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3. IPv6 Header Compression

In this section, we define the LOWPAN_IPHC encoding format for compressing the IPv6 header. To enable effective compression, LOWPAN_IPHC relies on information pertaining to the entire 6LoWPAN. LOWPAN_IPHC assumes the following will be the common case for 6LoWPAN communication: Version is 6; Traffic Class and Flow Label are both zero; Payload Length can be inferred from lower layers from either the 6LoWPAN Fragmentation header or the IEEE 802.15.4 header; Hop Limit will be set to a well-known value by the source; addresses assigned to 6LoWPAN interfaces will be formed using the link-local prefix or a small set of routable prefixes assigned to the entire 6LoWPAN; addresses assigned to 6LoWPAN interfaces are formed with an IID derived directly from either the 64-bit extended or the 16-bit short IEEE 802.15.4 addresses.

 +-------------------------------------+----------------------------
 | Dispatch + LOWPAN_IPHC (2-3 octets) | In-line IPv6 Header Fields
 +-------------------------------------+----------------------------

                    Figure 1: LOWPAN_IPHC Header

The LOWPAN_IPHC encoding utilizes 13 bits, 5 of which are taken from the rightmost bits of the dispatch type. The encoding may be extended by another octet to support additional contexts. Any information from the uncompressed IPv6 header fields carried in-line follow the LOWPAN_IPHC encoding, as shown in Figure 1. In the best case, the LOWPAN_IPHC can compress the IPv6 header down to two octets (the dispatch octet and the LOWPAN_IPHC encoding) with link-local communication.

When routing over multiple IP hops, LOWPAN_IPHC can compress the IPv6 header down to 7 octets (1-octet dispatch, 1-octet LOWPAN_IPHC, 1-octet Hop Limit, 2-octet Source Address, and 2-octet Destination Address). The Hop Limit may not be compressed because it needs to decremented at each hop and may take any value. Stateful address compression must be applied to the source and destination IPv6 addresses because they do not statelessly match the source and destination link-layer addresses on intermediate hops.

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