Re: [tcpm] 2nd WGLC for draft-ietf-tcpm-rfc8312bis

Yoshifumi Nishida <> Wed, 16 February 2022 07:43 UTC

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From: Yoshifumi Nishida <>
Date: Tue, 15 Feb 2022 23:42:57 -0800
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To: Markku Kojo <>
Cc: " Extensions" <>, tcpm-chairs <>
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Subject: Re: [tcpm] 2nd WGLC for draft-ietf-tcpm-rfc8312bis
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Hi Markku,

Thanks for the comments. I think these are very valid points.
However, I would like to check several things as a co-chair and a doc
shepherd before we discuss the points you've raised.

In my understanding (please correct me if I'm wrong), when this draft was
adopted as an WG item, I think the goal of the doc was some minor updates
from RFC8312 which include more clarifications, minor changes and bug
However, if we try to address your concerns, I think we'll need to invent a
new version of CUBIC something like CUBIC++ or NewCUBIC in the end.
I won't deny the value of such doc, but, this seems not to be what we
agreed on beforehand.
if we proceed in this direction, I think we will need to check the WG
consensus whether this should be a new goal for the doc.

So, I would like to check if this is what you intend for the doc or you
think we can address your points while aligning with the original goal.
Also, if someone has opinions on this, please share.


On Fri, Feb 11, 2022 at 9:34 PM Markku Kojo <> wrote:

