From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vijay Subramanian Subject: [PATCH net-next V2 1/1] tcp: Prevent needless syn-ack rexmt during TWHS Date: Fri, 26 Oct 2012 01:05:50 -0700 Message-ID: <1351238750-13611-1-git-send-email-subramanian.vijay@gmail.com> Cc: davem@davemloft.net, edumazet@google.com, ncardwell@google.com, Venkat Venkatsubra , Elliott Hughes , Vijay Subramanian To: netdev@vger.kernel.org Return-path: Received: from mail-pa0-f46.google.com ([209.85.220.46]:47227 "EHLO mail-pa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756912Ab2JZHr5 (ORCPT ); Fri, 26 Oct 2012 03:47:57 -0400 Received: by mail-pa0-f46.google.com with SMTP id hz1so1756776pad.19 for ; Fri, 26 Oct 2012 00:47:56 -0700 (PDT) Sender: netdev-owner@vger.kernel.org List-ID: Elliott Hughes saw strange behavior when server socket was not calling accept(). Client was receiving SYN-ACK back even when socket on server side was not yet available. Eric noted server sockets kept resending SYN_ACKS and further investigation revealed the following problem. If server socket is slow to accept() connections, request_socks can represent connections for which the three-way handshake is already done. From client's point of view, the connection is in ESTABLISHED state but on server side, socket is not in accept_queue or ESTABLISHED state. When the syn-ack timer expires, because of the order in which tests are performed, server can retransmit the synack repeatedly. Following patch prevents the server from retransmitting the synack needlessly (and prevents client from replying with ack). This reduces traffic when server is slow to accept() connections. If the server socket has received the third ack during connection establishment, this is remembered in inet_rsk(req)->acked. The request_sock will expire in around 30 seconds and will be dropped if it does not move into accept_queue. With help from Eric Dumazet. Reported-by: Eric Dumazet Acked-by: Neal Cardwell Tested-by: Neal Cardwell Acked-by: Eric Dumazet Signed-off-by: Vijay Subramanian --- Changes from V1: Changed Reported-by tag and commit message. Added Acked-by and Tested-by tags. Ignoring "WARNING: line over 80 characters" in the interest of readability. net/ipv4/inet_connection_sock.c | 5 ++--- 1 files changed, 2 insertions(+), 3 deletions(-) diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c index d34ce29..4e8e52e 100644 --- a/net/ipv4/inet_connection_sock.c +++ b/net/ipv4/inet_connection_sock.c @@ -598,9 +598,8 @@ void inet_csk_reqsk_queue_prune(struct sock *parent, &expire, &resend); req->rsk_ops->syn_ack_timeout(parent, req); if (!expire && - (!resend || - !req->rsk_ops->rtx_syn_ack(parent, req, NULL) || - inet_rsk(req)->acked)) { + (!resend || inet_rsk(req)->acked || + !req->rsk_ops->rtx_syn_ack(parent, req, NULL))) { unsigned long timeo; if (req->retrans++ == 0) -- 1.7.0.4