Re: [6tisch] MSF Shepherd review

"Pascal Thubert (pthubert)" <pthubert@cisco.com> Fri, 29 November 2019 15:23 UTC

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From: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
To: Tengfei Chang <tengfei.chang@gmail.com>
CC: "draft-ietf-6tisch-msf@ietf.org" <draft-ietf-6tisch-msf@ietf.org>, "6tisch@ietf.org" <6tisch@ietf.org>
Thread-Topic: [6tisch] MSF Shepherd review
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Date: Fri, 29 Nov 2019 15:23:17 +0000
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Subject: Re: [6tisch] MSF Shepherd review
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Please do not call him preferred parent that’s something specific in RPL, the best parent for forwarding up the dodag.

Why not just say “the parent “ explaining that the 6P protocol can be used in parallel with multiple parents?


Regards,

Pascal

Le 29 nov. 2019 à 16:19, Tengfei Chang <tengfei.chang@gmail.com> a écrit :


Hi Pascal,

For the preferred parent issue:

When running MSF, the node is deal with one parent at a time out of the parent set, which we called preferred parent.
It doesn't mean there is only one parent for each nodes.
The node may change its preferred parent to other parent, which responded in the switching_parent section in MSF.

For the sentence:

It is recommended to set MAX_NUMCELLS value at least 4x of the maximum link traffic load of the network with unit of packets per slotframe.

The following example helps to understand the meaning:

For example, a 2 packets/slotframe traffic load results an average 4 cells scheduled, using the value of double number of scheduled cells (which is 8) as MAX_NUM_CELLS gives a good resolution on cell usage calculation.

Any recommendation on the rephrasing?

Tengfei



On Wed, Nov 27, 2019 at 10:07 PM Pascal Thubert (pthubert) <pthubert@cisco.com<mailto:pthubert@cisco.com>> wrote:
Hello Tengfei


Please see below

Le 27 nov. 2019 à 21:44, Tengfei Chang <tengfei.chang@gmail.com<mailto:tengfei.chang@gmail.com>> a écrit :


Thanks a lot for the reviewing, I responded inline:

On Mon, Nov 25, 2019 at 6:42 PM Pascal Thubert (pthubert) <pthubert@cisco.com<mailto:pthubert@cisco.com>> wrote:
Dear all

Please find some comments below:




Please migrate to XML2RFC v3. This will save time in the future.

TC: got it! Will used in version 9.

:)



   However, an implementor MAY implement MSF without implementing

   Minimal 6TiSCH Configuration.


This is not helpful without explanations. What is the tradeoff? How does the  network operates in that case?

TC: Yes, the sentence is misleading. What we try to say is MSF can work with other specifications protocols, rather then minimal 6TiSCH configuration, as long as the protocols gives a way to communicate the EB and DIO among the network.


Those words in the draft will make me a happy shepherd...





For example, a Trickle Timer defined in

[RFC6550<https://tools.ietf.org/html/rfc6550>] MAY be applied on DIOs. However, this behavior is

implementation-specific which is out of the scope of MSF.



This is not for this spec to define. RPL already mandates trickle. Suggestion:


For example, the Trickle operation defined in [RFC6206]

is applied on DIO Messages [RFC6550<https://tools.ietf.org/html/rfc6550>]0>]. This behavior is

out of the scope of MSF.



TC: agreed!





MSF RECOMMENDS the use of 3 slotframes.

Discussion on slotframes and cells comes without an introduction to TSCH.
I’d suggest you add a few words on RFC 7554 appendix A and 6TiSCH architecture section 4.3.5. to introduce those concepts.
They should probably be normative references.

TC: I added the following text at beginning of section 2:
            In a TSCH network, time is sliced up into time slots.
            The time slots are grouped as one of more slotframes which repeat over time.
            The TSCH schedule instructs a node what to do at each time slots, such as transmit, receive or sleep <xref target="RFC7554"/>.
            In case of a slot to transmit or receive, a channel is assigned to the time slot.
            The tuple (slot, channel) is indicated as a cell of TSCH schedule.
            MSF is one of the policies defining how to manage the TSCH schedule.

