Re: [PATCH 4/4] tcp: Send TCP keepalive segments after a period of tap-side inactivity

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

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On Thu, Feb 05, 2026 at 01:17:45AM +0100, Stefano Brivio wrote:
> On Wed,  4 Feb 2026 21:41:37 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > There are several circumstances in which a live, but idle TCP connection
> > can be forgotten by a guest, with no "on the wire" indication that this has
> > happened.  The most obvious is if the guest abruptly reboots.  A more
> > subtle case can happen with a half-closed connection, specifically one
> > in FIN_WAIT_2 state on the guest.  A connection can, legitimately, remain
> > in this state indefinitely.  If however, a socket in this state is closed
> > by userspace, Linux at least will remove the kernel socket after 60s
> > (or as configured in the net.ipv4.tcp_fin_timeout sysctl).
> > 
> > Because there's no on the wire indication in these cases, passt will
> > pointlessly retain the connection in its flow table, at least until it is
> > removed by the inactivity timeout after several hours.
> > 
> > To avoid keeping connections around for so long in this state, add
> > functionality to periodically send TCP keepalive segments to the guest if
> > we've seen no activity on the tap interface.  If the guest is no longer
> > aware of the connection, it should respond with an RST which will let
> > passt remove the stale entry.
> > 
> > To do this we use a method similar to the inactivity timeout - a 1-bit
> > page replacement / clock algorithm, but with a shorter interval, and only
> > checking for tap side activity.  Currently we use a 300s interval, meaning
> > we'll send a keepalive after 5-10 minutes of (tap side) inactivity.
> > 
> > Link: https://bugs.passt.top/show_bug.cgi?id=179
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  tcp.c      | 39 +++++++++++++++++++++++++++++++++++++++
> >  tcp.h      |  2 ++
> >  tcp_conn.h |  2 ++
> >  3 files changed, 43 insertions(+)
> > 
> > diff --git a/tcp.c b/tcp.c
> > index acdac7df..bf57be23 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -206,6 +206,12 @@
> >   *   keepalives) will be removed between INACTIVITY_INTERVAL s and
> >   *   2*INACTIVITY_INTERVAL s after the last activity.
> >   *
> > + * - KEEPALIVE_INTERVAL: if a connection has had no tap-side activity for an
> > + *   entire interval, send a tap-side keepalive.  If the endpoint is no longer
> > + *   aware of the connection (due to a reboot, or a kernel timeout in FIN_WAIT_2
> > + *   state) that should trigger an RST, so we won't keep track of connections
> > + *   that the guest endpoint no longer cares about.
> > + *
> >   * Summary of data flows (with ESTABLISHED event)
> >   * ----------------------------------------------
> >   *
> > @@ -342,6 +348,7 @@ enum {
> >  #define RTO_INIT_AFTER_SYN_RETRIES	3		/* s, RFC 6298 */
> >  
> >  #define INACTIVITY_INTERVAL		7200		/* s */
> > +#define	KEEPALIVE_INTERVAL		30		/* s */
> 
> By the way, once we're done testing this, I'm not sure which value to
> use. MUST-28 in RFC 9293 (3.8.4. TCP Keep-Alives) says we should
> "default to no less than two hours". It looks a bit too long to be
> useful, to me.

I agree, and as we've discussed a bit, the purpose of these keepalives
differs a bit from more common reasons for sending TCP keepalives,
meaning we have different considerations for the interval.

Also note that while RFC 9293 is pretty recent, the "no less than two
hours" is unchanged since RFC 1122, from 1989.  In 1989, keepalives
every few minutes might have been a significant bandwidth concern,
now, not so much.  Especially since these are tap side only, which
usually be a local high-speed connections (it only won't be if the
guest routes to a VPN or something).

I was thinking of going with a 300s interval, so timeout would happen
after 5-10 minutes.  That has the worse case timeout being just one
(base 10) order of magnitude away from the default 60s value of
net.ipv4.tcp_fin_timeout.  I could be persuaded to increase the
interval to 900s (15-30m timeout).  Much longer than that and I don't
think it would really accomplish the purpose we're looking for in
bug179 - we'd still have stale connections hanging around for an hour
or more.

> >  
> >  #define LOW_RTT_TABLE_SIZE		8
> >  #define LOW_RTT_THRESHOLD		10 /* us */
> > @@ -2263,6 +2270,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
> >  	}
> >  
> >  	conn->inactive = false;
> > +	conn->tap_inactive = false;
> >  
> >  	if (th->ack && !(conn->events & ESTABLISHED))
> >  		tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq));
> > @@ -2884,6 +2892,36 @@ int tcp_init(struct ctx *c)
> >  	return 0;
> >  }
> >  
> > +/**
> > + * tcp_keepalive() - Send keepalives for connections which need it
> > + * @:	Execution context
> 
>  * @c:		Execution context
>  * @now:	Current timestamp

Fixed.

