Date: Mon, 12 Oct 2015 10:19:27 -0700 From: hiren panchasara <hiren@strugglingcoder.info> To: transport@FreeBSD.org Subject: Re: Setting congestion window on loss detection Message-ID: <20151012171927.GB92230@strugglingcoder.info> In-Reply-To: <20151007195445.GC42742@strugglingcoder.info> References: <20151007195445.GC42742@strugglingcoder.info>
next in thread | previous in thread | raw e-mail | index | archive | help
--Fba/0zbH8Xs+Fj9o Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 10/07/15 at 12:54P, hiren panchasara wrote: > Found this issue about a month ago and started a discussion on -net: > https://lists.freebsd.org/pipermail/freebsd-net/2015-September/043249.html >=20 > I feel this forum is a better place to discuss this further now. >=20 > Problem: We set cwnd to 1mss when we detect loss via arrivals of 3 dupack= s. > That is wrong as we severely underutilizing network capacity by doing > so. >=20 > Next question is, what should we set cwnd to? >=20 > RFC6675 (TCP SACK) suggests following on detecting loss: > ssthresh =3D cwnd =3D (FlightSize / 2) >=20 > RFC5681 (TCP Congestion control) suggest: > ssthresh =3D max (FlightSize / 2, 2*SMSS) > cwnd =3D (ssthresh + 3*SMSS) >=20 > (Here, FlightSize is bytes in flight.) >=20 > OR should we let whatever congestion control (CC) algo in control decide > that value? I also tried to look at what Linux does. It has PRR (Proportional Rate Reduction) RFC 6937 (something I plan to work on after these initial needed fixes/improvements) in place. Looking back pre-PRR code, linux seems to be doing following: cwnd =3D min(cwnd, FlightSize) Here, cwnd in the equation is adjusted as per rate-halving (draft-mathis-tcp-ratehalving-00) which says "the window is reduced by sending one data segment for each two segments which are acknowledged". (I am not very familiar with linux code so please correct me if that's not the case.) Basically, I think any of these approaches is better than what we have in the tree right now. Cheers, Hiren --Fba/0zbH8Xs+Fj9o Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQF8BAABCgBmBQJWG+ucXxSAAAAAAC4AKGlzc3Vlci1mcHJAbm90YXRpb25zLm9w ZW5wZ3AuZmlmdGhob3JzZW1hbi5uZXRBNEUyMEZBMUQ4Nzg4RjNGMTdFNjZGMDI4 QjkyNTBFMTU2M0VERkU1AAoJEIuSUOFWPt/lF/AH/35US0f2QyjOIvJFR/32gjGG d63SvM0t+jE9qcDQ4VlS7nPAfIVVs/noJfOk4EP1aJGnIpLSsok03iME+hoHHRX3 gJQY6AL956buoViFwdmlETXxznpbFHYnKofcEr14InrorjM6QJxv8ddezsapk8oa ggwS+ytRqV813x8PotZ88zJk1X8k9KtE6Quy3Rf0SIz6u60yHPg2mcAQjNo/3I6r srAboh42pbP/MMn35IPY8b/046n7ZhtPOEBgau1COyztAzWiKoe/4Bxj+jWF+iae 9NALXqEM5mbJTDfw+iTEA9DJ7ncqkJg3MOSBTG22Gq7aiHuiLY2eQlVcrwhWr/U= =J3iC -----END PGP SIGNATURE----- --Fba/0zbH8Xs+Fj9o--
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20151012171927.GB92230>