# ESP-NOW

[中文]

## Overview

ESP-NOW is a kind of connectionless Wi-Fi communication protocol that is defined by Espressif. In ESP-NOW, application data is encapsulated in a vendor-specific action frame and then transmitted from one Wi-Fi device to another without connection.

CTR with CBC-MAC Protocol (CCMP) is used to protect the action frame for security. ESP-NOW is widely used in smart light, remote controlling, sensor, etc.

## Frame Format

ESP-NOW uses a vendor-specific action frame to transmit ESP-NOW data. The default ESP-NOW bit rate is 1 Mbps.

Currently, ESP-NOW supports one version: v1.0. The maximum packet length supported by v1.0 devices is ESP_NOW_MAX_DATA_LEN bytes. The v1.0 devices can receive packets if the packet length is less than or equal to ESP_NOW_MAX_IE_DATA_LEN. For packets exceeding this length, the v1.0 devices will discard the packet entirely.

The format of the vendor-specific action frame is as follows:

    ------------------------------------------------------------------------------------------------------------
    | MAC Header | Category Code | Organization Identifier | Random Values | Vendor Specific Content |   FCS   |
    ------------------------------------------------------------------------------------------------------------
      24 bytes         1 byte              3 bytes               4 bytes             7-x bytes        4 bytes

- Category Code: The Category Code field is set to the value (127) indicating the vendor-specific category.

- Organization Identifier: The Organization Identifier contains a unique identifier (0x18fe34), which is the first three bytes of MAC address applied by Espressif.

- Random Value: The Random Value filed is used to prevents relay attacks.

- Vendor Specific Content: The Vendor Specific Content contains one vendor-specific element field, x = 257(250 + 7).

The format of the vendor-specific element frame is as follows:

    ------------------------------------------------------------------------------------------
    | Element ID | Length | Organization Identifier | Type | Reserved | Version |    Body    |
    ------------------------------------------------------------------------------------------
                                                             7~4 bits | 3~0 bits
        1 byte     1 byte            3 bytes         1 byte        1 byte         0-250 bytes

- Element ID: The Element ID field is set to the value (221), indicating the vendor-specific element.

- Length: The length is the total length of Organization Identifier, Type, Version and Body, the maximum value is 255.

- Organization Identifier: The Organization Identifier contains a unique identifier (0x18fe34), which is the first three bytes of MAC address applied by Espressif.

- Type: The Type field is set to the value (4) indicating ESP-NOW.

- Version: The Version field is set to the version of ESP-NOW.

- Body: The Body contains the actual ESP-NOW data to be transmitted.

As ESP-NOW is connectionless, the MAC header is a little different from that of standard frames. The FromDS and ToDS bits of FrameControl field are both 0. The first address field is set to the destination address. The second address field is set to the source address. The third address field is set to broadcast address (0xff:0xff:0xff:0xff:0xff:0xff).

## Security

ESP-NOW uses the CCMP method, which is described in IEEE Std. 802.11-2012, to protect the vendor-specific action frame. The Wi-Fi device maintains a Primary Master Key (PMK) and several Local Master Keys (LMKs, each paired device has one LMK). The lengths of both PMK and LMK are 16 bytes.

> - PMK is used to encrypt LMK with the AES-128 algorithm. Call `esp_now_set_pmk()` to set PMK. If PMK is not set, a default PMK will be used.

> - LMK of the paired device is used to encrypt the vendor-specific action frame with the CCMP method. If the LMK of the paired device is not set, the vendor-specific action frame will not be encrypted.

Encrypting multicast vendor-specific action frame is not supported.

## Initialization and Deinitialization

Call `esp_now_init()` to initialize ESP-NOW and `esp_now_deinit()` to de-initialize ESP-NOW. ESP-NOW data must be transmitted after Wi-Fi is started, so it is recommended to start Wi-Fi before initializing ESP-NOW and stop Wi-Fi after de-initializing ESP-NOW.

When `esp_now_deinit()` is called, all of the information of paired devices are deleted.

## Add Paired Device

Call `esp_now_add_peer()` to add the device to the paired device list before you send data to this device. If security is enabled, the LMK must be set. A device with a broadcast MAC address must be added before sending broadcast data.

You can send ESP-NOW data via both the Station and the SoftAP interface. Make sure that the interface is enabled before sending ESP-NOW data.

The range of the channel of paired devices is from 0 to 14. If the channel is set to 0, data will be sent on the current channel. Otherwise, the channel must be set as the channel that the local device is on.

For the receiving device, calling `esp_now_add_peer()` is not required. If no paired device is added, it can only receive broadcast packets and unencrypted unicast packets. To receive encrypted unicast packets, a paired device must be added, and the same LMK must be set.

