175 lines
8.0 KiB
ReStructuredText
175 lines
8.0 KiB
ReStructuredText
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=======
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6LoWPAN
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=======
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6LoWPAN Addressing
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------------------
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The current 6LoWPAN implementation uses only link local, MAC-based
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addressing addressing (as discussed in more detail below). Thus if you know
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the node addressing, then you know the IPv6 address (and vice-versa).
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As a configuration option, the 6LoWPAN implementation will use either the
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node's 2-byte short address or 8-byte extended address as the MAC address
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that the IPv6 address is based on. This is determined by the configuration
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setting ``CONFIG_NET_6LOWPAN_EXTENDEDADDR``. By default, the 2-byte short
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address is used for the IEEE802.15.4 MAC device's link layer address. If
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this option is selected, then an 8-byte extended address will be used,
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instead.
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All nodes operating on a network have unique, 8-byte extended address,
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that was assigned before the network is configured. 6LoWPAN will use
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either the extended address for direct communication within the PAN or
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the short 2-byte address. The short 2-byte address, however, is allocated
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by the PAN coordinator when the device associated. If short addresses are
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used, the network cannot be brought up until is is associated.
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Node addressing is modified through IOCTL calls from application logic.
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The network must be in the DOWN state when ever the node addressing is
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modified. The modified node addresses will have no effect on the reported
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IPv6 address until the network is brought to the UP state. The new IPv6
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MAC-based addresses are only instantiated when the network transitions
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from the DOWN to UP state.
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IPv6 Neighbor Discovery
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-----------------------
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IPv6 Neighbor Discovery is not supported. The current ICMPv6 and neighbor-
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related logic only works with Ethernet MAC. For 6LoWPAN, a new more
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conservative IPv6 neighbor discovery is provided by RFC 6775 which is not
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currently supported. With IPv6 neighbor discovery, any IPv6 address may be
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associated with any short or extended address. In fact, that is the whole
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purpose of the neighbor discover logic: It plays the same role as ARP in
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IPv4; it ultimately just manages a neighbor table that, like the arp table,
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provides the mapping between IP addresses and node addresses.
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The NuttX, Contiki-based 6LoWPAN implementation circumvents the need for
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the neighbor discovery logic by using only MAC-based addressing, i.e., the
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lower two or eight bytes of the IP address are the node address.
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Most of the 6LoWPAN compression algorithms exploit this kind of addressing
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to compress the IPv6 address to nothing but a single bit indicating that the
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IP address derives from the node address. In this use case, IPv6 neighbor
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discover is not useful: If we want to use IPv6 neighbor discovery, we could
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dispense with the all MAC based addressing. But if we want to retain the
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more compact MAC-based addressing, then we don't need IPv6 neighbor discovery.
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However, it would still be nice to have enough in place to support ping6.
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Full neighbor support would be necessary if we wanted to route 6LoWPAN frames
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outside of the WPAN.
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Optimal 6LoWPAN Configuration
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-----------------------------
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1. Link local IP addresses::
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128 112 96 80 64 48 32 16
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fe80 0000 0000 0000 xxxx xxxx xxxx xxxx
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2. MAC-based IP addresses::
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128 112 96 80 64 48 32 16
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---- ---- ---- ---- ---- ---- ---- ----
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AAAA xxxx xxxx xxxx xxxx 00ff fe00 MMMM 2-byte short address IEEE 48-bit MAC
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AAAA 0000 0000 0000 NNNN NNNN NNNN NNNN 8-byte extended address IEEE EUI-64
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Where MMM is the 2-byte short address XORed 0x0200. For example, the MAC
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address of 0xabcd would be 0xa9cd. And NNNN NNNN NNNN NNNN is the 8-byte
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extended address address XOR 02000 0000 0000 0000.
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For link-local address, AAAA is 0xfe80
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3. MAC based link-local addresses::
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128 112 96 80 64 48 32 16
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---- ---- ---- ---- ---- ---- ---- ----
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fe80 0000 0000 0000 0000 00ff fe00 MMMM 2-byte short address IEEE 48-bit MAC
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fe80 0000 0000 0000 NNNN NNNN NNNN NNNN 8-byte extended address IEEE EUI-64
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4. To be compressible, port numbers must be in the range 0xf0b0-0xf0bf,
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hexadecimal. That is 61616-61631 decimal.
