Re: [PATCH v7 4/5] tcp: Update data retransmission timeout

[Yumei Huang <yuhuang@redhat.com>]

On Mon, Nov 3, 2025 at 9:38 AM David Gibson <david@gibson.dropbear.id.au> 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 <yuhuang@redhat.com>
> > Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
>
> 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 <david@gibson.dropbear.id.au>
>
> 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.

>
> > -                     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