6LoWPAN - Transmission of IPv6 Packets over IEEE 802.15.4 Networks

6LoWPAN covered topics include the following:

 

• Frame format for transmission of IPv6 packets
• Method of forming IPv6 link-local addresses
• Statelessly autoconfigured addresses on IEEE 802.15.4 networks
• Simple header compression scheme using shared context
• Provisions for packet delivery in IEEE 802.15.4 meshes

 

IEEE 802.15.4 Mode for IP

 

• IEEE 802.15.4 defines four types of frames: beacon frames, MAC command frames, acknowledgement frames, and data frames.
• IPv6 packets MUST be carried on data frames.
• IEEE 802.15.4 networks can either be nonbeacon-enabled or beaconenabled.
• 6LoWPAN does not require that IEEE networks run in beacon-enabled mode.
• Beacons are still useful for link-layer device discovery to aid in association and disassociation events.
• 6LoWPAN requires both source and destination addresses be included in the IEEE 802.15.4 frame header.
• The source or destination PAN ID fields may also be included.

 

Addressing Modes

 

• IEEE 802.15.4 defines several addressing modes: IEEE 64-bit extended addresses or 16-bit short addresses unique within the PAN.
• 6LoWPAN supports both 64-bit extended addresses, and 16-bit short addresses.
• 6LoWPAN assumes that a PAN maps to a specific IPv6 link.
• Multicast is not supported natively in IEEE 802.15.4.
• IPv6 level multicast packets MUST be carried as link-layer broadcast frames in IEEE 802.15.4 networks.
• Broadcast frames are only heeded by devices within the specific PAN of the link：
  • A destination PAN identifier is included in the frame, and it MUST match the PAN ID of the link in question.
  • A short destination address is included in the frame, and it MUST match the broadcast address (0xffff).
• Hosts learn IPv6 prefixes via router advertisements.

 

Maximum Transmission Unit

 

• IEEE 802.15.4 frame sizes:
  • Maximum physical layer packet size of 127 octets (aMaxPHYPacketSize) –
  • Maximum frame overhead of 25 (aMaxFrameOverhead) –
  • Link-layer security overhead (21 octets in AES-CCM-128, 9 and 13 in AES-CCM-32 and AES-CCM-64)
  • leaves only 81 octets for IPv6 -
  • IPv6 header of 40 octets
  • leaves only 41 octets for upper-layer protocols, like UDP
  • UDP uses 8 octets in the header
  • leaves only 33 octets for application data.
  • fragmentation and reassembly layer will use even more octets.
• Fragmention and reassembly adaptation layer must be provided at the layer below IP.

 

LoWPAN Adaptation Layer and Frame Format

 

• All LoWPAN encapsulated datagrams transported over IEEE 802.15.4 are prefixed by an encapsulation header stack.
• LoWPAN header sequence is mesh (L2) addressing, hop-by-hop options (including L2 broadcast/multicast), fragmentation, and finally payload.

 

• When more than one LoWPAN header is used in the same packet, they MUST appear in the following order:
  • Mesh Addressing Header
  • Broadcast Header
  • Fragmentation Header

Dispatch Type and Header

、

Mesh Addressing Type and Header

Fragmentation Type and Header

 

• All link fragments for a datagram except the last one MUST be multiples of eight bytes in length.

 

Stateless Address Autoconfiguration

 

• All 802.15.4 devices have an IEEE EUI-64 address, but 16-bit short addresses are also possible.
• Interface Identifier is formed from the EUI-64 according to the "IPv6 over Ethernet" specification.
• When 16-bit short addressing is used, a "pseudo 48-bit address" is formed as follows:
  • First, the left-most 32 bits are formed by concatenating 16 zero bits to the 16-bit PAN ID (or 16 zero bits) - 16_bit_PAN:16_zero_bits.
  • Then, these 32 bits are concatenated with the 16-bit short address - 32_bits_as_specified_previously:16_bit_short_address.
• A different MAC address set manually or by software MAY be used to derive the Interface Identifier.

 

IPv6 Link Local Address

 

Unicast Address Mapping

 

• The Source/Target Link-layer Address option has the following forms when the link layer is IEEE 802.15.4 and the addresses are EUI-64 or 16-bit short addresses, respectively.
  

  

Multicast Address Mapping

 

• An IPv6 packet with a multicast destination address (DST), consisting of the sixteen octets DST[1] through DST[16], is transmitted to the following 802.15.4 16-bit multicast address:

 

Header Compression

Encoding of IPv6 Header Fields

• A packet is compressible via the LOWPAN_HC1 format by using a Dispatch value of LOWPAN_HC1 followed by a LOWPAN_HC1 header "HC1 encoding" field (8 bits) to encode the different combinations as shown below.
• This header may be preceded by a fragmentation header, which may be preceded by a mesh header.\

 

Encoding of UDP Header Fields