> > + */
> > +static void tcp_keepalive(struct ctx *c, const struct timespec *now)
> > +{
> > +	union flow *flow;
> > +
> > +	if (now->tv_sec - c->tcp.keepalive_run < KEEPALIVE_INTERVAL)
> > +		return;
> > +
> > +	c->tcp.keepalive_run = now->tv_sec;
> > +
> > +	flow_foreach(flow) {
> > +		struct tcp_tap_conn *conn = &flow->tcp;
> > +
> > +		if (flow->f.type != FLOW_TCP)
> > +			continue;
> > +
> > +		if (conn->tap_inactive) {
> > +			flow_dbg(conn, "No tap activity for least %us, send keepalive",
> 
> s/least// (or "for at least")

Oops.  "for at least" was what I intended, since that's more accurate.

> > +				 KEEPALIVE_INTERVAL);
> > +			tcp_send_flag(c, conn, KEEPALIVE);
> 
> Do we have to make sure we don't interpret replies to keepalives as
> duplicate ACKs (by setting some internal flow flag)? I haven't really
> checked that path.

That's... an interesting question.  I think the answer is "not in a
way that matters".  If we get here, we haven't heard anything
from the guest for at least 5 minutes (or whatever).  That implies either:

a) it's already acked everything we sent.  So, even if we see the
   keepalive reply as a dup ack, there's nothing to retransmit, so
   it's harmless.

or

b) it's not listening at all, in which case it's unlikely to respond
   to the keepalive either

or

c) we're already retransmitting, and have backed off to pretty long
   timeouts (greater than the keepalive interval), in which case a
   dup-ack is just a bit more data for us

> > +		}
> > +
> > +		/* Ready to check fot next interval */
> 
> for

Copypasta.  Fixed.

> > +		conn->tap_inactive = true;
> > +	}
> > +}
> > +
> >  /**
> >   * tcp_inactivity() - Scan for and close long-inactive connections
> >   * @:	Execution context
> > @@ -2927,6 +2965,7 @@ void tcp_timer(struct ctx *c, const struct timespec *now)
> >  	if (c->mode == MODE_PASTA)
> >  		tcp_splice_refill(c);
> >  
> > +	tcp_keepalive(c, now);
> >  	tcp_inactivity(c, now);
> >  }
> >  
> > diff --git a/tcp.h b/tcp.h
> > index e104d453..2739f309 100644
> > --- a/tcp.h
> > +++ b/tcp.h
> > @@ -38,6 +38,7 @@ extern bool peek_offset_cap;
> >   * @rto_max:		Maximum retry timeout (in s)
> >   * @syn_retries:	SYN retries using exponential backoff timeout
> >   * @syn_linear_timeouts: SYN retries before using exponential backoff timeout
> > + * @keepalive_run:	Time we last issued tap-side keepalives
> >   * @inactivity_run:	Time we last scanned for inactive connections
> >   */
> >  struct tcp_ctx {
> > @@ -48,6 +49,7 @@ struct tcp_ctx {
> >  	int rto_max;
> >  	uint8_t syn_retries;
> >  	uint8_t syn_linear_timeouts;
> > +	time_t keepalive_run;
> >  	time_t inactivity_run;
> >  };
> >  
> > diff --git a/tcp_conn.h b/tcp_conn.h
> > index 7197ff63..c82e1441 100644
> > --- a/tcp_conn.h
> > +++ b/tcp_conn.h
> > @@ -16,6 +16,7 @@
> >   * @ws_from_tap:	Window scaling factor advertised from tap/guest
> >   * @ws_to_tap:		Window scaling factor advertised to tap/guest
> >   * @tap_mss:		MSS advertised by tap/guest, rounded to 2 ^ TCP_MSS_BITS
> > + * @tapinactive:	No tao activity within the current KEEPALIVE_INTERVAL
> 
> tap_inactive, tap activity

The typos keep coming.  I'm going to blame the laptop keyboard and an
uncomfortable public library chair :).

> 
> >   * @inactive:		No activity within the current INACTIVITY_INTERVAL
> >   * @sock:		Socket descriptor number
> >   * @events:		Connection events, implying connection states
> > @@ -58,6 +59,7 @@ struct tcp_tap_conn {
> >  	(conn->rtt_exp = MIN(RTT_EXP_MAX, ilog2(MAX(1, rtt / RTT_STORE_MIN))))
> >  #define RTT_GET(conn)			(RTT_STORE_MIN << conn->rtt_exp)
> >  
> > +	bool		tap_inactive	:1;
> >  	bool		inactive	:1;
> >  
> >  	int		sock		:FD_REF_BITS;
> 
> The rest looks all good to me, but I didn't test it at all.
> 
> -- 
> 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

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