> Hi Yoshi, all,
> It seems to me that many issues that I raised have been solved, thanks.
> However, there are still a number of important issues that have not been
> addressed adequately. At least the following:
> #135 on W_max. Yoshi's observation was correct that this is not resolved:
> the co-authors and original developpers of CUBIC (@lisongxu and
> @sangtaeha) agreed in their last message that Wmax needs different
> treatment for slow start and congestion avoidance and plan comprehensive
> (new) evaluation of it. This is obviously an open issue but the issue
> was closed?
> #85 (& #86 with basically same issue and these two were combined) This
> (#85) is about ECN but the major issue is on using the same MD
> factor in slow start and in congestion avoidance when using
> loss-based CC. This (#85) remained closed even though I provided a
> thorough explanation why it is wrong and against the original theory and
> design by Van Jacobson, against the congestion control principles (RFC
> 2914) and two co-authors agreed on this in their same last message to
> #135 when they agreed on Wmax needing rework. This is an important issue
> that the wg should consider very carefully because it is not only
> updating RFC 5681 but also in conflict with RFC 2914. How can
> tcpm (and IETF) suggest and allow one CC algo to not follow congestion
> control principles as set in RFC 2914 while requiring all other CCs to
> follow RFC 2914 guidelines?
> The current draft does not provide any justification for using the same
> MD factor in slow start as in congestion avoidance. Nor am I
> awere of any experimental data that would support this change.
> The fact thet CUBIC has been long deployed does not alone provide any
> supporting evidence because CUBIC is likely to give good performance as
> it is overagressive and thereby unfair to competing traffic and users
> tend to be happy when measuring the performance of the sending CUBIC
> only, not the competing traffic that is badly impacted. HyStart++ is
> suggested as mitigation to the problem but it cannot; HyStart++ is only
> applicable during initial slow start, not during slow start after RTO!
> That is, the "SHOULD use HYStart++" text in Sec 4.10 is impossible
> to implement as I have pointed out in my comments earlier. Using a proper
> MD factor in slow start is even more important if loss is detectected
> during a RTO recovery because the sender is likely to face heavy
> congestion in such a case and it is very bad if the sender continues
> sending with overaggressive rate, stealing the capacity from and causing
> harm to coexisting flows. In addition, as I have explained, HyStart++
> does not remove the problem even for the initial slow start as it is not
> shown to work always. Instead, the results with the HyStart++ draft show
> that it reduces 50% of rexmits and only 36% RTOs, meaning that there is
> likely to be a notable percentage of cases when a sender is still
> in slow start when first loss is detected (i.e., HyStart++ had no effect)
> and a significant number of cases where a CUBIC sender is
> overaggressive continuing with a 1-40% larger cwnd than what is the
> available capacity. Note also that any delay-based heuristics like
> HyStart++ are known to work poorly in various wireless environmens where
> link delay tends to vary a lot. We may come up with some other MD factor
> that 0.5 when in slow start and HyStart++ is in use, but that is
> experimental, if not research, and definitely not ready for stds track.
> #114, #132, and #143 w.r.t flightsize vs. cwnd. The current text
>       does not quite correcly reflect what stacks that use cwnd do.
>       I'll comment in #143 separately.
> #96 & #98: The text added does not address the problems raised which are
> also evidenced in the paper pointed by Bob in #96. Even though CUBIC has
> been modified a bit after the paper was published, it does not
> automatically mean that the problem has been shown resolved: experimental
> evidence is required but not provided. The fact that CUBIC does not
> change MD factor for fast convergence is the root of the problem
> evidenced in the paper and remains so in the algo specified in this
> draft. This is also a significant problem when competing with Reno CC
> because CUBIC behaves much more aggressive than Reno CC when there is
> sudden congestion and all competing flows must converge fast down to a
> small fraction of the current cwnd to be fair to each other. This again
> cannot be evidenced not to be a problem by long deployment experience
> unless experimental data that measures the impact on competing traffic is
> presented to back the claims. Adjusting just Wmax for fast convergence is
> not enough and is even likely to be ineffective because there tend to be
> several losses when sudden congestion is hit, and particularly if NewReno
> is in use the sender stays several RTTs in Fast Recovery being
> overaggressive and then possibly continues at the same rate in CA which
> is unlikely to reach evan close to Wmax before a new loss hits
> the sender again. That is, lower Wmax and lower additive increase factor
> do not compensate the use of larger MD factor when sudden congestion is
> encountered.
> #93 & #94 & (#89) Sec 5.3 still does not address any difficult
> environments, in particular buffer-bloated paths (nor does Sec 5.4).
> We need evidence (results) that show CUBIC is fair towards other CCs
> (Reno) also in such environments. Note that CUBIC's decision to leave
> Reno-friendly region is based on the size of cwnd which tends to be
> incorrect with buffer-bloated bottlenecks because with huge buffers the
> cwnd can be many times larger than what is actually needed to fully
> utilize the available network bit-rate. Therefore, Reno CC has no problem
> in fully utilizing such bottleneck links and CUBIC must stay in
> Reno-friendly region longer but it leaves it too early because the same C
> is used as with non-bloated environments. We lack experiments showing
> CUBIC follows the congestion control principles and is fair to current
> standard TCP CC; to my understanding no experiments with buffer-bloated
> bottlenecks are cited to back up the claims even though buffer bloat is
> very well known to be a common (difficult) environment in today's
> Internet.
> #90 The current text on applying undo (a response to detected false
> fastrexmit) does not provide correct result if someone implements it.
> I have explained the problems there in github but seem to have not
> replied to latest comments by Neal. I'll reply and try to explain more.
> Again one major problem here is that the draft suggest a new response
> algo for false fast rexmits but does not provide any experimental data to
> support it. Long deployment experience has been suggested as
> justification but again without any carefully evaluated experimental
> data and evidence there is no meat. The issue is important to solve but is
> not specific to CUBIC. Instead, it is general problem for all TCP CC
> variants. IMHO, this is not ready for standards track but deserves a draft
> of its own so that it can be carefully evaluated and discussed ion the
> list. AFAIK there has been no discussion on this on the tcpm list, so
> those probably interested and having experience are likely to be unaware
> that this is part of CUBIC draft.
> #88 The problem with correctness of the AIMD model and setting alpha for
> CUBIC requires further consideration. Bob provided an analysis that
> leaves things still open. It seems that I never had time to review and
> comment the analysis and clarify why the model does not work. I'll do that
> separately as it is important to ensure CUBIC behaves fairly as intended
> for the Reno-friendly region.
> Best regards,
> /Markku
> On Mon, 31 Jan 2022, Yoshifumi Nishida wrote:
> > Hello,
> >
> > After some discussions among chairs, we decided to run the 2nd WGLC on
> draft-ietf-tcpm-rfc8312bis in
> > consideration of the importance of the draft.
> > We'll be grateful if you could send your feedback to the ML. The WGLC
> runs until *Feb 11*.
> >
> > If interested, you can check in-depth past discussions in the following
> URL.
> >
> >
> > Thank you so much!
> > --
> > tcpm co-chairs
> >
> >
> > On Wed, Jan 26, 2022 at 2:50 AM Lars Eggert <> wrote:
> >       Hi,
> >
> >       this -06 version rolls in all the changes requested during (and
> after) WGLC ended.
> >
> >       I'll leave it up to the chairs to decide if another WGLC is
> warranted or the document can
> >       progress as-is.
> >
> >       Thanks,
> >       Lars
> >
> >
> >       > On 2022-1-26, at 11:12, wrote:
> >       >
> >       >
> >       > A New Internet-Draft is available from the on-line
> Internet-Drafts directories.
> >       > This draft is a work item of the TCP Maintenance and Minor
> Extensions WG of the IETF.
> >       >
> >       >        Title           : CUBIC for Fast and Long-Distance
> Networks
> >       >        Authors         : Lisong Xu
> >       >                          Sangtae Ha
> >       >                          Injong Rhee
> >       >                          Vidhi Goel
> >       >                          Lars Eggert
> >       >       Filename        : draft-ietf-tcpm-rfc8312bis-06.txt
> >       >       Pages           : 35
> >       >       Date            : 2022-01-26
> >       >
> >       > Abstract:
> >       >   CUBIC is a standard TCP congestion control algorithm that uses
> a
> >       >   cubic function instead of a linear congestion window increase
> >       >   function to improve scalability and stability over fast and
> long-
> >       >   distance networks.  CUBIC has been adopted as the default TCP
> >       >   congestion control algorithm by the Linux, Windows, and Apple
> stacks.
> >       >
> >       >   This document updates the specification of CUBIC to include
> >       >   algorithmic improvements based on these implementations and
> recent
> >       >   academic work.  Based on the extensive deployment experience
> with
> >       >   CUBIC, it also moves the specification to the Standards Track,
> >       >   obsoleting RFC 8312.  This also requires updating RFC 5681, to
> allow
> >       >   for CUBIC's occasionally more aggressive sending behavior.
> >       >
> >       >
> >       > The IETF datatracker status page for this draft is:
> >       >
> >       >
> >       > There is also an HTML version available at:
> >       >
> >       >
> >       > A diff from the previous version is available at:
> >       >
> >       >
> >       >
> >       > Internet-Drafts are also available by rsync at
> :internet-drafts
> >       >
> >       >
> >       > _______________________________________________
> >       > tcpm mailing list
> >       >
> >       >
> >
> >       _______________________________________________
> >       tcpm mailing list
> >
> >
> >
> >
> >