Re: [6tisch] TSV-ART review of draft-ietf-6tisch-architecture

[G Fairhurst <[e-mail]>]

Please see my responses tagged GF1 below,


On 17/06/2019, 17:08, Pascal Thubert (pthubert) wrote:
>
> Interestingly, I received an echo of my response truncated, no idea 
> why. Just in case, attaching what I really sent here..
>
Hello Gorry:

Many thanks for your review, this is much appreciated.

Please see below

 > Intended status

 > -------------

 >

 > The document is proposed with standards track status, but appears to 
mainly

 > provide informational background about the architecture. There are no

 > normative requirements made. This could be published as informational.

It was originally intended informational. The discussion for the DetNet 
architecture lead to a different track because of external visibility - 
this spec is highly related to IEEE work.

As editor I'm agnostic, up to the A-Ds and I'll respect their decision.

[GF1]: There may be good reasons why this standards-track, but these 
should be clear. At least for me, this means my review will be more 
detailed.

 >

 > * Use of References:

 >

 > I suggest someone checks the presence of a large body of informative

 > references to current working group documents - rather than normative

 > citation of the published work.

 > I am concerned about citing individual work in progress within the 
body of the

 > document:

 >

 > Final para of Section 3.1 makes a reference to an individual draft 
that appears

 > to target the ROLL WG.

 >

This one is already solved: draft-thubert-roll-unaware-leaves is now 
https://tools.ietf.org/html/draft-ietf-roll-unaware-leaves .

It is a rather simple draft since it is the IGP counterpart of RFC 8505 
but really important for low power nodes that will not need to support RPL.

[GF1]: Sound like this has been resolved.

 > Section 3.2 makes reference to an individual draft to describe multicast

 > [ID.thubert…] Section 4.1.1 makes a reference to an individual draft that

 > appears to target the ROLL WG, mentioned twice as defining something,

 > Section 4.1.1 makes a reference to an individual draft that appears 
to target

 > the ROLL WG.

 >

[I-D.thubert-6lo-unicast-lookup] is an optimization. I cannot fathom its 
success as RFC. But it is interesting as an architectural construct.

Explaining that what this draft does is an architectural possibility is 
of interest in an architecture document, feels incomplete to ignore it. 
Note that the feature ships using non-standard methods (e.g., derived 
from LISP). I'd rather cite an open document, be it a draft.

The bottom line is that 6TiSCH has been trying to build a complete 
architecture and doing so, found overlaps and gaps between existing RFCs.

We started a number of document in several WGs and int and rtg and 
working hand in hand with experts in others e.g., sec.

Some of those documents are RFCs, e.g., 8505. Others passed WGLC like 
the backbone router and AP ND that are cited in this spec.

This one is only in inception but still addresses a gap that must be 
discussed in the architecture.

Maybe I could rephrase from:

"

A 6LBR located on the backbone may contribute to Duplicate Address

Detection as well as Address Lookup and save multicast operations

[I-D.thubert-6lo-unicast-lookup].

"

To

"

The use of multicast can also be reduced on the backbone with an optional

registrar that would contribute to Duplicate Address Detection as well as

Address Lookup using only unicast request/response exchanges.

<xref target="I-D.thubert-6lo-unicast-lookup"/> provides an example of how

that can be achieved with an extension of <xref target="RFC8505"/>, using a

6LBR as a SubNet-level registrar.

"

[GF1]: Thanks this is heading in a good direction, but I think your 
response makes me suggest the context of the citation needs to be clear. 
The word /optional/ doesn't really help. Is it possible to say something 
more like " this is a proposed method that presents an example of how to 
.... " - to clearly show this is NOT a thing yet being progressed in the 
IETF.

 > * References to unchartered individual work in Appendix.

 >

 > Appendix A describes a set of architectural dependencies that depend 
on non-

 > adopted (individual) documents or work not formally chartered by the 
IETF.

 > This description raises concerns about what happens when the present

 > document is published, and this work is not taken-up or some alternative

 > proposal is instead pursued, it would be safer to remove these 
references - or

 > at least to confine them to an appendix that clearly sets out that 
this work is

 > individual work in progress.

 >

 >

Makes sense. I took the latter approach.

The bottom line is that we could build interoperable 6TiSCH stacks 
without any of those features, IOW they are optional and future 
optimizations (e.g., zerotouch, broadcast reduction, etc…).

I had to rework the appendix to sort that out. The result looks like:

“

Appendix A. Related Work In Progress . . . . . . . . . . . . . . 61

A.1. Chartered IETF work items . . . . . . . . . . . . . . . . 61

A.2. Unchartered IETF work items . . . . . . . . . . . . . . . 62

A.2.1. 6TiSCH Zerotouch security . . . . . . . . . . . . . . 62

A.2.2. 6TiSCH Track Setup . . . . . . . . . . . . . . . . . 62

A.2.3. Using BIER in a 6TiSCH Network . . . . . . . . . . . 63

A.3. External (non-IETF) work items . . . . . . . . . . . . . 63

"

 > Transport issues

 > --------------

 >

 > * "Transport Mode"

 >

 > Section 4.7.1 describes a "transport mode”, but from the perspective 
of the

 > IETF transport area is not a transport. While there have been many

 > interpretations of “transports” by SDOs and IETF Specs, I suggest it 
would

 > beneficial to add a few words refining the meaning of the words in 
the present

 > usage: e.g. that this does not provide a “transport protocol”, but 
provides a

 > way to send and receive IPv6 packets that carry a transport protocol.

The language is inherited from IPSec 
http://www.firewall.cx/networking-topics/protocols/870-ipsec-modes.html

I agree we should migrate to a different terminology as DetNet did in a 
similar collision situation recently.

Would “native” work? By default I’ll migrate to that.


