Re: Genart last call review of draft-ietf-lpwan-ipv6-static-context-hc-21

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Hello Pete,

Please accept our sincere thanks for your time reviewing and commenting on
this draft.
Your comments do help us shape a better draft.
Please see answers and questions inlined in your text.
Best regards

Dominique


Le 07/08/19 05:13, « Pete Resnick via Datatracker » <noreply@xxxxxxxx> a
écrit :

>Reviewer: Pete Resnick
>Review result: Ready with Issues
>
>I am the assigned Gen-ART reviewer for this draft. The General Area
>Review Team (Gen-ART) reviews all IETF documents being processed
>by the IESG for the IETF Chair.  Please treat these comments just
>like any other last call comments.
>
>For more information, please see the FAQ at
>
><https://trac.ietf.org/trac/gen/wiki/GenArtfaq>.
>
>Document: draft-ietf-lpwan-ipv6-static-context-hc-21
>Reviewer: Pete Resnick
>Review Date: 2019-08-06
>IETF LC End Date: 2019-07-19
>IESG Telechat date: Not scheduled for a telechat
>
>Summary:
>
>Some minor issues, but otherwise looks good to me.
>
>My apologies for this very late review. I hope these comments are useful,
>but
>none of these are showstoppers in my opinion.
>
>Major issues:
>
>None.
>
>Minor issues:
>
>Section 5:
>
>   If the SCHC
>   Packet is to be fragmented, the optional SCHC Fragmentation MAY be
>   applied to the SCHC Packet.
>
>Don't you mean:
>
>   If the SCHC Packet is to be fragmented, the OPTIONAL SCHC
>   Fragmentation is applied to the SCHC Packet.
>
>or even just:
>
>   SCHC Fragmentation is applied if the SCHC Packet is to be fragmented.
>
>I think it's confusing to say that using SCHC is optional if the SCHC
>Packet is
>to be fragmented. If you're fragmenting, it's not optional, is it?
DB: some LPWAN technologies have their own fragmentation technique (e.g.
NB-IoT), and as part of their Profile, they might want to specify that
SCHC Fragmentation shall not be used.
In such a case, even though fragmentation is needed, SCHC Fragmentation is
not used.
How about
OLD TEXT
If the SCHC Packet is to be fragmented, the optional SCHC Fragmentation
MAY be applied to the SCHC Packet.
NEW TEXT
If needed by the underlying layer, the optional SCHC Fragmentation MAY be
applied to the SCHC Packet.



>
>Section 7.1 or 7.3:
>
>It took me a while to get that what you're looking for is a Rule in the
>list of
>Rules that has a function for *all* of the header fields given the DI and
>FP.
>It would be good to say some sort of overview thing like this explicitly,
>either in 7.1 or at the top of 7.3. It's possible this is obvious to
>someone
>versed in this topic, but it wasn't for me.
[DB] your proposed rephrasing is not quite accurate. We are looking for a
Rule that
has a function for all of the header fields and *no more* than the header
fields in the packet being compressed. This is reflected in the detailed
algorithm.
Regarding an overview statement, how about changing
OLD TEXT
the set of Rules is browsed to identify which Rule will be used to
compress the packet header.
The Rule is selected by matching the Fields Descriptions to the packet
header. The detailed steps are the following:
NEW TEXT
the general idea is to find in the Rule set a Rule that has a matching
Field Descriptor (given the DI and FP) for exactly each and every header
field of the packet being compressed. The detailed algorithm is the
following:

Also, I realised we use the word "steps" at two indentation levels. I
suggest we change
OLD TEXT
The compression/decompression process follows several steps
NEXT TEXT
The compression/decompression process follows several phases



>
>Section 7.3:
>
>Question: Is it possible for multiple Rules to match a given packet? What
>happens if you find more than one? That should probably be specified.
DB: indeed, this specification does not prevent multiple Rules from
matching. We have kept silent about this in order to keep the text small
and to avoid ensuing discussions.
But since you're asking, how about adding the following?
NEW TEXT
This specification does not prevent multiple Rules from matching. Whether
a multiple match is allowed or not, and what to do in the case where
multiple Rules match, is left to the implementation. A long as the same
Rule set is installed at both ends, this degree of freedom does not
constitute an interoperability issue.



>
>Section 7.5.2:
>
>This encoding seems to use more space than needed. I assume you kept the
>size
>in multiples of 4 to make it on nibble boundaries, but I don't understand
>why
>you'd want 28 bits instead of 24. The larger sizes could simply be 0xFF
>followed by the 16-bit value.
[DB] I don't quite understand his proposal. Is it a two-sized approach (4
bits and 24 bits), or a three-sized approach (4, 12 and 24 bits). In the
former case, we pay a high cost for value sizes 15 and upward. In the
latter case, the intermediate size (12 bits) is only applicable to value
size 15 (or 15-31?). I like the three-sized approach and suggest we don't
change our current spec. We expect the 4 and 12 bits encodings to be used
most of the time, and added the 24 bits encoding as a safety net.


>
>Nits/editorial comments:
>
>Section 7.3:
>
>In the last bullet of the Rule selection algorithm, it says:
>
>   Sending an uncompressed header may require SCHC F/R.
>
>Sending a compressed header may also require F/R, couldn't it? Seems to
>me this
>sentence is superfluous.
DB: you're right, in a pure logic sense. The aim of "may" was to attract
attention to the likeliness that fragmentation be required. Another
reviewer suggested 
OLD TEXT
Sending an uncompressed header may require SCHC F/R.

NEW TEXT
Sending an uncompressed header is likely to require SCHC F/R.


>
>Section 8.1, second paragraph:
>
>It seems like you'd want one or both occurrences of "optional" to be
>"OPTIONAL", in the 2119 sense. Is there a reason they're not?
DB: True, thanks for catching this. The second one should be capital
letters.

>
>I'm not sure I understand the last sentence of that paragraph. Do you
>simply
>mean, "You can ignore the rest of section 8"? That seems unnecessary to
>say.
DB: thanks for your opinion on this. We will remove this sentence.

>
>Section 8.2.2.2:
>
>Change:
>
>   o  their numbers MUST increase from 0 upward, from the start of the
>      SCHC Packet to its end.
>
>to:
>
>   o  their numbers MUST increase by 1 from 0 upward, from the start of
>      the SCHC Packet to its end.
>
>in order to avoid someone being inordinately cute (or stupid).
DB: Definitely true, this was a shortcoming of ours. We'll change this
text to say "increment by 1" or "increase by 1" as suggested.


>
>8.2.4:
>
>"The W field is optional" - Is OPTIONAL not appropriate here? If so, this
>appears in many places in section 8.
DB: True. I haven't paid enough attention to the use of capital OPTIONAL,
overall. I will give it another pass.


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