Re: [6tisch] 6P "design team" session this Friday, same time as interim meeting

[Yasuyuki Tanaka <[e-mail]>]

Thank you, Thomas.

Let me add comments on a couple of the ppt slides in advance, which
could save time of the coming meeting, I hope...:

   The ppt sides Thomas provided:
   https://bitbucket.org/6tisch/meetings/raw/c9bd4d78fa452a0588abefc104d08520d32e0c26/161209_webex/slides_161209_webex.ppt

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(6P unclarities 2 [1/4], slide-19)

ppt> Section 4.2.2: [Q] Why must the value of SeqNum increment *by
ppt> exactly one* at each new 6P request to a certain neighbor?
ppt>
ppt> -> Why not?

I think "MUST" is too strict. I thought an idea behind that
requirement was to try to have different SeqNum to previous
requests. If this was true, "incrementing by exactly one at each new
6P request" could be just an example of implementation.

In addition, there is no validation defined in the draft to see if the
SeqNum of a receiving request is off by one from the previous one. If
it was a really "MUST" thing, some validation should be in place,
shouldn't it? But, of course, such a validation is not practical at
all since a request could be lost in the air.

I may be wrong; just wanted to know the reason of the restriction,
"MUST increment by exactly one at each new 6P request issued to the
same neighbor."

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ppt> [C] I'd suggest listing only valid combinations of CellOption
ppt> bits and mentioning others are invalid.
ppt> -> isn’t that policy?

Indeed, 6P doesn't care about the meanings of bits in CellOptions in
its operation. However, the definitions in Figure 11 are RECOMMENDED
by the draft. I cannot see any reason to have ineffective CellOptions
values separately in the recommendation...

draft> 4.2.6.  6P CellOptions
draft> (snip)
draft> Figure 11 contains the RECOMMENDED meaning of the 6P
draft> CellOptions field for the 6P STATUS and 6P LIST requests.

In this context, we may have to define a return code to respond to
CellOptions invalid to a corresponding SF, something like
RC_ERR_CELL_OPTIONS. Or, should RC_ERR (generic error) be returned? In
any case, it'd be nice to have some text on checking CellOptions value
in Section 4.3.

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(6P unclarities 2 [4/4], slide-22)

ppt> [C] I'm not sure typical use cases of the LIST operation. When
ppt> does a SF use STATUS and LIST...? I think these commands would be
ppt> useful for the purpose of management or administration. But, it's
ppt> not within the scope of SF, is it? I'd be nice that a typical use
ppt> case of LIST is provided in the text.
ppt> -> Recover after reset

Such recovery doesn't work due to generation inconsistency.

Let's say, there are two 6P nodes, Node A and Node B. Node A resets
after having some Add or Delete operations with node B. Now, both of
GTX and GRX on Node A are 0b00. At least GTX or GRX on Node B is
either 0b01 or 0b10.

Then, Node A sends a Status request to Node B for recovery. Node B
responds with RC_ERR_GEN since GAB and GBA of the request are zero
while node-B has non-zero GTX and/or non-zero GRX. The Status request
fails. The same thing will happen to a List request. In the end, Node
A cannot recover the cells.

Here is an excerpt of Section 4.3.11.3 of draft-03:

draft> Upon receiving a 6P message, a node MUST do the following
draft> checks:
draft>
draft> o When node B receives a 6P Request of 6P confirmation from node A,
draft>   it verifies that GAB==GRX and GBA==GTX.
draft>
draft> (snip)
draft>
draft> If any of these comparisons is false, the node has detected a
draft> schedule generation inconsistency.
draft>
draft> When a schedule generation inconsistency is detected:
draft>
draft>    o If the code of the 6P Request is different from CMD_CLEAR, the
draft>      node MUST reply with error code RC_ERR_GEN.

This is related to the topic in the thread of "Handling Inconsistent
Allocation in 6P."

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ppt> - Is there any plan for 6P to support the following cells?
ppt>
ppt>   - a cell whose macNodeAddress is a group MAC address or a
ppt>     16-bit multicast address
ppt> -> No. Use case?

A use-case in my mind is allocating cells for IPv6 multicast. RFC 4944
defines the mapping rule between IPv6 multicast address and IEEE
802.15.4 16-bit multicast address.

# I'm not sure the exact meaning of "mesh-enabled LoWPAN" in Section 9
# of RFC 4944, though... I think Section 9 could be applied to a
# 6TiSCH network.

I think this has some potential to enable to dynamically allocate
cells dedicated to ff02::1a (all-RPL-node) or ff0X::fc
(ALL_MPL_FORWARDERS), for example.

Now, I understand this is out of scope of the draft at this point as
well as allocating cells for broadcast.

I'm a person interested in multicast, by the way ;-)

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ppt> - Is there any plan for 6P to support the following cells?
ppt> (snip)
ppt>    - a dedicated TX cell to multiple recipients
ppt> -> IEEE802.15.4e question, multiple cells?

Tero told me that such a TX cell is possible as per the IEEE 802.15.4
specification.

https://www.ietf.org/mail-archive/web/6tisch/current/msg04909.html

mail> > Therefore, in theory, we may have a dedicated (non-shared) TX
mail> > cell whose macNodeAddress is the broadcast address.
mail>
mail> Yes. I.e. you are the only one allowed to send to that link, but
mail> there are multiple listeneres in there. So, as it does not have
mail> shared bit on, there will not be other transmitters, but there
mail> can be multiple listeners on it.

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ppt> - Is there any plan for 6P to support the following cells?
ppt> (snip)
ppt>    - a RX cell shared with multiple senders
ppt> -> supported

OK, then another question comes up... how is a cell having multiple
senders (or multiple recipients) allocated or deallocated?

For example, one of senders may request to delete such a cell with a
recipient. However, the recipient is not supposed to delete the cell
by the request alone since there are other senders on the cell.

If it's supported, some explanation would be needed, about how it
works, although this kind of detail may be up to SF.

This could be related to the topic about what value is set to
"macNodeAddress" as a result of 6P Add transaction.

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That's it! Thanks.

Best,
Yatch