret) {
+        print_error("mbedtls_ssl_config_defaults", ret);
+        return false;
+    }
+    mbedtls_ssl_conf_rng(&conf_, mbedtls_ctr_drbg_random, &ctr_drbg_);
+    mbedtls_ssl_conf_authmode(&conf_, verify ? MBEDTLS_SSL_VERIFY_REQUIRED : MBEDTLS_SSL_VERIFY_NONE);
+    ret = mbedtls_ssl_conf_own_cert(&conf_, &public_cert_, &pk_key_);
+    if (ret) {
+        print_error("mbedtls_ssl_conf_own_cert", ret);
+        return false;
+    }
+    if (verify) {
+        mbedtls_ssl_conf_ca_chain(&conf_, &ca_cert_, nullptr);
+    }
+    ret = mbedtls_ssl_setup(&ssl_, &conf_);
+    if (ret) {
+        print_error("mbedtls_ssl_setup", ret);
+        return false;
+    }
+    return true;
+}
+
+void Tls::print_error(const char *function, int error_code)
+{
+    static char error_buf[100];
+    mbedtls_strerror(error_code, error_buf, sizeof(error_buf));
+
+    printf("%s() returned -0x%04X\n", function, -error_code);
+    printf("-0x%04X: %s\n", -error_code, error_buf);
+}
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "esp_log.h"
+int Tls::handshake()
+{
+    ESP_LOGI("TLS", "handshake");
+    int ret = 0;
+    mbedtls_ssl_set_bio(&ssl_, this, bio_write, bio_read, nullptr);
+
+    while ( ( ret = mbedtls_ssl_handshake( &ssl_ ) ) != 0 ) {
+        if ( ret != MBEDTLS_ERR_SSL_WANT_READ && ret != MBEDTLS_ERR_SSL_WANT_WRITE ) {
+            print_error( "mbedtls_ssl_handshake returned", ret );
+            return -1;
+        }
+        vTaskDelay(pdMS_TO_TICKS(500));
+    }
+    ESP_LOGI("TLS", "handshake done with %d", ret);
+    return ret;
+}
+
+int Tls::bio_write(void *ctx, const unsigned char *buf, size_t len)
+{
+    auto s = static_cast<Tls *>(ctx);
+    return s->send(buf, len);
+}
+
+int Tls::bio_read(void *ctx, unsigned char *buf, size_t len)
+{
+    auto s = static_cast<Tls *>(ctx);
+    return s->recv(buf, len);
+}
+
+int Tls::write(const unsigned char *buf, size_t len)
+{
+    return mbedtls_ssl_write( &ssl_, buf, len );
+}
+
+int Tls::read(unsigned char *buf, size_t len)
+{
+    return mbedtls_ssl_read( &ssl_, buf, len );
+}
+
+bool Tls::set_own_cert(const_buf crt, const_buf key)
+{
+    int ret = mbedtls_x509_crt_parse(&public_cert_, crt.first, crt.second);
+    if (ret < 0) {
+        print_error("mbedtls_x509_crt_parse", ret);
+        return false;
+    }
+    ret = mbedtls_pk_parse_key(&pk_key_, key.first, key.second, nullptr, 0);
+    if (ret < 0) {
+        print_error("mbedtls_pk_parse_keyfile", ret);
+        return false;
+    }
+    return true;
+}
+
+bool Tls::set_ca_cert(const_buf crt)
+{
+    int ret = mbedtls_x509_crt_parse(&ca_cert_, crt.first, crt.second);
+    if (ret < 0) {
+        print_error("mbedtls_x509_crt_parse", ret);
+        return false;
+    }
+    return true;
+}
+
+Tls::Tls()
+{
+    mbedtls_x509_crt_init(&public_cert_);
+    mbedtls_pk_init(&pk_key_);
+    mbedtls_x509_crt_init(&ca_cert_);
+}
+
+int Tls::mbedtls_pk_parse_key(mbedtls_pk_context *ctx, const unsigned char *key, size_t keylen, const unsigned char *pwd, size_t pwdlen)
+{
+
+    return ::mbedtls_pk_parse_key(ctx, key, keylen, pwd, pwdlen, nullptr, nullptr);
+}
+
+size_t Tls::get_available_bytes()
+{
+    return ::mbedtls_ssl_get_bytes_avail(&ssl_);
+}

components/esp_modem/examples/modem_tcp_client/components/extra_tcp_transports/tls_transport.cpp

