Re: [tcpm] CUBIC rfc8312bis / WGLC Issue 6 and proposed text

[Markku Kojo <[e-mail]>]

Hi Vidhi (Cc'ing also Gorry because he has better insight into RFC 7661),

Thanks for the text. I'll reply here to keep the discussion on the wg list 
so that anyone interested can more easily follow the discussion (jumping 
back and worth between ML and github may become intractable).

First, I want to stress that I am not expecting nor insisting to stick 
with my proposed text (with necessary changes to address 
the comments on the list) but it could possibly result in faster 
conclusion(?).

Second, I'd like to highlight that my proposed additional text was 
intended just to bring up the caveats of simply using cwnd and give 
advise that is in line with other standards track CC RFCs. It was not 
intended to specify any algorithm (or a part of an algorithm).

The new text in github IMO has the following issues:

1. It does not mention the caveat of letting cwnd to grow
    beyond rwnd which is a known implementation problem in
    the past (so it can easily be repeated by some new
    implementors) nor does it explain why simply using
    cwnd is incorrect.

2. Even though this draft is intended to be applicable
    with QUIC, it misses the pointer to RFC 9002 that
    articulates the corresponding requirement for QUIC.

3. It tries to create an ad-hoc algorithm in a single
    sentence that captures the RFC 7661 algorithm for
    those using cwnd. RFC 7661 is some 20 pages of which
    several pages are used to specify the algorithm.
    That is, the proposed text ignores several crucial
    actions of RFC 7661. And, my proposal was not to
    provide an algorithm for those using cwnd.

4. The sentence / rule "when flight_size is less than 1/2
    of the cwnd" is not capturing what was discussed earlier
    in github and now on the wg list, and it is also in
    conflict with RFC 9002 and effectively also with RFC
    5681. None of these effectively allows the sender not
    reacting to a congestion event but the "new" rule would
    allow it once the cwnd has grown more than 43% above the
    flightsize.

Apologies for finding so many issues with the proposed text.

Thanks,

/Markku

  On Sun, 17 Jul 2022, Vidhi Goel wrote:

