On Thu, Dec 04, 2025 at 08:45:36AM +0100, Stefano Brivio wrote:
> We correctly avoid doing that at the beginning of tcp_prepare_flags(),
> but we might clear the flag later on if we actually end up sending a
> "flag" segment.
> 
> Make sure we don't, otherwise we might delay window updates after a
> zero-window condition significantly, and significantly affect
> throughput.
> 
> In some cases, we're forcing peers to send zero-window probes or
> keep-alive segments.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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

> ---
>  tcp.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tcp.c b/tcp.c
> index b00b874..e4c5a5b 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1285,7 +1285,8 @@ int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
>  	th->fin = !!(flags & FIN);
>  
>  	if (th->ack) {
> -		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap))
> +		if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap) &&
> +		    conn->wnd_to_tap)
>  			conn_flag(c, conn, ~ACK_TO_TAP_DUE);
>  		else
>  			conn_flag(c, conn, ACK_TO_TAP_DUE);
> -- 
> 2.43.0
> 

-- 
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