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

6LoWPAN: IP for Wireless Sensor Networks and Smart Cooperating Objects 51-11
51.5.5.2 UDP Header Compression
Further, protocol-specific compressions in addition to the above-presented IPv6 header compression are
summarized in the RFC4944 HC2 encoding format. In the current version of the 6LoWPAN specification,
only the HC_UDP scheme is described for HC2 encoding.
A standard UDP header consists of 2 octets for the source port of the message, 2 octets for the destination
port, 2 octets indicating the length of the frame, and 2 octets for the checksum field. This sums
up in 8 octets, which can be compressed to 4 octets in 6LoWPAN networks. Like the Hop Limit field in
the HC1 header, the checksum field cannot be compressed and must be carried in-line full. According
to the IP length field, the UDP length field is omitted by referring to other length fields included in the
link layer or the mesh header. The source and destination port can be separately carried full in-line or be
compressed to 4 bits. If compressed, the 4 bits determine a port between 61,616 and 61,631. The resulting
overall length of the compressed RFC4944 UDP header is between 4 octets and 9 octets.
51.5.6 Scopes
The architecture of wireless sensor mesh networks differs significantly from existing IP-based networks.
Networks using wired technologies like Ethernet use one single-hop shared medium and differentiate
between unicast messages sent to only one endpoint or broadcast sent to all endpoints. Multicast messages
are hybrids carried to multiply endpoints, controlled by the routers in the network. The routers are also
responsible for forwarding packages depending on their target address into the correct subnets. Wireless
technologies like Wi-Fi achieve a similar behavior by abstraction and link emulation of the used wireless
channel. Wi-Fi endpoints can only be connected to one other endpoint (peer-to-peer) or to one access point
(AP). This causes the required abstraction and thus the applicability of existing IP-based protocols and
technologies. Furthermore, Wi-Fi devices are not subjected by the same energy constraints like WSN.
Emerging meshed sensor networks have more complex topologies with one common wireless communication
medium, but in contrast to Ethernet and Wi-Fi, also with limited communication range. Unicast
messages cannot be transmitted only to one endpoint without an effect on other endpoints in communication
range and might be forwarded by endpoints also and not only by dedicated routers. This has a deep
impact on active discussions on scopes and their effect as used in IPv6. IPv6 differs between link-local scope,
site-local scope, and global scope. Link-local scope addresses are not forwarded by routers. Global scope
addresses are globally unique and thus can be used as target address by every other endpoint. If routing of IP
packets is performed under IP layer (route under), the complete 6LoWPAN network might communicate in
one single link-local scope and new routing concepts must be developed. Especially for multicast messages, a
proper mapping must be found, to avoid flooding of the network. The 6LoWPAN header compression makes
use of existing information in the link layers to omit redundant data. Route under concepts require further
mappings of other link layers like Ethernet or Wi-Fi to fit the 6LoWPAN requirements or cross-layered solutions.
If routing is done on the IP layer (route over), existing concepts might be used but requires every node
in the 6LoWPAN, which is not an end node (leaf) in the routing tree, to perform routing. But route over concepts
limit the link-local scope to all endpoints in direct communication range. In current developments of
IETF ROLL working group, route over concepts are preferred, which would make the mesh header described
in this chapter and the broadcast/multicast mapping obsolete.
51.5.7 Summary of Frame Types and Compression Schemes
The miscellaneous frame formats and compression schemes result in various different frame types
and combinations depending, e.g., on the scenario, application, and scope of communication. A vital
requirement for IPv6 traffic on IEEE 802.15.4 is the adaption to fit into the maximum packet size of
102 byte that is defined for the MAC layer by 802.15.4. Keep in mind that with security on MAC layer
enabled, this size is decreased to 81 byte. In this section, different resulting frame header sizes and their
combination are summarized. The comparison is presented in Table 51.1 and derived from the currently
© 2011 by Taylor and Francis Group, LLC