From: David Gibson <david@gibson.dropbear.id.au>
To: passt-dev@passt.top, Stefano Brivio <sbrivio@redhat.com>
Cc: maxi.rostock@outlook.de, pholzing@redhat.com,
David Gibson <david@gibson.dropbear.id.au>
Subject: [PATCH 4/4] tcp: Unconditionally force ACK for all !SYN, !RST packets
Date: Tue, 26 Mar 2024 16:42:24 +1100 [thread overview]
Message-ID: <20240326054224.716874-5-david@gibson.dropbear.id.au> (raw)
In-Reply-To: <20240326054224.716874-1-david@gibson.dropbear.id.au>
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>
---
tcp.c | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)
diff --git a/tcp.c b/tcp.c
index b65ddeb5..28562b7f 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1593,8 +1593,6 @@ static void tcp_update_seqack_from_tap(const struct ctx *c,
*/
static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
{
- uint32_t prev_ack_to_tap = conn->seq_ack_to_tap;
- uint32_t prev_wnd_to_tap = conn->wnd_to_tap;
struct tcp4_l2_flags_buf_t *b4 = NULL;
struct tcp6_l2_flags_buf_t *b6 = NULL;
struct tcp_info tinfo = { 0 };
@@ -1675,9 +1673,7 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
*data++ = OPT_WS_LEN;
*data++ = conn->ws_to_tap;
} else if (!(flags & RST)) {
- if (conn->seq_ack_to_tap != prev_ack_to_tap ||
- !prev_wnd_to_tap)
- flags |= ACK;
+ flags |= ACK;
}
th->doff = (sizeof(*th) + optlen) / 4;
--
@@ -1593,8 +1593,6 @@ static void tcp_update_seqack_from_tap(const struct ctx *c,
*/
static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
{
- uint32_t prev_ack_to_tap = conn->seq_ack_to_tap;
- uint32_t prev_wnd_to_tap = conn->wnd_to_tap;
struct tcp4_l2_flags_buf_t *b4 = NULL;
struct tcp6_l2_flags_buf_t *b6 = NULL;
struct tcp_info tinfo = { 0 };
@@ -1675,9 +1673,7 @@ static int tcp_send_flag(struct ctx *c, struct tcp_tap_conn *conn, int flags)
*data++ = OPT_WS_LEN;
*data++ = conn->ws_to_tap;
} else if (!(flags & RST)) {
- if (conn->seq_ack_to_tap != prev_ack_to_tap ||
- !prev_wnd_to_tap)
- flags |= ACK;
+ flags |= ACK;
}
th->doff = (sizeof(*th) + optlen) / 4;
--
2.44.0
next prev parent reply other threads:[~2024-03-26 5:42 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-26 5:42 [PATCH 0/4] Corrections to seeting of ACK flag in TCP packets David Gibson
2024-03-26 5:42 ` [PATCH 1/4] tcp: Split handling of DUP_ACK from ACK David Gibson
2024-03-26 5:42 ` [PATCH 2/4] tcp: Rearrange logic for setting ACK flag in tcp_send_flag() David Gibson
2024-03-26 5:42 ` [PATCH 3/4] tcp: Never automatically add the ACK flag to RST packets David Gibson
2024-03-26 5:42 ` David Gibson [this message]
2024-03-26 8:54 ` [PATCH 0/4] Corrections to seeting of ACK flag in TCP packets Stefano Brivio
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