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

51-10 Industrial Communication Systems
opposite to 40 octets for standard IPv6 headers. Because the following described header compression
is not fully compliant with the IPv6 specification, compressed headers might be used for messaging
inside the 802.15.4 network only. For message exchange with external networks via dedicated edge routers,
compliant IPv6 headers might be used. If compressed headers are applied for external messaging
also, the edge router is responsible for translating the compressed headers in compliant IPv6 headers.
The current 6LoWPAN specification describes header compression for IPv6 and UDP headers also.
Compression schemes for TCP and further protocols are not described yet.
51.5.5.1 IPv6 Header Compression
The IPv6 header compression is described by the HC1 encoding. This HC1 encoding scheme is
independent from IPv6 and unique for 6LoWPANs. The 128 bit IPv6 transport addresses are separated
in 64 bit prefixes derived from the (edge) router of the subnet and 64 bit for the interface
identifier. It is possible to define separate compression for the four different address parts of originator
and destination: prefix of originator, interface identifier of originator, prefix of destination,
and interface identifier of destination. Because 6LoWPANs are estimated to operate within one
subnet with one unique prefix, it is not applicable to include the prefixes into the headers for internal
communication and can be elided. For communication out of the 6LoWPAN, prefixes can be
carried in-line. The interface identifier might be carried in-line also. If elided, in opposite to the
prefixes, the interface identifier for the destination address might be either derived from the link
layer directly or in meshed networks from the addressing fields in the according MESH header.
A compliant IPv6 header additionally includes the IPv6 version identifier, the traffic class field,
the flow label field, the hop limit field, and a payload length field. The length field is omitted and
derived from the 801.15.4 layers or from fragmentation frame header. The traffic class and the flow
label field can be compressed and thereby defined as zero. The IPv6 version identifier is omitted
in general. The 8 bits long Hop Limit field is the only field in the IPv6 header that cannot be compressed
and must be completely included in-line. The Hop Limit defines the maximum number
of IP hops a packet can be forwarded before it is dropped. One IP hop must not be equal with a
physical hop because not every intermediate node in a mesh network must be an IP-enabled node.
The additional next header field specifies the header type following the IP header, e.g., UDP, TCP,
or ICMP. Such next header fields are derived from the evolving IPv6 standard. In opposite to IPv4,
IPv6 does not always include all required parts, e.g., fragmentation and routing information in
the header. Headers, known as extension headers in IPv6, are added separately depending on the
scenario and refer to each other with defined next header fields, comparable to that ones used by
the 6LoWPAN compression formats (header chaining). To sum up, the complete RFC4944 HC1
encoded IPv6 header includes at least 2 octets for 8 bit Hop Limit and 8 bit for the LOWPAN_HC1
field (cf. Figure 51.8). The size of the complete header can be between 2 octets and 37 octets plus 4
bit (cf. MTU of IEEE 802.15.4 is 102 byte). The 4 bits are necessary to complete the octet with zero
padding and allow routers a more efficient and byte-size-optimized packet handling.
LOWPAN_HC1
Hop count
Following fields/headers/payload
Following header compressed per HC2 encoding or not
Next Header (UDP, ICMP, TCP, not compressed)
Traffic class/flow label zero or carried inline
IPv6 destination address
Preifx carried inline or elided
Interface identifier carried inline or derived from lower layers
IPv6 source address
Preifx carried inline or elided
Interface identifier carried inline or derived from lower layers
FIGURE 51.8
LOWPAN_HC1 encoding defined in RFC4944.
© 2011 by Taylor and Francis Group, LLC