On Fri, Oct 17, 2025 at 01:46:15PM +0800, Yumei Huang wrote: > On Fri, Oct 17, 2025 at 12:44 PM David Gibson > wrote: > > > > On Fri, Oct 17, 2025 at 12:27:42PM +0800, Yumei Huang wrote: > > > If a client connects while guest is not connected or ready yet, > > > resend SYN instead of just resetting connection after 10 seconds. > > > > > > Use the same backoff calculation for the timeout as linux kernel. > > > > > > Link: https://bugs.passt.top/show_bug.cgi?id=153 > > > Signed-off-by: Yumei Huang > > > --- > > > tcp.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++-------- > > > tcp.h | 5 +++++ > > > 2 files changed, 52 insertions(+), 8 deletions(-) > > > > > > diff --git a/tcp.c b/tcp.c > > > index 2ec4b0c..71e2da0 100644 > > > --- a/tcp.c > > > +++ b/tcp.c > > > @@ -179,9 +179,11 @@ > > > * > > > * Timeouts are implemented by means of timerfd timers, set based on flags: > > > * > > > - * - SYN_TIMEOUT: if no ACK is received from tap/guest during handshake (flag > > > - * ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, reset the > > > - * connection > > > + * - 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 > > > @@ -340,7 +342,7 @@ enum { > > > #define WINDOW_DEFAULT 14600 /* RFC 6928 */ > > > > > > #define ACK_INTERVAL 10 /* ms */ > > > -#define SYN_TIMEOUT 10 /* s */ > > > +#define SYN_TIMEOUT_INIT 1 /* s */ > > > #define ACK_TIMEOUT 2 > > > #define FIN_TIMEOUT 60 > > > #define ACT_TIMEOUT 7200 > > > @@ -365,6 +367,9 @@ uint8_t tcp_migrate_rcv_queue [TCP_MIGRATE_RCV_QUEUE_MAX]; > > > > > > #define TCP_MIGRATE_RESTORE_CHUNK_MIN 1024 /* Try smaller when above this */ > > > > > > +#define TCP_SYN_RETRIES "/proc/sys/net/ipv4/tcp_syn_retries" > > > +#define TCP_SYN_LINEAR_TIMEOUTS "/proc/sys/net/ipv4/tcp_syn_linear_timeouts" \ > > > + > > > /* "Extended" data (not stored in the flow table) for TCP flow migration */ > > > static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX]; > > > > > > @@ -581,8 +586,13 @@ 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)) > > > - it.it_value.tv_sec = SYN_TIMEOUT; > > > + if (!(conn->events & ESTABLISHED)) { > > > + if (conn->retries < c->tcp.syn_linear_timeouts) > > > + it.it_value.tv_sec = SYN_TIMEOUT_INIT; > > > + else > > > + it.it_value.tv_sec = SYN_TIMEOUT_INIT << > > > + (conn->retries - c->tcp.syn_linear_timeouts); > > > + } > > > else > > > it.it_value.tv_sec = ACK_TIMEOUT; > > > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > > > @@ -2409,8 +2419,17 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref) > > > tcp_timer_ctl(c, conn); > > > } else if (conn->flags & ACK_FROM_TAP_DUE) { > > > if (!(conn->events & ESTABLISHED)) { > > > - flow_dbg(conn, "handshake timeout"); > > > - tcp_rst(c, conn); > > > + if (conn->retries >= TCP_MAX_RETRIES || > > > + conn->retries >= (c->tcp.tcp_syn_retries + > > > + c->tcp.syn_linear_timeouts)) { > > > + flow_dbg(conn, "handshake timeout"); > > > + tcp_rst(c, conn); > > > + } else { > > > + flow_trace(conn, "SYN timeout, retry"); > > > + tcp_send_flag(c, conn, SYN); > > > + conn->retries++; > > > + tcp_timer_ctl(c, conn); > > > + } > > > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > > > flow_dbg(conn, "FIN timeout"); > > > tcp_rst(c, conn); > > > @@ -2766,6 +2785,24 @@ static socklen_t tcp_probe_tcp_info(void) > > > return sl; > > > } > > > > > > +/** > > > + * tcp_syn_params_init() - Get initial syn params for inbound connection > > > > Stefano's comment wasn't 100% clear, but I believe he was suggesting > > s/syn params/SYN parameters/ > > in the comment here for clarity. > > Oops, I missed this one. > > > > > + * @c: Execution context > > > +*/ > > > +void tcp_syn_params_init(struct ctx *c) > > > +{ > > > + intmax_t tcp_syn_retries, syn_linear_timeouts; > > > + > > > + tcp_syn_retries = read_file_integer(TCP_SYN_RETRIES, 8); > > > + syn_linear_timeouts = read_file_integer(TCP_SYN_LINEAR_TIMEOUTS, 1); > > > + > > > + c->tcp.tcp_syn_retries = MIN(tcp_syn_retries, UINT8_MAX); > > > + c->tcp.syn_linear_timeouts = MIN(syn_linear_timeouts, UINT8_MAX); > > > + > > > + debug("TCP SYN parameters: retries=%"PRIu8", linear_timeouts=%"PRIu8, > > > + c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts); > > > +} > > > + > > > /** > > > * tcp_init() - Get initial sequence, hash secret, initialise per-socket data > > > * @c: Execution context > > > @@ -2776,6 +2813,8 @@ int tcp_init(struct ctx *c) > > > { > > > ASSERT(!c->no_tcp); > > > > > > + tcp_syn_params_init(c); > > > + > > > tcp_sock_iov_init(c); > > > > > > memset(init_sock_pool4, 0xff, sizeof(init_sock_pool4)); > > > diff --git a/tcp.h b/tcp.h > > > index 234a803..bb58324 100644 > > > --- a/tcp.h > > > +++ b/tcp.h > > > @@ -59,12 +59,17 @@ union tcp_listen_epoll_ref { > > > * @fwd_out: Port forwarding configuration for outbound packets > > > * @timer_run: Timestamp of most recent timer run > > > * @pipe_size: Size of pipes for spliced connections > > > + * @tcp_syn_retries: Number of times initial SYNs during handshake > > > > Perhaps, > > > > "Number of SYN retries using exponential backoff" > > It's not actually retries using exponential backoff. It's the total > number of retries, including linear backoff and exponential backoff. That's what ip-sysctl.rst says, but as we discussed the other day, the kernel code and behaviour seem to treat it as just the retries after exhausting linear_timeouts. > I got the "Number of times initial SYNs" from the kernel doc > https://www.kernel.org/doc/Documentation/networking/ip-sysctl.rst. Right, but by cutting off the "will be retransmitted", it's no longer grammatical (admittedly it's a pretty long confusing sentence). > Maybe "Number of SYN retries during handshake" ? What do you think? That would be good if the description in ip-sysctl.rst was accurate, but it seems not to be. > > > > > + * @syn_linear_timeouts: Number of SYN retries using linear backoff timeout > > > + * before switching to exponential backoff timeout > > > */ > > > struct tcp_ctx { > > > struct fwd_ports fwd_in; > > > struct fwd_ports fwd_out; > > > struct timespec timer_run; > > > size_t pipe_size; > > > + uint8_t tcp_syn_retries; > > > + uint8_t syn_linear_timeouts; > > > }; > > > > > > #endif /* TCP_H */ > > > -- > > > 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 > > > > -- > 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