ESP-NOW

[英文]

概述

ESP-NOW 是一种由乐鑫公司定义的无连接 Wi-Fi 通信协议。在 ESP-NOW 中,应用程序数据被封装在各个供应商的动作帧中,然后在无连接的情况下,从一个 Wi-Fi 设备传输到另一个 Wi-Fi 设备。 CTR 与 CBC-MAC 协议 (CCMP) 可用来保护动作帧的安全。ESP-NOW 广泛应用于智能照明、远程控制、传感器等领域。

帧格式

ESP-NOW 使用各个供应商的动作帧传输数据,默认比特率为 1 Mbps。各个供应商的动作帧格式为:

-----------------------------------------------------------------------------------------
|   MAC 报头   |  分类代码  |  组织标识符  |  随机值  |  供应商特定内容  |   FCS   |
-----------------------------------------------------------------------------------------
   24 字节        1 字节        3 字节      4 字节      7~255 字节       4 字节
  • 分类代码:分类代码字段可用于指示各个供应商的类别(比如 127)。

  • 组织标识符:组织标识符包含一个唯一标识符 (比如 0x18fe34),为乐鑫指定的 MAC 地址的前三个字节。

  • 随机值:防止重放攻击。

  • 供应商特定内容:供应商特定内容包含供应商特定字段,如下所示:

----------------------------------------------------------------------------------------
|  元素 ID  |  长度  |  组织标识符  |  类型  |  版本  |     正文     |
----------------------------------------------------------------------------------------
   1 字节     1 字节     3 字节      1 字节   1 字节    0~250 字节
  • 元素 ID:元素 ID 字段可用于指示特定于供应商的元素。

  • 长度:长度是组织标识符、类型、版本和正文的总长度。

  • 组织标识符:组织标识符包含一个唯一标识符 (比如 0x18fe34),为乐鑫指定的 MAC 地址的前三个字节。

  • 类型:类型字段设置为 4,代表 ESP-NOW。

  • 版本:版本字段设置为 ESP-NOW 的版本。

  • 正文:正文包含 ESP-NOW 数据。

由于 ESP-NOW 是无连接的,因此 MAC 报头与标准帧略有不同。FrameControl 字段的 FromDS 和 ToDS 位均为 0。第一个地址字段用于配置目标地址。第二个地址字段用于配置源地址。第三个地址字段用于配置广播地址 (0xff:0xff:0xff:0xff:0xff:0xff)。

安全

ESP-NOW 采用 CCMP 方法保护供应商特定动作帧的安全,具体可参考 IEEE Std. 802.11-2012。Wi-Fi 设备维护一个初始主密钥 (PMK) 和若干本地主密钥 (LMK),长度均为 16 个字节。
  • PMK 可使用 AES-128 算法加密 LMK。请调用 esp_now_set_pmk() 设置 PMK。如果未设置 PMK,将使用默认 PMK。

  • LMK 可通过 CCMP 方法对供应商特定的动作帧进行加密,最多拥有 6 个不同的 LMK。如果未设置配对设备的 LMK,则动作帧不进行加密。

目前,不支持加密组播供应商特定的动作帧。

初始化和反初始化

调用 esp_now_init() 初始化 ESP-NOW,调用 esp_now_deinit() 反初始化 ESP-NOW。ESP-NOW 数据必须在 Wi-Fi 启动后传输,因此建议在初始化 ESP-NOW 之前启动 Wi-Fi,并在反初始化 ESP-NOW 之后停止 Wi-Fi。 当调用 esp_now_deinit() 时,配对设备的所有信息都将被删除。

添加配对设备

在将数据发送到其他设备之前,请先调用 esp_now_add_peer() 将其添加到配对设备列表中。配对设备的最大数量是 20。如果启用了加密,则必须设置 LMK。ESP-NOW 数据可以从 Station 或 Softap 接口发送。 确保在发送 ESP-NOW 数据之前已启用该接口。在发送广播数据之前必须添加具有广播 MAC 地址的设备。配对设备的信道范围是从 0 ~14。如果信道设置为 0,数据将在当前信道上发送。否则,必须使用本地设备所在的通道。

发送 ESP-NOW 数据

调用 esp_now_send() 发送 ESP-NOW 数据,调用 esp_now_register_send_cb 注册发送回调函数。如果 MAC 层成功接收到数据,则该函数将返回 ESP_NOW_SEND_SUCCESS 事件。否则,它将返回 ESP_NOW_SEND_FAIL。ESP-NOW 数据发送失败可能有几种原因,比如目标设备不存在、设备的信道不相同、动作帧在传输过程中丢失等。应用层并不一定可以总能接收到数据。如果需要,应用层可在接收 ESP-NOW 数据时发回一个应答 (ACK) 数据。如果接收 ACK 数据超时,则将重新传输 ESP-NOW 数据。可以为 ESP-NOW 数据设置序列号,从而删除重复的数据。