@@ -0,0 +1,132 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#include "esp_log.h"
+#include "esp_transport.h"
+#include "mbedtls_wrap.hpp"
+
+static const char *TAG = "tls_transport";
+
+class TlsTransport: public Tls {
+public:
+    explicit TlsTransport(esp_transport_handle_t parent) : Tls(), transport_(parent) {}
+    int send(const unsigned char *buf, size_t len) override;
+    int recv(unsigned char *buf, size_t len) override;
+    static bool set_func(esp_transport_handle_t tls_transport);
+
+private:
+    esp_transport_handle_t transport_{};
+    int connect(const char *host, int port, int timeout_ms);
+
+    struct priv {
+        static int connect(esp_transport_handle_t t, const char *host, int port, int timeout_ms);
+        static int read(esp_transport_handle_t t, char *buffer, int len, int timeout_ms);
+        static int write(esp_transport_handle_t t, const char *buffer, int len, int timeout_ms);
+        static int close(esp_transport_handle_t t);
+        static int poll_read(esp_transport_handle_t t, int timeout_ms);
+        static int poll_write(esp_transport_handle_t t, int timeout_ms);
+        static int destroy(esp_transport_handle_t t);
+    };
+};
+
+esp_transport_handle_t esp_transport_tls_init(esp_transport_handle_t parent)
+{
+    esp_transport_handle_t ssl = esp_transport_init();
+    auto *tls = new TlsTransport(parent);
+    esp_transport_set_context_data(ssl, tls);
+    TlsTransport::set_func(ssl);
+    return ssl;
+}
+
+int TlsTransport::send(const unsigned char *buf, size_t len)
+{
+    return esp_transport_write(transport_, reinterpret_cast<const char *>(buf), len, 0);
+}
+
+int TlsTransport::recv(unsigned char *buf, size_t len)
+{
+    int ret = esp_transport_read(transport_, reinterpret_cast<char *>(buf), len, 0);
+
+    if (ret == ERR_TCP_TRANSPORT_CONNECTION_TIMEOUT) {
+        return MBEDTLS_ERR_SSL_WANT_READ;
+    }
+    return ret == ERR_TCP_TRANSPORT_CONNECTION_CLOSED_BY_FIN ? 0 : ret;
+}
+
+bool TlsTransport::set_func(esp_transport_handle_t tls_transport)
+{
+    return esp_transport_set_func(tls_transport, TlsTransport::priv::connect, TlsTransport::priv::read, TlsTransport::priv::write, TlsTransport::priv::close, TlsTransport::priv::poll_read, TlsTransport::priv::poll_write, TlsTransport::priv::destroy) == ESP_OK;
+}
+
+int TlsTransport::connect(const char *host, int port, int timeout_ms)
+{
+    return esp_transport_connect(transport_, host, port, timeout_ms);
+}
+
+int TlsTransport::priv::connect(esp_transport_handle_t t, const char *host, int port, int timeout_ms)
+{
+    ESP_LOGI("tag", "SSL connect!");
+    auto tls = static_cast<TlsTransport *>(esp_transport_get_context_data(t));
+    tls->init(false, false);
+
+    ESP_LOGI("tag", "TCP connect!");
+    auto ret = tls->connect(host, port, timeout_ms);
+    if (ret < 0) {
+        ESP_LOGI("tag", "TCP connect fail!");
+        return ret;
+    }
+    return tls->handshake();
+}
+
+int TlsTransport::priv::read(esp_transport_handle_t t, char *buffer, int len, int timeout_ms)
+{
+    auto tls = static_cast<TlsTransport *>(esp_transport_get_context_data(t));
+    if (tls->get_available_bytes() <= 0) {
+        int poll = esp_transport_poll_read(t, timeout_ms);
+        if (poll == -1) {
+            return ERR_TCP_TRANSPORT_CONNECTION_FAILED;
+        }
+        if (poll == 0) {
+            return ERR_TCP_TRANSPORT_CONNECTION_TIMEOUT;
+        }
+    }
+    return tls->read(reinterpret_cast<unsigned char *>(buffer), len);
+}
+
+int TlsTransport::priv::write(esp_transport_handle_t t, const char *buffer, int len, int timeout_ms)
+{
+    int poll;
+    if ((poll = esp_transport_poll_write(t, timeout_ms)) <= 0) {
+        ESP_LOGW(TAG, "Poll timeout or error timeout_ms=%d", timeout_ms);
+        return poll;
+    }
+
+    auto tls = static_cast<TlsTransport *>(esp_transport_get_context_data(t));
+    return tls->write(reinterpret_cast<const unsigned char *>(buffer), len);
+}
+
+int TlsTransport::priv::close(esp_transport_handle_t t)
+{
+    auto tls = static_cast<TlsTransport *>(esp_transport_get_context_data(t));
+    return esp_transport_close(tls->transport_);
+}
+
+int TlsTransport::priv::poll_read(esp_transport_handle_t t, int timeout_ms)
+{
+    auto tls = static_cast<TlsTransport *>(esp_transport_get_context_data(t));
+    return esp_transport_poll_read(tls->transport_, timeout_ms);
+}
+
+int TlsTransport::priv::poll_write(esp_transport_handle_t t, int timeout_ms)
+{
+    auto tls = static_cast<TlsTransport *>(esp_transport_get_context_data(t));
+    return esp_transport_poll_write(tls->transport_, timeout_ms);
+}
+
+int TlsTransport::priv::destroy(esp_transport_handle_t t)
+{
+    auto tls = static_cast<TlsTransport *>(esp_transport_get_context_data(t));
+    return esp_transport_destroy(tls->transport_);
+}

