Re: [PATCH v6 4/4] tcp: Update data retransmission timeout

[Stefano Brivio <sbrivio@redhat.com>]

On Tue, 28 Oct 2025 16:09:03 +0800
Yumei Huang <yuhuang@redhat.com> wrote:

> On Fri, Oct 24, 2025 at 7:04 AM Stefano Brivio <sbrivio@redhat.com> wrote:
> >
> > On Fri, 17 Oct 2025 14:28:38 +0800
> > Yumei Huang <yuhuang@redhat.com> 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>
> > > ---
> > >  tcp.c | 27 ++++++++++++---------------
> > >  1 file changed, 12 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/tcp.c b/tcp.c
> > > index 9385132..dc0ec6c 100644
> > > --- a/tcp.c
> > > +++ b/tcp.c
> > > @@ -179,16 +179,14 @@
> > >   *
> > >   * 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. 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
> > > - *   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. If this persists too many times
> > > + *   in a row, reset the connection: TCP_MAX_RETRIES for established
> > > + *   connections, or (tcp_syn_retries + tcp_syn_linear_timeouts) during the
> > > + *   handshake.
> > >   *
> > >   * - 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 +340,7 @@ enum {
> > >  #define WINDOW_DEFAULT                       14600           /* RFC 6928 */
> > >
> > >  #define ACK_INTERVAL                 10              /* ms */
> > > -#define SYN_TIMEOUT_INIT             1               /* s */
> > > -#define ACK_TIMEOUT                  2
> > > +#define RTO_INIT                     1               /* s, RFC 6298 */
> > >  #define FIN_TIMEOUT                  60
> > >  #define ACT_TIMEOUT                  7200
> > >
> > > @@ -588,13 +585,13 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
> > >       } else if (conn->flags & ACK_FROM_TAP_DUE) {
> > >               if (!(conn->events & ESTABLISHED)) {
> > >                       if (conn->retries < c->tcp.syn_linear_timeouts)
> > > -                             it.it_value.tv_sec = SYN_TIMEOUT_INIT;
> > > +                             it.it_value.tv_sec = RTO_INIT;
> > >                       else
> > > -                             it.it_value.tv_sec = SYN_TIMEOUT_INIT <<
> > > +                             it.it_value.tv_sec = RTO_INIT <<
> > >                                       (conn->retries - c->tcp.syn_linear_timeouts);
> > >               }
> > >               else
> > > -                     it.it_value.tv_sec = ACK_TIMEOUT;
> > > +                     it.it_value.tv_sec = RTO_INIT << conn->retries;  
> >
> > Same as on 3/4, but here it's clearly more convenient: just assign
> > RTO_INIT, and multiply as needed in the if / else clauses.  
> 
> I guess we can't just assign RTO_INIT.  Maybe assign it only when
> retries==0, otherwise multiply as it.it_value.tv_sec <<=1.

Why can't you do that? Say:

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

but anyway, see below.

> But it seems more complicated. What do you think?

Or maybe, building on my latest comment to 3/4:

		int factor = conn->retries;

		if (!(conn->events & ESTABLISHED))
			factor -= c->tcp.syn_linear_timeouts;

		it.it_value.tv_sec = RTO_INIT << MAX(factor, 0);

?

> >  
> > >       } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
> > >               it.it_value.tv_sec = FIN_TIMEOUT;
> > >       } else {  
> >
> > The rest of the series looks good to me.
> >
> > It might be slightly more practical to factor in directly the RTO
> > clamp, and I don't think it's complicated now that you have the helper
> > from 2/4, but it's not a strong preference from my side, as the series
> > makes sense in any case.  
> 
> Reading tcp_rto_max_ms can be easy with the helper. My concern is
> about the way we get the total time for retries.
> 
> I used to do it like this in v2,
> https://archives.passt.top/passt-dev/20251010074700.22177-4-yuhuang@redhat.com/:
> 
> +#define RETRY_ELAPSED(timeout_init, retries) \
> + ((timeout_init) * ((1 << ((retries) + 1)) - 2))
> 
> Though the formula is not quite right, we could refine it as below:
> 
> #define RETRY_ELAPSED(retries) ((RTO_INIT) * ((1 << ((retries) + 1)) - 1))
> 
> Does it make sense to get the time this way?

Well, it also depends on c->tcp.syn_linear_timeouts, right?

But in any case, do you really need to calculate that explicitly? I was
thinking that you can just clamp the value when you use it in
tcp_timer_ctl().

If c->tcp.rto_max is DIV_ROUND_CLOSEST(x, 1000), where 'x' is the value
you read from the kernel, then I guess it's just:

---
		it.it_value.tv_sec = MIN(it.it_value.tv_sec, c->tcp.rto_max);
	} ...

	if (timerfd_settime(conn->timer, 0, &it, NULL))
		flow_perror(conn, "failed to set timer");
---

-- 
Stefano