Re: [PATCH v6 3/4] tcp: Resend SYN for inbound connections

[Yumei Huang <yuhuang@redhat.com>]

On Fri, Oct 24, 2025 at 7:04 AM Stefano Brivio <sbrivio@redhat.com> wrote:
>
> On Fri, 17 Oct 2025 14:28:37 +0800
> Yumei Huang <yuhuang@redhat.com> 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.
>
> Linux.
>
> >
> > 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..9385132 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
>
> "If this persists" makes sense for the existing ACK_TIMEOUT
> description but not here, because it looks like it refers to "starting
> timeout".
>
> Coupled with the next patch, it becomes increasingly difficult to
> understand what "this" persisting thing is.
>
> Maybe directly say "Retry for ..., then reset the connection"? It's
> shorter and clearer.
>
> > + *   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 */
>
> Maybe mention RFC 6928 as done above? That's where this value comes
> from.
>
> I just noticed you do that in 4/4, so it's slightly nicer if you do
> that right away here for ease of future reference, but not really
> needed.
>
> >  #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"                                            \
>
> This uses 121 columns. I'm not sure where all those tabs and \ come
> from.

My bad. I renamed the macro and it fits in 80 columns but I forgot to
remove the '\' and the spaces before that.
>
> > +
> >  /* "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);
>
> Probably more readable, but I haven't tried: always start from
> SYN_TIMEOUT_INIT, then multiply/shift if
> conn->retries >= c->tcp.syn_linear_timeouts.

I guess you meant:

    if (!(conn->events & ESTABLISHED)) {
            it.it_value.tv_sec = SYN_TIMEOUT_INIT;
            if (conn->retries >= c->tcp.syn_linear_timeouts)
                    it.it_value.tv_sec = SYN_TIMEOUT_INIT <<
                          (conn->retries - c->tcp.syn_linear_timeouts);

other than something like:

             it.it_value.tv_sec <<= 1

in above second if block, since the latter would cause
it.it_value.tv_sec=2 always when retries>=syn_linear_timeouts, as
it.it_value.tv_sec is set to SYN_TIMEOUT_INIT before the if condition.

If I'm correct, I'm not sure if it's more readable to change to that.
I feel the if/else clause quite matches the way the kernel handles it.
What do you think?

>
> > +             }
> >               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++;
>
> I think I already raised this point on a previous revision: this needs
> to be zeroed as the connection is established, but I don't see that in
> the current version.
>
> > +                             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 parameters for inbound connection
>
> They're not initial, they'll be used for all the connections if I
> understand correctly.
>
> Maybe "Get SYN retries sysctl values"?
> I think the _init() in the
> function name is also somewhat misleading.

Do you have a better idea for the function name? I named it just like
other functions called in tcp_init(), such as tcp_sock_iov_init(),
tcp_splice_init().

>
> > + * @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);
>
> Why 8? Perhaps a #define would help?

The 8 actually comes from your suggestion in v1:
"
  (optional, and might be in another series, but keeping it together
   with the rest might be more convenient): read tcp_syn_retries, limit
   to 8, and also read tcp_syn_linear_timeouts
"
I guess it can be replaced with 6 as it's the default value according to
Documentation/networking/ip-sysctl.rst.

We already have TCP_SYN_RETRIES for the sysctl file, maybe have
another TCP_SYN_RETRIES_DEFAULT for the number?

>
> > +     syn_linear_timeouts = read_file_integer(TCP_SYN_LINEAR_TIMEOUTS, 1);

The default value is 4 as Documentation/networking/ip-sysctl.rst. I
don't quite remember where the 1 is from. Should we update it as well
and add the similar macro as above?

> > +
> > +     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,
>
> Similar to the comment above: these are not parameters of SYN segments
> (which would seem to imply TCP options, such as the MSS).
>
> We typically don't print C assignments, rather human-readable messages,
> so that could be "Read sysctl values tcp_syn_retries: ...,
> syn_linear_timeouts: ...".

Got it.
>
>
> > +           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..4369b52 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 SYN retries during handshake
> > + * @syn_linear_timeouts: Number of SYN retries using linear backoff timeout
> > + *                     before switching to exponential backoff timeout
>
> Maybe more compact:
>
>  * @syn_linear_timeouts:        SYN retries before using exponential timeout

So we remove the '"Number of" as well? I guess we should make it
consistent for both tcp_syn_retries and syn_linear_timeouts.

>
> >   */
> >  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 */
>
> --
> Stefano
>


-- 
Thanks,

Yumei Huang