Re: [PATCH v3 4/4] tcp: Update data retransmission timeout

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

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On Tue, Oct 14, 2025 at 03:38:36PM +0800, Yumei Huang wrote:
> According to RFC 2988 and RFC 6298, we should use an exponential
> backoff timeout for data retransmission starting from one second
> (see Appendix A in RFC 6298), and limit it to about 60 seconds
> as allowed by the same RFC:
> 
>    (2.5) A maximum value MAY be placed on RTO provided it is at
>          least 60 seconds.

The interpretation of this isn't entirely clear to me.  Does it mean
if the total retransmit delay exceeds 60s we give up and RST (what
this patch implements)?  Or does it mean that if the retransmit delay
reaches 60s we keep retransmitting, but don't increase the delay any
further?

Looking at tcp_bound_rto() and related code in the kernel suggests the
second interpretation.

> Combine the macros defining the initial timeout for both SYN and ACK.
> And add a macro ACK_RETRIES to limit the total timeout to about 60s.
> 
> Signed-off-by: Yumei Huang <yuhuang@redhat.com>
> ---
>  tcp.c | 32 ++++++++++++++++----------------
>  1 file changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 3ce3991..84da069 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
> + * - ACK_TIMEOUT_INIT: if no ACK segment was received from tap/guest, eiher
> + *   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. If this persists for more than 
> + *   allowed times in a row, 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 +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 ACK_TIMEOUT_INIT		1		/* s, RFC 6298 */

I'd suggest calling this RTO_INIT to match the terminology used in the
RFCs.

>  #define FIN_TIMEOUT			60
>  #define ACT_TIMEOUT			7200
>  
> @@ -352,6 +347,11 @@ enum {
>  
>  #define ACK_IF_NEEDED	0		/* See tcp_send_flag() */
>  
> +/* Number of retries calculated from the exponential backoff formula, limited
> + * by a total timeout of about 60 seconds.
> + */
> +#define ACK_RETRIES		5
> +

As noted above, I think this is based on a misunderstanding of what
the RFC is saying.  TCP_MAX_RETRIES should be fine as it is, I think.
We could implement the clamping of the RTO, but it's a "MAY" in the
RFC, so we don't have to, and I don't really see a strong reason to do
so.

>  #define CONN_IS_CLOSING(conn)						\
>  	(((conn)->events & ESTABLISHED) &&				\
>  	 ((conn)->events & (SOCK_FIN_RCVD | TAP_FIN_RCVD)))
> @@ -589,13 +589,13 @@ 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 = ACK_TIMEOUT_INIT;
>  			else
> -				it.it_value.tv_sec = SYN_TIMEOUT_INIT <<
> +				it.it_value.tv_sec = ACK_TIMEOUT_INIT <<
>  					(conn->retries - c->tcp.syn_linear_timeouts);
>  		}
>  		else
> -			it.it_value.tv_sec = ACK_TIMEOUT;
> +			it.it_value.tv_sec = ACK_TIMEOUT_INIT << conn->retries;
>  	} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
>  		it.it_value.tv_sec = FIN_TIMEOUT;
>  	} else {
> @@ -2433,7 +2433,7 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref)
>  		} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
>  			flow_dbg(conn, "FIN timeout");
>  			tcp_rst(c, conn);
> -		} else if (conn->retries == TCP_MAX_RETRIES) {
> +		} else if (conn->retries >= ACK_RETRIES) {
>  			flow_dbg(conn, "retransmissions count exceeded");
>  			tcp_rst(c, conn);
>  		} 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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