[PATCH v2 0/3] Retry SYNs for inbound connections

[PATCH v2 0/3] Retry SYNs for inbound connections

[Yumei Huang]

When a client connects, SYN would be sent to guest only once. If the 
guest is not connected or ready at that time, the connection will be
reset in 10s. These patches introduce the SYN retry mechanism using
the similar backoff timeout as linux kernel. Also update the data
retransmission timeout.

Yumei Huang (3):
  tcp: Rename "retrans" to "retries"
  tcp: Resend SYN for inbound connections
  tcp: Update data retransmission timeout

 tcp.c      | 132 +++++++++++++++++++++++++++++++++++++++++++++--------
 tcp.h      |   2 +
 tcp_conn.h |  12 ++---
 3 files changed, 120 insertions(+), 26 deletions(-)

-- 
2.47.0

[PATCH v2 1/3] tcp: Rename "retrans" to "retries"

[Yumei Huang]

Rename "retrans" to "retries" so it can be used for SYN retries.

Signed-off-by: Yumei Huang <yuhuang@redhat.com>
---
 tcp.c      | 12 ++++++------
 tcp_conn.h | 12 ++++++------
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/tcp.c b/tcp.c
index 0f9e9b3..2ec4b0c 100644
--- a/tcp.c
+++ b/tcp.c
@@ -186,7 +186,7 @@
  * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending
  *   data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the
  *   socket and reset sequence to what was acknowledged. If this persists for
- *   more than TCP_MAX_RETRANS times in a row, reset the connection
+ *   more than TCP_MAX_RETRIES times in a row, reset the connection
  *
  * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (flag ACK_FROM_TAP_DUE
  *   with TAP_FIN_SENT event), and no ACK is received within this time, reset
@@ -1127,7 +1127,7 @@ static void tcp_update_seqack_from_tap(const struct ctx *c,
 		if (SEQ_LT(seq, conn->seq_to_tap))
 			conn_flag(c, conn, ACK_FROM_TAP_DUE);
 
-		conn->retrans = 0;
+		conn->retries = 0;
 		conn->seq_ack_from_tap = seq;
 	}
 }
@@ -2414,7 +2414,7 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref)
 		} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
 			flow_dbg(conn, "FIN timeout");
 			tcp_rst(c, conn);
-		} else if (conn->retrans == TCP_MAX_RETRANS) {
+		} else if (conn->retries == TCP_MAX_RETRIES) {
 			flow_dbg(conn, "retransmissions count exceeded");
 			tcp_rst(c, conn);
 		} else {
@@ -2423,7 +2423,7 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref)
 			if (!conn->wnd_from_tap)
 				conn->wnd_from_tap = 1; /* Zero-window probe */
 
-			conn->retrans++;
+			conn->retries++;
 			if (tcp_rewind_seq(c, conn))
 				return;
 
@@ -3382,7 +3382,7 @@ static int tcp_flow_repair_opt(const struct tcp_tap_conn *conn,
 int tcp_flow_migrate_source(int fd, struct tcp_tap_conn *conn)
 {
 	struct tcp_tap_transfer t = {
-		.retrans		= conn->retrans,
+		.retries		= conn->retries,
 		.ws_from_tap		= conn->ws_from_tap,
 		.ws_to_tap		= conn->ws_to_tap,
 		.events			= conn->events,
@@ -3662,7 +3662,7 @@ int tcp_flow_migrate_target(struct ctx *c, int fd)
 	memcpy(&flow->f.side, &t.side, sizeof(flow->f.side));
 	conn = FLOW_SET_TYPE(flow, FLOW_TCP, tcp);
 
-	conn->retrans			= t.retrans;
+	conn->retries			= t.retries;
 	conn->ws_from_tap		= t.ws_from_tap;
 	conn->ws_to_tap			= t.ws_to_tap;
 	conn->events			= t.events;
diff --git a/tcp_conn.h b/tcp_conn.h
index 38b5c54..e5c8146 100644
--- a/tcp_conn.h
+++ b/tcp_conn.h
@@ -13,7 +13,7 @@
  * struct tcp_tap_conn - Descriptor for a TCP connection (not spliced)
  * @f:			Generic flow information
  * @in_epoll:		Is the connection in the epoll set?
- * @retrans:		Number of retransmissions occurred due to ACK_TIMEOUT
+ * @retries:		Number of retries occurred due to timeouts
  * @ws_from_tap:	Window scaling factor advertised from tap/guest
  * @ws_to_tap:		Window scaling factor advertised to tap/guest
  * @tap_mss:		MSS advertised by tap/guest, rounded to 2 ^ TCP_MSS_BITS
@@ -38,9 +38,9 @@ struct tcp_tap_conn {
 
 	bool		in_epoll	:1;
 