components/esp_modem/examples/modem_tcp_client/main/CMakeLists.txt

@@ -6,6 +6,7 @@ endif()
 
 idf_component_register(SRCS "modem_client.cpp"
                             "sock_dce.cpp"
+                            "tcp_transport_at.cpp"
                             "${device_srcs}"
                        INCLUDE_DIRS ".")
 target_compile_options(${COMPONENT_LIB} PRIVATE "-Wno-format")

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

@@ -1,5 +1,10 @@
 menu "Example Configuration"
 
+    config EXAMPLE_CUSTOM_TCP_TRANSPORT
+        bool "Custom TCP transport"
+        help
+            Use custom TCP transport to connect to MQTT broker
+
     choice EXAMPLE_MODEM_DEVICE
         prompt "Choose supported modem device (DCE)"
         default EXAMPLE_MODEM_DEVICE_BG96

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

@@ -15,14 +15,17 @@
 #include "freertos/FreeRTOS.h"
 #include "freertos/event_groups.h"
 #include "esp_netif.h"
-#include "esp_netif_ppp.h"
 #include "mqtt_client.h"
 #include "esp_modem_config.h"
 #include "cxx_include/esp_modem_api.hpp"
 #include "sock_dce.hpp"
 #include "esp_log.h"
+#include "tcp_transport_mbedtls.h"
+#include "tcp_transport_at.h"
 
 #define BROKER_URL "mqtt.eclipseprojects.io"
+#define BROKER_PORT 8883
+
 
 static const char *TAG = "modem_client";
 static EventGroupHandle_t event_group = NULL;
@@ -70,6 +73,7 @@ static void mqtt_event_handler(void *handler_args, esp_event_base_t base, int32_
     }
 }
 
+
 extern "C" void app_main(void)
 {
 
@@ -101,24 +105,29 @@ extern "C" void app_main(void)
 
     /* create the DCE and initialize network manually (using AT commands) */
     auto dce = sock_dce::create(&dce_config, std::move(dte));
-    if (!dce->init_network()) {
+    if (!dce->init()) {
         ESP_LOGE(TAG,  "Failed to setup network");
         return;
     }
 
-    dce->init_sock(8883);
     esp_mqtt_client_config_t mqtt_config = {};
-#if ESP_IDF_VERSION >= ESP_IDF_VERSION_VAL(5, 0, 0)
-    mqtt_config.broker.address.uri = "mqtts://127.0.0.1";
+    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 = "mqtts://127.0.0.1";
+    dce->start_listening(BROKER_PORT);
 #else
-    mqtt_config.uri = "mqtt://127.0.0.1";
-    mqtt_config.message_retransmit_timeout = 10000;
+    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;
 #endif
     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, NULL);
     esp_mqtt_client_start(mqtt_client);
-    if (!dce->start(BROKER_URL, 8883)) {
+#ifndef CONFIG_EXAMPLE_CUSTOM_TCP_TRANSPORT
+    if (!dce->connect(BROKER_URL, BROKER_PORT)) {
         ESP_LOGE(TAG, "Failed to start DCE");
         return;
     }
@@ -128,13 +137,16 @@ extern "C" void app_main(void)
         }
         ESP_LOGE(TAG, "Loop exit.. retrying");
         // handle disconnections errors
-        if (!dce->init_network()) {
+        if (!dce->init()) {
             ESP_LOGE(TAG, "Failed to reinit network");
             return;
         }
-        if (!dce->start("test.mosquitto.org", 1883)) {
+        if (!dce->connect(BROKER_URL, BROKER_PORT)) {
             ESP_LOGI(TAG, "Network reinitialized, retrying");
         }
     }
+#else
+    vTaskDelay(portMAX_DELAY);
+#endif
 
