Re: [PATCH v5 3/4] tcp: Resend SYN for inbound connections

[Yumei Huang <yuhuang@redhat.com>]

On Fri, Oct 17, 2025 at 3:48 PM David Gibson
<david@gibson.dropbear.id.au> wrote:
>
> On Fri, Oct 17, 2025 at 01:46:15PM +0800, Yumei Huang wrote:
> > On Fri, Oct 17, 2025 at 12:44 PM David Gibson
> > <david@gibson.dropbear.id.au> 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 <yuhuang@redhat.com>
> > > > ---
> > > >  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.

You are right. I forgot about that.
>
> > 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.

I probably should wait a bit before sending the v6.
Anyway, I will wait and send the v7 next week to see if more comments
from Stefano.

Thanks for all the reviews and comments.
>
> > >
> > > > + * @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



-- 
Thanks,

Yumei Huang