The maximum number of paired devices is 20, and the paired encryption devices are no more than 4, the default is 2. If you want to change the number of paired encryption devices, set CONFIG_ESP_WIFI_ESPNOW_MAX_ENCRYPT_NUM in the Wi-Fi component configuration menu.

## Send ESP-NOW Data

Call `esp_now_send()` to send ESP-NOW data and `esp_now_register_send_cb()` to register sending callback function. It will return ESP_NOW_SEND_SUCCESS in sending callback function if the data is received successfully on the MAC layer. Otherwise, it will return ESP_NOW_SEND_FAIL. Several reasons can lead to ESP-NOW fails to send data. For example, the destination device does not exist; the channels of the devices are not the same; the action frame is lost when transmitting on the air, etc. It is not guaranteed that application layer can receive the data. If necessary, send back ack data when receiving ESP-NOW data. If receiving ack data timeouts, retransmit the ESP-NOW data. A sequence number can also be assigned to ESP-NOW data to drop the duplicate data.

If there is a lot of ESP-NOW data to send, call `esp_now_send()` to send less than or equal to 250 bytes of data once a time. Note that too short interval between sending two ESP-NOW data may lead to disorder of sending callback function. So, it is recommended that sending the next ESP-NOW data after the sending callback function of the previous sending has returned. The sending callback function runs from a high-priority Wi-Fi task. So, do not do lengthy operations in the callback function. Instead, post the necessary data to a queue and handle it from a lower priority task.

## Receiving ESP-NOW Data

Call `esp_now_register_recv_cb()` to register receiving callback function. Call the receiving callback function when receiving ESP-NOW. The receiving callback function also runs from the Wi-Fi task. So, do not do lengthy operations in the callback function. Instead, post the necessary data to a queue and handle it from a lower priority task.

## Config ESP-NOW Rate

Call `esp_wifi_config_espnow_rate()` to config ESP-NOW rate of specified interface. Make sure that the interface is enabled before config rate. This API should be called after `esp_wifi_start()`.

## Config ESP-NOW Power-saving Parameter

Sleep is supported only when ESP32-C2 is configured as station.

Call `esp_now_set_wake_window()` to configure Window for ESP-NOW RX at sleep. The default value is the maximum, which allowing RX all the time.

If Power-saving is needed for ESP-NOW, call `esp_wifi_connectionless_module_set_wake_interval()` to configure Interval as well.

Please refer to connectionless module power save to get more detail.

## Application Examples

- Example of sending and receiving ESP-NOW data between two devices: wifi/espnow.

- For more application examples of how to use ESP-NOW, please visit ESP-NOW repository.

## API Reference

### Header File

- components/esp_wifi/include/esp_now.h

- This header file can be included with:

  >     #include "esp_now.h"

- This header file is a part of the API provided by the `esp_wifi` component. To declare that your component depends on `esp_wifi`, add the following to your CMakeLists.txt:

  >     REQUIRES esp_wifi

  > or

  >     PRIV_REQUIRES esp_wifi

### Functions

esp_err_t esp_now_init(void)

  Initialize ESPNOW function.

  Returns:

    - ESP_OK : succeed

    - ESP_ERR_ESPNOW_INTERNAL : Internal error

esp_err_t esp_now_deinit(void)

  De-initialize ESPNOW function.

  Returns:

    - ESP_OK : succeed

esp_err_t esp_now_get_version(uint32_t *version)

  Get the version of ESPNOW. Currently, ESPNOW supports one version: v1.0.

             The v1.0 devices can receive packets if the packet length is less than or equal to ESP_NOW_MAX_IE_DATA_LEN.
             For packets exceeding this length, the v1.0 devices will discard the packet entirely.

  Parameters:

    version -- ESPNOW version

  Returns:

    - ESP_OK : succeed

    - ESP_ERR_ESPNOW_ARG : invalid argument

esp_err_t esp_now_register_recv_cb(esp_now_recv_cb_t cb)

  Register callback function of receiving ESPNOW data.

  Parameters:

    cb -- callback function of receiving ESPNOW data

  Returns:

    - ESP_OK : succeed

    - ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

    - ESP_ERR_ESPNOW_INTERNAL : internal error

esp_err_t esp_now_unregister_recv_cb(void)

  Unregister callback function of receiving ESPNOW data.

  Returns:

    - 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.

  Parameters:

    cb -- callback function of sending ESPNOW data

  Returns:

    - ESP_OK : succeed

    - ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

    - ESP_ERR_ESPNOW_INTERNAL : internal error

esp_err_t esp_now_unregister_send_cb(void)

  Unregister callback function of sending ESPNOW data.

