COAP協議 - arduino ESP32 M2M（端對端）通訊與代碼詳解

前言

最近我在研究 COAP 協議，在嘗試使用 COAP 協議找了到了一個能在ESP32上用的coap-simple庫，雖然庫並不完善關於loop處理的部分應該是沒寫完，但是對於第一次接觸COAP的朋友來說更容易理解，方便學習，需要的朋友可以去下面下載：

https://github.com/hirotakaster/CoAP-simple-library

我之前使用 IOT PI 的 COAP 能和 PC node coap 通訊，但是因為 coap-simple 庫不完善，正常的無法與 node coap 通訊，只能和同樣使用這個庫設備通訊，這次就來嘗試 ESP32 之間的 M2M 通訊。

獲取庫

使用 arduino IDE 就能下載到這個庫：

如果沒有看到這個庫，可以去首選項添加一下附加開發板管理器網址：

https://github.com/espressif/arduino-esp32/releases/download/1.0.5/package_esp32_index.json

具體使用可以參考的我 arduino 超詳細的開發入門指導 或者直接通過我上面發的 GitHub 網址下載。

代碼解析

以下代碼為了方便講解，可能經過了調換了順序或者裁剪。

這個 demo 是客戶端、服務端一體的，只需要注冊對應的回調函數就行。

初始化部分

這部分包括了設備初始化，協議初始化等部分，重點在服務器/客戶端的回調函數部分。和 SDDC 官方demo類似，在這注冊回調函數之后，通過對應的端點找到對應的回調函數。

#include <WiFi.h>
#include <WiFiUdp.h>
#include <coap-simple.h>

void setup() {
  Serial.begin(115200);

  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
      delay(500);
      Serial.print(".");
  }

  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println("IP address: ");
  Serial.println(WiFi.localIP());

  // LED State
  pinMode(9, OUTPUT);
  digitalWrite(9, HIGH);
  LEDSTATE = true;
  
  // 添加服務器url端點.
  // 可以添加多個端點url.
  //      coap.server(callback_switch, "switch");
  //      coap.server(callback_env, "env/temp");
  //      coap.server(callback_env, "env/humidity");
  Serial.println("Setup Callback Light");
  // 其實就是注冊服務器處理回調函數
  // 將處理函數指針與url添加到 uri.add 中 
  coap.server(callback_light, "light");

  // 注冊客戶端響應的回調函數。
  // this endpoint is single callback.
  Serial.println("Setup Response Callback");
  // 很上面一樣，其實就是把回調函數指針注冊到resp里
  coap.response(callback_response);

  // 使用默認端口5683 啟動  coap server/client 
  coap.start();
}

void loop() {
  // 作為客戶端時向coap服務器發送GET或PUT coap請求.
  // 可以發送給另外一個 ESP32 
  // msgid = coap.put(IPAddress(192, 168, 128, 101), 5683, "light", "0");
  // msgid = coap.get(IPAddress(192, 168, 128, 101), 5683, "light");

  delay(1000);
  coap.loop();
}

回調函數

// CoAP 服務器端點 URL ,對客戶端發過來的命令進行處理並且回應
void callback_light(CoapPacket &packet, IPAddress ip, int port) 
{
  // 這是一個模擬控燈的回調函數，通過接收的命令
  Serial.println("[Light] ON/OFF");
  Serial.println(packet.messageid);

  // 發送響應
  char p[packet.payloadlen + 1];
  memcpy(p, packet.payload, packet.payloadlen);
  p[packet.payloadlen] = NULL;
  
  String message(p);

  if (message.equals("0"))
    LEDSTATE = false;
  else if(message.equals("1"))
    LEDSTATE = true;
      
  if (LEDSTATE) {
    digitalWrite(9, HIGH) ; 
      Serial.println("[Light] ON");

    coap.sendResponse(ip, port, packet.messageid, "1");
  } else { 
    digitalWrite(9, LOW) ; 
    Serial.println("[Light] OFF");
    coap.sendResponse(ip, port, packet.messageid, "0");
  }
}

// CoAP客戶端響應回調
void callback_response(CoapPacket &packet, IPAddress ip, int port) 
{
  Serial.println("[Coap Response got]");
  
  char p[packet.payloadlen + 1];
  memcpy(p, packet.payload, packet.payloadlen);
  p[packet.payloadlen] = NULL;
  