-#define TCP_RETRANS_BITS		3
-	unsigned int	retrans		:TCP_RETRANS_BITS;
-#define TCP_MAX_RETRANS			MAX_FROM_BITS(TCP_RETRANS_BITS)
+#define TCP_RETRIES_BITS		3
+	unsigned int	retries		:TCP_RETRIES_BITS;
+#define TCP_MAX_RETRIES			MAX_FROM_BITS(TCP_RETRIES_BITS)
 
 #define TCP_WS_BITS			4	/* RFC 7323 */
 #define TCP_WS_MAX			14
@@ -102,7 +102,7 @@ struct tcp_tap_conn {
  * struct tcp_tap_transfer - Migrated TCP data, flow table part, network order
  * @pif:		Interfaces for each side of the flow
  * @side:		Addresses and ports for each side of the flow
- * @retrans:		Number of retransmissions occurred due to ACK_TIMEOUT
+ * @retries:		Number of retries occurred due to timeouts
  * @ws_from_tap:	Window scaling factor advertised from tap/guest
  * @ws_to_tap:		Window scaling factor advertised to tap/guest
  * @events:		Connection events, implying connection states
@@ -122,7 +122,7 @@ struct tcp_tap_transfer {
 	uint8_t		pif[SIDES];
 	struct flowside	side[SIDES];
 
-	uint8_t		retrans;
+	uint8_t		retries;
 	uint8_t		ws_from_tap;
 	uint8_t		ws_to_tap;
 	uint8_t		events;
-- 
2.47.0

[PATCH v2 2/3] tcp: Resend SYN for inbound connections

[Yumei Huang]

If a client connects while guest is not connected or ready yet,
resend SYN instead of just resetting connection after 10 seconds.

Use the same backoff calculation for the timeout as linux kernel.

Signed-off-by: Yumei Huang <yuhuang@redhat.com>
---
 tcp.c | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
 tcp.h |  2 ++
 2 files changed, 89 insertions(+), 8 deletions(-)

diff --git a/tcp.c b/tcp.c
index 2ec4b0c..85bbdac 100644
--- a/tcp.c
+++ b/tcp.c
@@ -179,9 +179,11 @@
  *
  * Timeouts are implemented by means of timerfd timers, set based on flags:
  *
- * - SYN_TIMEOUT: if no ACK is received from tap/guest during handshake (flag
- *   ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, reset the
- *   connection
+ * - SYN_TIMEOUT_INIT: if no ACK is received from tap/guest during handshake
+ *   (flag ACK_FROM_TAP_DUE without ESTABLISHED event) within this time, resend
+ *   SYN. It's the starting timeout for the first SYN retry. If this persists
+ *   for more than TCP_MAX_RETRIES or (tcp_syn_retries +
+ *   tcp_syn_linear_timeouts) times in a row, reset the connection
  *
  * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending
  *   data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the
@@ -309,6 +311,7 @@
 #include "tcp_internal.h"
 #include "tcp_buf.h"
 #include "tcp_vu.h"
+#include "lineread.h"
 
 /*
  * The size of TCP header (including options) is given by doff (Data Offset)
@@ -340,7 +343,7 @@ enum {
 #define WINDOW_DEFAULT			14600		/* RFC 6928 */
 
 #define ACK_INTERVAL			10		/* ms */
-#define SYN_TIMEOUT			10		/* s */
+#define SYN_TIMEOUT_INIT		1		/* s */
 #define ACK_TIMEOUT			2
 #define FIN_TIMEOUT			60
 #define ACT_TIMEOUT			7200
@@ -365,6 +368,10 @@ uint8_t tcp_migrate_rcv_queue		[TCP_MIGRATE_RCV_QUEUE_MAX];
 
 #define TCP_MIGRATE_RESTORE_CHUNK_MIN	1024 /* Try smaller when above this */
 
+#define TCP_SYN_RETRIES_SYSCTL		"/proc/sys/net/ipv4/tcp_syn_retries"
+#define TCP_SYN_LINEAR_TIMEOUTS_SYSCTL						\
+	"/proc/sys/net/ipv4/tcp_syn_linear_timeouts"
+
 /* "Extended" data (not stored in the flow table) for TCP flow migration */
 static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX];
 