 }

components/esp_modem/examples/modem_tcp_client/main/sock_commands.hpp

@@ -13,8 +13,6 @@
 
 namespace sock_commands {
 
-//using namespace esp_modem;
-
 #define ESP_MODEM_DECLARE_DCE_COMMAND(name, return_type, num, ...) \
         esp_modem::return_type name(esp_modem::CommandableIf *t, ## __VA_ARGS__);
 

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

@@ -6,7 +6,6 @@
 
 #include <charconv>
 #include <cstring>
-#include <sys/socket.h>
 #include "sock_commands.hpp"
 #include "cxx_include/esp_modem_command_library_utils.hpp"
 #include "sock_dce.hpp"
@@ -132,7 +131,7 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
     if (data_to_recv == 0) {
         const std::string_view head = "+QIRD: ";
         auto head_pos = std::search(recv_data, recv_data + len, head.begin(), head.end());
-        if (head_pos == nullptr) {
+        if (head_pos == recv_data + len) {
             return ret::FAIL;
         }
 
@@ -160,17 +159,17 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
         recv_data = next_nl + 1;
         auto first_data_len = len - (recv_data - (char *)data) /* minus size of the command marker */;
         if (actual_len > first_data_len) {
-            ::send(sock, recv_data, first_data_len, 0);
+            on_read(recv_data, first_data_len);
             data_to_recv = actual_len - first_data_len;
             return ret::NEED_MORE_DATA;
         }
-        ::send(sock, recv_data, actual_len, 0);
+        on_read(recv_data, actual_len);
     } else if (data_to_recv > len) {    // continue sending
-        ::send(sock, recv_data, len, 0);
+        on_read(recv_data, len);
         data_to_recv -= len;
         return ret::NEED_MORE_DATA;
     } else if (data_to_recv <= len) {    // last read -> looking for "OK" marker
-        ::send(sock, recv_data, data_to_recv, 0);
+        on_read(recv_data, data_to_recv);
         actual_len = data_to_recv;
     }
 

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

@@ -6,7 +6,6 @@
 
 #include <charconv>
 #include <cstring>
-#include <sys/socket.h>
 #include "sock_commands.hpp"
 #include "cxx_include/esp_modem_command_library_utils.hpp"
 #include "sock_dce.hpp"
@@ -218,7 +217,7 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
     if (data_to_recv == 0) {
         static constexpr std::string_view head = "+CIPRXGET: 2,0,";
         auto head_pos = std::search(recv_data, recv_data + len, head.begin(), head.end());
-        if (head_pos == nullptr) {
+        if (head_pos == recv_data + len) {
             return ret::FAIL;
         }
 
@@ -246,7 +245,7 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
             ESP_LOGE(TAG, "TOO BIG");
             return ret::FAIL;
         }
-        size_t total_len = 0;
+        total_len = 0;
         if (std::from_chars(next_comma + 1, next_nl - 1, total_len).ec == std::errc::invalid_argument) {
             ESP_LOGE(TAG, "cannot convert");
             return ret::FAIL;
@@ -255,17 +254,17 @@ Responder::ret Responder::recv(uint8_t *data, size_t len)
         recv_data = next_nl + 1;
         auto first_data_len = len - (recv_data - (char *)data) /* minus size of the command marker */;
         if (actual_len > first_data_len) {
-            ::send(sock, recv_data, first_data_len, 0);
+            on_read(recv_data, first_data_len);
             data_to_recv = actual_len - first_data_len;
             return ret::NEED_MORE_DATA;
         }
-        ::send(sock, recv_data, actual_len, 0);
+        on_read(recv_data, actual_len);
     } else if (data_to_recv > len) {    // continue sending
-        ::send(sock, recv_data, len, 0);
+        on_read(recv_data, len);
         data_to_recv -= len;
         return ret::NEED_MORE_DATA;
     } else if (data_to_recv <= len) {    // last read -> looking for "OK" marker
-        ::send(sock, recv_data, data_to_recv, 0);
+        on_read(recv_data, data_to_recv);
         actual_len = data_to_recv;
     }
 

