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ESP-NOW

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 two versions: v1.0 and v2.0. The maximum packet length supported by v2.0 devices is 1490 (ESP_NOW_MAX_DATA_LEN_V2) bytes, while the maximum packet length supported by v1.0 devices is 250 (ESP_NOW_MAX_DATA_LEN) bytes.

The v2.0 devices are capable of receiving packets from both v2.0 and v1.0 devices. In contrast, v1.0 devices can only receive packets from other v1.0 devices.

However, v1.0 devices can receive v2.0 packets if the packet length is less than or equal to 250 (ESP_NOW_MAX_IE_DATA_LEN).For packets exceeding this length, the v1.0 devices will either truncate the data to the first 250 (ESP_NOW_MAX_IE_DATA_LEN) bytes or discard the packet entirely.

For detailed behavior, please refer to the documentation corresponding to the specific IDF version.

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 several (at least one) vendor-specific element fields. For version v2.0, x = 1532(1490 + 6*7), for version v1.0, x = 257(250 + 7).

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

ESP-NOW v1.0:
------------------------------------------------------------------------------------------
| 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

ESP-NOW v2.0:
-----------------------------------------------------------------------------------------------------
| Element ID | Length | Organization Identifier | Type | Reserved | More data |Version |    Body    |
-----------------------------------------------------------------------------------------------------
                                                         7~5 bits |   1 bit   |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. 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.

A device with a broadcast MAC address must be added before sending broadcast 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.

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

Config ESP-NOW Power-saving Parameter

Sleep is supported only when ESP32-C5 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

wifi/espnow demonstrates how to use the ESPNOW feature of ESP32-C5's Wi-Fi, including starting Wi-Fi, initializing ESP-NOW, registering ESP-NOW sending or receiving callback function, adding ESP-NOW peer information, and sending and receiving ESP-NOW data between two devices.

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 two versions: v1.0 and v2.0.

       The v2.0 devices are capable of receiving packets from both v2.0 and v1.0 devices. In contrast, v1.0 devices can only receive packets from other v1.0 devices.
       However, v1.0 devices can receive v2.0 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 either truncate the data to the first ESP_NOW_MAX_IE_DATA_LEN bytes or discard the packet entirely.
       For detailed behavior, please refer to the documentation corresponding to the specific IDF version.

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

struct esp_now_rate_config

ESPNOW rate config.

Public Members

wifi_phy_mode_t phymode: ESPNOW phymode of specified interface

wifi_phy_rate_t rate: ESPNOW rate of specified interface

bool ersu: ESPNOW using ERSU to send frame, ERSU is a transmission mode related to 802.11 ax. ERSU is always used in long distance transmission, and its frame has lower rate compared with SU mode

bool dcm: ESPNOW using dcm rate to send frame

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

ESP_NOW_MAX_DATA_LEN_V2: Maximum length of data sent in each ESPNOW transmission for v2.0

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 struct esp_now_rate_config 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

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