ESP-NETIF

The purpose of the ESP-NETIF library is twofold:

It provides an abstraction layer for the application on top of the TCP/IP stack. This allows applications to choose between IP stacks in the future.
The APIs it provides are thread-safe, even if the underlying TCP/IP stack APIs are not.

ESP-IDF currently implements ESP-NETIF for the lwIP TCP/IP stack only. However, the adapter itself is TCP/IP implementation-agnostic and allows different implementations.

It is also possible to use a custom TCP/IP stack with ESP-IDF, provided it implements BSD API. For more information on building ESP-IDF without lwIP, please refer to components/esp_netif_stack/README.md.

Some ESP-NETIF API functions are intended to be called by application code, for example, to get or set interface IP addresses, and configure DHCP. Other functions are intended for internal ESP-IDF use by the network driver layer.

In many cases, applications do not need to call ESP-NETIF APIs directly as they are called by the default network event handlers.

ESP-NETIF Architecture

                       |          (A) USER CODE                 |
                       |                 Apps                   |
      .................| init          settings      events     |
      .                +----------------------------------------+
      .                   .                |           *
      .                   .                |           *
  --------+            +===========================+   *     +-----------------------+
          |            | new/config   get/set/apps |   *     | init                  |
          |            |                           |...*.....| Apps (DHCP, SNTP)     |
          |            |---------------------------|   *     |                       |
    init  |            |                           |****     |                       |
    start |************|  event handler            |*********|  DHCP                 |
    stop  |            |                           |         |                       |
          |            |---------------------------|         |                       |
          |            |                           |         |    NETIF              |
    +-----|            |                           |         +-----------------+     |
    | glue|---<----|---|  esp_netif_transmit       |--<------| netif_output    |     |
    |     |        |   |                           |         |                 |     |
    |     |--->----|---|  esp_netif_receive        |-->------| netif_input     |     |
    |     |        |   |                           |         + ----------------+     |
    |     |...<....|...|  esp_netif_free_rx_buffer |...<.....| packet buffer         |
    +-----|     |  |   |                           |         |                       |
          |     |  |   |                           |         |         (D)           |
    (B)   |     |  |   |          (C)              |         +-----------------------+
  --------+     |  |   +===========================+
COMMUNICATION   |  |                                               NETWORK STACK
DRIVER          |  |           ESP-NETIF
                |  |                                         +------------------+
                |  |   +---------------------------+.........| open/close       |
                |  |   |                           |         |                  |
                |  -<--|  l2tap_write              |-----<---|  write           |
                |      |                           |         |                  |
                ---->--|  esp_vfs_l2tap_eth_filter |----->---|  read            |
                       |                           |         |                  |
                       |            (E)            |         +------------------+
                       +---------------------------+
                                                                   USER CODE
                             ESP-NETIF L2 TAP

Data and Event Flow in the Diagram

........ Initialization line from user code to ESP-NETIF and communication driver
--<--->-- Data packets going from communication media to TCP/IP stack and back
******** Events aggregated in ESP-NETIF propagate to the driver, user code, and network stack
| User settings and runtime configuration

ESP-NETIF Interaction

A) User Code, Boilerplate

Overall application interaction with a specific IO driver for communication media and configured TCP/IP network stack is abstracted using ESP-NETIF APIs and is outlined as below:

Initialization code

Initializes IO driver

Creates a new instance of ESP-NETIF and configure it with

ESP-NETIF specific options (flags, behavior, name)

Network stack options (netif init and input functions, not publicly available)

IO driver specific options (transmit, free rx buffer functions, IO driver handle)

Attaches the IO driver handle to the ESP-NETIF instance created in the above steps

Configures event handlers

Use default handlers for common interfaces defined in IO drivers; or define a specific handler for customized behavior or new interfaces

Register handlers for app-related events (such as IP lost or acquired)

Interaction with network interfaces using ESP-NETIF API

Gets and sets TCP/IP-related parameters (DHCP, IP, etc)

Receives IP events (connect or disconnect)

Controls application lifecycle (set interface up or down)

B) Communication Driver, IO Driver, and Media Driver

Communication driver plays these two important roles in relation to ESP-NETIF:

Event handlers: Defines behavior patterns of interaction with ESP-NETIF (e.g., ethernet link-up -> turn netif on)
Glue IO layer: Adapts the input or output functions to use ESP-NETIF transmit, receive, and free receive buffer