如果有大量 ESP-NOW 数据要发送,则调用 esp_now_send() 一次性发送不大于 250 字节的数据。 请注意,两个 ESP-NOW 数据包的发送间隔太短可能导致回调函数返回混乱。因此,建议在等到上一次回调函数返回 ACK 后再发送下一个 ESP-NOW 数据。发送回调函数从高优先级的 Wi-Fi 任务中运行。因此,不要在回调函数中执行冗长的操作。相反,将必要的数据发布到队列,并交给优先级较低的任务处理。

接收 ESP-NOW 数据

调用 esp_now_register_recv_cb 注册接收回调函数。当接收 ESP-NOW 数据时,需要调用接收回调函数。接收回调函数也在 Wi-Fi 任务任务中运行。因此,不要在回调函数中执行冗长的操作。 相反,将必要的数据发布到队列,并交给优先级较低的任务处理。

应用示例

API 参考

Functions

esp_err_t esp_now_init(void)

Initialize ESPNOW function.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_INTERNAL : Internal error

esp_err_t esp_now_deinit(void)

De-initialize ESPNOW function.

Return

  • ESP_OK : succeed

esp_err_t esp_now_get_version(uint32_t *version)

Get the version of ESPNOW.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_ARG : invalid argument

Parameters
  • version: ESPNOW version

esp_err_t esp_now_register_recv_cb(esp_now_recv_cb_t cb)

Register callback function of receiving ESPNOW data.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_INTERNAL : internal error

Parameters
  • cb: callback function of receiving ESPNOW data

esp_err_t esp_now_unregister_recv_cb(void)

Unregister callback function of receiving ESPNOW data.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

esp_err_t esp_now_register_send_cb(esp_now_send_cb_t cb)

Register callback function of sending ESPNOW data.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_INTERNAL : internal error

Parameters
  • cb: callback function of sending ESPNOW data

esp_err_t esp_now_unregister_send_cb(void)

Unregister callback function of sending ESPNOW data.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

esp_err_t esp_now_send(const uint8_t *peer_addr, const uint8_t *data, size_t len)

Send ESPNOW data.

Attention

1. If peer_addr is not NULL, send data to the peer whose MAC address matches peer_addr

Attention

2. If peer_addr is NULL, send data to all of the peers that are added to the peer list

Attention

3. The maximum length of data must be less than ESP_NOW_MAX_DATA_LEN

Attention

4. The buffer pointed to by data argument does not need to be valid after esp_now_send returns

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_ARG : invalid argument

  • ESP_ERR_ESPNOW_INTERNAL : internal error

  • ESP_ERR_ESPNOW_NO_MEM : out of memory

  • ESP_ERR_ESPNOW_NOT_FOUND : peer is not found

  • ESP_ERR_ESPNOW_IF : current WiFi interface doesn’t match that of peer

Parameters
  • peer_addr: peer MAC address

  • data: data to send

  • len: length of data

esp_err_t esp_now_add_peer(const esp_now_peer_info_t *peer)

Add a peer to peer list.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_ARG : invalid argument

  • ESP_ERR_ESPNOW_FULL : peer list is full

  • ESP_ERR_ESPNOW_NO_MEM : out of memory

  • ESP_ERR_ESPNOW_EXIST : peer has existed

Parameters
  • peer: peer information

esp_err_t esp_now_del_peer(const uint8_t *peer_addr)

Delete a peer from peer list.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_ARG : invalid argument

  • ESP_ERR_ESPNOW_NOT_FOUND : peer is not found

Parameters
  • peer_addr: peer MAC address

esp_err_t esp_now_mod_peer(const esp_now_peer_info_t *peer)

Modify a peer.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_ARG : invalid argument

  • ESP_ERR_ESPNOW_FULL : peer list is full

Parameters
  • peer: peer information

esp_err_t esp_now_get_peer(const uint8_t *peer_addr, esp_now_peer_info_t *peer)

Get a peer whose MAC address matches peer_addr from peer list.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_ARG : invalid argument

  • ESP_ERR_ESPNOW_NOT_FOUND : peer is not found

Parameters
  • peer_addr: peer MAC address

  • peer: peer information

esp_err_t esp_now_fetch_peer(bool from_head, esp_now_peer_info_t *peer)

Fetch a peer from peer list. Only return the peer which address is unicast, for the multicast/broadcast address, the function will ignore and try to find the next in the peer list.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_ARG : invalid argument

  • ESP_ERR_ESPNOW_NOT_FOUND : peer is not found

Parameters
  • from_head: fetch from head of list or not

  • peer: peer information

bool esp_now_is_peer_exist(const uint8_t *peer_addr)

Peer exists or not.

