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tcp: Acknowledge keep-alive segments, ignore them for the rest

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RFC 9293, 3.8.4 says:

   Implementers MAY include "keep-alives" in their TCP implementations
   (MAY-5), although this practice is not universally accepted.  Some
   TCP implementations, however, have included a keep-alive mechanism.
   To confirm that an idle connection is still active, these
   implementations send a probe segment designed to elicit a response
   from the TCP peer.  Such a segment generally contains SEG.SEQ =
   SND.NXT-1 and may or may not contain one garbage octet of data.  If
   keep-alives are included, the application MUST be able to turn them
   on or off for each TCP connection (MUST-24), and they MUST default to
   off (MUST-25).

but currently, tcp_data_from_tap() is not aware of this and will
schedule a fast re-transmit on the second keep-alive (because it's
also a duplicate ACK), ignoring the fact that the sequence number was
rewinded to SND.NXT-1.

ACK these keep-alive segments, reset the activity timeout, and ignore
them for the rest.

At some point, we could think of implementing an approximation of
keep-alive segments on outbound sockets, for example by setting
TCP_KEEPIDLE to 1, and a large TCP_KEEPINTVL, so that we send a single
keep-alive segment at approximately the same time, and never reset the
connection. That's beyond the scope of this fix, though.

Reported-by: Tim Besard <tim.besard@gmail.com>
Link: https://github.com/containers/podman/discussions/24572
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
This commit is contained in:
Stefano Brivio 2024-11-19 20:53:44 +01:00
parent af464c4ffb
commit 238c69f9af
1 changed files with 14 additions and 0 deletions
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14
tcp.c
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@ -1763,6 +1763,20 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
continue;
seq = ntohl(th->seq);
if (SEQ_LT(seq, conn->seq_from_tap) && len <= 1) {
flow_trace(conn,
"keep-alive sequence: %u, previous: %u",
seq, conn->seq_from_tap);
tcp_send_flag(c, conn, ACK);
tcp_timer_ctl(c, conn);
if (p->count == 1)
return 1;
continue;
}
ack_seq = ntohl(th->ack_seq);
if (th->ack) {