wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

6LoWPAN: IP for Wireless Sensor Networks and Smart Cooperating Objects 51-9
The mesh frame includes address information in addition to the MAC and PHY layers address information.
This is required because in multi-hop network topologies, every forwarding node of a mesh frame has to
adapt the source and destination addresses in the link layer. The overall originator address and the overall
final destination address are given by the mesh frame address field and stay unchanged. Before transmitting,
the forwarding node analyzes the mesh final destination address field to determine the overall
destination. If the node itself is not the final destination, the Hops Left field is reduced. If the result is zero,
the packet is discarded. Otherwise, the node invokes the link layer routing table to retrieve the next node
toward the final destination. This address of the next hop is put into the link layer address field. The source
address in the link layer is changed to the address of the current node and the packet is transmitted.
The mesh frame type can be used by routing protocols to be developed in particular by the IETF
ROLL working group. Later on (cf. Section 51.5.6), applicability of the mesh header and, thus, routing
strategies of IP packets in 6LoWPAN networks are discussed.
51.5.4 Broadcast and Multicast Address Mapping
As IPv6 makes extensive use of multicast messaging, 6LoWPAN specifies an additional compressed
multicast mapping as well. There are no multicast concepts defined in IEEE 802.15.4 natively. IPv6 multicasts
must be carried by 802.15.4 link layer broadcast messages, if no routing on IP layer is available.
The broadcast message includes a specific PAN-ID. Thus, the messages are forwarded by the routers to
the PAN in question only. IEEE 802.15.4 defines 0xFFFF as 16 bit short address for broadcast messaging.
In addition to this broadcast mapping of multicast messages, for mesh-enabled networks, support
of further address transformations are possible. Like all IPv6 addresses, the multicast addresses have a
size of 128 bit (16 octets). For 6LoWPAN messaging, these addresses are converted to 16 bit addresses
to allow integration into the short address scheme of IEEE 802.15.4 (cf. Figure 51.7). Each of these 16 bit
addresses starts with the predefined 3 bit preamble (100, should be common for all multicast addresses
in 6LoWPANs), followed by the last 5 bits of the 15th octet of the IPv6 multicast address. The last 8 bits
of the mapped address are copied from the 16th octet of the 128 bit address. The full specification of
a multicast mesh routing algorithm is out of scope of the 6LoWPAN specification. Nevertheless, such
a specification is a vital requirement to avoid flooding low-power WSNs with multicasts mapped on
broadcasts (cf. broadcast storm) and will be focused in particular by the ROLL working group.
51.5.5 Header Compression
The transport-specific IPv6 addresses, which are used in 6LoWPAN networks, can be generated by the
EUI-64 link layer identifier that is unique for every device. Hence, the additional addressing information
in the IP layer are redundant. Thus, 6LoWPAN provides header compression schemes for address
information and additional fields like length indications to avoid redundant data in the different layers.
All header fields, which are already present in the 802.15.4 fields, are elided in further headers, which
is a key design decision in 6LoWPAN. Basically, the header compression is applied without assuming
context information for the payload or additional information about the frame (stateless header compression).
A full compressed 6LoWPAN header compliant with RFC4944 has a size of only 2 octets in
IPv6 128 bit
Octet 1–14 Octet 15 Octet 16
...
Mapped 16 bit multicast
1 0 0
FIGURE 51.7
Derive 16 bit short address for multicast addressing.
© 2011 by Taylor and Francis Group, LLC