Subject: Re: [lp-wan] Mirja Kühlewind's No Objection on draft-ietf-lpwan-ipv6-static-context-hc-21: (with COMMENT)

From: <[email-masked]>
Date: Sat, 2 Nov 2019 07:45:32 +0000

Hello Mirja,

Thanks for having taken the time to review our draft and to provide your
comments.
We have just published -22, which should be a big step closer to an RFC.
See our responses to your comments inline.
Best regards

Dominique

Le 21/08/19 18:38, « Mirja Kühlewind via Datatracker » <[email-masked]>
a écrit :

>Mirja Kühlewind has entered the following ballot position for
>draft-ietf-lpwan-ipv6-static-context-hc-21: No Objection
>
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>
>
>----------------------------------------------------------------------
>COMMENT:
>----------------------------------------------------------------------
>
>Thanks for the well-written document!
>
>A few quick comments:
>
>1) This text on ECN in sec 10.3:
>    " ECN functionality depends on both bits of the ECN field, which are
>      the 2 LSBs of this field, hence sending only a single LSB of this
>      field is NOT RECOMMENDED."
>probably belongs in section 10.2 instead, no?
[DB] Indeed, good catch, thanks! Moved to 10.2.

>
>2) If I understand the protocol correctly, I believe it should be a
>normative
>requirement that the Inactivity Timer is always larger than the
>Retransmission
>Timer? Related to this, Appendix F says: "The initial value of the
>Inactivity
>timer is
>   expected to be greater than that of the Retransmission timer, in
>   order to make sure that a (buffered) SCHC Fragment to be
>   retransmitted can find an opportunity for that transmission.  One
>   exception to this recommendation is the special case of the All-1
>   SCHC Fragment transmission."
>I think this section should be moved into the body of the document.
>Further, if
>you need a different timer value for the All-1 SCHC Fragment, you should
>give
>this timer a distinct name and value.
[DB] We went a different way of thinking. Remembering that this is the
specification of the generic SCHC mechanism, we simply stated that the
ACK-Always and ACK-on-Error are bidirectional protocols that require
bi-directional links. Some LPWANs only provide quasi-bidirectional links
(Sigfox, LoRaWAN Class A), some others provide full bidirectional links
(NB-IoT).
They are many ways the quasi-bidirectional links can be made bidirectional
(keep-alive uplink messages at the application layer or other protocol
stack layers, etc.).
Rather that trying in this draft to solve specificities of some use-cases
over some LPWAN technologies, we just deferred it to the schc-over-foo
draft to specify how quasi-bidirectional links (if they exist in the
technology considered) can be made bidirectional. We kept the prior text
as an example that schc-over-foo draft can copy-paste from.
And yes, we gave the new timer a distinct name!

>
>3) I also agree with Alvaro that appendix E should be moved into the body
>of
>the document if you intent to specify something normatively there.
[DB] the capitalised MUST that remained in App E was a mistake.
We rewrote Appendix E such that it is clearly non-normative, but purely
educational.

>
>4) Editorial in Sec 10.11:
>"...with a strength that is identical or better to the UDP
>   checksum."
>Not sure what you mean by "better"...?
[DB] by "identical or better strength", we mean "no weaker", i.e. that it
catches all errors that the UDP checksum does catch.
Do you have a clearer description in mind, that we could use?

>
>5) I wondering if you want to say maybe something about pacing out
>different
>fragments (mostly when a window is used). I know that any recommendation
>would
>be technology dependent but in some cases sending a burst of packets e.g.
>could
>overload some network queues; therefore it could be good to at least
>mention
>it...?
[DB] Thanks for you suggestion.
Following the same line of thinking than for App F, we thought this would
be a good thing to do in a schc-over-foo document, and not in this one.

>
>6) Without having thought too hard about it: is it possible with this
>compression scheme to specify a rule that increases a counter/sequence
>number
>by 1 (given the initial value is known)? Would maybe be nice to say
>something
>about this in the document...
[DB] SCHC was really designed to use a static context. Being able to
compress an incrementing counter field would mean maintaining a dynamic
context, which leads to many difficulties in case of transmission errors,
etc. This is a Pandora box we don't feel like opening.

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