Date: Wed, 19 Aug 2020 08:48:41 -0400 From: Liang Tian <l.tian.email@gmail.com> To: Michael Tuexen <Michael.Tuexen@lurchi.franken.de> Cc: freebsd-net <freebsd-net@freebsd.org> Subject: Re: Appropriate Byte Counting during Congestion Avoidance Message-ID: <CAJhigrjAjeqyUMrN7jbL5xnNEbJTvmJwEA2GrfRMSWU0Ad3dJQ@mail.gmail.com> In-Reply-To: <51C01869-3FC2-43A8-AC9D-4A6F961F93B7@lurchi.franken.de> References: <CAJhigrgajo%2BXAfY7PQA5zPr_HWOqTjfThnGkX9St2NqbgiTprg@mail.gmail.com> <51C01869-3FC2-43A8-AC9D-4A6F961F93B7@lurchi.franken.de>
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Great, thanks!
On Wed, Aug 19, 2020 at 4:14 AM Michael Tuexen
<Michael.Tuexen@lurchi.franken.de> wrote:
>
> > On 19. Aug 2020, at 06:51, Liang Tian <l.tian.email@gmail.com> wrote:
> >
> > Hi everyone,
> >
> > We noticed CWND is growing much slower than expected during congestion
> > avoidance with new reno, and we came to this piece of code in
> > cc_ack_received() at tcp_input.c:353
> >
> > if (type == CC_ACK) {
> > ....
> > if (tp->snd_cwnd > tp->snd_ssthresh) {
> > tp->t_bytes_acked += min(tp->ccv->bytes_this_ack,
> > nsegs * V_tcp_abc_l_var * tcp_maxseg(tp));
> > if (tp->t_bytes_acked >= tp->snd_cwnd) {
> > tp->t_bytes_acked -= tp->snd_cwnd;
> > tp->ccv->flags |= CCF_ABC_SENTAWND;
> > }
> > The increment of t_bytes_acked is capped at 2*maxseg.
> > The description of the sysctl variable tcp_abc_l_var(default value 2) is
> > "Cap the max cwnd increment during slow-start to this number of segments"
> > After reading RFC3465, it doesn't look like this cap should be applied
> > here since this is clearly not during slow-start.
> > We've seen in some cases the receiver is ACKing every 16 packets, and
> > CWND is growing at 1/8 of the expected rate because of this.
> >
> > I would appreciate your opinion on this. Thanks a lot.
> Hi Liang,
>
> thanks for bringing this up. I agree. A patch is under review:
> https://reviews.freebsd.org/D26120
>
> Best regards
> Michael
> >
> > Regards,
> > Liang
> > _______________________________________________
> > freebsd-net@freebsd.org mailing list
> > https://lists.freebsd.org/mailman/listinfo/freebsd-net
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>
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