Linux IEEE 802.15.4 implementation
  
  
  Introduction
  ============

  The IEEE 802.15.4 working group focuses on standardization of bottom

  two layers: Medium Access Control (MAC) and Physical (PHY). And there

  are mainly two options available for upper layers:

   - ZigBee - proprietary protocol from the ZigBee Alliance
   - 6LoWPAN - IPv6 networking over low rate personal area networks

  The linux-wpan project goal is to provide a complete implementation
  of the IEEE 802.15.4 and 6LoWPAN protocols. IEEE 802.15.4 is a stack

  of protocols for organizing Low-Rate Wireless Personal Area Networks.

  The stack is composed of three main parts:
   - IEEE 802.15.4 layer;  We have chosen to use plain Berkeley socket API,
     the generic Linux networking stack to transfer IEEE 802.15.4 messages
     and a special protocol over genetlink for configuration/management
   - MAC - provides access to shared channel and reliable data delivery
   - PHY - represents device drivers

  
  
  Socket API
  ==========
  
  int sd = socket(PF_IEEE802154, SOCK_DGRAM, 0);
  .....
  
  The address family, socket addresses etc. are defined in the

  include/net/af_ieee802154.h header or in the special header

  in the userspace package (see either http://wpan.cakelab.org/ or the
  git tree at https://github.com/linux-wpan/wpan-tools).

  
  One can use SOCK_RAW for passing raw data towards device xmit function. YMMV.

  Kernel side
  =============
  
  Like with WiFi, there are several types of devices implementing IEEE 802.15.4.
  1) 'HardMAC'. The MAC layer is implemented in the device itself, the device
     exports MLME and data API.
  2) 'SoftMAC' or just radio. These types of devices are just radio transceivers
     possibly with some kinds of acceleration like automatic CRC computation and
     comparation, automagic ACK handling, address matching, etc.
  
  Those types of devices require different approach to be hooked into Linux kernel.
  
  
  HardMAC
  =======

  See the header include/net/ieee802154_netdev.h. You have to implement Linux

  net_device, with .type = ARPHRD_IEEE802154. Data is exchanged with socket family

  code via plain sk_buffs. On skb reception skb->cb must contain additional
  info as described in the struct ieee802154_mac_cb. During packet transmission
  the skb->cb is used to provide additional data to device's header_ops->create

  function. Be aware that this data can be overridden later (when socket code

  submits skb to qdisc), so if you need something from that cb later, you should
  store info in the skb->data on your own.

  
  To hook the MLME interface you have to populate the ml_priv field of your

  net_device with a pointer to struct ieee802154_mlme_ops instance. The fields
  assoc_req, assoc_resp, disassoc_req, start_req, and scan_req are optional.
  All other fields are required.

  
  SoftMAC
  =======

  The MAC is the middle layer in the IEEE 802.15.4 Linux stack. This moment it
  provides interface for drivers registration and management of slave interfaces.
  
  NOTE: Currently the only monitor device type is supported - it's IEEE 802.15.4
  stack interface for network sniffers (e.g. WireShark).
  
  This layer is going to be extended soon.

  See header include/net/mac802154.h and several drivers in
  drivers/net/ieee802154/.

  
  Device drivers API
  ==================
  
  The include/net/mac802154.h defines following functions:
   - struct ieee802154_dev *ieee802154_alloc_device
     (size_t priv_size, struct ieee802154_ops *ops):
     allocation of IEEE 802.15.4 compatible device
  
   - void ieee802154_free_device(struct ieee802154_dev *dev):
     freeing allocated device
  
   - int ieee802154_register_device(struct ieee802154_dev *dev):
     register PHY in the system
  
   - void ieee802154_unregister_device(struct ieee802154_dev *dev):
     freeing registered PHY
  
  Moreover IEEE 802.15.4 device operations structure should be filled.
  
  Fake drivers
  ============

  In addition there is a driver available which simulates a real device with
  SoftMAC (fakelb - IEEE 802.15.4 loopback driver) interface. This option
  provides possibility to test and debug stack without usage of real hardware.

  See sources in drivers/net/ieee802154 folder for more details.

  6LoWPAN Linux implementation
  ============================

  The IEEE 802.15.4 standard specifies an MTU of 127 bytes, yielding about 80

  octets of actual MAC payload once security is turned on, on a wireless link
  with a link throughput of 250 kbps or less.  The 6LoWPAN adaptation format
  [RFC4944] was specified to carry IPv6 datagrams over such constrained links,
  taking into account limited bandwidth, memory, or energy resources that are
  expected in applications such as wireless Sensor Networks.  [RFC4944] defines
  a Mesh Addressing header to support sub-IP forwarding, a Fragmentation header
  to support the IPv6 minimum MTU requirement [RFC2460], and stateless header
  compression for IPv6 datagrams (LOWPAN_HC1 and LOWPAN_HC2) to reduce the
  relatively large IPv6 and UDP headers down to (in the best case) several bytes.
  
  In Semptember 2011 the standard update was published - [RFC6282].
  It deprecates HC1 and HC2 compression and defines IPHC encoding format which is
  used in this Linux implementation.

  All the code related to 6lowpan you may find in files: net/6lowpan/*
  and net/ieee802154/6lowpan/*

  
  To setup 6lowpan interface you need (busybox release > 1.17.0):
  1. Add IEEE802.15.4 interface and initialize PANid;
  2. Add 6lowpan interface by command like:
     # ip link add link wpan0 name lowpan0 type lowpan
  3. Set MAC (if needs):
     # ip link set lowpan0 address de:ad:be:ef:ca:fe:ba:be
  4. Bring up 'lowpan0' interface