On Mon, Nov 03, 2025 at 10:57:27AM +0800, Yumei Huang wrote: > On Mon, Nov 3, 2025 at 9:38 AM David Gibson wrote: > > > > 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). > > I'm not sure, but I just googled it. IIUC, the uint8_t (conn->retries > and c->tcp.syn_linear_timeouts) will go through integer promotion > before subtraction. So the line is like: > > int exp = (int) conn->retries - (int) c->tcp.syn_linear_timeouts; > > Please correct me if I'm wrong. Huh, I thought it would only be promoted if one of the operands was an int. But C promotion rules are really confusing, so I could well be wrong. > > > > > > - 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 > > > > -- > Thanks, > > Yumei Huang > -- 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