Re: [PATCH v9 4/5] tcp: Update data retransmission timeout

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 25 Nov 2025 15:26:37 +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 | 31 ++++++++++++-------------------
>  1 file changed, 12 insertions(+), 19 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 2887f2c..b3aa064 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 SYN,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 (syn_retries + 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 (syn_retries + syn_linear_timeouts) times
> + *   during the handshake, reset the connection

Same as David's comment on 3/5: ", then reset the connection".

Otherwise, after saying "Retry x times, or y times, ..." is looks like
"reset the connection" is another possibility. No, we do that in any
case, but later (after retrying), hence "then".

Sorry, I just realised this was already like that starting from v7 but
we missed it.

>   *
>   * - 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
>  
> @@ -594,13 +590,10 @@ 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;
> -			exp = (int)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 {

-- 
Stefano