From patchwork Fri Jan 31 12:24:20 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: SeongJae Park X-Patchwork-Id: 11359829 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 4093A14B4 for ; Fri, 31 Jan 2020 12:25:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1F1CD217BA for ; Fri, 31 Jan 2020 12:25:11 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=amazon.com header.i=@amazon.com header.b="hRQnojC/" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728608AbgAaMZG (ORCPT ); Fri, 31 Jan 2020 07:25:06 -0500 Received: from smtp-fw-9102.amazon.com ([207.171.184.29]:19839 "EHLO smtp-fw-9102.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728479AbgAaMZG (ORCPT ); Fri, 31 Jan 2020 07:25:06 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazon201209; t=1580473506; x=1612009506; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version; bh=Zk3wI8pneK5JMxV2EImoUPA7OfrNe6Lhhf3JHIUUWqY=; b=hRQnojC/oaqgQ+X76gJT6GVaCC/0OKnYU0nwztqm9rz8wrFtEhQX5+7C u97T80bi05uYAJkRAgiYDOH85FtouhpnaGDYdR5JtxzYa6MiVes0Uuwo6 jAcmnaDnhTdwwmDcQyDXcUdulNZXxplOq3fcSTMoPqq8osGA/GWeEMasC E=; IronPort-SDR: Np1qsOA8+sTvpoqKlzTfcuNbFkT0rEFe4IoX6P/b3gjbU+BnT4Ox6rykIMoLiQr3zVcW4JyFiH ndG8wwzXOpvQ== X-IronPort-AV: E=Sophos;i="5.70,385,1574121600"; d="scan'208";a="22232066" Received: from sea32-co-svc-lb4-vlan3.sea.corp.amazon.com (HELO email-inbound-relay-2a-1c1b5cdd.us-west-2.amazon.com) ([10.47.23.38]) by smtp-border-fw-out-9102.sea19.amazon.com with ESMTP; 31 Jan 2020 12:24:54 +0000 Received: from EX13MTAUEA002.ant.amazon.com (pdx4-ws-svc-p6-lb7-vlan2.pdx.amazon.com [10.170.41.162]) by email-inbound-relay-2a-1c1b5cdd.us-west-2.amazon.com (Postfix) with ESMTPS id C9323A1E27; Fri, 31 Jan 2020 12:24:53 +0000 (UTC) Received: from EX13D31EUA001.ant.amazon.com (10.43.165.15) by EX13MTAUEA002.ant.amazon.com (10.43.61.77) with Microsoft SMTP Server (TLS) id 15.0.1236.3; Fri, 31 Jan 2020 12:24:53 +0000 Received: from u886c93fd17d25d.ant.amazon.com (10.43.162.50) by EX13D31EUA001.ant.amazon.com (10.43.165.15) with Microsoft SMTP Server (TLS) id 15.0.1367.3; Fri, 31 Jan 2020 12:24:48 +0000 From: To: , CC: , , , , , , SeongJae Park Subject: [PATCH 2/3] tcp: Reduce SYN resend delay if a suspicous ACK is received Date: Fri, 31 Jan 2020 13:24:20 +0100 Message-ID: <20200131122421.23286-3-sjpark@amazon.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20200131122421.23286-1-sjpark@amazon.com> References: <20200131122421.23286-1-sjpark@amazon.com> MIME-Version: 1.0 X-Originating-IP: [10.43.162.50] X-ClientProxiedBy: EX13D24UWA003.ant.amazon.com (10.43.160.195) To EX13D31EUA001.ant.amazon.com (10.43.165.15) 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 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 | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 2a976f57f7e7..b168e29e1ad1 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -5893,8 +5893,12 @@ 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 ignore. */ + 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,