Date: Mon, 10 Oct 2016 09:56:24 -0400 From: Randall Stewart <rrs@netflix.com> To: hiren panchasara <hiren@strugglingcoder.info> Cc: FreeBSD Transports <transport@FreeBSD.org>, lstewart@FreeBSD.org Subject: Re: Exiting from loss recovery Message-ID: <1BC53E8D-08B9-4C4C-8E7B-6346CA83F66E@netflix.com> In-Reply-To: <20161007002211.GO50669@strugglingcoder.info> References: <20161007002211.GO50669@strugglingcoder.info>
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Hiren:
I have a bit of experience now with this code.
If you exit recovery where Lawerence has marked, then when you go
down a few lines and credit the ack to the cwnd it can be a very
large =E2=80=9Cstretch=E2=80=9D ack.. making cwnd spike up incorrectly =
by standard
new-reno terms=E2=80=A6=20
If you do move exit recover to there, you probably need to have a limit
on how big the cwnd can move=E2=80=A6 i.e. some sort of segment limit.
R
> On Oct 6, 2016, at 8:22 PM, hiren panchasara =
<hiren@strugglingcoder.info> wrote:
>=20
> In tcp_do_segment():
>=20
> /*
> * If the congestion window was inflated to account
> * for the other side's cached packets, retract it.
> */
> if (IN_FASTRECOVERY(tp->t_flags)) {
> if (SEQ_LT(th->th_ack, tp->snd_recover)) {
> if (tp->t_flags & TF_SACK_PERMIT)
> tcp_sack_partialack(tp, th);=20
> else
> tcp_newreno_partial_ack(tp, =
th);=20
> } else=20
> cc_post_recovery(tp, th);=20
> }
>=20
> Here, if we get an ack that marks recovery from loss i.e. >=3D
> snd_recovery, we call cc_post_recovery() which in-turn calls CC =
specific
> post_recovery routine. But we don't reset TF_FASTRECOVERY |
> TF_CONGRECOVERY flags by calling EXIT_RECOVERY()=20
>=20
> Later in the code we do this check again in 'process_ACK:'
>=20
> /* XXXLAS: Can this be moved up into cc_post_recovery? =
*/
> if (IN_RECOVERY(tp->t_flags) &&
> SEQ_GEQ(th->th_ack, tp->snd_recover)) {
> EXIT_RECOVERY(tp->t_flags);
> }
>=20
> And as it can be seen, Lawrence marked it as something that could
> possibly be done here and at the end of cc_post_recovery().=20
>=20
> So, should we do it? i.e call EXIT_RECOVERY() at the end of
> cc_post_recovery() and remove the block from 'process_ACK' section? or
> there is something subtle I am not seeing?
>=20
> Cheers,
> Hiren
--------
Randall Stewart
rrs@netflix.com
803-317-4952
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