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

@@ -192,14 +192,8 @@ bool DCE::accept_sock()
     return false;
 }
 
-void DCE::init_sock(int port)
+void DCE::start_listening(int port)
 {
-    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);
-
     listen_sock = socket(AF_INET, SOCK_STREAM, IPPROTO_IP);
     if (listen_sock < 0) {
         ESP_LOGE(TAG, "Unable to create socket: errno %d", errno);
@@ -228,7 +222,7 @@ void DCE::init_sock(int port)
 
 }
 
-bool DCE::start(std::string host, int port)
+bool DCE::connect(std::string host, int port)
 {
     dte->on_read(nullptr);
     tcp_close();
@@ -245,8 +239,14 @@ bool DCE::start(std::string host, int port)
     return true;
 }
 
-bool DCE::init_network()
+bool DCE::init()
 {
+    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;

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

@@ -9,6 +9,7 @@
 #include <cxx_include/esp_modem_dce_factory.hpp>
 #include "socket_commands.inc"
 #include "sock_commands.hpp"
+#include <sys/socket.h>
 
 #pragma  once
 
@@ -46,9 +47,36 @@ public:
     {
         return buffer_size;
     }
+
+    void clear_offsets()
+    {
+        actual_read = 0;
+    }
+
+    size_t data_available()
+    {
+        return actual_read;
+    }
+
+    size_t has_data()
+    {
+        return total_len;
+    }
+
 private:
     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);
+#else
+        ::memcpy(&buffer[actual_read], data, len);
+        actual_read += len;
+#endif
+        return true;
+    }
+
     ret recv(uint8_t *data, size_t len);
     ret send(uint8_t *data, size_t len);
     ret send(std::string_view response);
@@ -59,6 +87,9 @@ private:
     }
     std::array<uint8_t, buffer_size> buffer;
     size_t data_to_recv = 0;
+    size_t actual_read = 0;
+    size_t total_len = 0;
+
     bool read_again = false;
     int &sock;
     int &data_ready_fd;
@@ -78,13 +109,101 @@ esp_modem::return_type name(__VA_ARGS__);
 
 #undef ESP_MODEM_DECLARE_DCE_COMMAND
 
-    bool init_network();
-    bool start(std::string host, int port);
+    bool init();
+    bool connect(std::string host, int port);
 
-    void init_sock(int port);
+    void start_listening(int port);
 
     bool perform_sock();
 
+    void set_idle()
+    {
+        signal.set(IDLE);
+    }
+
+    bool wait_to_idle(uint32_t ms)
+    {
+        if (!signal.wait(IDLE, ms)) {
+            ESP_LOGE("dce", "Failed to get idle");
+            return false;
+        }
+        if (state != status::IDLE) {
+            ESP_LOGE("dce", "Unexpected state %d", state);
+            return false;
+        }
+        return true;
+    }
+
+    int sync_recv(char *buffer, int len, int timeout_ms)
+    {
+        if (!wait_to_idle(timeout_ms)) {
+            return 0;
+        }
+        at.clear_offsets();
+        state = status::RECEIVING;
+        uint64_t data;
+        read(data_ready_fd, &data, sizeof(data));
+        int max_len = std::min(len, (int)at.get_buf_len());
+        at.start_receiving(max_len);
+        if (!signal.wait(IDLE, 500 + timeout_ms)) {
+            return 0;
+        }
+        int ret = at.data_available();
+        if (ret > 0) {
+            memcpy(buffer, at.get_buf(), ret);
+        }
+        set_idle();
+        return ret;
+    }
+
+    int sync_send(const char *buffer, size_t len, int timeout_ms)
+    {
+        int len_to_send = std::min(len, at.get_buf_len());
+        if (!wait_to_idle(timeout_ms)) {
+            return -1;
+        }
+        state = status::SENDING;
+        memcpy(at.get_buf(), buffer, len_to_send);
+        ESP_LOG_BUFFER_HEXDUMP("dce", at.get_buf(), len, ESP_LOG_VERBOSE);
+        at.start_sending(len_to_send);
+        if (!signal.wait(IDLE, timeout_ms + 1000)) {
+            if (state == status::PENDING) {
+                state = status::IDLE;
+            } else {
+                return -1;
+            }
+        }
+        set_idle();
+        return len_to_send;
+    }
+
+    int wait_to_read(uint32_t ms)
+    {
+        if (at.has_data() > 0) {
+            ESP_LOGD("dce",  "Data buffered in modem (len=%d)", at.has_data());
+            return 1;
+        }
+        struct timeval tv = {
+            .tv_sec = static_cast<time_t>(ms / 1000),
+            .tv_usec = 0,
+        };
+        fd_set fdset;
+        FD_ZERO(&fdset);
+        FD_SET(data_ready_fd, &fdset);
+        int s = select(data_ready_fd + 1, &fdset, nullptr, nullptr, &tv);
+        if (s == 0) {
+            return 0;
+        } else if (s < 0) {
+            ESP_LOGE("dce",  "select error %d", errno);
+            return -1;
+        }
+        if (FD_ISSET(data_ready_fd, &fdset)) {
+            ESP_LOGD("dce", "select read: modem data available");
+            return 1;
+        }
+        return -1;
+    }
+
 private:
     esp_modem::SignalGroup signal;
 