Installs driver_transmit to the appropriate ESP-NETIF object so that outgoing packets from the network stack are passed to the IO driver

Calls esp_netif_receive() to pass incoming data to the network stack

C) ESP-NETIF

ESP-NETIF serves as an intermediary between an IO driver and a network stack, connecting the packet data path between the two. It provides a set of interfaces for attaching a driver to an ESP-NETIF object at runtime and configures a network stack during compiling. Additionally, a set of APIs is provided to control the network interface lifecycle and its TCP/IP properties. As an overview, the ESP-NETIF public interface can be divided into six groups:

Initialization APIs (to create and configure ESP-NETIF instance)
Input or Output API (for passing data between IO driver and network stack)
Event or Action API

Used for network interface lifecycle management

ESP-NETIF provides building blocks for designing event handlers

Setters and Getters API for basic network interface properties
Network stack abstraction API: enabling user interaction with TCP/IP stack

Set interface up or down

DHCP server and client API

DNS API

SNTP API

Driver conversion utilities API

D) Network Stack

The network stack has no public interaction with application code with regard to public interfaces and shall be fully abstracted by ESP-NETIF API.

E) ESP-NETIF L2 TAP Interface

The ESP-NETIF L2 TAP interface is a mechanism in ESP-IDF used to access Data Link Layer (L2 per OSI/ISO) for frame reception and transmission from the user application. Its typical usage in the embedded world might be the implementation of non-IP-related protocols, e.g., PTP, Wake on LAN. Note that only Ethernet (IEEE 802.3) is currently supported.

From a user perspective, the ESP-NETIF L2 TAP interface is accessed using file descriptors of VFS, which provides file-like interfacing (using functions like open(), read(), write(), etc). To learn more, refer to Virtual Filesystem Component.

There is only one ESP-NETIF L2 TAP interface device (path name) available. However multiple file descriptors with different configurations can be opened at a time since the ESP-NETIF L2 TAP interface can be understood as a generic entry point to the Layer 2 infrastructure. What is important is then the specific configuration of the particular file descriptor. It can be configured to give access to a specific Network Interface identified by if_key (e.g., ETH_DEF) and to filter only specific frames based on their type (e.g., Ethernet type in the case of IEEE 802.3). Filtering only specific frames is crucial since the ESP-NETIF L2 TAP needs to exist along with the IP stack and so the IP-related traffic (IP, ARP, etc.) should not be passed directly to the user application. Even though this option is still configurable, it is not recommended in standard use cases. Filtering is also advantageous from the perspective of the user's application, as it only gets access to the frame types it is interested in, and the remaining traffic is either passed to other L2 TAP file descriptors or to the IP stack.

ESP-NETIF L2 TAP Interface Usage Manual

Initialization

To be able to use the ESP-NETIF L2 TAP interface, it needs to be enabled in Kconfig by CONFIG_ESP_NETIF_L2_TAP first and then registered by esp_vfs_l2tap_intf_register() prior usage of any VFS function.

`open()`

Once the ESP-NETIF L2 TAP is registered, it can be opened at path name "/dev/net/tap". The same path name can be opened multiple times up to CONFIG_ESP_NETIF_L2_TAP_MAX_FDS and multiple file descriptors with a different configuration may access the Data Link Layer frames.

The ESP-NETIF L2 TAP can be opened with the O_NONBLOCK file status flag to make sure the read() does not block. Note that the write() may block in the current implementation when accessing a Network interface since it is a shared resource among multiple ESP-NETIF L2 TAP file descriptors and IP stack, and there is currently no queuing mechanism deployed. The file status flag can be retrieved and modified using fcntl().

On success, open() returns the new file descriptor (a nonnegative integer). On error, -1 is returned, and errno is set to indicate the error.

`ioctl()`

The newly opened ESP-NETIF L2 TAP file descriptor needs to be configured prior to its usage since it is not bounded to any specific Network Interface and no frame type filter is configured. The following configuration options are available to do so:

L2TAP_S_INTF_DEVICE - bounds the file descriptor to a specific Network Interface that is identified by its if_key. ESP-NETIF Network Interface if_key is passed to ioctl() as the third parameter. Note that default Network Interfaces if_key's used in ESP-IDF can be found in esp_netif/include/esp_netif_defaults.h.

