Atmel AVR2070 - Manual (Page 63)

AVR2070
63
8240B-AVR-06/09
Traffic Class and Flow Label encoding
A message will be properly parsed by end-nodes regardless if traffic class and flow
label is compressed or uncompressed. However the value itself is not read, hence to
avoid wasting space all messages should have the traffic class and flow label set to
zero.
Address Compression
The code supports carrying all 128 bytes; or compressing an address down to 64, 16,
or 0 bytes of extra payload.
Next Header Compression
Only a UDP packet will have the next header field compressed, any other type will
have the next header field carried in full.
Hop Limit Compression
A hop limit of 255, 64, or 1 will result in the hop limit being compressed. Any other
value will result in the hop limit field being carried in full.
UDP Header Compression
UDP Header compression is supported by this implementation.
C.6 6LoWPAN Compressed Header
If sniffing a 6LoWPAN network, it can be useful to understand the header. Note that
the 802.15.4 payload will be the RUM frame, which includes some additional fields.
The 6LoWPAN header described here is actually the RUM payload. Table C-6-1
illustrates this relationship.
Table C-6-1 Understanding the relationship between 6LoWPAN, RUM, and 802.15.4 Frames.
802.15.4 Frame
802.15.4 Header 802.15.4 Payload
RUM Data Header RUM Data Payload
FCF Seq PID Dest Src
Final
Dest
Origin Type 6LoWPAN Frame
0x8821 0x12 0x1234 0x0006 0x0007 0x0000 0x0007 0x05 …..
The 6LoWPAN Frame contains three bytes of interest at the beginning. The first is the
‘dispatch’ which is always set to 0x03, which corresponds to the header compression
used. The HC01 Encoding is specified in two bytes, and a detailed description of this
field is given at
http://tools.ietf.org/html/draft-ietf-6lowpan-hc-01 in section C.5.1.
Additionally the source code for the avr_sixlowpan.c file in the
sixlowpan_hc01_process() function provides a reference for decoding the
compressed 6LoWPAN header.
Table C-6-2 6LoWPAN Frame
6LoWPAN
Dispatch
HC01
Encoding Compressed IPv6 Header / IPv6 Payload
1 byte 2 bytes Variable