[tsvwg] Re: WGLC for Convergence of Congestion Control from Retained State (draft-ietf-tsvwg-careful-resume)

[Gorry Fairhurst <[e-mail]>]

On 15/03/2025 17:23, Kazuho Oku wrote:
> Hello,
>
> I know it's really late but I finally had a chance to read -15 (the 
> most recent version). I see lots of improvements from -12, -15 looks 
> really great. Thank you for all the efforts.
>
> When reading -15, I had the following questions floating inside my 
> head. I could very well be missing something, or it could be the case 
> that the same question has already been asked and answered. If so, 
> please forgive me for the noise.
These are good points.
>
> Section 3.1:
> > If the measured CWND is less than four times the Initial Window (IW) 
> a sender can choose to not save the CC parameters, because the 
> additional actions associated with performing Careful Resume for a 
> small CWND would not justify its use.
>
> Isn't it two times, assuming that RTT is known during connection 
> establishment?

I think the benefit comes when the saved_cwnd is x4, i.e., the jump_cwnd 
 > 2 *IW.

>
> Section 3.6:
>
> This section talks about RTO, but QUIC does not have RTO. QUIC uses 
> Probe Timeouts (PTOs) instead; see RFC 9002 Section 4.7.  Considering 
> that the I-D gives specific advices to QUIC implementers in other 
> sections, it might make sense to refer to PTO here.

Yes, to make this better suited to QUIC, I suggest:

"A sender that experiences persisent congestion (e.g., a Retransmission 
Time Out, RTO, expiry in TCP) ceases to use Careful Resume."

>
> Section 4.1:
>
> > At the end of slow start, the CWND can be significantly larger than 
> needed to fully utilise the path (i.e., a CWND overshoot). It is 
> inappropriate to use an overshoot in the CWND as a basis for 
> estimating the capacity. In most cases, the CWND will converge to a 
> stable value after several more RTTs. One mitigation could be to set 
> the saved_cwnd based on the flight_size, or an averaged CWND.
>
> I-D suggests the use of flight_size, but doesn’t flight_size share the 
> same problem? I think what we might want to suggest here is to use the 
> confirmed pipe size (i.e., CWND / 2) if the connection is closed 
> during Slow Start.
>
I suggest:

One mitigation when a connection is in Slow Start could be to set the
saved_cwnd based on the validated pipe size (i.e., CWND / 2).

Thanks again, do let me know if these changes would address your comments,

Gorry

>
> 2025年1月19日(日) 21:25 Marten Seemann <[e-mail]>:
>
>     This email starts a 2 week WG Last Call call to determine if the
>     following TSVWG ID is ready to publish:
>
>     https://datatracker.ietf.org/doc/draft-ietf-tsvwg-careful-resume/
>
>     This document targets Proposed Standard.
>
>     The WGLC will conclude on February 2, 2025.
>
>     Please do read the draft, and send any comments/concerns to the
>     TSVWG mailing list, including notes on whether these are ready to
>     publish (or send an email directly to the chairs).
>
>     Best wishes,
>     Gorry and Marten
>     (tsvwg co-chairs)
>
>
>
> -- 
> Kazuho Oku