Re: [Tsv-art] [Lwip] Tsvart telechat review of draft-ietf-lwig-tcp-constrained-node-networks-11

[Carles Gomez Montenegro <[e-mail]>]

Hi Mirja,

Thank you very much for your review!

We just submitted revision -12, which aims at addressing the comments
received from the IESG and related reviewers, including yours:
https://tools.ietf.org/html/draft-ietf-lwig-tcp-constrained-node-networks-12

Please find below our inline responses:

> Reviewer: Mirja Kühlewind
> Review result: Ready
>
> This document has been reviewed as part of the transport area review
> team's
> ongoing effort to review key IETF documents. These comments were written
> primarily for the transport area directors, but are copied to the
> document's
> authors and WG to allow them to address any issues raised and also to the
> IETF
> discussion list for information.
>
> When done at the time of IETF Last Call, the authors should consider this
> review as part of the last-call comments they receive. Please always CC
> [e-mail] if you reply to or forward this review.
>
> Thanks for the well-written document! This is good to go, I only have some
> optional suggestions below:

Thanks for your kind words and for your constructive review.

> In section 4.1.2. you could potentially also mention
> draft-ietf-tcpm-generalized-ecn as connections with small windows might
> especially benefit when the ACK is also ECN-capable.

Agreed. We added the following before the last sentence of former 4.1.2
(now 3.1.2):

NEW:
   As of the writing, there is on-going work to extend the types of TCP
   packets that are ECN-capable, including pure ACKs [draft-ietf-tcpm-
   generalized-ecn]. Such a feature may further increase the benefits of
   ECN in CNN environments.

> In section 4.2.1: "In that case, both congestion and flow control
> implementation are quite simple." I guess this is even an understatement.
> Basically this would be a static configuration and there is nothing left
> to
> "control".

We see your point, but we understand that at least some sort of flow
control is still possible for a very simple TCP/IP stack. For instance, a
TCP receiver can still send segments with zero receive window (RWND=0).
Regarding congestion control, the RTO backoff applies, which is some basic
form of congestion control. The wording "quite simple" in the document
aims to capture that. However, please let us know if you have a different
opinion.

> In section 4.2.3: "Disabling Delayed ACKs at the sender allows an
> immediate ACK
> for the data segment carrying the response." I guess you can in addition,
> however, explain that keeping the delayed ACK could help to piggy-back the
> ACK
> with maybe some additional data to send instead of sending the ACK in a
> separate TCP packet (of course depending on the traffic pattern of the
> application). I think this is roughly mentioned later again but seems to
> belong
> her as well.

If we understand it correctly, your suggested addition is already present
in the sentence right before the one that you focused on in this comment.
If we misunderstood your comment, please let us know.

> Section 4.3.1.1: is this sentence correct? It least it did confuse me a
> bit
> when reading first: “A receiver supporting SACK will need to keep track
> of the
> SACK blocks that need to be received.” Maybe “A receiver supporting
> SACK will
> need to keep track of the data blocks that need to be received.” ?

Agreed.

> Section 4.3.2: Maybe s/it may make sense to use a small timeout/it may
> make
> sense to use a smaller timeout/ or s/it may make sense to use a small
> timeout/it may make sense to use a very small timeout/ ?

Agreed (we used the first proposal).

> Section 4.3.3: “with an IW setting of 10 MSS, there is significant
> buffer
> overflow risk” I think it would be good to explain slightly better which
> buffer
> you mean. Is there an assumption that network buffer sizes are known in
> CNNs as
> managed by the same entity than the constraint devices? Maybe the
> recommendation should be to make this parameter configurable? I guess most
> stacks provide an option for this but not sure if all do…

In the highlighted text, “buffer” may actually refer to a buffer at any
layer (e.g. a “radio buffer”, or an upper layer buffer), depending on the
specific context. For example, some constrained device hardware platforms
support a send buffer of at most one MSS of data.  We hope that the
following update can help clarify the intent:

OLD:
    with an IW setting of 10 MSS, there is significant buffer overflow risk,

NEW:
    with an IW setting of 10 MSS, there is significant buffer overflow risk,
    since many CNN devices support network or radio buffers of a size
    smaller than 10 MSS.

> Section 6: maybe provide a reference to RFC8446 TLS1.3..?

Agreed.

> Also section 6: You mention TCP-OA but you don’t give a really
> recommendation
> if or when it should be used or not.

We now aim at highlighting the benefits of TCP-AO, along with the
trade-off that needs to be assessed by the implementer. We updated the
first two paragraphs in Section 5 (formerly, Section 6), as follows:

NEW:
   Best current practice for securing TCP and TCP-based communication
   also applies to CNN.  As example, use of Transport Layer Security
   (TLS) [RFC 8446] is strongly recommended if it is applicable. However,
   note that TLS protects only the contents of the data segments.

   There are TCP options which can actually protect the transport layer.
   One example is the TCP Authentication Option (TCP-AO) [RFC5925]. However,
   this option adds overhead and complexity.  TCP-AO typically has a size of
   16-20 bytes. An implementer needs to asses the trade-off between security
   and performance when using TCP-AO, considering the characteristics (in
   terms of energy, bandwidth and computational power) of the environment
   where TCP will be used.

> I assume section 8 is intended to be kept for publication. I support that
> as
> it’s interesting information.

Thanks for your support!

Yes, we (the authors) would like to keep that former section 8 (now,
section 7) for publication.

Greetings,

Carles (on behalf of the authors)