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

@@ -0,0 +1,74 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Unlicense OR CC0-1.0
+ */
+#include "sock_dce.hpp"
+#include "esp_log.h"
+#include "esp_transport.h"
+
+
+static int tcp_connect(esp_transport_handle_t t, const char *host, int port, int timeout_ms)
+{
+    auto *dce = (sock_dce::DCE *)esp_transport_get_context_data(t);
+    if (!dce->connect(host, port)) {
+        return -1;
+    }
+    if (dce->wait_to_idle(timeout_ms)) {
+        dce->set_idle();
+        return 1;
+    }
+    return -1;
+}
+
+static int tcp_read(esp_transport_handle_t t, char *buffer, int len, int timeout_ms)
+{
+    auto *dce = (sock_dce::DCE *)esp_transport_get_context_data(t);
+    auto ret = dce->wait_to_read(1000);
+    if (ret > 0) {
+        return dce->sync_recv(buffer, len, timeout_ms);
+    }
+    return ret;
+}
+
+static int tcp_write(esp_transport_handle_t t, const char *buffer, int len, int timeout_ms)
+{
+
+    auto *dce = (sock_dce::DCE *)esp_transport_get_context_data(t);
+    return dce->sync_send(buffer, len, timeout_ms);
+
+}
+
+static int base_close(esp_transport_handle_t t)
+{
+    auto *dce = (sock_dce::DCE *)esp_transport_get_context_data(t);
+    return dce->tcp_close() == esp_modem::command_result::OK ? 0 : -1;
+}
+
+static int base_poll_read(esp_transport_handle_t t, int timeout_ms)
+{
+    auto *dce = (sock_dce::DCE *)esp_transport_get_context_data(t);
+    return dce->wait_to_read(timeout_ms);
+}
+
+static int base_poll_write(esp_transport_handle_t t, int timeout_ms)
+{
+    // expect the socket is always writable
+    return 1;
+}
+
+static int base_destroy(esp_transport_handle_t transport)
+{
+    return 0;
+}
+
+esp_transport_handle_t esp_transport_at_init(sock_dce::DCE *dce)
+{
+    esp_transport_handle_t at = esp_transport_init();
+    if (!at) {
+        return nullptr;
+    }
+    esp_transport_set_context_data(at, (void *)dce);
+    esp_transport_set_func(at, tcp_connect, tcp_read, tcp_write, base_close, base_poll_read, base_poll_write, base_destroy);
+    return at;
+}

components/esp_modem/examples/modem_tcp_client/main/tcp_transport_at.h

@@ -0,0 +1,17 @@
+/*
+ * SPDX-FileCopyrightText: 2023 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+#pragma once
+
+#include "esp_transport.h"
+#include "sock_dce.hpp"
+
+/**
+ * @brief Initializes TCP transport based on AT commands
+ *
+ * @param DCE
+ * @return Transport handle on success
+ */
+esp_transport_handle_t esp_transport_at_init(sock_dce::DCE *dce);