From owner-freebsd-net@freebsd.org Wed Aug 19 04:51:56 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 2A9AF3B1934 for ; Wed, 19 Aug 2020 04:51:56 +0000 (UTC) (envelope-from l.tian.email@gmail.com) Received: from mail-ej1-x62f.google.com (mail-ej1-x62f.google.com [IPv6:2a00:1450:4864:20::62f]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256 client-signature RSA-PSS (2048 bits) client-digest SHA256) (Client CN "smtp.gmail.com", Issuer "GTS CA 1O1" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 4BWb3Y5Rchz4Bps for ; Wed, 19 Aug 2020 04:51:53 +0000 (UTC) (envelope-from l.tian.email@gmail.com) Received: by mail-ej1-x62f.google.com with SMTP id p24so24658808ejf.13 for ; Tue, 18 Aug 2020 21:51:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:from:date:message-id:subject:to; bh=rVsqpwySu/JZPhjzHdesB0EQrTO4OUPdokvHypoQwFI=; b=gR9F7a3gsY7B8Hzyod7b1wP/xyrm+azH+HKqAObQAvcpg2cwMkIecxjyFtHkcg43+2 NGj6ZJIbpdSVYG3jA21M1nyW6GGF2ChAV0izj2WrCbNKucRIPR8FHVGX5MNz0gxK170D tvIfznnf8fEVk9mhpY+z78j8gpwm0atJQiJ1tocwyyFoK4dvtJEXQJYQcbIJdeQZSrl+ 47qtubWuBtegFyQSl0K1ijQctWm1GIJWj6cShcdeIMRcbNdqHFj8zJspOGphElKAME5d WOr8w+SwJNGqIpi9WkU9HtE23Xj5AZpS5xZqJzOR8C6o0heK2YgdgZ1IxmnLgk6e3LbF FhAg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:from:date:message-id:subject:to; bh=rVsqpwySu/JZPhjzHdesB0EQrTO4OUPdokvHypoQwFI=; b=uNyZ5cLkdiW+uIS/zgjruIvhtxKevM8mj1wIiaWL2QFPSQ4a8ZFREnoULa3fWI38Lx cq5bb5JyKjExnKdbj/JhYicrdQMo6HcgtDFstXPkRaXjr5VCn6FO7HVuO5XyFgeXz/Bc 0ohe0fvLHVh/d3/s0ODqHYPwTWQbLHsyPWVe4/q5n3LB9ISH/BXPQiHU8hvpyjZHmMiA SXV4sX1NOUuioYAhBnsGGxJzI8xRC4ylvI6gps83dd/ij5lCn7HGd2HTxtkWH/xv/UY3 YPUvJKxIgtCM9AS8lcGm0mP8VZgjJnsH2KEIK36FPXR1+ysM5hFA6vN7At60PApoq5du 5sxw== X-Gm-Message-State: AOAM530MwDrsOerS8gtz3pLc0+dSXDG9wcqP0B7zabSlZluS+//Hi1ik eqyn6K+MV7hchioTFaiOkQ4A+1BBjVd+atYHGWZOB5GgfZM= X-Google-Smtp-Source: ABdhPJyDMXKOWPYKCyM4chbcNaHRqJVckXPfURS0FERedkb+1k1F12azlWuNNbnKR4rCL4gFRWTqaAl7mKW9kygoqEM= X-Received: by 2002:a17:906:1b04:: with SMTP id o4mr24659164ejg.332.1597812711399; Tue, 18 Aug 2020 21:51:51 -0700 (PDT) MIME-Version: 1.0 From: Liang Tian Date: Wed, 19 Aug 2020 00:51:40 -0400 Message-ID: Subject: Appropriate Byte Counting during Congestion Avoidance To: freebsd-net Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: 4BWb3Y5Rchz4Bps X-Spamd-Bar: - Authentication-Results: mx1.freebsd.org; dkim=pass header.d=gmail.com header.s=20161025 header.b=gR9F7a3g; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (mx1.freebsd.org: domain of ltianemail@gmail.com designates 2a00:1450:4864:20::62f as permitted sender) smtp.mailfrom=ltianemail@gmail.com X-Spamd-Result: default: False [-1.44 / 15.00]; ARC_NA(0.00)[]; R_DKIM_ALLOW(-0.20)[gmail.com:s=20161025]; RCVD_TLS_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; DWL_DNSWL_NONE(0.00)[gmail.com:dkim]; R_SPF_ALLOW(-0.20)[+ip6:2a00:1450:4000::/36:c]; FREEMAIL_FROM(0.00)[gmail.com]; MIME_GOOD(-0.10)[text/plain]; PREVIOUSLY_DELIVERED(0.00)[freebsd-net@freebsd.org]; RCPT_COUNT_ONE(0.00)[1]; TO_DN_ALL(0.00)[]; DKIM_TRACE(0.00)[gmail.com:+]; DMARC_POLICY_ALLOW(-0.50)[gmail.com,none]; RCVD_IN_DNSWL_NONE(0.00)[2a00:1450:4864:20::62f:from]; NEURAL_HAM_SHORT(-0.44)[-0.442]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; FREEMAIL_ENVFROM(0.00)[gmail.com]; ASN(0.00)[asn:15169, ipnet:2a00:1450::/32, country:US]; TAGGED_FROM(0.00)[]; MAILMAN_DEST(0.00)[freebsd-net]; RCVD_COUNT_TWO(0.00)[2] 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 04:51:56 -0000 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. Regards, Liang