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 :idf_file:`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.
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:
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:
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 :doc:`/api-reference/storage/vfs`.
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.
To be able to use the ESP-NETIF L2 TAP interface, it needs to be enabled in Kconfig by :ref:`CONFIG_ESP_NETIF_L2_TAP` first and then registered by :cpp:func:`esp_vfs_l2tap_intf_register()` prior usage of any VFS function.
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 :ref:`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()``.
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 :component_file:`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., :cpp:type:`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.
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.
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.
``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.
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 :ref:`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.
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:
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.
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.
You can find a brief introduction to SNTP in general, its initialization code, and basic modes in Section :ref:`system-time-sntp-sync` in :doc:`System Time </api-reference/system/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:
2) Start the service via :cpp:func:`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.
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:
First of all, we have to enable the lwIP configuration option :ref:`CONFIG_LWIP_DHCP_GET_NTP_SRV`. Then we have to initialize the SNTP module with the DHCP option and without the NTP server:
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.
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).
The initialization code as well as registering event handlers for default interfaces, such as softAP and station, are provided in separate APIs to facilitate simple startup code for most applications:
Please note that these functions return the ``esp_netif`` handle, i.e., a pointer to a network interface object allocated and configured with default settings, as a consequence, which means that:
* The created object has to be destroyed if a network de-initialization is provided by an application using :cpp:func:`esp_netif_destroy_default_wifi()`.
