From nobody Tue May 02 09:14:04 2023 X-Original-To: freebsd-net@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 4Q9ZCD0c1bz48kx2 for ; Tue, 2 May 2023 09:14:16 +0000 (UTC) (envelope-from hps@selasky.org) Received: from mail.turbocat.net (turbocat.net [88.99.82.50]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4Q9ZCC3GSXz3Hrx for ; Tue, 2 May 2023 09:14:15 +0000 (UTC) (envelope-from hps@selasky.org) Authentication-Results: mx1.freebsd.org; none Received: from [10.36.2.154] (unknown [46.212.121.255]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mail.turbocat.net (Postfix) with ESMTPSA id A7ED52600D5; Tue, 2 May 2023 11:14:06 +0200 (CEST) Message-ID: Date: Tue, 2 May 2023 11:14:04 +0200 List-Id: Networking and TCP/IP with FreeBSD List-Archive: https://lists.freebsd.org/archives/freebsd-net List-Help: List-Post: List-Subscribe: List-Unsubscribe: Sender: owner-freebsd-net@freebsd.org MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:102.0) Gecko/20100101 Thunderbird/102.10.1 Subject: Re: Cwnd grows slowly during slow-start due to LRO of the receiver side. To: Chen Shuo , freebsd-net@freebsd.org References: Content-Language: en-US From: Hans Petter Selasky In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 4Q9ZCC3GSXz3Hrx X-Spamd-Bar: ---- X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[]; ASN(0.00)[asn:24940, ipnet:88.99.0.0/16, country:DE] X-Rspamd-Pre-Result: action=no action; module=replies; Message is reply to one we originated X-ThisMailContainsUnwantedMimeParts: N Hi Chen! The FreeBSD mbufs carry the number of ACKs that have been joined together into the following field: m->m_pkthdr.lro_nsegs Can this value be of any use to cc_newreno ? --HPS On 5/2/23 09:46, Chen Shuo wrote: > As per newreno_ack_received() in sys/netinet/cc/cc_newreno.c, > FreeBSD TCP sender strictly follows RFC 5681 with RFC 3465 extension > That is, during slow-start, when receiving an ACK of 'bytes_acked' > > cwnd += min(bytes_acked, abc_l_var * SMSS); // abc_l_var = 2 dflt > > As discussed in sec3.2 of RFC 3465, L=2*SMSS bytes exactly balances > the negative impact of the delayed ACK algorithm. RFC 5681 also > requires that a receiver SHOULD generate an ACK for at least every > second full-sized segment, so bytes_acked per ACK is at most 2 * SMSS. > If both sender and receiver follow it. cwnd should grow exponentially > during slow-slow: > > cwnd *= 2 (per RTT) > > However, LRO and TSO are widely used today, so receiver may generate > much less ACKs than it used to do. As I observed, Both FreeBSD and > Linux generates at most one ACK per segment assembled by LRO/GRO. > The worst case is one ACK per 45 MSS, as 45 * 1448 = 65160 < 65535. > > Sending 1MB over a link of 100ms delay from FreeBSD 13.2: > > 0.000 IP sender > sink: Flags [S], seq 205083268, win 65535, options > [mss 1460,nop,wscale 10,sackOK,TS val 495212525 ecr 0], length 0 > 0.100 IP sink > sender: Flags [S.], seq 708257395, ack 205083269, win > 65160, options [mss 1460,sackOK,TS val 563185696 ecr > 495212525,nop,wscale 7], length 0 > 0.100 IP sender > sink: Flags [.], ack 1, win 65, options [nop,nop,TS > val 495212626 ecr 563185696], length 0 > // TSopt omitted below for brevity. > > // cwnd = 10 * MSS, sent 10 * MSS > 0.101 IP sender > sink: Flags [.], seq 1:14481, ack 1, win 65, length 14480 > > // got one ACK for 10 * MSS, cwnd += 2 * MSS, sent 12 * MSS > 0.201 IP sink > sender: Flags [.], ack 14481, win 427, length 0 > 0.201 IP sender > sink: Flags [.], seq 14481:31857, ack 1, win 65, length 17376 > > // got ACK of 12*MSS above, cwnd += 2 * MSS, sent 14 * MSS > 0.301 IP sink > sender: Flags [.], ack 31857, win 411, length 0 > 0.301 IP sender > sink: Flags [.], seq 31857:52129, ack 1, win 65, length 20272 > > // got ACK of 14*MSS above, cwnd += 2 * MSS, sent 16 * MSS > 0.402 IP sink > sender: Flags [.], ack 52129, win 395, length 0 > 0.402 IP sender > sink: Flags [P.], seq 52129:73629, ack 1, win 65, > length 21500 > 0.402 IP sender > sink: Flags [.], seq 73629:75077, ack 1, win 65, length 1448 > > As a consequence, instead of growing exponentially, cwnd grows > more-or-less quadratically during slow-start, unless abc_l_var is > set to a sufficiently large value. > > NewReno took more than 20 seconds to ramp up throughput to 100Mbps > over an emulated 100ms delay link. While Linux took ~2 seconds. > I can provide the pcap file if anyone is interested. > > Switching to CUBIC won't help, because it uses the logic in NewReno > ack_received() for slow start. > > Is this a well-known issue and abc_l_var is the only cure for it? > https://calomel.org/freebsd_network_tuning.html > > Thank you! > > Best, > Shuo Chen >