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3.2. IPv6 Header Encoding

Fields carried in-line (in part or in whole) appear in the same order as they do in the IPv6 header format [RFC2460]. The Version field is always elided. Unicast IPv6 addresses may be compressed to 64 or 16 bits or completely elided. Multicast IPv6 addresses may be compressed to 8, 32, or 48 bits. The IPv6 Payload Length field MUST always be elided and inferred from lower layers using the 6LoWPAN Fragmentation header or the IEEE 802.15.4 header.

3.2.1. Traffic Class and Flow Label Compression​

The Traffic Class field in the IPv6 header comprises 6 bits of Diffserv extension [RFC2474] and 2 bits of Explicit Congestion Notification (ECN) [RFC3168]. The TF field in the LOWPAN_IPHC encoding indicates whether the Traffic Class and Flow Label are carried in-line in the compressed IPv6 header. When Flow Label is included while the Traffic Class is compressed, an additional 4 bits are included to maintain byte alignment. Two of the 4 bits contain the ECN bits from the Traffic Class field.

To ensure that the ECN bits appear in the same location for all encodings that include them, the Traffic Class field is rotated right by 2 bits in the compressed IPv6 header. The encodings are shown below:

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|ECN|   DSCP    |  rsv  |             Flow Label                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 Figure 4: TF = 00: Traffic Class and Flow Label carried in-line

                     1                   2
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|ECN|rsv|             Flow Label                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 5: TF = 01: Flow Label carried in-line

 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|ECN|   DSCP    |
+-+-+-+-+-+-+-+-+

        Figure 6: TF = 10: Traffic Class carried in-line

3.2.2. Deriving IIDs from the Encapsulating Header​

LOWPAN_IPHC elides the IIDs of source or destination addresses when SAM = 3 or DAM = 3, respectively. In this mode, the IID is derived from the encapsulating header. When the encapsulating header carries IPv6 addresses, bits for the source and destination addresses are copied from the source and destination addresses of the encapsulating IPv6 header.

The remainder of this section defines the mapping from IEEE 802.15.4 [IEEE802.15.4] link-layer addresses to IIDs for both short and extended IEEE 802.15.4 addresses. IID bits not covered by the context information MAY be elided if they match the link-layer address mapping and MUST NOT be elided if they do not.

An extended IEEE 802.15.4 address takes the form of an IEEE EUI-64 address. Generating an IID from an extended address is identical to that defined in Appendix A of [RFC4291]. The only change needed to transform an IEEE EUI-64 identifier to an interface identifier is to invert the universal/local bit.

A short IEEE 802.15.4 address is 16 bits in length. Short addresses are mapped into the restricted space of IEEE EUI-64 addresses by setting the middle 16 bits to 0xfffe, the bottom 16 bits to the short address, and all other bits to zero. As a result, an IID generated from a short address has the form:

0000:00ff:fe00:XXXX

where XXXX carries the short address. The universal/local bit is zero to indicate local scope.

This mapping for non-EUI-64 identifiers differs from that presented in Appendix A of [RFC4291]. Using the restricted space ensures no overlap with IIDs generated from unrestricted IEEE EUI-64 addresses. Also, including 0xfffe in the middle of the IID helps avoid overlap with other locally managed IIDs.

This mapping from a short IEEE 802.15.4 address to 64-bit IIDs is also used to reconstruct any part of an IID not covered by context information.

3.2.3. Stateless Multicast Address Compression​

LOWPAN_IPHC supports stateless compression of multicast addresses when M = 1 and DAC = 0. An IPv6 multicast address may be compressed down to 48, 32, or 8 bits using stateless compression. The format supports compression of the Solicited-Node Multicast Address (ff02::1:ffXX:XXXX) as well as any IPv6 multicast address where the upper bits of the multicast group identifier are zero. The 8-bit compressed form only carries the least-significant bits of the multicast group identifier. The 48- and 32-bit compressed forms carry the multicast scope and flags in-line, in addition to the least-significant bits of the multicast group identifier.

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Scope |              Group Identifier                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Group Identifier       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 7: DAM = 01. 48-bit Compressed Multicast Address
                     (ffFS::00GG:GGGG:GGGG)

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Scope |              Group Identifier                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 8: DAM = 10. 32-bit Compressed Multicast Address
                        (ffFS::00GG:GGGG)

 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|   Group ID    |
+-+-+-+-+-+-+-+-+

Figure 9: DAM = 11. 8-bit Compressed Multicast Address (ff02::GG)

3.2.4. Stateful Multicast Address Compression​

LOWPAN_IPHC supports stateful compression of multicast addresses when M = 1 and DAC = 1. This document currently defines DAM = 00: context-based compression of Unicast-Prefix-based IPv6 Multicast Addresses [RFC3306][RFC3956]. In particular, the Prefix Length and Network Prefix can be taken from a context. As a result, LOWPAN_IPHC can compress a Unicast-Prefix-based IPv6 Multicast Address down to 6 octets by only carrying the 4-bit Flags, 4-bit Scope, 8-bit Rendezvous Point Interface ID (RIID), and 32-bit Group Identifier in-line.

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Scope | Rsvd / RIID   |       Group Identifier        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|        Group Identifier       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 10: DAM = 00.  Unicast-Prefix-based IPv6 Multicast
                       Address Compression

Note that the Reserved field MUST carry the reserved bits from the multicast address format as described in [RFC3306]. When a Rendezvous Point is encoded in the multicast address as described in [RFC3956], the Reserved field carries the RIID bits in-line.

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