.. _net_l2_interface: L2 Layer Management ################### .. contents:: :local: :depth: 2 Overview ******** The L2 stack is designed to hide the whole networking link-layer part and the related device drivers from the upper network stack. This is made through a :c:struct:`net_if` declared in :zephyr_file:`include/zephyr/net/net_if.h`. The upper layers are unaware of implementation details beyond the net_if object and the generic API provided by the L2 layer in :zephyr_file:`include/zephyr/net/net_l2.h` as :c:struct:`net_l2`. Only the L2 layer can talk to the device driver, linked to the net_if object. The L2 layer dictates the API provided by the device driver, specific for that device, and optimized for working together. Currently, there are L2 layers for :ref:`Ethernet `, :ref:`IEEE 802.15.4 Soft-MAC `, :ref:`Bluetooth IPSP `, :ref:`CANBUS `, :ref:`OpenThread `, Wi-Fi, and a dummy layer example that can be used as a template for writing a new one. L2 layer API ************ In order to create an L2 layer, or a driver for a specific L2 layer, one needs to understand how the L3 layer interacts with it and how the L2 layer is supposed to behave. See also :ref:`network stack architecture ` for more details. The generic L2 API has these functions: - ``recv()``: All device drivers, once they receive a packet which they put into a :c:type:`net_pkt`, will push this buffer to the network stack via :c:func:`net_recv_data`. At this point, the network stack does not know what to do with it. Instead, it passes the buffer along to the L2 stack's ``recv()`` function for handling. The L2 stack does what it needs to do with the packet, for example, parsing the link layer header, or handling link-layer only packets. The ``recv()`` function will return ``NET_DROP`` in case of an erroneous packet, ``NET_OK`` if the packet was fully consumed by the L2, or ``NET_CONTINUE`` if the network stack should then handle it. - ``send()``: Similar to receive function, the network stack will call this function to actually send a network packet. All relevant link-layer content will be generated and added by this function. The ``send()`` function returns the number of bytes sent, or a negative error code if there was a failure sending the network packet. - ``enable()``: This function is used to enable/disable traffic over a network interface. The function returns ``<0`` if error and ``>=0`` if no error. - ``get_flags()``: This function will return the capabilities of an L2 driver, for example whether the L2 supports multicast or promiscuous mode. Network Device drivers ********************** Network device drivers fully follows Zephyr device driver model as a basis. Please refer to :ref:`device_model_api`. There are, however, two differences: - The driver_api pointer must point to a valid :c:struct:`net_if_api` pointer. - The network device driver must use :c:macro:`NET_DEVICE_INIT_INSTANCE()` or :c:macro:`ETH_NET_DEVICE_INIT()` for Ethernet devices. These macros will call the :c:macro:`DEVICE_DEFINE()` macro, and also instantiate a unique :c:struct:`net_if` related to the created device driver instance. Implementing a network device driver depends on the L2 stack it belongs to: :ref:`Ethernet `, :ref:`IEEE 802.15.4 `, etc. In the next section, we will describe how a device driver should behave when receiving or sending a network packet. The rest is hardware dependent and is not detailed here. Ethernet device driver ====================== On reception, it is up to the device driver to fill-in the network packet with as many data buffers as required. The network packet itself is a :c:type:`net_pkt` and should be allocated through :c:func:`net_pkt_rx_alloc_with_buffer`. Then all data buffers will be automatically allocated and filled by :c:func:`net_pkt_write`. After all the network data has been received, the device driver needs to call :c:func:`net_recv_data`. If that call fails, it will be up to the device driver to unreference the buffer via :c:func:`net_pkt_unref`. On sending, the device driver send function will be called, and it is up to the device driver to send the network packet all at once, with all the buffers. Each Ethernet device driver will need, in the end, to call ``ETH_NET_DEVICE_INIT()`` like this: .. code-block:: c ETH_NET_DEVICE_INIT(..., CONFIG_ETH_INIT_PRIORITY, &the_valid_net_if_api_instance, 1500); IEEE 802.15.4 device driver =========================== Device drivers for IEEE 802.15.4 L2 work basically the same as for Ethernet. What has been described above, especially for ``recv()``, applies here as well. There are two specific differences however: - It requires a dedicated device driver API: :c:struct:`ieee802154_radio_api`, which overloads :c:struct:`net_if_api`. This is because 802.15.4 L2 needs more from the device driver than just ``send()`` and ``recv()`` functions. This dedicated API is declared in :zephyr_file:`include/zephyr/net/ieee802154_radio.h`. Each and every IEEE 802.15.4 device driver must provide a valid pointer on such relevantly filled-in API structure. - Sending a packet is slightly different than in Ethernet. IEEE 802.15.4 sends relatively small frames, 127 bytes all inclusive: frame header, payload and frame checksum. Buffers are meant to fit such frame size limitation. But a buffer containing an IPv6/UDP packet might have more than one fragment. IEEE 802.15.4 drivers handle only one buffer at a time. This is why the :c:struct:`ieee802154_radio_api` requires a tx function pointer which differs from the :c:struct:`net_if_api` send function pointer. Instead, the IEEE 802.15.4 L2, provides a generic :c:func:`ieee802154_radio_send` meant to be given as :c:type:`net_if` send function. It turn, the implementation of :c:func:`ieee802154_radio_send` will ensure the same behavior: sending one buffer at a time through :c:type:`ieee802154_radio_api` tx function, and unreferencing the network packet only when all the transmission were successful. Each IEEE 802.15.4 device driver, in the end, will need to call ``NET_DEVICE_INIT_INSTANCE()`` that way: .. code-block:: c NET_DEVICE_INIT_INSTANCE(..., the_device_init_prio, &the_valid_ieee802154_radio_api_instance, IEEE802154_L2, NET_L2_GET_CTX_TYPE(IEEE802154_L2), 125); API Reference ************* .. doxygengroup:: net_l2