1 2 Linux IEEE 802.15.4 implementation 3 4 5Introduction 6============ 7The IEEE 802.15.4 working group focuses on standardization of the bottom 8two layers: Medium Access Control (MAC) and Physical access (PHY). And there 9are mainly two options available for upper layers: 10 - ZigBee - proprietary protocol from the ZigBee Alliance 11 - 6LoWPAN - IPv6 networking over low rate personal area networks 12 13The goal of the Linux-wpan is to provide a complete implementation 14of the IEEE 802.15.4 and 6LoWPAN protocols. IEEE 802.15.4 is a stack 15of protocols for organizing Low-Rate Wireless Personal Area Networks. 16 17The stack is composed of three main parts: 18 - IEEE 802.15.4 layer; We have chosen to use plain Berkeley socket API, 19 the generic Linux networking stack to transfer IEEE 802.15.4 data 20 messages and a special protocol over netlink for configuration/management 21 - MAC - provides access to shared channel and reliable data delivery 22 - PHY - represents device drivers 23 24 25Socket API 26========== 27 28int sd = socket(PF_IEEE802154, SOCK_DGRAM, 0); 29..... 30 31The address family, socket addresses etc. are defined in the 32include/net/af_ieee802154.h header or in the special header 33in the userspace package (see either http://wpan.cakelab.org/ or the 34git tree at https://github.com/linux-wpan/wpan-tools). 35 36 37Kernel side 38============= 39 40Like with WiFi, there are several types of devices implementing IEEE 802.15.4. 411) 'HardMAC'. The MAC layer is implemented in the device itself, the device 42 exports a management (e.g. MLME) and data API. 432) 'SoftMAC' or just radio. These types of devices are just radio transceivers 44 possibly with some kinds of acceleration like automatic CRC computation and 45 comparation, automagic ACK handling, address matching, etc. 46 47Those types of devices require different approach to be hooked into Linux kernel. 48 49 50HardMAC 51======= 52 53See the header include/net/ieee802154_netdev.h. You have to implement Linux 54net_device, with .type = ARPHRD_IEEE802154. Data is exchanged with socket family 55code via plain sk_buffs. On skb reception skb->cb must contain additional 56info as described in the struct ieee802154_mac_cb. During packet transmission 57the skb->cb is used to provide additional data to device's header_ops->create 58function. Be aware that this data can be overridden later (when socket code 59submits skb to qdisc), so if you need something from that cb later, you should 60store info in the skb->data on your own. 61 62To hook the MLME interface you have to populate the ml_priv field of your 63net_device with a pointer to struct ieee802154_mlme_ops instance. The fields 64assoc_req, assoc_resp, disassoc_req, start_req, and scan_req are optional. 65All other fields are required. 66 67 68SoftMAC 69======= 70 71The MAC is the middle layer in the IEEE 802.15.4 Linux stack. This moment it 72provides interface for drivers registration and management of slave interfaces. 73 74NOTE: Currently the only monitor device type is supported - it's IEEE 802.15.4 75stack interface for network sniffers (e.g. WireShark). 76 77This layer is going to be extended soon. 78 79See header include/net/mac802154.h and several drivers in 80drivers/net/ieee802154/. 81 82 83Device drivers API 84================== 85 86The include/net/mac802154.h defines following functions: 87 - struct ieee802154_hw * 88 ieee802154_alloc_hw(size_t priv_data_len, const struct ieee802154_ops *ops): 89 allocation of IEEE 802.15.4 compatible hardware device 90 91 - void ieee802154_free_hw(struct ieee802154_hw *hw): 92 freeing allocated hardware device 93 94 - int ieee802154_register_hw(struct ieee802154_hw *hw): 95 register PHY which is the allocated hardware device, in the system 96 97 - void ieee802154_unregister_hw(struct ieee802154_hw *hw): 98 freeing registered PHY 99 100Moreover IEEE 802.15.4 device operations structure should be filled. 101 102Fake drivers 103============ 104 105In addition there is a driver available which simulates a real device with 106SoftMAC (fakelb - IEEE 802.15.4 loopback driver) interface. This option 107provides a possibility to test and debug the stack without usage of real hardware. 108 109See sources in drivers/net/ieee802154 folder for more details. 110 111 1126LoWPAN Linux implementation 113============================ 114 115The IEEE 802.15.4 standard specifies an MTU of 127 bytes, yielding about 80 116octets of actual MAC payload once security is turned on, on a wireless link 117with a link throughput of 250 kbps or less. The 6LoWPAN adaptation format 118[RFC4944] was specified to carry IPv6 datagrams over such constrained links, 119taking into account limited bandwidth, memory, or energy resources that are 120expected in applications such as wireless Sensor Networks. [RFC4944] defines 121a Mesh Addressing header to support sub-IP forwarding, a Fragmentation header 122to support the IPv6 minimum MTU requirement [RFC2460], and stateless header 123compression for IPv6 datagrams (LOWPAN_HC1 and LOWPAN_HC2) to reduce the 124relatively large IPv6 and UDP headers down to (in the best case) several bytes. 125 126In September 2011 the standard update was published - [RFC6282]. 127It deprecates HC1 and HC2 compression and defines IPHC encoding format which is 128used in this Linux implementation. 129 130All the code related to 6lowpan you may find in files: net/6lowpan/* 131and net/ieee802154/6lowpan/* 132 133To setup a 6LoWPAN interface you need: 1341. Add IEEE802.15.4 interface and set channel and PAN ID; 1352. Add 6lowpan interface by command like: 136 # ip link add link wpan0 name lowpan0 type lowpan 1373. Bring up 'lowpan0' interface 138