L2TAP_S_DEVICE_DRV_HNDL - is another way to bound the file descriptor to a specific Network Interface. In this case, the Network interface is identified directly by IO Driver handle (e.g., esp_eth_handle_t in case of Ethernet). The IO Driver handle is passed to ioctl() as the third parameter.

L2TAP_S_RCV_FILTER - sets the filter to frames with the type to be passed to the file descriptor. In the case of Ethernet frames, the frames are to be filtered based on the Length and Ethernet type field. In case the filter value is set less than or equal to 0x05DC, the Ethernet type field is considered to represent IEEE802.3 Length Field, and all frames with values in interval <0, 0x05DC> at that field are passed to the file descriptor. The IEEE802.2 logical link control (LLC) resolution is then expected to be performed by the user's application. In case the filter value is set greater than 0x05DC, the Ethernet type field is considered to represent protocol identification and only frames that are equal to the set value are to be passed to the file descriptor.

All above-set configuration options have a getter counterpart option to read the current settings.

Warning

The file descriptor needs to be firstly bounded to a specific Network Interface by L2TAP_S_INTF_DEVICE or L2TAP_S_DEVICE_DRV_HNDL to make L2TAP_S_RCV_FILTER option available.

Note

VLAN-tagged frames are currently not recognized. If the user needs to process VLAN-tagged frames, they need a set filter to be equal to the VLAN tag (i.e., 0x8100 or 0x88A8) and process the VLAN-tagged frames in the user application.

Note

L2TAP_S_DEVICE_DRV_HNDL is particularly useful when the user's application does not require the usage of an IP stack and so ESP-NETIF is not required to be initialized too. As a result, Network Interface cannot be identified by its if_key and hence it needs to be identified directly by its IO Driver handle.

On success, ioctl() returns 0. On error, -1 is returned, and errno is set to indicate the error.
* EBADF - not a valid file descriptor.
* EACCES - options change is denied in this state (e.g., file descriptor has not been bounded to Network interface yet).
* EINVAL - invalid configuration argument. Ethernet type filter is already used by other file descriptors on that same Network interface.
* ENODEV - no such Network Interface which is tried to be assigned to the file descriptor exists.
* ENOSYS - unsupported operation, passed configuration option does not exist.

`fcntl()`

fcntl() is used to manipulate with properties of opened ESP-NETIF L2 TAP file descriptor.

The following commands manipulate the status flags associated with the file descriptor:

F_GETFD - the function returns the file descriptor flags, and the third argument is ignored.

F_SETFD - sets the file descriptor flags to the value specified by the third argument. Zero is returned.

On success, ioctl() returns 0. On error, -1 is returned, and errno is set to indicate the error.
* EBADF - not a valid file descriptor.
* ENOSYS - unsupported command.

`read()`

Opened and configured ESP-NETIF L2 TAP file descriptor can be accessed by read() to get inbound frames. The read operation can be either blocking or non-blocking based on the actual state of the O_NONBLOCK file status flag. When the file status flag is set to blocking, the read operation waits until a frame is received and the context is switched to other tasks. When the file status flag is set to non-blocking, the read operation returns immediately. In such case, either a frame is returned if it was already queued or the function indicates the queue is empty. The number of queued frames associated with one file descriptor is limited by CONFIG_ESP_NETIF_L2_TAP_RX_QUEUE_SIZE Kconfig option. Once the number of queued frames reached a configured threshold, the newly arrived frames are dropped until the queue has enough room to accept incoming traffic (Tail Drop queue management).

On success, read() returns the number of bytes read. Zero is returned when the size of the destination buffer is 0. On error, -1 is returned, and errno is set to indicate the error.
* EBADF - not a valid file descriptor.
* EAGAIN - the file descriptor has been marked non-blocking (O_NONBLOCK), and the read would block.

