From patchwork Sat Feb 1 07:18:58 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: SeongJae Park X-Patchwork-Id: 208835 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH, MAILING_LIST_MULTI, SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 845B8C35246 for ; Sat, 1 Feb 2020 07:19:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5AEC920723 for ; Sat, 1 Feb 2020 07:19:24 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="TNKN/2nF" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726106AbgBAHTO (ORCPT ); Sat, 1 Feb 2020 02:19:14 -0500 Received: from mail-wr1-f68.google.com ([209.85.221.68]:46101 "EHLO mail-wr1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726115AbgBAHTO (ORCPT ); Sat, 1 Feb 2020 02:19:14 -0500 Received: by mail-wr1-f68.google.com with SMTP id z7so11192360wrl.13; Fri, 31 Jan 2020 23:19:12 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=from:to:cc:subject:date:message-id:in-reply-to:references; bh=eke5sBTLy6OgiheiuxHRk3qgDygD3NTMWAJknxyaymw=; b=TNKN/2nFPUQzkhyZmQ8cOlu4SKJUNAlhH5McodWv3mtwh59+UMcbd1TT2lP0NrGe6/ lros/X9McKFfhCFJFT8LtYnvbXY5jVMHtUqH+RIffqj6Dv20gNnhU7K51d3kca1WmiYC hSoHbA906qPZXtGNzK5k8/QG0jP1FGIoDCHJazCvQHiui0FsjQjOkDW7UfE/tOzA9RIX 3ZJjRqISoVNNOMWOXfSis+9q+/zTSwy1ohBXaKTgU2OibvJ7e94cKTs8oTgstFMcb9Ro 2NsoZBpjc38tgIslJLsEuPJ7ngq3J7qIrb7FsKnox9DwRwD61FsX7mhVauVN5rjg3gI5 6NOA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references; bh=eke5sBTLy6OgiheiuxHRk3qgDygD3NTMWAJknxyaymw=; b=Ab3te8aiyGbAKR1k/Sst0AYRTcCLqRjvi4N9MmODoCFrbdwN5nDflhIfis7tpsMLTS oFHrxOrBdqhYc0BUZmGa2hxCa0mtFj4LkEScyRKcSK22Lte5ajgwWzHScbJAnymo3cLW 2NEd5rA22JSGYOsKo+Js+6zFM+DJt7sOaRMGW88GNQEDr/K2DWhtW/6QRtnSPCn4ifIg HD77UDH8Ov/ceN6CrQ+x6jc6Rq/imB5C5i3mZ7/C8vS/062m8YpDaj2CWxhIXKrBv8IR 74JlpNI8tNTXcG82NMMeHh+syJ29VRdIhwn6DHR510WN2M4nvuSNxIBbVJS5p1RgopmL n3NA== X-Gm-Message-State: APjAAAX5FOkEsptIKQFVpBE9fnAqK1Xsy+BD9W/ajJ42rE+ZFgCDNI4R PqMuqIsx8Jqs2tXtwsY7/MsKFNdR X-Google-Smtp-Source: APXvYqwR0qofoMoeKrmpnjGW2PFx1dUNqMR4jfpQBbrzXP0deaIb/Wv+zKjuM8cyS6MMaXcofLQhbw== X-Received: by 2002:a5d:66cc:: with SMTP id k12mr3052135wrw.72.1580541552100; Fri, 31 Jan 2020 23:19:12 -0800 (PST) Received: from localhost.localdomain (cable-158-181-93-24.cust.telecolumbus.net. [158.181.93.24]) by smtp.gmail.com with ESMTPSA id o4sm664286wrw.15.2020.01.31.23.19.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 31 Jan 2020 23:19:11 -0800 (PST) From: sj38.park@gmail.com To: eric.dumazet@gmail.com, edumazet@google.com Cc: davem@davemloft.net, aams@amazon.com, ncardwell@google.com, netdev@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, shuah@kernel.org, ycheng@google.com, David.Laight@ACULAB.COM, sj38.park@gmail.com, SeongJae Park Subject: [PATCH v2 1/2] tcp: Reduce SYN resend delay if a suspicous ACK is received Date: Sat, 1 Feb 2020 07:18:58 +0000 Message-Id: <20200201071859.4231-2-sj38.park@gmail.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20200201071859.4231-1-sj38.park@gmail.com> References: <20200201071859.4231-1-sj38.park@gmail.com> Sender: linux-kselftest-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org From: SeongJae Park When closing a connection, the two acks that required to change closing socket's status to FIN_WAIT_2 and then TIME_WAIT could be processed in reverse order. This is possible in RSS disabled environments such as a connection inside a host. For example, expected state transitions and required packets for the disconnection will be similar to below flow. 00 (Process A) (Process B) 01 ESTABLISHED ESTABLISHED 02 close() 03 FIN_WAIT_1 04 ---FIN--> 05 CLOSE_WAIT 06 <--ACK--- 07 FIN_WAIT_2 08 <--FIN/ACK--- 09 TIME_WAIT 10 ---ACK--> 11 LAST_ACK 12 CLOSED CLOSED In some cases such as LINGER option applied socket, the FIN and FIN/ACK will be substituted to RST and RST/ACK, but there is no difference in the main logic. The acks in lines 6 and 8 are the acks. If the line 8 packet is processed before the line 6 packet, it will be just ignored as it is not a expected packet, and the later process of the line 6 packet will change the status of Process A to FIN_WAIT_2, but as it has already handled line 8 packet, it will not go to TIME_WAIT and thus will not send the line 10 packet to Process B. Thus, Process B will left in CLOSE_WAIT status, as below. 00 (Process A) (Process B) 01 ESTABLISHED ESTABLISHED 02 close() 03 FIN_WAIT_1 04 ---FIN--> 05 CLOSE_WAIT 06 (<--ACK---) 07 (<--FIN/ACK---) 08 (fired in right order) 09 <--FIN/ACK--- 10 <--ACK--- 11 (processed in reverse order) 12 FIN_WAIT_2 Later, if the Process B sends SYN to Process A for reconnection using the same port, Process A will responds with an ACK for the last flow, which has no increased sequence number. Thus, Process A will send RST, wait for TIMEOUT_INIT (one second in default), and then try reconnection. If reconnections are frequent, the one second latency spikes can be a big problem. Below is a tcpdump results of the problem: 14.436259 IP 127.0.0.1.45150 > 127.0.0.1.4242: Flags [S], seq 2560603644 14.436266 IP 127.0.0.1.4242 > 127.0.0.1.45150: Flags [.], ack 5, win 512 14.436271 IP 127.0.0.1.45150 > 127.0.0.1.4242: Flags [R], seq 2541101298 /* ONE SECOND DELAY */ 15.464613 IP 127.0.0.1.45150 > 127.0.0.1.4242: Flags [S], seq 2560603644 This commit mitigates the problem by reducing the delay for the next SYN if the suspicous ACK is received while in SYN_SENT state. Following commit will add a selftest, which can be also helpful for understanding of this issue. Signed-off-by: SeongJae Park --- net/ipv4/tcp_input.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 2a976f57f7e7..980bd04b9d95 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -5893,8 +5893,14 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb, * the segment and return)" */ if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) || - after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) + after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) { + /* Previous FIN/ACK or RST/ACK might be ignored. */ + if (icsk->icsk_retransmits == 0) + inet_csk_reset_xmit_timer(sk, + ICSK_TIME_RETRANS, TCP_ATO_MIN, + TCP_RTO_MAX); goto reset_and_undo; + } if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr && !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,