Re: [nwcrg] RG Last Call for "BATS Coding Scheme for Multi-hop Data Transport"

Vincent Roca <vincent.roca@inria.fr> Tue, 14 September 2021 08:01 UTC

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From: Vincent Roca <vincent.roca@inria.fr>
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Date: Tue, 14 Sep 2021 10:01:43 +0200
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Cc: Vincent Roca <vincent.roca@inria.fr>, Marie-Jose Montpetit <marie@mjmontpetit.com>
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Subject: Re: [nwcrg] RG Last Call for "BATS Coding Scheme for Multi-hop Data Transport"
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Dear Authors, everybody,

As promised (a bit late however, sorry), here is my review of version -01 (draft-irtf-nwcrg-bats-01).

There is still some work to be done on this I-D before I can say it’s ready for IRSG review, but we are converging.
Authors, could you update your document accordingly? Thanks in advance.


# Main comments:

* Section 2.1
  Could Fig. 1 be made more explicit? Just to give you an idea of what I have in mind
  (to be improved, completed, and probably corrected):

  |
  | {set of source packets}
  v
+-+-+-+-+-+-+-+
| source node | outer coding: create batches of M BATS/coded packets each
|             | inner coding: recode each BATS/coded packets to form DDP packets
|             | transmit DDP packets
+-+-+-+-+-+-+-+
  |
  |     {set of DDP packets}
  v
+-+-+-+-+-+-+-+-+-+-+
| intermediate node | inner coding: recode DDP packets (if needed)
|                   | transmit incoming and/or recoded DDP packets
+-+-+-+-+-+-+-+-+-+-+
  ...

  The textual description of what happens is not cristal clear I'd say, the above figure could help.
  One question I have is about the partionning of the incoming set of source packets into batches: is there a fixed number of source packets per batch, is there an overlapping between consecutive batches or not?
  It also depends on the "degree" but I understand the degree varies according to a certain distribution, so different BATS packets in a given batch will depend on a different number of source packets…
  It’s confused in my mind, I don't have the answer to my question, could you clarify?
  Also, in section 1.2, the "degree" definition could suggest (ambiguous) that this degree may be fixed for a given batch.
  It did not help me.


* Section 2.4
  Strange to see a DDP that does not define any protocol version number, no session identifier (if there are multiple flows between the same endpoints).
  A full DDP should also define what happens when the small 12-bit BID space reaches its maximum value value (wrapping to zero I guess) and if it is an issue (e.g., with a high throughput link that consummes this space very rapidly), etc.
  I understand this is only a minimum example of DDP, not a full featured DDP and that this I-D is above all meant to specify the BATS scheme, not the DDP.
  It should be reminded that several simplifications have been made.


* Section 3.2:
  Function DegreeSampler() computes a degree distribution table in line with a predefined distribution, and returns a random degree that matches this distribution.
  However, looking at Fig. 7, I have the feeling that if K < MAX_DEG, there could be a bias in the actual distribution because of:
        return min(d,K)
  d matches the desired distribution but not: min(d,K) that will over-represent K in that case.
  I don't know if it's an issue.


* Last paragraph of Section 3.3:
  I fully agree, especially as forwarding systematic recoded packets immediately will reduce transmission latency.
  On the opposite, if only the linear combination approach is used by each intermediate node, latency will accumulate linearly with the number of nodes.
  Is it realistic? I'm surprised it's not discussed.


* Section 3: interoperability considerations
  The I-D should specify clearly which GF(256) to consider, i.e., its irreducible polynomial.
  This is what we did (we received this comment) in RFC 8681, section  3.7.1. Finite Field Definitions
  Do not hesitate to refer to (if meaningful):
        https://www.rfc-editor.org/rfc/rfc8681.html#name-finite-field-operations


* Section 4.1:
  - What is meant by throughput in: "The BATS code specification in Section 3 has nearly optimal throughput"?
    I guess: BATS approaches ideal codes.

  - The sentence is a bit strange: "The belief propagation decoder in Section 3.4 guarantees the recovery..."
    There is no guaranty. If the loss rate is too high the decoder will face problems even to recover a subset of the source packets.
    Maybe: "the BP usually enables the recovery of (at least) a fraction of the source packets".


** Section 6: intro.
  I find the use of the term « confidentiality » excessive.
  If an eavesdropper can collect a sufficient number of DDP packets, he can  decode them and recover the source packets.
  There is no hidden, encrypted, info (e.g., the coefficients or some key info) that could prevent it.
  Okay if the eavesdropper only captures a few DDP packets, but this is not a realistic attacker model.
  Even if we admit random coefficients are encrypted, it remains that the "confidentiality" depends on the message content (imagine a long message that contains a 32-bit secret and which is padded by thousands of null bytes).
  This is not in line with what the security community understands by confidentiality.
  This is also why [Bhattad05] that you refer to uses the adjective « Weakly » in the title.
  Obfuscation is preferable IMHO.


* Section 6.1: there are two instances of "must" (lower case).
  Since this is not normative language as per RFC 2119, I don't understand what is meant.
  Is a strong MUST more appropriate, or should it be "SHOULD"?


* Section 6.2, item 3:
  - Typo in "Original authentication". I guess it's "source" or "origin" authentication.
  - Additionally, I think you mean "origin authentication and message integrity".


* It's good practice to have an "Acknowledgement" section ;-)


* A reference to rfc 8406 "Taxonomy of Coding Techniques for Efficient Network Communications" would be meaningful since this is the NWCRG foundations.
        https://www.rfc-editor.org/rfc/rfc8406.html


Minor comments:

* Section 2.4.1:
  - Mq also refers the O value. To be added to:
      "Mq: 4-bit unsigned integer to specify the value of M and q as Table 1."
  - Also, I don't understand why the table is not organized with increasing values of Mq? It’s not wrong, I’m just surprised.
  - You could also say if value 0 is invalid (?) and whether values 8 and above are unused in this version but left for future evolutions.

* section 3.2: s/return/returns/ in "Define a function called DegreeSampler that return an integer d"

* section 3.4: s/batches/batch/ in   "Find a batches j that is decodable."

* section 4.1: s/call/called/ in: "which is also call multicast"

* section 4.3: s/techinques/techniques/


I hope it will help.
Regards,


   Vincent


> Le 30 juil. 2021 à 12:44, roca <vincent.roca@inria.fr> a écrit :
> 
> Dear all,
> 
> Following the recent update of the I-D and in line with IETF111 discussion, we would like to officially start a RG Last Call for:
> 	"BATS Coding Scheme for Multi-hop Data Transport » / draft-irtf-nwcrg-bats-01
> 	https://datatracker.ietf.org/doc/draft-irtf-nwcrg-bats/ <https://datatracker.ietf.org/doc/draft-irtf-nwcrg-bats/>
> 
> Since many participants may be on vacation, the call will end on Monday September 6th (5 weeks).
> 
> Please read it and provide feedback on the mailing list. Thanks in advance.
> 
> Regards,
> 
>     Marie-Jose and Vincent
>