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L2 Layer Management

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 net_if declared in 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 include/zephyr/net/net_l2.h as 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 Ethernet, IEEE 802.15.4 Soft-MAC, CANBUS, 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 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 net_pkt, will push this buffer to the network stack via 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 Device Driver Model.

There are, however, two differences:

Implementing a network device driver depends on the L2 stack it belongs to: Ethernet, 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 net_pkt and should be allocated through net_pkt_rx_alloc_with_buffer(). Then all data buffers will be automatically allocated and filled by net_pkt_write().

After all the network data has been received, the device driver needs to call net_recv_data(). If that call fails, it will be up to the device driver to unreference the buffer via 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:

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: ieee802154_radio_api, which overloads 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 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. Most IEEE 802.15.4 PHYs support relatively small frames only, 127 bytes all inclusive: frame header, payload and frame checksum. Buffers to be sent over the radio will often not fit this frame size limitation, e.g. a buffer containing an IPv6 packet will often have to be split into several fragments and IP6 packet headers and fragments need to be compressed using a protocol like 6LoWPAN before being passed on to the radio driver. Additionally the IEEE 802.15.4 standard defines medium access (e.g. CSMA/CA), frame retransmission, encryption and other pre-processing procedures (e.g. addition of information elements) that individual radio drivers should not have to care about. This is why the ieee802154_radio_api requires a tx function pointer which differs from the net_if_api send function pointer. Zephyr’s native IEEE 802.15.4 L2 implementation provides a generic ieee802154_send() instead, meant to be given as net_if send function. The implementation of ieee802154_send() takes care of IEEE 802.15.4 standard packet preparation procedures, splitting the packet into possibly compressed, encrypted and otherwise pre-processed fragment buffers, sending one buffer at a time through ieee802154_radio_api tx function and unreferencing the network packet only when the transmission as a whole was either successful or failed.

Interaction between IEEE 802.15.4 radio device drivers and L2 is bidirectional:

  • L2 -> L1: Methods as ieee802154_send() and several IEEE 802.15.4 net management calls will call into the driver, e.g. to send a packet over the radio link or re-configure the driver at runtime. These incoming calls will all be handled by the methods in the ieee802154_radio_api.

  • L1 -> L2: There are several situations in which the driver needs to initiate calls into the L2/MAC layer. Zephyr’s IEEE 802.15.4 L1 -> L2 adaptation API employs an “inversion-of-control” pattern in such cases avoids duplication of complex logic across independent driver implementations and ensures implementation agnostic loose coupling and clean separation of concerns between MAC (L2) and PHY (L1) whenever reverse information transfer or close co-operation between hardware and L2 is required. During driver initialization, for example, the driver calls ieee802154_init() to pass the interface’s MAC address as well as other hardware-related configuration to L2. Similarly, drivers may indicate performance or timing critical radio events to L2 that require close integration with the hardware (e.g. ieee802154_handle_ack()). Calls from L1 into L2 are not implemented as methods in ieee802154_radio_api but are standalone functions declared and documented as such in include/zephyr/net/ieee802154_radio.h. The API documentation will clearly state which functions must be implemented by all L2 stacks as part of the L1 -> L2 “inversion-of-control” adaptation API.

Note: Standalone functions in include/zephyr/net/ieee802154_radio.h that are not explicitly documented as callbacks are considered to be helper functions within the PHY (L1) layer implemented independently of any specific L2 stack, see for example ieee802154_is_ar_flag_set().

As all net interfaces, IEEE 802.15.4 device driver implementations will have to call NET_DEVICE_INIT_INSTANCE() in the end:

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

Network L2 Abstraction Layer

Related code samples