netdev - Re: Missing TCP SYN on loopback, retransmits after 1s

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Message-ID: <1322028802.1298.10.camel@edumazet-laptop>
Date:	Wed, 23 Nov 2011 07:13:22 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	John Heffner <johnwheffner@...il.com>
Cc:	Jesse Young <jlyo@...o.org>, netdev@...r.kernel.org
Subject: Re: Missing TCP SYN on loopback, retransmits after 1s

Le mercredi 23 novembre 2011 à 07:09 +0100, Eric Dumazet a écrit :
> Le mercredi 23 novembre 2011 à 06:58 +0100, Eric Dumazet a écrit :
> 
> > First connection went well.
> > 
> > Now we try to reuse tuple  (ports 49374, 8009 on loopback) while a socket is in TIMEWAIT, and first
> > SYN packet (time 06:48:20.337335) is dropped (considered as a packet part of previous session)
> > 
> > Now why the first SYN packet is dropped and not the second one, I dont know yet.
> 
> 
> in netstat -s output, the suspect increasing counter is :
> 
> "45 congestion windows recovered without slow start after partial ack"
> 
> 

# vi +2831 net/ipv4/tcp_input.c

/* Undo during loss recovery after partial ACK. */
static int tcp_try_undo_loss(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (tcp_may_undo(tp)) {
                struct sk_buff *skb;
                tcp_for_write_queue(skb, sk) {
                        if (skb == tcp_send_head(sk))
                                break;
                        TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
                }

                tcp_clear_all_retrans_hints(tp);

                DBGUNDO(sk, "partial loss");
                tp->lost_out = 0;
                tcp_undo_cwr(sk, true);
HERE:            NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSSUNDO);
                inet_csk(sk)->icsk_retransmits = 0;
                tp->undo_marker = 0;
                if (tcp_is_sack(tp))
                        tcp_set_ca_state(sk, TCP_CA_Open);
                return 1;
        }
        return 0;
}


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