[tcpm] Comments for draft-ietf-tcpm-rfc8312bis-02

Yoshifumi Nishida <nsd.ietf@gmail.com> Tue, 01 June 2021 08:35 UTC

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From: Yoshifumi Nishida <nsd.ietf@gmail.com>
Date: Tue, 1 Jun 2021 01:35:09 -0700
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Subject: [tcpm] Comments for draft-ietf-tcpm-rfc8312bis-02
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Hello, I have the following comments on this draft.

1: While this draft focuses on only TCP,  CUBIC can actually be applied to
other transport protocols
    such as QUIC, SCTP. Do we want to mention it in the draft or is it out
of scope?

2: Section 3.1 "After a window reduction in response to a congestion event
is

     detected by duplicate ACKs or Explicit Congestion Notification-Echo
     (ECN-Echo, ECE) ACKs [RFC3168
<https://datatracker.ietf.org/doc/html/rfc3168>], CUBIC remembers the
congestion window..."


   I think the events detected by RACK (or PTO for QUIC) can also be
included here.


3: Section 4.1.1

    It seems that the text version of the draft uses b_cubic and
a_aimd, while the pdf version of the

    draft uses different names. But, I prefer to use the same names in
both versions to avoid confusion.


4: Section 4.2  I personally prefer to refer the equations in the
draft as 'Equation' rather than 'Figure'  if there's

    no strong reason for it.


5: Section 4.3  It seems to me that there are two meanings for a_aimd
in this section.

    One is the additive factor for CUBIC and the other is a generic
parameter for AIMD() function.

    This looks a bit confusing to me.


6: Section 4.3  The description for P is required for Figure 3.


7: Section 4.3  I think The analysis in [FHP00] doesn't include
delayed ACK factor. So, the AIMD TCP model here

   can be a bit aggressive compared to a TCP that doesn't enable ABC
and uses delayed ACK.

  This is fine, but I think it might be good to clarify it.


8: Section 4.3  I am not very sure why segments_acked is used rather
than byted_acked here. What is the benefit of it?

    How do we calculate when the acks are split?

   Also, I think it should be clarified that cwnd is expressed in
segments here in this case.


9: Section 4.3  "Note that once _W_est_ reaches _W_max_, that is,
_W_est_ >= _W_max_, ..."

   I might miss something, but I'm not sure why  a_aimd can be set to
1 to be compatible with AIMD TCP.

   Does this mean b_cubic is also updated? If not, why this can be compatible?


10: Section 4.5: " The convex profile ensure that the window increases
very slowly at the beginning.."

   I am wondering how much this part is accurate. Because of Principal
2,  even though cwnd is increased

   through convex profile, I think it will be overridden by linear
growth by AIMD.


11: Section 4.7: "we update w_max as follows, before the window
reduction as described in section 4.6"

   I am wondering if reducing w_max is the right approach here.
Because if we reduce w_max, CUBIC

   can exit from convex region earlier than the case where fast
convergence is not used.

   It seems to me that keeping w_max and reducing only cwnd (using
smaller value than b_cubic)

   look more conservative.


12: Section 5.2 and Section 5.3. Do these results are based on the
algorithms and the parameter values

   described in the draft?  If there're differences, I think it should
be described.

   Thanks,

--

Yoshi



On Thu, May 13, 2021 at 1:19 AM Yoshifumi Nishida <nsd.ietf@gmail.com>
wrote:

> Hi,
>
> Sorry. The link for the draft was not correct. Please use this one.
> https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-rfc8312bis-02
> --
> Yoshi
>
> On Thu, May 13, 2021 at 12:57 AM Yoshifumi Nishida <nsd.ietf@gmail.com>
> wrote:
>
>> Hello everyone,
>>
>> This e-mail initiates the WGLC for
>> https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-rfc8312bis-01
>> Please send your feedback to the ML.
>> This WGLC runs until * Monday Jun 6 *
>>
>> The intended status of the draft is Proposed Standard.
>> We appreciate your cooperation.
>>
>> Thanks,
>> --
>> tcpm-chairs
>>
>>