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

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

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On Fri, Oct 31, 2025 at 01:42:41PM +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>

As reported, the carried over R-b was a minor mistake, since the code
has changed, but here's a new one:

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Small comment below, though.

> ---
>  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;

I didn't spot it in the previous patch, but this is (theoretically)
buggy.  conn->retries is unsigned, so the subtraction will be
performed unsigned and only then cast to signed.  I think that will
probably do the right thing in practice, but I don't think that's
guaranteed by the C standard (and might even be UB).

> -			it.it_value.tv_sec = SYN_TIMEOUT_INIT << MAX(exp, 0);
> -		}
> -		else
> -			it.it_value.tv_sec = ACK_TIMEOUT;
> +		int exp = conn->retries;

This change fixes it, by forcing the cast to a signed int before the
subtraction.  It also removes the minor style error I noted in the
previous patch.  Given that, I don't think we need to worry about
either of them.

> +		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
> 

-- 
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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