[6tisch] Compression Reference and Coalescence

[Tengfei Chang <[e-mail]>]

Dear all,

I have some concern on the compression Reference and Coalescence.

In the 6lorh draft, all the hops in RH3 are compressed according to the
compression reference (the source address, root usually). And when doing
Coalescence, the process is kind of like taking the first address in first
RH3 as the reference. The inconsistency between compression reference and
Coalescense may waste some bytes in some cases.

For example:
A packet is issued by root with a compressed RH3 along an A->B->C->D
source route path.

The nodes address are:
root:      bbbb::0000:0000:0000:0001
node A: bbbb::1111:2222:3333:0001
node B: bbbb::1111:2222:3333:0002
node C: bbbb::1111:2222:3333:0003
node D: bbbb::1111:2222:3333:0004

According to the 6lorh draft: all hops in RH3 will be compressed according
to reference, which is the root. The  Packet received by node A is:
----------------------------

Type 3 RH3-6LoRH Size = 2  1111 2222 3333 0001
                                           1111 2222 3333 0002
                                           1111 2222 3333 0003


And which maybe more reasonable packet would be like :
----------------------------

Type 3 RH3-6LoRH Size = 0  1111 2222 3333 0001
Type 0 RH3-6LoRH Size = 1  02
                                            03

Which means the first hop in first RH3 entry is compressed according to the
reference(which is the root in this case) and the rest hops are compressed
according to the first hop in first RH3 entry. For me, this compression way
is more consistent  with the way when doing coalescence.

What do you think?
Tengfei