Re: [PATCH v6 4/4] tcp: Update data retransmission timeout

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 17 Oct 2025 14:28:38 +0800
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>
> ---
>  tcp.c | 27 ++++++++++++---------------
>  1 file changed, 12 insertions(+), 15 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 9385132..dc0ec6c 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -179,16 +179,14 @@
>   *
>   * 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. If this persists
> - *   for more than TCP_MAX_RETRIES or (tcp_syn_retries +
> - *   tcp_syn_linear_timeouts) times in a row, 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. If this persists too many times
> + *   in a row, reset the connection: TCP_MAX_RETRIES for established
> + *   connections, or (tcp_syn_retries + tcp_syn_linear_timeouts) during the
> + *   handshake.
>   *
>   * - 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 +340,7 @@ enum {
>  #define WINDOW_DEFAULT			14600		/* RFC 6928 */
>  
>  #define ACK_INTERVAL			10		/* ms */
> -#define SYN_TIMEOUT_INIT		1		/* s */
> -#define ACK_TIMEOUT			2
> +#define RTO_INIT			1		/* s, RFC 6298 */
>  #define FIN_TIMEOUT			60
>  #define ACT_TIMEOUT			7200
>  
> @@ -588,13 +585,13 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
>  	} else if (conn->flags & ACK_FROM_TAP_DUE) {
>  		if (!(conn->events & ESTABLISHED)) {
>  			if (conn->retries < c->tcp.syn_linear_timeouts)
> -				it.it_value.tv_sec = SYN_TIMEOUT_INIT;
> +				it.it_value.tv_sec = RTO_INIT;
>  			else
> -				it.it_value.tv_sec = SYN_TIMEOUT_INIT <<
> +				it.it_value.tv_sec = RTO_INIT <<
>  					(conn->retries - c->tcp.syn_linear_timeouts);
>  		}
>  		else
> -			it.it_value.tv_sec = ACK_TIMEOUT;
> +			it.it_value.tv_sec = RTO_INIT << conn->retries;

Same as on 3/4, but here it's clearly more convenient: just assign
RTO_INIT, and multiply as needed in the if / else clauses.

>  	} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
>  		it.it_value.tv_sec = FIN_TIMEOUT;
>  	} else {

The rest of the series looks good to me.

It might be slightly more practical to factor in directly the RTO
clamp, and I don't think it's complicated now that you have the helper
from 2/4, but it's not a strong preference from my side, as the series
makes sense in any case.

-- 
Stefano