`write()`

A raw Data Link Layer frame can be sent to Network Interface via opened and configured ESP-NETIF L2 TAP file descriptor. The user's application is responsible to construct the whole frame except for fields which are added automatically by the physical interface device. The following fields need to be constructed by the user's application in case of an Ethernet link: source/destination MAC addresses, Ethernet type, actual protocol header, and user data. The length of these fields is as follows:

Destination MAC	Source MAC	Type/Length	Payload (protocol header/data)
6 B	6 B	2 B	0-1486 B

In other words, there is no additional frame processing performed by the ESP-NETIF L2 TAP interface. It only checks the Ethernet type of the frame is the same as the filter configured in the file descriptor. If the Ethernet type is different, an error is returned and the frame is not sent. Note that the write() may block in the current implementation when accessing a Network interface since it is a shared resource among multiple ESP-NETIF L2 TAP file descriptors and IP stack, and there is currently no queuing mechanism deployed.

On success, write() returns the number of bytes written. Zero is returned when the size of the input buffer is 0. On error, -1 is returned, and errno is set to indicate the error.
* EBADF - not a valid file descriptor.
* EBADMSG - The Ethernet type of the frame is different from the file descriptor configured filter.
* EIO - Network interface not available or busy.

`close()`

Opened ESP-NETIF L2 TAP file descriptor can be closed by the close() to free its allocated resources. The ESP-NETIF L2 TAP implementation of close() may block. On the other hand, it is thread-safe and can be called from a different task than the file descriptor is actually used. If such a situation occurs and one task is blocked in the I/O operation and another task tries to close the file descriptor, the first task is unblocked. The first's task read operation then ends with an error.

On success, close() returns zero. On error, -1 is returned, and errno is set to indicate the error.

* EBADF - not a valid file descriptor.

`select()`

Select is used in a standard way, just CONFIG_VFS_SUPPORT_SELECT needs to be enabled to make the select() function available.

SNTP API

You can find a brief introduction to SNTP in general, its initialization code, and basic modes in Section SNTP Time Synchronization in System Time.

This section provides more details about specific use cases of the SNTP service, with statically configured servers, or use the DHCP-provided servers, or both. The workflow is usually very simple:

Initialize and configure the service using esp_netif_sntp_init().
Start the service via esp_netif_sntp_start(). This step is not needed if we auto-started the service in the previous step (default). It is useful to start the service explicitly after connecting if we want to use the DHCP-obtained NTP servers. Please note, this option needs to be enabled before connecting, but the SNTP service should be started after.
Wait for the system time to synchronize using esp_netif_sntp_sync_wait() (only if needed).
Stop and destroy the service using esp_netif_sntp_deinit().

Basic Mode with Statically Defined Server(s)

Initialize the module with the default configuration after connecting to the network. Note that it is possible to provide multiple NTP servers in the configuration struct:

esp_sntp_config_t config = ESP_NETIF_SNTP_DEFAULT_CONFIG_MULTIPLE(2,
                           ESP_SNTP_SERVER_LIST("time.windows.com", "pool.ntp.org" ) );
esp_netif_sntp_init(&config);

Note

If we want to configure multiple SNTP servers, we have to update the lwIP configuration CONFIG_LWIP_SNTP_MAX_SERVERS.

Use DHCP-Obtained SNTP Server(s)

First of all, we have to enable the lwIP configuration option CONFIG_LWIP_DHCP_GET_NTP_SRV. Then we have to initialize the SNTP module with the DHCP option and without the NTP server:

esp_sntp_config_t config = ESP_NETIF_SNTP_DEFAULT_CONFIG_MULTIPLE(0, {} );
config.start = false;                       // start the SNTP service explicitly
config.server_from_dhcp = true;             // accept the NTP offer from the DHCP server
esp_netif_sntp_init(&config);

Then, once we are connected, we could start the service using:

esp_netif_sntp_start();

Note

It is also possible to start the service during initialization (default config.start=true). This would likely to cause the initial SNTP request to fail (since we are not connected yet) and lead to some back-off time for subsequent requests.

Use Both Static and Dynamic Servers

Very similar to the scenario above (DHCP provided SNTP server), but in this configuration, we need to make sure that the static server configuration is refreshed when obtaining NTP servers by DHCP. The underlying lwIP code cleans up the rest of the list of NTP servers when the DHCP-provided information gets accepted. Thus the ESP-NETIF SNTP module saves the statically configured server(s) and reconfigures them after obtaining a DHCP lease.

