Re: [PATCH] tcp: Lower the initial RTO to 1s as per draft RFC 2988bis-02.

[Yuchung Cheng <ycheng@google.com>]

Hi Benoit,

AFAICT, the passive open side would not fall back the
RTO to 3sec in this change because SYNACK timeouts are not
recorded in icsk_retransmits but reqsk->retrans?

Yuchung

On Wed, May 18, 2011 at 11:36 PM, Benoit Sigoure <tsunanet@gmail.com> wrote:
>
> From: Benoit Sigoure <tsuna@stumbleupon.com>
>
> 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.
> ---
>
> On Wed, May 18, 2011 at 10:46 PM, David Miller <davem@davemloft.net> wrote:
> > From: tsuna <tsunanet@gmail.com>
> > Date: Wed, 18 May 2011 21:33:21 -0700
> >
> >> On Wed, May 18, 2011 at 9:14 PM, David Miller <davem@davemloft.net> wrote:
> >>> I really would rather see the initial RTO be static and be set to 1
> >>> with fallback RTO of 3.
> >>
> >> I can also provide a simple patch for this if you want to start from
> >> there.  And then maybe we can discuss having a runtime knob some more
> >> :-)
> >
> > Yeah why don't we do that :-)
>
> Alright, here we go.
>
>
>  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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