tcp: Send TCP keepalive segments after a period of tap-side inactivityHEADmaster

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>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

0 files changed, 0 insertions, 0 deletions