Re: [PATCH v4 4/4] tcp: Update data retransmission timeout

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Oct 16, 2025 at 10:34:23AM +0800, Yumei Huang 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>

Except for some minor English clarity notes below.

> ---
>  tcp.c | 25 ++++++++++---------------
>  1 file changed, 10 insertions(+), 15 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 3003333..3254d67 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -179,16 +179,12 @@
>   *
>   * 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. It's the
> + *   starting timeout for the first retry.

For clarity, I'd suggest:
	This is the timeout for the first retry, in seconds.

> If this persists for more than
> + *   allowed times in a row, reset the connection

I'd suggest:

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 +338,7 @@ 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
>  
> @@ -589,13 +584,13 @@ 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;
>  	} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
>  		it.it_value.tv_sec = FIN_TIMEOUT;
>  	} else {
> -- 
> 2.47.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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