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

On Tue, 25 Nov 2025 15:26:37 +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 Reviewed-by: David Gibson --- 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