[GF1]: "Native" is fine, explaining "transport mode" could also fine 
(it's too late to claim we can fix this definition consistenly in all 
documents, so I am only insisting that you explain your meaning of the term.

 >

 > * The IPv6 Flow Label may be zero

 >

 > Section 4.7.1.1 seems to imply that IPv6 packets carry a non-zero 
flow label

 > value. Whilst I would support that desire, there are still cases 
where no flow

 > label is set, and this casts should be described (or

 > noted) in the text.

In general hope it’s zero since a non-zero cannot be compressed : )

But for a Track we need a flow ID. DetNet has immensely progressed on IP 
and now

We have a reference for the words in that section, so I changed to:

“

In native mode, the Protocol Data Unit (PDU) is associated

with flow-dependent meta-data that refers uniquely to the Track,

so the 6top sublayer can place the frame in the appropriate cell

without ambiguity. In the case of IPv6 traffic, this flow

identification may be done using a 6-tuple as discussed in

[I-D.ietf-detnet-ip].

The flow identification is validated at egress before restoring

the destination MAC address (DMAC) and punting to the upper layer.

“

[GF1]: The new text is fine. I would also not have objected to 
"Implementations of this document SHOULD support identification of 
DetNet flows based on the IPv6 Flow Label field" - esepcillay, if you 
wish endpoints to set a non-zero value.

 >

 > * Assumption about IPv6 MTU

 >

 > Section 4.7.3 notes the IPv6 MTU is 1280 B. This is not true, please 
rewrite this

 > sentence. It could be that the intentional was to state the minimum 
IPv6 MTU

 > or something else was intended?

The 6LoWPAN MTU is 1280, though it is the IPv6 Minimal MTU. Proposed change:

“

    Considering that 6LoWPAN packets can be as large as 1280 bytes (the

    IPv6 MTU)

“

->

“

    Considering that per section 4 of [RFC4944] 6LoWPAN packets can be as

    large as 1280 bytes (the IPv6 minimum MTU)

“

[GF1]: This addresses my comment.

 >

 > * Possible PHB - reference to not chartered transport work.

 >

 > Section 4.8.3 describes a transport feature (diffserv PHB) and appears to

 > promote an individual draft. The particular drafts appear to target 
tsvwg, but I

 > do not recall these being proposed to that WG for adoption, and 
therefore this

 > seems a little presumptuous to appear in the main body of text or 
within the

 > architecture. The reference is also out-of-date.

 > - I suggest it would be sufficient to explain that a future document 
could define

 > a PHB for this purpose and refer to the IANA registry as the place 
where IETF-

 > defined PHBs are listed.

 > - Please this without citing a ref to an individual spec.?

 > - In addition, I think if this sections retained, it needs to be 
moved to the

 > appendix.

Ack. The section now reads:

“

  

  

4.8.3.  Differentiated Services Per-Hop-Behavior

  

    A future document could define a PHB for Deterministic Flows, to be

    indicated in the IANA registry where IETF-defined PHBs are listed.

“

The reference was removed.

[GF1]: Sure. (By the way, should there be consensus, such work could be 
proposed in TSVWG.)

 >

 >

 > Editorial Issues

 > -------------

 >

 > The following editorial issues should be addressed:

 >

 > The document makes frequent use of the phrase “in order to”, which in 
most

 > (all?) cases could be replaced more simply by “to” without loss of 
meaning.

Scanned and applied

 > This would remove several cases where the sentence could currently be 
mis-

 > interpreted as requiring temporal ordering of the actions.

Good!

 >

 > Section 4.3.3: “infoirmation” should be “information”.

Applied

 >

 > Section 1, Sentence “Distributed is a concurrent model that..”. i 
could not

 > understand the intended meaning of this sentence.

 >

Proposed change

“

In contrast, Distributed Routing refers to a model that relies on the

concurrent peer to peer protocol exchanges for TSCH resource allocation

and routing operations.

“

[GF1]: OK.

 > “such a Advance metering”. “a” maybe should be “as”? Why is Advanced

 > capitalised, I think this sentence lacks some necessary explanation, 
please add,

 > and provide a reference.

Great point. I modified the sentence as follows, adding a link to DoE.

“

    The architecture defines mechanisms to establish and maintain routing

    and scheduling in a centralized, distributed, or mixed fashion, for

    use in multiple OT environments.  It is applicable in particular to

    highly scalable solutions such as used in the Advanced Metering

    Infrastructure [AMI] solutions that leverage distributed routing to

    enable multipath forwarding over large LLN meshes.

“

    [AMI]      US Department of Energy, "Advanced Metering Infrastructure

               and Customer Systems", 2006,

               <https://www.energy.gov/sites/prod/files/2016/12/f34/

               AMI%20Summary%20Report_09-26-16.pdf>.

[GF1]: OK.

 >

 > CoJP - no reference is provided to where this is specified.

JP is now introduced as:

“

    CoJP (Constrained Join Protocol):  The Constrained Join Protocol

                (CoJP) enables a pledge to securely join a 6TiSCH network

                and obtain network parameters over a secure channel.

                Minimal Security Framework for 6TiSCH

                [I-D.ietf-6tisch-minimal-security] defines the minimal CoJP

                setup with pre-shared keys defined.  In that mode, CoJP

                can operate with a single round trip exchange.

“

[GF1]: Thanks, OK.

Please let me know if the changes are OK and I’ll publish a revision.

Again, many thanks Gorry!

Pascal

---

[GF1]: Thank you Pascal for providing these responses. I think you now 
have good text to address the issues I noted. Theer are one or two 
points that above you may wish to refine your update.

The following issue remains, and will need someone else to assess this:

Intended document status - as standards track or informational?

Gorry

(cc'ed to TSV-ART for the benefit of others tracking this review.)