  Returns:

    - 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

  Parameters:

    - peer_addr -- peer MAC address

    - data -- data to send

    - len -- length of data

  Returns:

    - 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, when this happens, you can delay a while before sending the next data

    - ESP_ERR_ESPNOW_NOT_FOUND : peer is not found

    - ESP_ERR_ESPNOW_IF : current Wi-Fi interface doesn't match that of peer

    - ESP_ERR_ESPNOW_CHAN: current Wi-Fi channel doesn't match that of peer

esp_err_t esp_now_add_peer(const esp_now_peer_info_t *peer)

  Add a peer to peer list.

  Parameters:

    peer -- peer information

  Returns:

    - 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

esp_err_t esp_now_del_peer(const uint8_t *peer_addr)

  Delete a peer from peer list.

  Parameters:

    peer_addr -- peer MAC address

  Returns:

    - 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

esp_err_t esp_now_mod_peer(const esp_now_peer_info_t *peer)

  Modify a peer.

  Parameters:

    peer -- peer information

  Returns:

    - 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_t esp_wifi_config_espnow_rate(wifi_interface_t ifx, wifi_phy_rate_t rate)

  Config ESPNOW rate of specified interface.

  Deprecated:

    please use esp_now_set_peer_rate_config() instead.

  Attention

    1. This API should be called after esp_wifi_start().

  Attention

    2. This API only work when not use Wi-Fi 6 and esp_now_set_peer_rate_config() not called.

  Parameters:

    - ifx -- Interface to be configured.

    - rate -- Phy rate to be configured.

  Returns:

    - ESP_OK: succeed

    - others: failed

esp_err_t esp_now_set_peer_rate_config(const uint8_t *peer_addr, esp_now_rate_config_t *config)

  Set ESPNOW rate config for each peer.

  Attention

    1. This API should be called after esp_wifi_start() and esp_now_init().

  Parameters:

    - peer_addr -- peer MAC address

    - config -- rate config to be configured.

  Returns:

    - 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_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.

  Parameters:

    - peer_addr -- peer MAC address

    - peer -- peer information

  Returns:

    - 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

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.

  Parameters:

    - from_head -- fetch from head of list or not

    - peer -- peer information

  Returns:

    - 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

bool esp_now_is_peer_exist(const uint8_t *peer_addr)

  Peer exists or not.

  Parameters:

    peer_addr -- peer MAC address

  Returns:

    - true : peer exists

    - false : peer not exists

esp_err_t esp_now_get_peer_num(esp_now_peer_num_t *num)

  Get the number of peers.

  Parameters:

    num -- number of peers

  Returns:

    - ESP_OK : succeed

    - ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

    - ESP_ERR_ESPNOW_ARG : invalid argument

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

  Parameters:

    pmk -- primary master key

  Returns:

    - ESP_OK : succeed

    - ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

    - ESP_ERR_ESPNOW_ARG : invalid argument

esp_err_t esp_now_set_wake_window(uint16_t window)

  Set wake window for esp_now to wake up in interval unit.

  Attention

    1. This configuration could work at connected status. When ESP_WIFI_STA_DISCONNECTED_PM_ENABLE is enabled, this configuration could work at disconnected status.

  Attention

    2. Default value is the maximum.

  Parameters:

    window -- Milliseconds would the chip keep waked each interval, from 0 to 65535.

  Returns:

    - ESP_OK : succeed

    - ESP_ERR_ESPNOW_NOT_INIT : ESPNOW is not initialized

### 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

struct esp_now_recv_info

  ESPNOW packet information.

  Public Members

  uint8_t *src_addr

    Source address of ESPNOW packet

  uint8_t *des_addr

    Destination address of ESPNOW packet

  wifi_pkt_rx_ctrl_t *rx_ctrl

    Rx control info of ESPNOW packet

### 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_ERR_ESPNOW_CHAN

  Channel 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_IE_DATA_LEN

  Maximum data length in a vendor-specific element

ESP_NOW_MAX_DATA_LEN

  Maximum length of data sent in each ESPNOW transmission for v1.0

### 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 struct esp_now_recv_info esp_now_recv_info_t

  ESPNOW packet information.

typedef wifi_tx_rate_config_t esp_now_rate_config_t

  ESPNOW rate config.

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

  Callback function of receiving ESPNOW data.

  Attention

    esp_now_info is a local variable，it can only be used in the callback.

  Param esp_now_info:

    received ESPNOW packet information

  Param data:

    received data

  Param 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.

  Param mac_addr:

    peer MAC address

  Param status:

    status of sending ESPNOW data (succeed or fail)

### Enumerations

enum esp_now_send_status_t

  Status of sending ESPNOW data .

  Values:

  enumerator ESP_NOW_SEND_SUCCESS

    Send ESPNOW data successfully

  enumerator ESP_NOW_SEND_FAIL

    Send ESPNOW data fail

Provide feedback about this document