Date: Tue, 25 Aug 2020 11:43:45 +0200 From: Michael Tuexen <Michael.Tuexen@lurchi.franken.de> To: Liang Tian <l.tian.email@gmail.com> Cc: freebsd-net <freebsd-net@freebsd.org> Subject: Re: Appropriate Byte Counting during Congestion Avoidance Message-ID: <2854347B-8C13-485A-AA23-B895B6ACD714@lurchi.franken.de> 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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> On 19. Aug 2020, at 10:14, Michael Tuexen =
<Michael.Tuexen@lurchi.franken.de> wrote:
>=20
>> On 19. Aug 2020, at 06:51, Liang Tian <l.tian.email@gmail.com> wrote:
>>=20
>> Hi everyone,
>>=20
>> 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
>>=20
>> if (type =3D=3D CC_ACK) {
>> ....
>> if (tp->snd_cwnd > tp->snd_ssthresh) {
>> tp->t_bytes_acked +=3D min(tp->ccv->bytes_this_ack,
>> nsegs * V_tcp_abc_l_var * tcp_maxseg(tp));
>> if (tp->t_bytes_acked >=3D tp->snd_cwnd) {
>> tp->t_bytes_acked -=3D tp->snd_cwnd;
>> tp->ccv->flags |=3D 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.
>>=20
>> I would appreciate your opinion on this. Thanks a lot.
> Hi Liang,
>=20
> thanks for bringing this up. I agree. A patch is under review:
> https://reviews.freebsd.org/D26120
Now fixed in https://svnweb.freebsd.org/changeset/base/364754
Thanks again for reporting.
Best regards
Michael
>=20
> Best regards
> Michael
>>=20
>> Regards,
>> Liang
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>=20
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