From patchwork Sun Feb 2 03:38:26 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: SeongJae Park X-Patchwork-Id: 11361521 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 178A1924 for ; Sun, 2 Feb 2020 03:38:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DEEB42082E for ; Sun, 2 Feb 2020 03:38:54 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="QDZVyZmc" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726921AbgBBDiy (ORCPT ); Sat, 1 Feb 2020 22:38:54 -0500 Received: from mail-pg1-f193.google.com ([209.85.215.193]:37854 "EHLO mail-pg1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726789AbgBBDiy (ORCPT ); Sat, 1 Feb 2020 22:38:54 -0500 Received: by mail-pg1-f193.google.com with SMTP id q127so5852896pga.4; Sat, 01 Feb 2020 19:38:52 -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=HqlOpqfDxlkvlmzjeW5jC7mYHnmnoACp+7MDfXGCRjk=; b=QDZVyZmc6a+j5r7LhYgz+9HrPZQrGeZ9S+Ai2TQCiODhsHhSnql4II0S4TL1P6lXZu zc+3bZJZzlfFU+z6nYQlxC8mfcTiPrm4AFDTVBARVzwGSQoL/W+pPJ/v3vsk5ZMyuRdo +BeOWRR6ZyRo0p461vhw+ORUjxugQRpMOYve4r+0QfQMfSJmPvDtx1tMcGB+lNSqRKhO qhQZ71ellBxdNPataR/zmWwlduV1EbJGiWFpCL4/tJXxWUkIbwbsEySQYNZbCpCtQ+qg ex9y4WTw8vBDFeVvNlbgc1uoqN4WEVn78bzGo2DkdZPCdQz0q0aZOJ3baYE1VKv+bxhU mnnw== 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=HqlOpqfDxlkvlmzjeW5jC7mYHnmnoACp+7MDfXGCRjk=; b=YWP0Ag66JjXdT9Do3sCkXE+fSAz2A/3Kko1K9NECJirMmKd0vP5WIXOvN2HdgPuPxZ NWyrLhYCI2Nl9ZaCc8tzM7x5XOPtFfjBdn2C/hFf+QXioSWh0XfgxTMHughrozifIx4E dy0GbKU7k7Qfn2c2XF7bn9tvZKqGT+WPIs8ArKfXPD55d9odEqPRyi089N4/aIlEtpUn 2iOx8gmhoz6gAvLamzc7y8Er9h6iuKkJA1hUUHXFbt7mPMOZV6tI2rpyGpEVpApOJMBt wC/3xgCvJ8scbnXUS0dV/rH08ATwR5lkQatE3GKlDdZbcrdGx8QsWHdQjdyhd7qISb1R qE8g== X-Gm-Message-State: APjAAAULVYNQCQx2I4pXOV6OcIINSMWfOKNMgYq2RCvBnysuxNv92YDJ hvAncLmgBD3HctOP6TnR8Hk= X-Google-Smtp-Source: APXvYqyR2esvd01q2mklM6OsZY4/Xnwd3Bpmb2WGRxDsF2XF2T0wlX9R+sZFi9gNb997AXZvVCOG3Q== X-Received: by 2002:a63:ff52:: with SMTP id s18mr18579544pgk.253.1580614731989; Sat, 01 Feb 2020 19:38:51 -0800 (PST) Received: from localhost.localdomain ([116.84.110.10]) by smtp.gmail.com with ESMTPSA id iq22sm15705334pjb.9.2020.02.01.19.38.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 01 Feb 2020 19:38:51 -0800 (PST) From: sj38.park@gmail.com To: edumazet@google.com Cc: sj38.park@gmail.com, David.Laight@aculab.com, aams@amazon.com, davem@davemloft.net, eric.dumazet@gmail.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, ncardwell@google.com, shuah@kernel.org, sjpark@amazon.de Subject: [PATCH v3 1/2] tcp: Reduce SYN resend delay if a suspicous ACK is received Date: Sun, 2 Feb 2020 03:38:26 +0000 Message-Id: <20200202033827.16304-2-sj38.park@gmail.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20200202033827.16304-1-sj38.park@gmail.com> References: <20200202033827.16304-1-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: Eric Dumazet 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..baa4fee117f9 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_TIMEOUT_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,