  Serial.println(p);
}

庫代碼

報文結構定義：

// 確定消息類型，在 coap 消息層
typedef enum {
    COAP_CON = 0,     // 可靠傳輸
    COAP_NONCON = 1,  // 不可靠傳輸
    COAP_ACK = 2,     // 回復
    COAP_RESET = 3    // 報文異常后的被動重發請求
} COAP_TYPE;
// 命令執行的動作，在請求/響應層
typedef enum {
    COAP_GET = 1,
    COAP_POST = 2,    // 主動的重發命令
    COAP_PUT = 3,
    COAP_DELETE = 4
} COAP_METHOD;
// 響應碼，相當於函數返回值或者err碼之類的，在請求/響應層
typedef enum {
    COAP_CREATED = RESPONSE_CODE(2, 1),
    COAP_DELETED = RESPONSE_CODE(2, 2),
    COAP_VALID = RESPONSE_CODE(2, 3),
    COAP_CHANGED = RESPONSE_CODE(2, 4),
    COAP_CONTENT = RESPONSE_CODE(2, 5),
    COAP_BAD_REQUEST = RESPONSE_CODE(4, 0),
    COAP_UNAUTHORIZED = RESPONSE_CODE(4, 1),
    COAP_BAD_OPTION = RESPONSE_CODE(4, 2),
    COAP_FORBIDDEN = RESPONSE_CODE(4, 3),
    COAP_NOT_FOUNT = RESPONSE_CODE(4, 4),
    COAP_METHOD_NOT_ALLOWD = RESPONSE_CODE(4, 5),
    COAP_NOT_ACCEPTABLE = RESPONSE_CODE(4, 6),
    COAP_PRECONDITION_FAILED = RESPONSE_CODE(4, 12),
    COAP_REQUEST_ENTITY_TOO_LARGE = RESPONSE_CODE(4, 13),
    COAP_UNSUPPORTED_CONTENT_FORMAT = RESPONSE_CODE(4, 15),
    COAP_INTERNAL_SERVER_ERROR = RESPONSE_CODE(5, 0),
    COAP_NOT_IMPLEMENTED = RESPONSE_CODE(5, 1),
    COAP_BAD_GATEWAY = RESPONSE_CODE(5, 2),
    COAP_SERVICE_UNAVALIABLE = RESPONSE_CODE(5, 3),
    COAP_GATEWAY_TIMEOUT = RESPONSE_CODE(5, 4),
    COAP_PROXYING_NOT_SUPPORTED = RESPONSE_CODE(5, 5)
} COAP_RESPONSE_CODE;
// Option 編號 ，在 coap 消息層
typedef enum {
    COAP_IF_MATCH = 1,
    COAP_URI_HOST = 3,
    COAP_E_TAG = 4,
    COAP_IF_NONE_MATCH = 5,
    COAP_URI_PORT = 7,
    COAP_LOCATION_PATH = 8,
    COAP_URI_PATH = 11,
    COAP_CONTENT_FORMAT = 12,
    COAP_MAX_AGE = 14,
    COAP_URI_QUERY = 15,
    COAP_ACCEPT = 17,
    COAP_LOCATION_QUERY = 20,
    COAP_PROXY_URI = 35,
    COAP_PROXY_SCHEME = 39
} COAP_OPTION_NUMBER;
// 內容類型和 Accept 用於表示CoAP負載的媒體格式
typedef enum {
    COAP_NONE = -1,
    COAP_TEXT_PLAIN = 0,
    COAP_APPLICATION_LINK_FORMAT = 40,
    COAP_APPLICATION_XML = 41,
    COAP_APPLICATION_OCTET_STREAM = 42,
    COAP_APPLICATION_EXI = 47,
    COAP_APPLICATION_JSON = 50,
    COAP_APPLICATION_CBOR = 60
} COAP_CONTENT_TYPE;

class CoapOption {
    public:
    uint8_t number;
    uint8_t length;
    uint8_t *buffer;
};

class CoapPacket {
    public:
		uint8_t type = 0;
		uint8_t code = 0;
		const uint8_t *token = NULL;
		uint8_t tokenlen = 0;
		const uint8_t *payload = NULL;
		size_t payloadlen = 0;
		uint16_t messageid = 0;
		uint8_t optionnum = 0;
		CoapOption options[COAP_MAX_OPTION_NUM];

		void addOption(uint8_t number, uint8_t length, uint8_t *opt_payload);
};

組包發送：
在這里填寫包的UDP需要地址，端口，端點等路徑相關信息以及 COAP 請求/響應層的信息

uint16_t Coap::send(IPAddress ip, int port, const char *url, COAP_TYPE type, COAP_METHOD method, const uint8_t *token, uint8_t tokenlen, const uint8_t *payload, size_t payloadlen, COAP_CONTENT_TYPE content_type) {

