Re: [PATCH v7 4/5] tcp: Update data retransmission timeout

[Yumei Huang <yuhuang@redhat.com>]

On Fri, Oct 31, 2025 at 1:43 PM Yumei Huang <yuhuang@redhat.com> wrote:
>
> Use an exponential backoff timeout for data retransmission according
> to RFC 2988 and RFC 6298. Set the initial RTO to one second as discussed
> in Appendix A of RFC 6298.
>
> Also combine the macros defining the initial RTO for both SYN and ACK.
>
> Signed-off-by: Yumei Huang <yuhuang@redhat.com>
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

I probably should have removed the reviewed-by since there are some
code changes.

> ---
>  tcp.c | 30 ++++++++++++------------------
>  1 file changed, 12 insertions(+), 18 deletions(-)
>
> diff --git a/tcp.c b/tcp.c
> index bada88a..96ee56a 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -179,16 +179,13 @@
>   *
>   * Timeouts are implemented by means of timerfd timers, set based on flags:
>   *
> - * - SYN_TIMEOUT_INIT: if no ACK is received from tap/guest during handshake
> - *   (flag ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, resend
> - *   SYN. It's the starting timeout for the first SYN retry. Retry for
> - *   TCP_MAX_RETRIES or (tcp_syn_retries + tcp_syn_linear_timeouts) times,
> - *   reset the connection
> - *
> - * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending
> - *   data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the
> - *   socket and reset sequence to what was acknowledged. If this persists for
> - *   more than TCP_MAX_RETRIES times in a row, reset the connection
> + * - RTO_INIT: if no ACK segment was received from tap/guest, either during
> + *   handshake (flag ACK_FROM_TAP_DUE without ESTABLISHED event) or after
> + *   sending data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data
> + *   from the socket and reset sequence to what was acknowledged. This is the
> + *   timeout for the first retry, in seconds. Retry for TCP_MAX_RETRIES times
> + *   for established connections, or (tcp_syn_retries +
> + *   tcp_syn_linear_timeouts) times during the handshake, reset the connection
>   *
>   * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (flag ACK_FROM_TAP_DUE
>   *   with TAP_FIN_SENT event), and no ACK is received within this time, reset
> @@ -342,8 +339,7 @@ enum {
>  #define WINDOW_DEFAULT                 14600           /* RFC 6928 */
>
>  #define ACK_INTERVAL                   10              /* ms */
> -#define SYN_TIMEOUT_INIT               1               /* s, RFC 6928 */
> -#define ACK_TIMEOUT                    2
> +#define RTO_INIT                       1               /* s, RFC 6298 */
>  #define FIN_TIMEOUT                    60
>  #define ACT_TIMEOUT                    7200
>
> @@ -589,12 +585,10 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
>         if (conn->flags & ACK_TO_TAP_DUE) {
>                 it.it_value.tv_nsec = (long)ACK_INTERVAL * 1000 * 1000;
>         } else if (conn->flags & ACK_FROM_TAP_DUE) {
> -               if (!(conn->events & ESTABLISHED)) {
> -                       int exp = conn->retries - c->tcp.syn_linear_timeouts;
> -                       it.it_value.tv_sec = SYN_TIMEOUT_INIT << MAX(exp, 0);
> -               }
> -               else
> -                       it.it_value.tv_sec = ACK_TIMEOUT;
> +               int exp = conn->retries;
> +               if (!(conn->events & ESTABLISHED))
> +                       exp -= c->tcp.syn_linear_timeouts;
> +               it.it_value.tv_sec = RTO_INIT << MAX(exp, 0);
>         } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
>                 it.it_value.tv_sec = FIN_TIMEOUT;
>         } else {
> --
> 2.49.0
>


-- 
Thanks,

Yumei Huang