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Sat, 22 Aug 2020 15:14:02 -0700 (PDT) MIME-Version: 1.0 From: Liang Tian Date: Sat, 22 Aug 2020 18:13:51 -0400 Message-ID: Subject: Fast recovery ssthresh value To: freebsd-net Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: 4BYt2j4Q9jz3dcX X-Spamd-Bar: - Authentication-Results: mx1.freebsd.org; dkim=pass header.d=gmail.com header.s=20161025 header.b=lKd5ZKb6; 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]; RCVD_TLS_ALL(0.00)[]; ARC_NA(0.00)[]; R_DKIM_ALLOW(-0.20)[gmail.com:s=20161025]; FROM_HAS_DN(0.00)[]; DWL_DNSWL_NONE(0.00)[gmail.com:dkim]; FREEMAIL_FROM(0.00)[gmail.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; 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)[-0.445]; R_SPF_ALLOW(-0.20)[+ip6:2a00:1450:4000::/36]; 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]; RCVD_COUNT_TWO(0.00)[2]; TAGGED_FROM(0.00)[]; MAILMAN_DEST(0.00)[freebsd-net] 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: Sat, 22 Aug 2020 22:14:06 -0000 Hi all, When 3 dupacks are received and TCP enter fast recovery, if SACK is used, the CWND is set to maxseg: 2593 if (tp->t_flags & TF_SACK_PERMIT) { 2594 TCPSTAT_INC( 2595 tcps_sack_recovery_episode); 2596 tp->snd_recover = tp->snd_nxt; 2597 tp->snd_cwnd = maxseg; 2598 (void) tp->t_fb->tfb_tcp_output(tp); 2599 goto drop; 2600 } Otherwise(SACK is not in use), CWND is set to maxseg before tcp_output() and then set back to snd_ssthresh+inflation 2601 tp->snd_nxt = th->th_ack; 2602 tp->snd_cwnd = maxseg; 2603 (void) tp->t_fb->tfb_tcp_output(tp); 2604 KASSERT(tp->snd_limited <= 2, 2605 ("%s: tp->snd_limited too big", 2606 __func__)); 2607 tp->snd_cwnd = tp->snd_ssthresh + 2608 maxseg * 2609 (tp->t_dupacks - tp->snd_limited); 2610 if (SEQ_GT(onxt, tp->snd_nxt)) 2611 tp->snd_nxt = onxt; 2612 goto drop; I'm wondering in the SACK case, should CWND be set back to ssthresh(which has been slashed in cc_cong_signal() a few lines above) before line 2599, like non-SACK case, instead of doing slow start from maxseg? I read rfc6675 and a few others, and it looks like that's the case. I appreciate your opinion, again. Thanks, Liang