Re: [nwcrg] nwcrg Digest, Vol 65, Issue 2

Salvatore Signorello <ssignorello@ciencias.ulisboa.pt> Fri, 08 March 2019 17:32 UTC

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From: Salvatore Signorello <ssignorello@ciencias.ulisboa.pt>
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Subject: Re: [nwcrg] nwcrg Digest, Vol 65, Issue 2
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Hi Janus,

many thanks to you for taking the time to go through/answer all my 
comments/questions. Follow a few more comments inline only for the sake 
of further clarification.

Best regards,

Salvatore

On 07/03/19 08:23, Janus Heide wrote:
>
> Hi Salvatore,
>
>
> Thanks for taking the time to read the draft and provide comments, and 
> great to hear that it contained information that was useful for you! 
> Below some thoughts / comments inline marked with <JH></JH>
>
> Best Janus
>
>
> W.r.t draft-heide-nwcrg-rlnc-background-00:
>
> - I was personally unaware of the precise definition of partially 
> decoding, so I had to go through some external references to 
> understand that. I think coded, recoded and systematic definitions are 
> much more intuitive. So, I was wondering if the "partially decoded 
> symbols" (used only twice throughout this draft) might need further 
> explanation somewhere.
>
> <JH> Generally I do not think think it is necessary/helpful to 
> distinguish between coded symbols and partially decoded symbols, since 
> recoding over symbols before or after the partial decoding will not 
> change the maximal rank the output "recoded" symbol can have, so I 
> think "partially decoded symbols" could be removed. </JH>
>
> - concerning sub-section 2.1.2, there are several related works which 
> have already explored the pros and cons of using different specific 
> generation/symbol/field size values. So, I was wondering if this 
> section could include more concrete details (yet not drawing any 
> application-specific guideline) complemented with the proper 
> bibliographic references.
>
> <JH> Hinting at good parameters is not easy, as both the reliability, 
> delay and computational impact needs to be factored into the system 
> performance. But I agree that it would be good to at-least provide 
> some useful references, which could also include the impact on 
> throughput and maybe also add some basic equations. </JH>
>
>
> W.r.t draft-heide-nwcrg-rlnc-01:
>
> - Sec. 2.1: "...a coding vector as described in Section 1.1 is first 
> generated...", there is no Section 1.1 within the current draft. This 
> most likely refers to Sec. 1.1 of draft-heide-nwcrg-rlnc-background-00 
> and has not updated during the transition from the previous single 
> draft to the two actual ones.
>
> <JH> Correct, to me referencing that section in the background 
> document is not helpful, I think it should be removed or replaced with 
> a reference that is more formal in its definition of encoding. </JH>
>
>
> - "If the TYPE is ’2’, all symbols included in this symbol 
> representation are coded, with coding vectors generated using the 
> included SEED and the ENCODER RANK."
>
> Not sure the above could be re-phrased better, yet my understanding is 
> that the Encoder Rank here only defines the number of coefficients per 
> coding vector. So, something along the line of the following could 
> work better:
>
> <JH> The rank defines the number of symbols over which encoding was 
> performed, so when coding over a block that would mean that only the 
> first ENCODER RANK coefficients can be non-zero where the remaining 
> coefficients must be zero. Appreciate your suggestion to make it 
> clearer, will try yo incorporate in the next iteration. </JH>
>
> "If the TYPE is ’2’, all symbols included in this symbol 
> representation are coded, with coding vectors of ENCODER RANK 
> coefficients each which are generated using the included SEED."
>
> - "If the TYPE is ’3’, all symbols included in this symbol 
> representation are either coded or recoded, with the coding 
> coefficients included constituting ENCODER RANK coefficients each."
>
> May be, using coding vectors could avoid the repetition of the word 
> coefficients and make the sentence clearer.
>
> <JH> a coding vector is usually defined as SYMBOLS coding coefficients 
> != ENCODER RANK coefficients. So if we only use coding vector and your 
> suggestions above we will end up operating over a set of vectors with 
> unequal length (all the unspecified coefficients are zero), I think 
> this would make it even hard to provide an unambiguous definition </JH>
>
*<SS>* here, sorry, but I am bit confused. Assuming, as you said, /"a 
coding vector is usually defined as SYMBOLS coding coefficients != 
ENCODER RANK coefficients"/, (here, SYMBOLS in capital letters means the 
header field value to me) then the following sentence might be a bit 
misleading.

     - "/For example, if ENCODER RANK is 12, then coding vector for the 
first symbol in this symbol representation is coefficients 0 through 11 
generated by the pseudo-random generator seeded by SEED,.../" . Even 
though you later specify "/If generation/window size is larger than 
ENCODER RANK, the remaining coefficients in the coding vector are zero./"

and the following one would seem to contradict your initial definition:

     - /"Each coding vector comprises ENCODER RANK number of coding 
coefficients, each coding coefficient having a predetermined field size."/

I understand there is a difference between what a coding vector is 
supposed to be (full vector including the zeros when GEN/WIN_SIZE > 
ENCODE_RANK) and what you are supposed to carry in the packet (which I 
guess is only made of ENCODER RANK coefficients).

Am I missing anything? If so, sorry for that, I would be grateful if you 
could further clarify this point. *</SS>*


> - "To ensure that SEED can be interpreted correctly at the receiver, 
> the same pseudo-random number generator MUST be used by the sender and 
> a recoding or receiving node. Otherwise, more than one SEED field 
> would need to be used."
>
> Does this raise any requirement (e.g., an additional field) for a 
> potential outer header?
>
> <JH> Depends on what flexibility you want, if a static PRNG is ok then 
> no, if you want to use different PRNGs for different sessions then you 
> should define it in an outer header (and maybe negotiate it at 
> initialization), same if you want to change the PRNG between symbol 
> representations - although I would question if there would be a 
> use-case for doing that? </JH>
>
*<SS>* I had no specific use-case in mind. Though, I still think using 
the seed field in the packet format for the symbol representation 
implicitly requires having an analogous packet field in an outer header 
to negotiate parameters related to the use of it. Here, my concern would 
be more whether or not there might be the need to amend the other draft 
when you say /"At the minimum, an outer header should indicate..."/ *</SS>*
>
> - I deem the examples of section 2.3.1 very useful, yet I do not see 
> yet the additional value/information the illustrations of packets for 
> small and large encoding window sub-sections bring. Could anyone 
> please elaborate a bit on this?
>
> <JH> Normally I would say that a generation size of 1024 or smaller 
> would provide acceptable flexibility. But there could be instances 
> where you want to support larger generation sizes. The reason why not 
> to just always go with the larger 18 bits is that this would 
> constitute a larger overhead on every packet - and one that would be 
> larger than the max target overhead we had </JH>
>
*<SS> *sorry, I should probably have elaborated more this point.  I know 
you may need smaller/bigger generation sizes and so a smaller/bigger 
encoding rank field may be needed for that purpose. So, it makes sense 
to me to enunciate the difference between Small and Large encoding 
windows. Yet, personally, I do not see the additional value of 
replicating the three illustrations (Fig. 8-9-10) of the same packet 
format where you only change the Encoder Rank field size (from Fig. 2-3-4).

*</SS>*

>
> Overall, I second the choice of moving the background information into 
> a separate draft. That makes the overall presentation of the content 
> neater.
>
> <JH> Great then it worked as hoped, and thanks again for your comments 
> </JH>
>
>
>
>
>
> On 05.03.2019 21.00, nwcrg-request@irtf.org wrote:
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>>     1. comments on the drafts nwcrg-rnlc-background00 and rlnc-01
>>        (Salvatore Todo Signorello)
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