[PATCH 3/4] tcp: Extend tcp_send_flag() to send TCP keepalive segments

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

TCP keepalives aren't technically a flag, but they are a zero-data segment
so they can be generated with only a small modification to
tcp_{buf,vu}_send_flag().  Implement this, using a new "pseudo-flag"
value (similar to DUP_ACK), KEEPALIVE.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp_buf.c      | 4 ++++
 tcp_internal.h | 2 ++
 tcp_vu.c       | 3 +++
 3 files changed, 9 insertions(+)

diff --git a/tcp_buf.c b/tcp_buf.c
index d2925410..18bdce81 100644
--- a/tcp_buf.c
+++ b/tcp_buf.c
@@ -227,6 +227,10 @@ int tcp_buf_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
 	tcp_frame_conns[tcp_payload_used++] = conn;
 	l4len = optlen + sizeof(struct tcphdr);
 	iov[TCP_IOV_PAYLOAD].iov_len = l4len;
+
+	if (flags & KEEPALIVE)
+		seq--;
+
 	tcp_l2_buf_fill_headers(c, conn, iov, NULL, seq, false);
 
 	tcp_l2_buf_pad(iov);
diff --git a/tcp_internal.h b/tcp_internal.h
index 5f8fb358..36f443b4 100644
--- a/tcp_internal.h
+++ b/tcp_internal.h
@@ -38,6 +38,8 @@
 
 /* Flags for internal usage */
 #define DUP_ACK		(1 << 5)
+#define KEEPALIVE	(1 << 6)
+
 #define OPT_EOL		0
 #define OPT_NOP		1
 #define OPT_MSS		2
diff --git a/tcp_vu.c b/tcp_vu.c
index b9e9b55e..f2d38aa5 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -135,6 +135,9 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
 	flags_elem[0].in_sg[0].iov_len = hdrlen + optlen;
 	payload = IOV_TAIL(flags_elem[0].in_sg, 1, hdrlen);
 
+	if (flags & KEEPALIVE)
+		seq--;
+
 	tcp_fill_headers(c, conn, NULL, eh, ip4h, ip6h, th, &payload,
 			 NULL, seq, !*c->pcap);
 
-- 
2.52.0