    // make packet
    CoapPacket packet;

    packet.type = type;
    packet.code = method;
    packet.token = token;
    packet.tokenlen = tokenlen;
    packet.payload = payload;
    packet.payloadlen = payloadlen;
    packet.optionnum = 0;
    packet.messageid = rand();

    // use URI_HOST UIR_PATH
    char ipaddress[16] = "";
    sprintf(ipaddress, "%d.%d.%d.%d", ip[0], ip[1], ip[2], ip[3]);
    packet.addOption(COAP_URI_HOST, strlen(ipaddress), (uint8_t *)ipaddress);

    // parse url
    int idx = 0;
    for (int i = 0; i < strlen(url); i++) {
        if (url[i] == '/') {
			packet.addOption(COAP_URI_PATH, i-idx, (uint8_t *)(url + idx));
            idx = i + 1;
        }
    }

    if (idx <= strlen(url)) {
		packet.addOption(COAP_URI_PATH, strlen(url)-idx, (uint8_t *)(url + idx));
    }

	// if Content-Format option
	uint8_t optionBuffer[2] {0};
	if (content_type != COAP_NONE) {
		optionBuffer[0] = ((uint16_t)content_type & 0xFF00) >> 8;
		optionBuffer[1] = ((uint16_t)content_type & 0x00FF) ;
		packet.addOption(COAP_CONTENT_FORMAT, 2, optionBuffer);
	}

    // send packet
    return this->sendPacket(packet, ip, port);
}

在這里的組裝 coap 包消息層的數據

uint16_t Coap::sendPacket(CoapPacket &packet, IPAddress ip, int port) {
    uint8_t buffer[COAP_BUF_MAX_SIZE];
    uint8_t *p = buffer;
    uint16_t running_delta = 0;
    uint16_t packetSize = 0;

    // 制作coap包基頭
    *p = 0x01 << 6;
    *p |= (packet.type & 0x03) << 4;
    *p++ |= (packet.tokenlen & 0x0F);
    *p++ = packet.code;
    *p++ = (packet.messageid >> 8);
    *p++ = (packet.messageid & 0xFF);
    p = buffer + COAP_HEADER_SIZE;
    packetSize += 4;

    // make token
    if (packet.token != NULL && packet.tokenlen <= 0x0F) {
        memcpy(p, packet.token, packet.tokenlen);
        p += packet.tokenlen;
        packetSize += packet.tokenlen;
    }

    // make option header
    for (int i = 0; i < packet.optionnum; i++)  {
        uint32_t optdelta;
        uint8_t len, delta;

        if (packetSize + 5 + packet.options[i].length >= COAP_BUF_MAX_SIZE) {
            return 0;
        }
        optdelta = packet.options[i].number - running_delta;
        COAP_OPTION_DELTA(optdelta, &delta);
        COAP_OPTION_DELTA((uint32_t)packet.options[i].length, &len);

        *p++ = (0xFF & (delta << 4 | len));
        if (delta == 13) {
            *p++ = (optdelta - 13);
            packetSize++;
        } else if (delta == 14) {
            *p++ = ((optdelta - 269) >> 8);
            *p++ = (0xFF & (optdelta - 269));
            packetSize+=2;
        } if (len == 13) {
            *p++ = (packet.options[i].length - 13);
            packetSize++;
        } else if (len == 14) {
            *p++ = (packet.options[i].length >> 8);
            *p++ = (0xFF & (packet.options[i].length - 269));
            packetSize+=2;
        }

        memcpy(p, packet.options[i].buffer, packet.options[i].length);
        p += packet.options[i].length;
        packetSize += packet.options[i].length + 1;
        running_delta = packet.options[i].number;
    }

    // make payload
    if (packet.payloadlen > 0) {
        if ((packetSize + 1 + packet.payloadlen) >= COAP_BUF_MAX_SIZE) {
            return 0;
        }
        *p++ = 0xFF;
        memcpy(p, packet.payload, packet.payloadlen);
        packetSize += 1 + packet.payloadlen;
    }

    _udp->beginPacket(ip, port);
    _udp->write(buffer, packetSize);
    _udp->endPacket();

    return packet.messageid;
}

因為這個庫解包的loop部分沒做完所以這里就先不說了

結果展示

COAP 客戶端發送了ID 為20125,24157,12868的三個消息，然后服務器端返回了這三個消息，並帶上了數據，客戶端也got 到了需要的數據。

總結

感覺很怪？怪就對了，這個 demo 並不完善，只是這個庫比較簡單方便理解，同時有一個基本框架，看懂這個代碼更容易理解 COAP 。