Excellent




Section 4 has numerous SHOULD. Trouble is, when SHOULD is used, the author SHOULD explain the alternate, what if the SHOULD is not followed.
Sometimes it’s quite obvious, like when using random in 4.2. But SHOULD use minimal is less obvious. Please consider adding text after the SHOULDs.

TC: agreed!  I have resolved this SHOULD issues in a new version. either the unnecessaries are removed or alternative explanation is added

I’ll review once you published




   field it contains, the presence and contents of the IE defined in

   [I-D.richardson-6tisch-join-enhanced-beacon<https://tools.ietf.org/html/draft-ietf-6tisch-msf-08#ref-I-D.richardson-6tisch-join-enhanced-beacon>]n>], or the key used to

   authenticate it.

The reference is now draft-ietf.. I agree that it should be normative; no worries the draft is already submitted for publication.
More important: Please move the reference to 6tisch-dtsecurity-zerotouch-join to informational. This is a DOWNREF today and your draft may be stuck in MISSREF in the future.

TC: I have updated  richardson-6tisch-join-enhanced-beacon to  ietf-6tisch-enrollment-enhanced-beacon.
I didn't get it how "move the reference to 6tisch-dtsecurity-zerotouch-join to informational" is done in the draft?


Sorry I was unclear. The draft is currently listed as a normative reference. This means that MSF will be held forever in miss ref at the RFC editor. Please move the link to the reference in the informational references section.




   After selected a preferred parent, the joined node MUST generate a 6P

Grammar: “After selecting” or “once it has selected” sound better.

TC: the latter sounds better! Thanks!



Section Section 8<https://tools.ietf.org/html/draft-ietf-6tisch-msf-08#section-8>

The <xref …> already generates the word “section”. If you write it too, it becomes duplicated as above.

TC: agreed!




For a node, this translates into

   monitoring the current usage of the cells it has to its preferred

   parent:


This is disturbing. MSF should not be used only with preferred parents. The whole game of doing a DODAG is to have and possibly use multiple parents.
A node can for instance send a NSM DAO with multiple transit options to the root. Also, it could be good to clarify that the child manages both directions.
Proposal:


For a node, this translates into

   monitoring the current usage of the cells it has to the parents it uses

   at this point of time for sending and receiving traffic:

Later there a numerous references to “preferred parent” => I’d suggest you use just “selected parent” or “active parent” or  something in that vein.
TC: I think "preferred parent" is same with "selected parent".  it indicates one preferred parent out of multiple. Isn't it right?

My problem is that there’s only one preferred parent, but a node may use several parents for data traffic. This is why we build dodags in the first place.

 I believe that the node may allocate cells with all of those “selected parents” if it likes. The use of “preferred parent” in that text would prevent this.

Please make sure your text does not limit to one parent...




Cell installed at initial


Not sure this is correct. Maybe “at init time”

TC: Applied!





It is recommended to set MAX_NUMCELLS value at

   least 4 times than the maximum link traffic load of the network in

   packets per slotframe.




This does not parse. Can you please rephrase?

TC: it's rephrased as "It is recommended to set MAX_NUMCELLS value at least 4x of the maximum link traffic load of the network with unit of packets per slotframe."

I still have a hard time

Do you mean “4 times the maximum number of used cells in a slot frame in recent history” ?




Section 8 does not try to avoid collisions with autocells. But it’s easy to compute the slot offset of autocells for self and parent and avoids those. Why not do that?

TC: agreed! Will apply in the next version.



Section 16 will require more attention, either now or during secdir review, probably both. You should start now. Think, say, what if an attacker claims many cells to all its neighbors? Can it attack someone’s autocells to block him?

TC: That's a good question! It may have a chance to do so. We need discuss internally on this section.
Thanks for belling ahead!

Speaking from experience with secdir. Better be prepared they will be coming for you ; )

Take care

Pascal



Voila!

Pascal as shepherd.






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--
——————————————————————————————————————

Dr. Tengfei, Chang
Postdoctoral Research Engineer, Inria

www.tchang.org/<http://www.tchang.org/>
——————————————————————————————————————


--
——————————————————————————————————————

Dr. Tengfei, Chang
Postdoctoral Research Engineer, Inria

www.tchang.org/<http://www.tchang.org/>
——————————————————————————————————————