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

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 Reviewed-by: David Gibson

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 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