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

6LoWPAN: IP for Wireless Sensor Networks and Smart Cooperating Objects 51-7
51.5 6LoWPAN
In general, IEEE 802.15.4 defines four types of frame formats: beacon frames, MAC command frames,
acknowledge frames, and data frames. For 6LoWPAN-compliant transmission, only data frames are
used. Furthermore, 802.15.4 provides beaconed and non-beaconed data transmission. It is recommended
to use the beacon-enabled mode because the beacons are useful for the device discovery with
upper layers. Nevertheless, no dedicated mapping of beacons on device discovery protocols is described
by 6LoWPAN.
The following subsections are derived from the RFC4944, submitted by the 6LoWPAN working
group. The concepts and meanings are explained and additional comments are added. At the end of this
section, the different described frame and header types are summarized and compared. Further, header
compression and improved multicast mapping will be defined by the working group in their currently
active drafts. Because the drafts are not matured in the time of writing this section, they cannot be fully
examined here.
51.5.1 address Autoconfiguration
An IPv6-compliant interface address is 128 bit wide with 64 bit for the interface identifier and a 64 bit
prefix. Two different address types are defined by 802.15.4 that can be used to generate an IPv6-compliant
interface identifier: the 16 bit short address and the extended IEEE 64 bit address. The 64 bit interface
identifier may base on the—for every device unique—EUI-64 identifier [EUI64] but can also be changed
by the software network stack. If this 64 bit address is applied, the interface identifier can be formed in
accordance to the “IPv6 over Ethernet” (RFC2464) specification [RFC2464]. The shorter 16 bit addresses
are used by 802.15.4 nodes inside a specific PAN after an association event with the coordinator. If 16 bit
short addresses are applied, a 48 bit address can be generated, which in turn is converted into a 64 bit address
following RFC2464 also [RFC2464].
Generating a 64 bit interface identifier out of 16 bit short addresses cannot grant that the derived
addresses are globally unique, which has to be announced by setting the universal/local bit—the seventh
bit in an IPv6 interface identifier—to zero. When generating an interface identifier out of the 16 bit short
address used in one 802.15.4 PAN, the derived address is limited to the lifetime of the connection with
the PAN coordinator. Thus, it might be applicable to use 64 bit addresses directly if possible.
The 64 bit prefix for the IPv6 address is obtained by the (edge) router in accordance to RFC4862 “IPv6
Stateless Address Autoconfiguration” [RFC4862]. “Neighbor Discovery (ND) for IP version 6 (IPv6)”
[RFC4861] e.g., duplicated address detection and finding of routers, makes extensive use of IP multicast.
There is no native support for multicast in IEEE 802.15.4 and thus further efforts are required.
Furthermore, RFC4861 assumes all devices of a subnet communicating on one single link. These two
points make standard ND not applicable to be applied in 6LoWPAN networks. It should be noted that
there is an active document of the 6LoWPAN working group, which describes an alternative ND for
6LoWPAN networks. The routers advertise their presence with periodical multicast messages. This
requires almost all nodes to wake up periodically for power consuming receiving and/or forwarding of
multicast messages. The implicit unreliable communication in LoWPANs and the absence of multicast
messaging capabilities on the link layer make standard ND protocols not practical.
51.5.2 Frame Types and Fragmentation
The 802.15.4 specification defines a maximum packet size of 127 byte for physical packets. The number
of available octets is further reduced by the MAC layer header and footer. IPv6 requires a minimum
MTU of 1280 octets. To fit the required IPv6 MTU, a separate fragmentation and reassembly layer has
to be provided on top over MAC layer and below IP layer (known as layer 2.5). Thus, it is required to
define additional frame formats to the standard IPv6 frame. All frame types are prefixed by an 8 bit wide
© 2011 by Taylor and Francis Group, LLC