From owner-freebsd-net@freebsd.org Wed Aug 19 08:14:39 2020 Return-Path: Delivered-To: freebsd-net@mailman.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mailman.nyi.freebsd.org (Postfix) with ESMTP id 33FB83B689C for ; Wed, 19 Aug 2020 08:14:39 +0000 (UTC) (envelope-from Michael.Tuexen@lurchi.franken.de) Received: from drew.franken.de (mail-n.franken.de [193.175.24.27]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client CN "*.franken.de", Issuer "Sectigo RSA Domain Validation Secure Server CA" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4BWgYV44g3z4P6h for ; Wed, 19 Aug 2020 08:14:38 +0000 (UTC) (envelope-from Michael.Tuexen@lurchi.franken.de) Received: from [IPv6:2a02:8109:1140:c3d:8108:5ab5:4ce3:914d] (unknown [IPv6:2a02:8109:1140:c3d:8108:5ab5:4ce3:914d]) (Authenticated sender: lurchi) by mail-n.franken.de (Postfix) with ESMTPSA id C1756721BE014; Wed, 19 Aug 2020 10:14:35 +0200 (CEST) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 13.4 \(3608.120.23.2.1\)) Subject: Re: Appropriate Byte Counting during Congestion Avoidance From: Michael Tuexen In-Reply-To: Date: Wed, 19 Aug 2020 10:14:32 +0200 Cc: freebsd-net Content-Transfer-Encoding: 7bit Message-Id: <51C01869-3FC2-43A8-AC9D-4A6F961F93B7@lurchi.franken.de> References: To: Liang Tian X-Mailer: Apple Mail (2.3608.120.23.2.1) X-Spam-Status: No, score=-2.9 required=5.0 tests=ALL_TRUSTED,BAYES_00 autolearn=disabled version=3.4.1 X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on mail-n.franken.de X-Rspamd-Queue-Id: 4BWgYV44g3z4P6h X-Spamd-Bar: + Authentication-Results: mx1.freebsd.org; dkim=none; dmarc=none; spf=none (mx1.freebsd.org: domain of Michael.Tuexen@lurchi.franken.de has no SPF policy when checking 193.175.24.27) smtp.mailfrom=Michael.Tuexen@lurchi.franken.de X-Spamd-Result: default: False [1.38 / 15.00]; RCVD_TLS_ALL(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; FROM_HAS_DN(0.00)[]; MV_CASE(0.50)[]; NEURAL_SPAM_SHORT(0.08)[0.082]; TAGGED_RCPT(0.00)[]; MIME_GOOD(-0.10)[text/plain]; ARC_NA(0.00)[]; AUTH_NA(1.00)[]; DMARC_NA(0.00)[franken.de]; TO_MATCH_ENVRCPT_SOME(0.00)[]; TO_DN_ALL(0.00)[]; RCPT_COUNT_TWO(0.00)[2]; R_SPF_NA(0.00)[no SPF record]; FREEMAIL_TO(0.00)[gmail.com]; FROM_EQ_ENVFROM(0.00)[]; R_DKIM_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; ASN(0.00)[asn:680, ipnet:193.174.0.0/15, country:DE]; RCVD_COUNT_TWO(0.00)[2]; MAILMAN_DEST(0.00)[freebsd-net]; RCVD_IN_DNSWL_LOW(-0.10)[193.175.24.27:from] X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.33 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 19 Aug 2020 08:14:39 -0000 > On 19. Aug 2020, at 06:51, Liang Tian 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 > To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org"