Re: [tcpm] [iccrg] Cwnd growth after fast convergence in CUBIC

[Vidhi Goel <[e-mail]>]

 +Christoph

> On Nov 6, 2020, at 7:27 PM, Vidhi Goel <vidhi_goel=[e-mail]> wrote:
> 
> ++
> 
>> Agreed, and this is basically what Vidhi has suggested as well. In my option it is slightly
>> more clear to introduce cwnd into the calculation of K as Vidhi has done. I suppose it comes 
>> down to where in a particular implementation one calculates K.
>> 
>> One note: If W_last_max were to be incorporated into K, the special case mentioned in section 
>> 4.8 about exiting hybrid slow start would also need to be updated to mention setting 
>> W_last_max = W_max = cwnd, as W_last_max  would not have been set in such a case. 
> 
> The way we have implemented Cubic, using W_last_max won’t work for us either. Previous suggestion of using `cwnd` works for us,
> K = cbrt(W_max - cwnd / C)
> 
> 
>> That said, the intent was to document what the Linux code was doing at the time. Is there a difference between the RFC and what Linux has been doing or is doing now? If yes, it might be time to revise the RFC.
>> 
>> (I think Martin Thomson also found some nits recently.)
> 
> 
> Revising the RFC would be good. While we are on the topic, there is one more thing that could use a discussion.
> 
> Christoph (cc’ed) and I had discussed this earlier regarding alpha_aimd used for TCP friendly region. 
> 
> Currently, alpha_aimd = 3*(1-beta_cubic)/(1+beta_cubic) // for the entire TCP friendly region
> 
> But we think that once the cwnd in TF region reaches W_max, we should set the alpha_aimd to 1 to have similar behavior as standard TCP congestion algorithm (eg. NewReno)
> 
> if (W_est < W_max)
> 	alpha_aimd = 3*(1-beta_cubic)/(1+beta_cubic)
> else
> 	alpha_aimd = 1
> 
> 
> Let me know your thoughts on this. I'd be happy to work on this.
> 
> 
> Thanks,
> Vidhi
> 
>> On Nov 6, 2020, at 12:34 PM, Rosenblum, Wesley <[e-mail] <mailto:[e-mail]>> wrote:
>> 
>> Agreed, and this is basically what Vidhi has suggested as well. In my option it is slightly
>> more clear to introduce cwnd into the calculation of K as Vidhi has done. I suppose it comes 
>> down to where in a particular implementation one calculates K.
>> 
>> One note: If W_last_max were to be incorporated into K, the special case mentioned in section 
>> 4.8 about exiting hybrid slow start would also need to be updated to mention setting 
>> W_last_max = W_max = cwnd, as W_last_max  would not have been set in such a case. 
>> 
>> Thanks,
>> Wesley
>> 
>> On 11/6/20, 6:10 AM, "Neal Cardwell" <ncardwell=[e-mail] <mailto:ncardwell=[e-mail]>> wrote:
>>> AFAICT,  changing Eq 2 to the following might fix the RFC and make it
>>> match the Linux TCP CUBIC behavior:
>>> 
>>>        K = cubic_root(W_max - W_last_max*beta_cubic)/C) (Eq. 2)
>> 
>>    best,
>>    neal
>> 
> 
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