Return

  • true : peer exists

  • false : peer not exists

Parameters
  • peer_addr: peer MAC address

esp_err_t esp_now_get_peer_num(esp_now_peer_num_t *num)

Get the number of peers.

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_ARG : invalid argument

Parameters
  • num: number of peers

esp_err_t esp_now_set_pmk(const uint8_t *pmk)

Set the primary master key.

Attention

1. primary master key is used to encrypt local master key

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

  • ESP_ERR_ESPNOW_ARG : invalid argument

Parameters
  • pmk: primary master key

esp_err_t esp_now_set_wake_window(uint16_t window)

Set esp_now wake window for sta_disconnected power management.

Attention

1. Only when ESP_WIFI_STA_DISCONNECTED_PM_ENABLE is enabled, this configuration could work

Attention

2. This configuration only work for station mode and disconnected status

Attention

3. If more than one module has configured its wake_window, chip would choose the largest one to stay waked

Attention

4. If the gap between interval and window is smaller than 5ms, the chip would keep waked all the time

Attention

5. If never configured wake_window, the chip would keep waked at disconnected once it uses esp_now

Return

  • ESP_OK : succeed

  • ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

Parameters
  • window: how much microsecond would the chip keep waked each interval, vary from 0 to 65535

Structures

struct esp_now_peer_info

ESPNOW peer information parameters.

Public Members

uint8_t peer_addr[ESP_NOW_ETH_ALEN]

ESPNOW peer MAC address that is also the MAC address of station or softap

uint8_t lmk[ESP_NOW_KEY_LEN]

ESPNOW peer local master key that is used to encrypt data

uint8_t channel

Wi-Fi channel that peer uses to send/receive ESPNOW data. If the value is 0, use the current channel which station or softap is on. Otherwise, it must be set as the channel that station or softap is on.

wifi_interface_t ifidx

Wi-Fi interface that peer uses to send/receive ESPNOW data

bool encrypt

ESPNOW data that this peer sends/receives is encrypted or not

void *priv

ESPNOW peer private data

struct esp_now_peer_num

Number of ESPNOW peers which exist currently.

Public Members

int total_num

Total number of ESPNOW peers, maximum value is ESP_NOW_MAX_TOTAL_PEER_NUM

int encrypt_num

Number of encrypted ESPNOW peers, maximum value is ESP_NOW_MAX_ENCRYPT_PEER_NUM

Macros

ESP_ERR_ESPNOW_BASE

ESPNOW error number base.

ESP_ERR_ESPNOW_NOT_INIT

ESPNOW is not initialized.

ESP_ERR_ESPNOW_ARG

Invalid argument

ESP_ERR_ESPNOW_NO_MEM

Out of memory

ESP_ERR_ESPNOW_FULL

ESPNOW peer list is full

ESP_ERR_ESPNOW_NOT_FOUND

ESPNOW peer is not found

ESP_ERR_ESPNOW_INTERNAL

Internal error

ESP_ERR_ESPNOW_EXIST

ESPNOW peer has existed

ESP_ERR_ESPNOW_IF

Interface error

ESP_NOW_ETH_ALEN

Length of ESPNOW peer MAC address

ESP_NOW_KEY_LEN

Length of ESPNOW peer local master key

ESP_NOW_MAX_TOTAL_PEER_NUM

Maximum number of ESPNOW total peers

ESP_NOW_MAX_ENCRYPT_PEER_NUM

Maximum number of ESPNOW encrypted peers

ESP_NOW_MAX_DATA_LEN

Maximum length of ESPNOW data which is sent very time

Type Definitions

typedef struct esp_now_peer_info esp_now_peer_info_t

ESPNOW peer information parameters.

typedef struct esp_now_peer_num esp_now_peer_num_t

Number of ESPNOW peers which exist currently.

typedef void (*esp_now_recv_cb_t)(const uint8_t *mac_addr, const uint8_t *data, int data_len)

Callback function of receiving ESPNOW data.

Parameters
  • mac_addr: peer MAC address

  • data: received data

  • data_len: length of received data

typedef void (*esp_now_send_cb_t)(const uint8_t *mac_addr, esp_now_send_status_t status)

Callback function of sending ESPNOW data.

Parameters
  • mac_addr: peer MAC address

  • status: status of sending ESPNOW data (succeed or fail)

Enumerations

enum esp_now_send_status_t

Status of sending ESPNOW data .

Values:

ESP_NOW_SEND_SUCCESS = 0

Send ESPNOW data successfully

ESP_NOW_SEND_FAIL

Send ESPNOW data fail