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

[David Gibson <david@gibson.dropbear.id.au>]

[-- Attachment #1: Type: text/plain, Size: 6440 bytes --]

On Fri, Oct 24, 2025 at 01:04:31AM +0200, Stefano Brivio 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.

Yeah.  This was my suggested wording, based on the existing wording
for ACK_TIMEOUT.  It's not great, but I struggled a bit to find better
wording.

> Maybe directly say "Retry for ..., then reset the connection"? It's
> shorter and clearer.

There it is :).  "Retry (NNN) times, then reset the connection".

[snip]
> > @@ -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.

Yes, you raised that, but then I realised it's already handled.  I
think I put that in the thread, not just direct to Yumei, but maybe
not?  Or it just got lost in the minutiae.

When we receive a SYN-ACK, it will have th->ack_seq advanced a byte
acknowledging the SYN.  tcp_tap_handler() calls
tcp_update_seqack_from_tap() in the !ESTABLISHED case which will see
the new ack_seq and clear retries (retrans before this series).

> > +				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.

"Get host kernel RTO parameters"?  Since we're thinking of extending
this to cover the RTO upper bound as well as the SYN specific
parameters.

> 
> > + * @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?
> 
> > +	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,
> 
> 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: ...".
> 
> 
> > +	      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
> 
> >   */
> >  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
> 

-- 
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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]