tcp: Unconditionally force ACK for all !SYN, !RST packets2024_03_26.4988e2b

Currently we set ACK on flags packets only when the acknowledged byte
pointer has advanced, or we hadn't previously set a window.  This means
in particular that we can send a window update with no ACK flag, which
doesn't appear to be correct.  RFC 9293 requires a receiver to ignore such
a packet [0], and indeed it appears that every non-SYN, non-RST packet
should have the ACK flag.

The reason for the existing logic, rather than always forcing an ACK seems
to be to avoid having the packet mistaken as a duplicate ACK which might
trigger a fast retransmit.  However, earlier tests in the function mean we
won't reach here if we don't have either an advance in the ack pointer -
which will already set the ACK flag, or a window update - which shouldn't
trigger a fast retransmit.

[0] https://www.ietf.org/rfc/rfc9293.html#section-3.10.7.4-2.5.2.1

Link: https://github.com/containers/podman/issues/22146
Link: https://bugs.passt.top/show_bug.cgi?id=84
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