On Fri, Nov 14, 2025 at 01:00:53AM +0100, Stefano Brivio wrote: > On Mon, 10 Nov 2025 21:46:55 +1100 > David Gibson wrote: > > > On Mon, Nov 10, 2025 at 05:31:35PM +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 > > > > Reviewed-by: David Gibson > > > > Though I have one small concern remaining > > > > > --- > > > tcp.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++--------- > > > tcp.h | 4 ++++ > > > 2 files changed, 57 insertions(+), 9 deletions(-) > > > > > > diff --git a/tcp.c b/tcp.c > > > index 2f49327..da40a99 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 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 > > > @@ -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, RFC 6928 */ > > > #define ACK_TIMEOUT 2 > > > #define FIN_TIMEOUT 60 > > > #define ACT_TIMEOUT 7200 > > > @@ -365,6 +367,13 @@ uint8_t tcp_migrate_rcv_queue [TCP_MIGRATE_RCV_QUEUE_MAX]; > > > > > > #define TCP_MIGRATE_RESTORE_CHUNK_MIN 1024 /* Try smaller when above this */ > > > > > > +#define SYN_RETRIES "/proc/sys/net/ipv4/tcp_syn_retries" > > > +#define SYN_LINEAR_TIMEOUTS "/proc/sys/net/ipv4/tcp_syn_linear_timeouts" > > > + > > > +#define SYN_RETRIES_DEFAULT 6 > > > +#define SYN_LINEAR_TIMEOUTS_DEFAULT 4 > > > +#define MAX_SYNCNT 127 /* derived from kernel's limit */ > > > + > > > /* "Extended" data (not stored in the flow table) for TCP flow migration */ > > > static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX]; > > > > > > @@ -585,10 +594,13 @@ 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; > > > - else > > > + if (!(conn->events & ESTABLISHED)) { > > > + int exp = conn->retries - c->tcp.syn_linear_timeouts; > > > > As discussed in the thread on the last version, the subtraction will > > be done unsigned, so the final result depends on behaviour of casting > > a "negative" (that is, close to max value) unsigned value into a > > signed variable. I'm pretty sure that will do the right thing in > > practice, but I don't believe that behaviour is guaranteed by the C > > standard. > > Surprisingly to me, C99 6.2.6.2 "Integer types" admits three > interpretations of the sign bit for signed types, quoting: > > — the corresponding value with sign bit 0 is negated (sign and > magnitude); > — the sign bit has the value −(2N ) (two’s complement); > — the sign bit has the value −(2N − 1) (ones’ complement). > > and this, together with the conversion rule from 6.3.1.3 "Signed > and unsigned integers": > > Otherwise, the new type is signed and the value cannot be represented > in it; either the result is implementation-defined or an > implementation-defined signal is raised. > > seems to confirm your interpretation, even though I'm really having > a hard time convincing myself that we should actually do stuff like: > > int exp; > > exp = (int)conn->retries - c->tcp.syn_linear_timeouts; > > and I still have the suspicion we're missing something. Is this syntax > what you're suggesting, by the way? Yes, that's what I'm suggesting. > > This probably isn't worth a respin, though. > > Maybe not but: > > > > + it.it_value.tv_sec = SYN_TIMEOUT_INIT << MAX(exp, 0); > > > + } > > > + else { > > this is another bit I would need to change. The coding style dictates: > > if (x) { > ... > } else { > ... > } > > *not*: > > if (x) { > ... > } > else { > ... > } > > ...on the other hand, it goes away in 5/6, so I don't care > particularly. Right, that was my reasoning too. Mind you might as well fix it if there's going to be a respin anyway. > It still matters a very tiny bit because if we need to revert the > commit resulting from 5/6 for any reason we'll end up with broken > coding style. But that's not a realistic scenario and it's very minor > anyway. > > > > it.it_value.tv_sec = ACK_TIMEOUT; > > > + } > > > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > > > it.it_value.tv_sec = FIN_TIMEOUT; > > > } else { > > > @@ -2425,8 +2437,18 @@ 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); > > > + int max; > > > + max = c->tcp.syn_retries + c->tcp.syn_linear_timeouts; > > > + max = MIN(TCP_MAX_RETRIES, max); > > > + if (conn->retries >= max) { > > > + 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); > > > @@ -2782,6 +2804,26 @@ static socklen_t tcp_probe_tcp_info(void) > > > return sl; > > > } > > > > > > +/** > > > + * tcp_get_rto_params() - Get host kernel RTO parameters > > > + * @c: Execution context > > > + */ > > > +void tcp_get_rto_params(struct ctx *c) > > > +{ > > > + intmax_t v; > > > + > > > + v = read_file_integer(SYN_RETRIES, SYN_RETRIES_DEFAULT); > > > + c->tcp.syn_retries = MIN(v, MAX_SYNCNT); > > > + > > > + v = read_file_integer(SYN_LINEAR_TIMEOUTS, SYN_LINEAR_TIMEOUTS_DEFAULT); > > > + c->tcp.syn_linear_timeouts = MIN(v, MAX_SYNCNT); > > > + > > > + debug("Read sysctl values syn_retries: %"PRIu8 > > > + ", syn_linear_timeouts: %"PRIu8, > > > + c->tcp.syn_retries, > > > + c->tcp.syn_linear_timeouts); > > > +} > > > + > > > /** > > > * tcp_init() - Get initial sequence, hash secret, initialise per-socket data > > > * @c: Execution context > > > @@ -2792,6 +2834,8 @@ int tcp_init(struct ctx *c) > > > { > > > ASSERT(!c->no_tcp); > > > > > > + tcp_get_rto_params(c); > > > + > > > tcp_sock_iov_init(c); > > > > > > memset(init_sock_pool4, 0xff, sizeof(init_sock_pool4)); > > > diff --git a/tcp.h b/tcp.h > > > index 0082386..37d7758 100644 > > > --- a/tcp.h > > > +++ b/tcp.h > > > @@ -60,12 +60,16 @@ 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 > > > + * @syn_retries: SYN retries using exponential backoff timeout > > > + * @syn_linear_timeouts: SYN retries before using 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 syn_retries; > > > + uint8_t syn_linear_timeouts; > > > }; > > > > > > #endif /* TCP_H */ > > > -- > > > 2.51.0 > > -- > Stefano > -- 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