The typical configuration now looks as per below, providing the specific IP_EVENT to update the config and index of the first server to reconfigure (for example setting config.index_of_first_server=1 would keep the DHCP provided server at index 0, and the statically configured server at index 1).

esp_sntp_config_t config = ESP_NETIF_SNTP_DEFAULT_CONFIG("pool.ntp.org");
config.start = false;                       // start the SNTP service explicitly (after connecting)
config.server_from_dhcp = true;             // accept the NTP offers from DHCP server
config.renew_servers_after_new_IP = true;   // let esp-netif update the configured SNTP server(s) after receiving the DHCP lease
config.index_of_first_server = 1;           // updates from server num 1, leaving server 0 (from DHCP) intact
config.ip_event_to_renew = IP_EVENT_STA_GOT_IP;  // IP event on which we refresh the configuration

Then we start the service normally with esp_netif_sntp_start().

ESP-NETIF Programmer's Manual

Please refer to the following example to understand the initialization process of the default interface:

Ethernet: ethernet/basic/main/ethernet_example_main.c
L2 TAP: protocols/l2tap/main/l2tap_main.c

Wi-Fi Access Point: wifi/getting_started/softAP/main/softap_example_main.c

For more specific cases, please consult this guide: ESP-NETIF Custom I/O Driver.

API Reference

Header File

components/esp_netif/include/esp_netif.h
This header file can be included with:
#include "esp_netif.h"
This header file is a part of the API provided by the esp_netif component. To declare that your component depends on esp_netif, add the following to your CMakeLists.txt:
REQUIRES esp_netif
or
PRIV_REQUIRES esp_netif

Functions

esp_err_t esp_netif_init(void)

Initialize the underlying TCP/IP stack.

Note

This function should be called exactly once from application code, when the application starts up.

Returns

ESP_OK on success
ESP_FAIL if initializing failed

esp_err_t esp_netif_deinit(void)

Deinitialize the esp-netif component (and the underlying TCP/IP stack)

     Note: Deinitialization is not supported yet

Returns

ESP_ERR_INVALID_STATE if esp_netif not initialized
ESP_ERR_NOT_SUPPORTED otherwise

esp_netif_t *esp_netif_new(const esp_netif_config_t *esp_netif_config)

Creates an instance of new esp-netif object based on provided config.

Parameters

esp_netif_config -- pointer esp-netif configuration

Returns

pointer to esp-netif object on success
NULL otherwise

void esp_netif_destroy(esp_netif_t *esp_netif)

Destroys the esp_netif object.

Parameters: esp_netif -- [in] pointer to the object to be deleted

esp_err_t esp_netif_set_driver_config(esp_netif_t *esp_netif, const esp_netif_driver_ifconfig_t *driver_config)

Configures driver related options of esp_netif object.

Parameters

esp_netif -- [inout] pointer to the object to be configured
driver_config -- [in] pointer esp-netif io driver related configuration

Returns

ESP_OK on success
ESP_ERR_ESP_NETIF_INVALID_PARAMS if invalid parameters provided

esp_err_t esp_netif_attach(esp_netif_t *esp_netif, esp_netif_iodriver_handle driver_handle)

Attaches esp_netif instance to the io driver handle.

Calling this function enables connecting specific esp_netif object with already initialized io driver to update esp_netif object with driver specific configuration (i.e. calls post_attach callback, which typically sets io driver callbacks to esp_netif instance and starts the driver)

Parameters

esp_netif -- [inout] pointer to esp_netif object to be attached
driver_handle -- [in] pointer to the driver handle

Returns

ESP_OK on success
ESP_ERR_ESP_NETIF_DRIVER_ATTACH_FAILED if driver's pot_attach callback failed

esp_err_t esp_netif_receive(esp_netif_t *esp_netif, void *buffer, size_t len, void *eb)

Passes the raw packets from communication media to the appropriate TCP/IP stack.

This function is called from the configured (peripheral) driver layer. The data are then forwarded as frames to the TCP/IP stack.

Parameters

esp_netif -- [in] Handle to esp-netif instance
buffer -- [in] Received data
len -- [in] Length of the data frame
eb -- [in] Pointer to internal buffer (used in Wi-Fi driver)

Returns

ESP_OK

void esp_netif_action_start(void *esp_netif, esp_event_base_t base, int32_t event_id, void *data)

Default building block for network interface action upon IO driver start event Creates network interface, if AUTOUP enabled turns the interface on, if DHCPS enabled starts dhcp server.

Note