On Mon, Oct 20, 2025 at 07:11:07AM +0200, Stefano Brivio wrote: > On Mon, 20 Oct 2025 11:20:19 +1100 > David Gibson wrote: > > > On Fri, Oct 17, 2025 at 08:28:12PM +0200, Stefano Brivio wrote: > > > On Thu, 16 Oct 2025 09:54:25 +1100 > > > David Gibson wrote: > > > > > > > On Wed, Oct 15, 2025 at 02:31:27PM +0800, Yumei Huang wrote: > > > > > On Wed, Oct 15, 2025 at 8:05 AM David Gibson > > > > > wrote: > > > > > > > > > > > > On Tue, Oct 14, 2025 at 03:38:36PM +0800, Yumei Huang wrote: > > > > > > > According to RFC 2988 and RFC 6298, we should use an exponential > > > > > > > backoff timeout for data retransmission starting from one second > > > > > > > (see Appendix A in RFC 6298), and limit it to about 60 seconds > > > > > > > as allowed by the same RFC: > > > > > > > > > > > > > > (2.5) A maximum value MAY be placed on RTO provided it is at > > > > > > > least 60 seconds. > > > > > > > > > > > > The interpretation of this isn't entirely clear to me. Does it mean > > > > > > if the total retransmit delay exceeds 60s we give up and RST (what > > > > > > this patch implements)? Or does it mean that if the retransmit delay > > > > > > reaches 60s we keep retransmitting, but don't increase the delay any > > > > > > further? > > > > > > > > > > > > Looking at tcp_bound_rto() and related code in the kernel suggests the > > > > > > second interpretation. > > > > > > > > > > > > > Combine the macros defining the initial timeout for both SYN and ACK. > > > > > > > And add a macro ACK_RETRIES to limit the total timeout to about 60s. > > > > > > > > > > > > > > Signed-off-by: Yumei Huang > > > > > > > --- > > > > > > > tcp.c | 32 ++++++++++++++++---------------- > > > > > > > 1 file changed, 16 insertions(+), 16 deletions(-) > > > > > > > > > > > > > > diff --git a/tcp.c b/tcp.c > > > > > > > index 3ce3991..84da069 100644 > > > > > > > --- a/tcp.c > > > > > > > +++ b/tcp.c > > > > > > > @@ -179,16 +179,12 @@ > > > > > > > * > > > > > > > * 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 > > > > > > > + * - ACK_TIMEOUT_INIT: if no ACK segment was received from tap/guest, eiher > > > > > > > + * 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. It's the > > > > > > > + * starting timeout for the first retry. If this persists for more than > > > > > > > + * allowed times in a row, 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 +338,7 @@ enum { > > > > > > > #define WINDOW_DEFAULT 14600 /* RFC 6928 */ > > > > > > > > > > > > > > #define ACK_INTERVAL 10 /* ms */ > > > > > > > -#define SYN_TIMEOUT_INIT 1 /* s */ > > > > > > > -#define ACK_TIMEOUT 2 > > > > > > > +#define ACK_TIMEOUT_INIT 1 /* s, RFC 6298 */ > > > > > > > > > > > > I'd suggest calling this RTO_INIT to match the terminology used in the > > > > > > RFCs. > > > > > > > > > > Sure. > > > > > > > > > > > > > #define FIN_TIMEOUT 60 > > > > > > > #define ACT_TIMEOUT 7200 > > > > > > > > > > > > > > @@ -352,6 +347,11 @@ enum { > > > > > > > > > > > > > > #define ACK_IF_NEEDED 0 /* See tcp_send_flag() */ > > > > > > > > > > > > > > +/* Number of retries calculated from the exponential backoff formula, limited > > > > > > > + * by a total timeout of about 60 seconds. > > > > > > > + */ > > > > > > > +#define ACK_RETRIES 5 > > > > > > > + > > > > > > > > > > > > As noted above, I think this is based on a misunderstanding of what > > > > > > the RFC is saying. TCP_MAX_RETRIES should be fine as it is, I think. > > > > > > We could implement the clamping of the RTO, but it's a "MAY" in the > > > > > > RFC, so we don't have to, and I don't really see a strong reason to do > > > > > > so. > > > > > > > > > > If we use TCP_MAX_RETRIES and not clamping RTO, the total timeout > > > > > could be 255 seconds. > > > > > > > > > > Stefano mentioned "Retransmitting data after 256 seconds doesn't make > > > > > a lot of sense to me" in the previous comment. > > > > > > > > That's true, but it's pretty much true for 60s as well. For the local > > > > link we usually have between passt and guest, even 1s is an eternity. > > > > > > Rather than the local link I was thinking of whatever monitor or > > > liveness probe in KubeVirt which might have a 60-second period, or some > > > firewall agent, or how long it typically takes for guests to stop and > > > resume again in KubeVirt. > > > > Right, I hadn't considered those. Although.. do those actually re-use > > a single connection? I would have guessed they use a new connection > > each time, making the timeouts here irrelevant. > > It depends on the definition of "each time", because we don't time out > host-side connections immediately. Hm, ok. Is your concern that getting a negative answer from the probe will take too long? > Pretending passt isn't there, the timeout would come from the default > values for TCP connections. It looks like there's no specific > SO_SNDTIMEO value set for those probes, and you can't configure the > timeout, at least according to: > > https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-a-tcp-liveness-probe My guess would be that the probe would probably time out at the application level long before the TCP layer times out, but I don't know for sure. > and for tcp_syn_retries, tcp(7) says: > > The default value is 6, which corresponds to retrying for up to > approximately 127 seconds. > > In this series, to make things transparent, we read out those values, > so that part is fine. But does the Linux kernel clamp the RTO? > > It turns out that yes, it does, TCP_RTO_MAX_SEC is 120 seconds (before > 1280c26228bd ("tcp: add tcp_rto_max_ms sysctl") that was TCP_RTO_MAX, > same value), and it's used by tcp_retransmit_timer() via tcp_rto_max(). > That change makes it configurable. > > I'm tempted to suggest that we should read out that value as well > (with a 120-second fallback for older kernels) to make our behaviour > as transparent as possible. > > It's slightly more complicated and perhaps not strictly needed, but > we've been bitten a few times by cases where applications and users > expect us to behave like the Linux kernel, and we didn't... so maybe > we could do this as well while at it? Given the rest of this series, > it looks like a relatively small addition to it. I think that's a good idea. It's a bit more work, but it doesn't greatly increase the conceptual complexity and will more closely match the kernel's behaviour. -- 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