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    Subject[PATCH] tcp: Lower the initial RTO to 1s as per draft RFC 2988bis-02.
    Date
    Draft RFC 2988bis-02 recommends that the initial RTO be lowered
    from 3 seconds down to 1 second, and that in case of a timeout
    during the TCP 3WHS, the RTO should fallback to 3 seconds when
    data transmission begins.

    Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>
    ---

    Apologies for the spam, I sent this patch from the wrong address and without
    sob'ing it. I build the Linux kernel in a 15G tmpfs (it's faster this way :D)
    and I lost my .git/config after a reboot.

    include/net/tcp.h | 5 ++++-
    net/ipv4/tcp_input.c | 13 +++++++++----
    2 files changed, 13 insertions(+), 5 deletions(-)

    diff --git a/include/net/tcp.h b/include/net/tcp.h
    index cda30ea..274d761 100644
    --- a/include/net/tcp.h
    +++ b/include/net/tcp.h
    @@ -122,7 +122,10 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
    #endif
    #define TCP_RTO_MAX ((unsigned)(120*HZ))
    #define TCP_RTO_MIN ((unsigned)(HZ/5))
    -#define TCP_TIMEOUT_INIT ((unsigned)(3*HZ)) /* RFC 1122 initial RTO value */
    +/* The next 2 values come from Draft RFC 2988bis-02. */
    +#define TCP_TIMEOUT_INIT ((unsigned)(1*HZ)) /* initial RTO value */
    +#define TCP_TIMEOUT_INIT_FALLBACK ((unsigned)(3*HZ)) /* initial RTO to fallback to when
    + * a timeout happens during the 3WHS. */

    #define TCP_RESOURCE_PROBE_INTERVAL ((unsigned)(HZ/2U)) /* Maximal interval between probes
    * for local resources.
    diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
    index bef9f04..a36bc35 100644
    --- a/net/ipv4/tcp_input.c
    +++ b/net/ipv4/tcp_input.c
    @@ -868,6 +868,11 @@ static void tcp_init_metrics(struct sock *sk)
    {
    struct tcp_sock *tp = tcp_sk(sk);
    struct dst_entry *dst = __sk_dst_get(sk);
    + /* If we had to retransmit anything during the 3WHS, use
    + * the initial fallback RTO as per draft RFC 2988bis-02.
    + */
    + int init_rto = inet_csk(sk)->icsk_retransmits ?
    + TCP_TIMEOUT_INIT_FALLBACK : TCP_TIMEOUT_INIT;

    if (dst == NULL)
    goto reset;
    @@ -890,7 +895,7 @@ static void tcp_init_metrics(struct sock *sk)
    if (dst_metric(dst, RTAX_RTT) == 0)
    goto reset;

    - if (!tp->srtt && dst_metric_rtt(dst, RTAX_RTT) < (TCP_TIMEOUT_INIT << 3))
    + if (!tp->srtt && dst_metric_rtt(dst, RTAX_RTT) < (init_rto << 3))
    goto reset;

    /* Initial rtt is determined from SYN,SYN-ACK.
    @@ -916,7 +921,7 @@ static void tcp_init_metrics(struct sock *sk)
    tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
    }
    tcp_set_rto(sk);
    - if (inet_csk(sk)->icsk_rto < TCP_TIMEOUT_INIT && !tp->rx_opt.saw_tstamp) {
    + if (inet_csk(sk)->icsk_rto < init_rto && !tp->rx_opt.saw_tstamp) {
    reset:
    /* Play conservative. If timestamps are not
    * supported, TCP will fail to recalculate correct
    @@ -924,8 +929,8 @@ reset:
    */
    if (!tp->rx_opt.saw_tstamp && tp->srtt) {
    tp->srtt = 0;
    - tp->mdev = tp->mdev_max = tp->rttvar = TCP_TIMEOUT_INIT;
    - inet_csk(sk)->icsk_rto = TCP_TIMEOUT_INIT;
    + tp->mdev = tp->mdev_max = tp->rttvar = init_rto;
    + inet_csk(sk)->icsk_rto = init_rto;
    }
    }
    tp->snd_cwnd = tcp_init_cwnd(tp, dst);
    --
    1.7.0.4


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