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5. IOBs: Must be big enough to hold one IEEE802.15.4 frame (typically 127).
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There must be enough IOBs to decompose the largest IPv6 packet
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(``CONFIG_NET_6LOWPAN_PKTSIZE``, default 1294, plus per frame overhead).
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Fragmentation Headers
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---------------------
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A fragment header is placed at the beginning of the outgoing packet just
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after the MAC header when the payload is too large to fit in a single IEEE
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802.15.4 frame. The fragment header contains three fields: Datagram size,
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datagram tag and datagram offset.
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1. Datagram size describes the total (un-fragmented) payload.
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2. Datagram tag identifies the set of fragments and is used to match
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fragments of the same payload.
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3. Datagram offset identifies the fragment’s offset within the un-
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fragmented payload (in units of 8 bytes).
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The length of the fragment header length is four bytes for the first header
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(FRAG1) and five bytes for all subsequent headers (FRAGN). For example,
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this is a HC1 compressed first frame of a packet::
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41 88 2a cefa 3412 cdab ### 9-byte MAC header
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c50e 000b ### 4-byte FRAG1 header
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42 ### SIXLOWPAN_DISPATCH_HC1
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fb ### SIXLOWPAN_HC1_HC_UDP_HC1_ENCODING
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e0 ### SIXLOWPAN_HC1_HC_UDP_UDP_ENCODING
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00 ### SIXLOWPAN_HC1_HC_UDP_TTL
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10 ### SIXLOWPAN_HC1_HC_UDP_PORTS
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0000 ### SIXLOWPAN_HC1_HC_UDP_CHKSUM
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104 byte Payload follows:
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4f4e452064617920 48656e6e792d7065 6e6e792077617320 7069636b696e6720
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757020636f726e20 696e207468652063 6f726e7961726420 7768656e2d2d7768
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61636b212d2d736f 6d657468696e6720 6869742068657220 75706f6e20746865
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20686561642e2027
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This is the second frame of the same transfer::
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41 88 2b cefa 3412 cdab ### 9-byte MAC header
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e50e 000b 0d ### 5 byte FRAGN header
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42 ### SIXLOWPAN_DISPATCH_HC1
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fb ### SIXLOWPAN_HC1_HC_UDP_HC1_ENCODING
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e0 ### SIXLOWPAN_HC1_HC_UDP_UDP_ENCODING
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00 ### SIXLOWPAN_HC1_HC_UDP_TTL
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10 ### SIXLOWPAN_HC1_HC_UDP_PORTS
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0000 ### SIXLOWPAN_HC1_HC_UDP_CHKSUM
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104 byte Payload follows:
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476f6f646e657373 2067726163696f75 73206d6521272073 6169642048656e6e
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792d70656e6e793b 202774686520736b 79277320612d676f 696e6720746f2066
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616c6c3b2049206d 75737420676f2061 6e642074656c6c20 746865206b696e67
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2e270a0a536f2073
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The payload length is encoded in the LS 11-bits of the first 16-bit value:
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In this example the payload size is 0x050e or 1,294. The tag is 0x000b. In
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the second frame, the fifth byte contains the offset 0x0d which is 13 << 3 =
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104 bytes, the size of the payload on the first packet.
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Star Configuration
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------------------
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The 6LoWPAN stack can be specially configured as member in a star topology;
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either as a endpoint on the star os the star hub. The endpoint is
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created with the following settings in the configuration file::
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CONFIG_NET_STAR=y
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CONFIG_NET_STARPOINT=y
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The ``CONFIG_NET_STARPOINT`` selection informs the endpoint 6LoWPAN stack that
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it must send all frames to the hub of the star, rather than directly to the
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recipient. The star hub is assumed to be the coordinator.
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The star hub configuration, on the other hand, uses these setting::
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CONFIG_NET_STAR=y
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CONFIG_NET_STARHUB=y
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CONFIG_NET_IPFORWARD=y
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The ``CONFIG_NET_IPFORWARD`` selection informs the hub that if it receives any
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packets that are not destined for the hub, it should forward those packets
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appropriately. This affects the behavior of IPv6 packet reception logic but
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does not change the behavior of the 6LoWPAN stack.
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