142 lines
5.4 KiB
Plaintext
142 lines
5.4 KiB
Plaintext
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Linux IEEE 802.15.4 implementation
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Introduction
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============
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The IEEE 802.15.4 working group focuses on standardization of bottom
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two layers: Medium Access Control (MAC) and Physical (PHY). And there
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are mainly two options available for upper layers:
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- ZigBee - proprietary protocol from the ZigBee Alliance
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- 6LoWPAN - IPv6 networking over low rate personal area networks
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The linux-wpan project goal is to provide a complete implementation
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of the IEEE 802.15.4 and 6LoWPAN protocols. IEEE 802.15.4 is a stack
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of protocols for organizing Low-Rate Wireless Personal Area Networks.
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The stack is composed of three main parts:
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- IEEE 802.15.4 layer; We have chosen to use plain Berkeley socket API,
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the generic Linux networking stack to transfer IEEE 802.15.4 messages
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and a special protocol over genetlink for configuration/management
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- MAC - provides access to shared channel and reliable data delivery
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- PHY - represents device drivers
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Socket API
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==========
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int sd = socket(PF_IEEE802154, SOCK_DGRAM, 0);
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.....
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The address family, socket addresses etc. are defined in the
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include/net/af_ieee802154.h header or in the special header
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in the userspace package (see either http://wpan.cakelab.org/ or the
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git tree at https://github.com/linux-wpan/wpan-tools).
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One can use SOCK_RAW for passing raw data towards device xmit function. YMMV.
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Kernel side
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=============
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Like with WiFi, there are several types of devices implementing IEEE 802.15.4.
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1) 'HardMAC'. The MAC layer is implemented in the device itself, the device
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exports MLME and data API.
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2) 'SoftMAC' or just radio. These types of devices are just radio transceivers
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possibly with some kinds of acceleration like automatic CRC computation and
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comparation, automagic ACK handling, address matching, etc.
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Those types of devices require different approach to be hooked into Linux kernel.
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HardMAC
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=======
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See the header include/net/ieee802154_netdev.h. You have to implement Linux
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net_device, with .type = ARPHRD_IEEE802154. Data is exchanged with socket family
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code via plain sk_buffs. On skb reception skb->cb must contain additional
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info as described in the struct ieee802154_mac_cb. During packet transmission
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the skb->cb is used to provide additional data to device's header_ops->create
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function. Be aware that this data can be overridden later (when socket code
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submits skb to qdisc), so if you need something from that cb later, you should
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store info in the skb->data on your own.
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To hook the MLME interface you have to populate the ml_priv field of your
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net_device with a pointer to struct ieee802154_mlme_ops instance. The fields
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assoc_req, assoc_resp, disassoc_req, start_req, and scan_req are optional.
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All other fields are required.
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SoftMAC
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=======
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The MAC is the middle layer in the IEEE 802.15.4 Linux stack. This moment it
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provides interface for drivers registration and management of slave interfaces.
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NOTE: Currently the only monitor device type is supported - it's IEEE 802.15.4
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stack interface for network sniffers (e.g. WireShark).
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This layer is going to be extended soon.
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See header include/net/mac802154.h and several drivers in
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drivers/net/ieee802154/.
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Device drivers API
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==================
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The include/net/mac802154.h defines following functions:
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- struct ieee802154_dev *ieee802154_alloc_device
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(size_t priv_size, struct ieee802154_ops *ops):
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allocation of IEEE 802.15.4 compatible device
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- void ieee802154_free_device(struct ieee802154_dev *dev):
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freeing allocated device
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- int ieee802154_register_device(struct ieee802154_dev *dev):
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register PHY in the system
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- void ieee802154_unregister_device(struct ieee802154_dev *dev):
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freeing registered PHY
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Moreover IEEE 802.15.4 device operations structure should be filled.
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Fake drivers
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============
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In addition there is a driver available which simulates a real device with
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SoftMAC (fakelb - IEEE 802.15.4 loopback driver) interface. This option
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provides possibility to test and debug stack without usage of real hardware.
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See sources in drivers/net/ieee802154 folder for more details.
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6LoWPAN Linux implementation
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============================
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The IEEE 802.15.4 standard specifies an MTU of 127 bytes, yielding about 80
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octets of actual MAC payload once security is turned on, on a wireless link
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with a link throughput of 250 kbps or less. The 6LoWPAN adaptation format
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[RFC4944] was specified to carry IPv6 datagrams over such constrained links,
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taking into account limited bandwidth, memory, or energy resources that are
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expected in applications such as wireless Sensor Networks. [RFC4944] defines
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a Mesh Addressing header to support sub-IP forwarding, a Fragmentation header
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to support the IPv6 minimum MTU requirement [RFC2460], and stateless header
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compression for IPv6 datagrams (LOWPAN_HC1 and LOWPAN_HC2) to reduce the
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relatively large IPv6 and UDP headers down to (in the best case) several bytes.
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In Semptember 2011 the standard update was published - [RFC6282].
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It deprecates HC1 and HC2 compression and defines IPHC encoding format which is
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used in this Linux implementation.
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All the code related to 6lowpan you may find in files: net/6lowpan/*
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and net/ieee802154/6lowpan/*
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To setup 6lowpan interface you need (busybox release > 1.17.0):
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1. Add IEEE802.15.4 interface and initialize PANid;
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2. Add 6lowpan interface by command like:
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# ip link add link wpan0 name lowpan0 type lowpan
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3. Set MAC (if needs):
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# ip link set lowpan0 address de:ad:be:ef:ca:fe:ba:be
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4. Bring up 'lowpan0' interface
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