> Thank you everyone for the discussion. Since some of the text was becoming duplicate, I moved the below text a
> few lines up and added a restriction for using cwnd directly. And the recommendation to use RFC 7661 now
> applies to both approaches.
> Some implementations of CUBIC currently
> use *cwnd* ….
> 
> 
> Here is the PR - https://github.com/NTAP/rfc8312bis/pull/148
> 
> I would really appreciate if folks can review it so that we can close this issue before meeting at IETF 114
> next week.
> 
> Thanks,
> Vidhi
>
>       On Jul 17, 2022, at 8:33 AM, Yuchung Cheng <[e-mail]> wrote:
> 
> Glad we agree on improving the text. I think Vidhi will come up with a good diff that captures our
> discussion.
> 
> On Thu, Jul 14, 2022 at 4:47 PM Markku Kojo <kojo=[e-mail]> wrote:
>       On Thu, 14 Jul 2022, Yuchung Cheng wrote:
>
>       > On Thu, Jul 14, 2022 at 4:12 PM Markku Kojo
>       > <kojo=[e-mail]> wrote:
>       >>
>       >> Hi Yuchung,
>       >>
>       >> On Tue, 12 Jul 2022, Yuchung Cheng wrote:
>       >>
>       >>> AFAIK, the latest (and previous) Linux Cubic implementation does not follow
>       >>> "The implementations that use _cwnd_ MUST use other measures to avoid
>       >>> _cwnd_ from growing beyond the receive window"
>       >>>
>       >>> I don't see the need to add that check in Linux as the effective
>       >>> window is always the min of cwnd and rwnd.
>       >>
>       >> That does not help. If cwnd is allowed to grow way beyond rwnd (more than
>       >> 43% for CUBIC or more than 100% for Reno CC), a sender would not respond
>       >> to an arriving congestion signal effectively at all (and may even
>       >> increase the cwnd). That is against the very basic rule in congestion
>       >> control principles we have.
>       >>
>       >>> On the other hand, Linux does restrict cwnd growth if flight size is
>       >>> below cwnd but the
>       >>> actual logic is more sophisticated than a "cwnd < inflight" check to
>       >>> work w/ TSO chunking issue well.
>       >>> https://elixir.bootlin.com/linux/latest/source/net/ipv4/tcp_output.c#L1881
>       >>> (Neal cc'd here is the inventor of the advanced check)
>       >>
>       >> I'm not sure I follow your logic. Doesn't that effectively restrict cwnd
>       >> from growing beyond rwnd? To my understanding with that check Linux
>       >> applies "other measures to avoid _cwnd_ from growing beyond the receive
>       >> window". It does not matter whether the conncetion is
>       >> receiver-application limited via rwnd or sender-application limited. If
>       >> rwnd limits flight size, cwnd will not grow more. Maybe I am missing
>       >> something?
>       > That's right. Limiting cwnd growth on flight size automatically
>       > prevents cwnd goes far beyond rwnd for the congestion case you are
>       > concerned about.
>       >
>       > Your suggested text "The implementations that use _cwnd_ MUST use
>       > other measures to avoid _cwnd_ from growing beyond the receive window"
>       > is easily interpreted as a line of code "cwnd = min(cwnd, rwnd)"
>
>       That was not the intent. So, I think we agree.
>
>       > What I am saying is that this sentence is not necessary and overly
>       > strict. Limiting cwnd growth based on inflight which is based on the
>       > effective sending window (= min(cwnd, rwnd)) already prevents cwnd
>       > bloats over rwnd.
>
>       Sure, I misinterepted your previous text ;)
>
>       > I don't see why Cubic needs to have more strict
>       > rules than other C.C.s.
>
>       It does not have to be more strict rule. So, to my understanding we just
>       need to tweak the text such that it is not misinterpreted the way you
>       thought but still clearly involves the message that the sender needs to
>       prevent cwnd to grow beyond rwnd.
>
>       Maybe(?):
>
>         "... The implementations that use _cwnd_ MUST use [other|alternative]
>          measures to not allow _cwnd_ to grow when bytes in flight is
>          smaller than cwnd_. That also effectively avoids _cwnd_ from
>          growing beyond the receive window. Such measures are important
>          to prevent a CUBIC sender from using an arbitrarily ..."
>
>       Thanks,
>
>       /Markku
>
>       >>
>       >> Thanks,
>       >>
>       >> /Markku
>       >>
>       >>> Just want to reflect a major implementation status.
>       >>>
>       >>>
>       >>> On Tue, Jul 12, 2022 at 2:01 PM Vidhi Goel
>       >>> <vidhi_goel=[e-mail]> wrote:
>       >>>>
>       >>>> Thank you Markku for proposing the text. Some of this is already covered in the latest
>       draft but I can do some edits to your proposed text and create a PR.
>       >>>> I spoke to other co-authors about this suggestion as well and we are mostly ok with it.
>       >>>>
>       >>>>
>       >>>> Vidhi
>       >>>>
>       >>>>> On Jul 11, 2022, at 5:37 PM, Markku Kojo <kojo=[e-mail]> wrote:
>       >>>>>
>       >>>>> Hi all,
>       >>>>>
>       >>>>> I promised to propose some text to some of the remaining issues.
>       >>>>> This thread starts the discussion on the issue 6 and proposes text to solve the issue:
>       >>>>>
>       >>>>> Issue 6) Flightsize:
>       >>>>>
>       >>>>>   The current text in Sec 4.6 w.r.t using FlightSize vs. cwnd for
>       >>>>>   calculating multiplicative decrease is fine except that it does
>       >>>>>   not quite correcly reflect what stacks that use cwnd instead of
>       >>>>>   flightsize should do and actually do. AFAIK and what was
>       >>>>>   discussed in github all stacks apply some sort of restrictions
>       >>>>>   to not allow cwnd to grow beyond rwnd and to not use an
>       >>>>>   arbitrarily high (old) cwnd value to calculate new cwnd
>       >>>>>   when a congestion event occurs.
>       >>>>>
>       >>>>>
>       >>>>> Current text in Sec 4.6::
>       >>>>>
>       >>>>>  Some implementations of CUBIC currently use _cwnd_ instead
>       >>>>>  of _flight_size_ when calculating a new _ssthresh_ using Figure 5.
>       >>>>>
>       >>>>> Proposed new text:
>       >>>>>
>       >>>>>  Some implementations of CUBIC currently use _cwnd_ instead
>       >>>>>  of _flight_size_ when calculating a new _ssthresh_ using Figure 5.
>       >>>>>  The implementations that use _cwnd_ MUST use other measures to
>       >>>>>  avoid _cwnd_ from growing beyond the receive window and to not
>       >>>>>  allow _cwnd_ to grow when bytes in flight is smaller than
>       >>>>>  _cwnd_. This prevents a CUBIC sender from using an arbitrarily
>       >>>>>  high _cwnd_ value in calculating the new value for _ssthresh_
>       >>>>>  and _cwnd_ when a congestion event is signalled, but it is not
>       >>>>>  as robust as the mechanisms described in [RFC7661].
>       >>>>>  [Many|Most|All] TCP implementations of CUBIC that use _cwnd_ apply
>       >>>>>  such measures. Likewise, a QUIC sender that also uses congestion
>       >>>>>  window to calculate a new value for the congestion window and
>       >>>>>  slow-start threshold is required to apply similar mechanisms
>       >>>>>  [RFC 9002].
>       >>>>>
>       >>>>>
>       >>>>> Any comments and help in formulating the text are welcome.
>       >>>>>
>       >>>>> Need also some guidance from TCP implementations of CUBIC to finish up the second but
>       last sentence.
>       >>>>>
>       >>>>> Thanks,
>       >>>>>
>       >>>>> /Markku
>       >>>>>
>       >>>>> _______________________________________________
>       >>>>> tcpm mailing list
>       >>>>> [e-mail]
>       >>>>> https://www.ietf.org/mailman/listinfo/tcpm
>       >>>>
>       >>>> _______________________________________________
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>       >>>> https://www.ietf.org/mailman/listinfo/tcpm
>       >>>
>       >
> 
> 
> 
>