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

Hi VIdhi,

On Tue, 12 Jul 2022, Vidhi Goel 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.

Thanks. Good, but please let's keep the discussion on the issue on the wg 
list, thank you.

Thanks,

/Markku

> 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=40cs.helsinki.fi@dmarc.ietf.org> 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
>>
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>
>