@@ -581,8 +588,13 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
 	if (conn->flags & ACK_TO_TAP_DUE) {
 		it.it_value.tv_nsec = (long)ACK_INTERVAL * 1000 * 1000;
 	} else if (conn->flags & ACK_FROM_TAP_DUE) {
-		if (!(conn->events & ESTABLISHED))
-			it.it_value.tv_sec = SYN_TIMEOUT;
+		if (!(conn->events & ESTABLISHED)) {
+			if (conn->retries < c->tcp.syn_linear_timeouts)
+				it.it_value.tv_sec = SYN_TIMEOUT_INIT;
+			else
+				it.it_value.tv_sec = SYN_TIMEOUT_INIT <<
+					(conn->retries - c->tcp.syn_linear_timeouts);
+		}
 		else
 			it.it_value.tv_sec = ACK_TIMEOUT;
 	} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
@@ -1961,6 +1973,7 @@ static void tcp_conn_from_sock_finish(const struct ctx *c,
 	}
 
 	tcp_send_flag(c, conn, ACK);
+	conn->retries = 0;
 
 	/* The client might have sent data already, which we didn't
 	 * dequeue waiting for SYN,ACK from tap -- check now.
@@ -2409,8 +2422,16 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref)
 		tcp_timer_ctl(c, conn);
 	} else if (conn->flags & ACK_FROM_TAP_DUE) {
 		if (!(conn->events & ESTABLISHED)) {
-			flow_dbg(conn, "handshake timeout");
-			tcp_rst(c, conn);
+			if (conn->retries == MIN(TCP_MAX_RETRIES,
+				(c->tcp.tcp_syn_retries + c->tcp.tcp_syn_retries))) {
+				flow_dbg(conn, "handshake timeout");
+				tcp_rst(c, conn);
+			} else {
+				flow_dbg(conn, "SYN timeout, retry");
+				tcp_send_flag(c, conn, SYN);
+				conn->retries++;
+				tcp_timer_ctl(c, conn);
+			}
 		} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
 			flow_dbg(conn, "FIN timeout");
 			tcp_rst(c, conn);
@@ -2766,6 +2787,62 @@ static socklen_t tcp_probe_tcp_info(void)
 	return sl;
 }
 
+/**
+ * get_tcp_syn_param() - Read SYN parameters from /proc/sys
+ * @path: Path to the sysctl file
+ * @fallback: Default value if file can't be read
+ *
+ * Return: Parameter value, fallback on failure
+*/
+int get_tcp_syn_param(const char *path, int fallback)
+{
+	char *line, *end;
+	struct lineread lr;
+	long value;
+	ssize_t len;
+	int fd;
+
+	fd = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		debug("Unable to open %s", path);
+		return fallback;
+	}
+
+	lineread_init(&lr, fd);
+	len = lineread_get(&lr, &line);
+	close(fd);
+
+	if (len < 0) {
+		debug("Unable to read %s", path);
+		return fallback;
+	}
+
+	errno = 0;
+	value = strtol(line, &end, 10);
+	if (*end && *end != '\n') {
+		debug("Invalid format in %s", path);
+		return fallback;
+	}
+	if (errno || value < 0 || value > INT_MAX) {
+		debug("Invalid value in %s: %ld", path, value);
+		return fallback;
+	}
+	return (int)value;
+}
+
+/**
+ * tcp_syn_params_init() - Get initial syn params for inbound connection
+ * @c:		Execution context
+*/
+void tcp_syn_params_init(struct ctx *c)
+{
+	c->tcp.tcp_syn_retries = get_tcp_syn_param(TCP_SYN_RETRIES_SYSCTL, 8);
+	c->tcp.syn_linear_timeouts =
+		get_tcp_syn_param(TCP_SYN_LINEAR_TIMEOUTS_SYSCTL, 1);
+	debug("TCP SYN parameters: retries=%d, linear_timeouts=%d",
+		  c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts);
+}
+
 /**
  * tcp_init() - Get initial sequence, hash secret, initialise per-socket data
  * @c:		Execution context
@@ -2776,6 +2853,8 @@ int tcp_init(struct ctx *c)
 {
 	ASSERT(!c->no_tcp);
 
+	tcp_syn_params_init(c);
+
 	tcp_sock_iov_init(c);
 
 	memset(init_sock_pool4,		0xff,	sizeof(init_sock_pool4));
diff --git a/tcp.h b/tcp.h
index 234a803..df699a4 100644
--- a/tcp.h
+++ b/tcp.h
@@ -65,6 +65,8 @@ struct tcp_ctx {
 	struct fwd_ports fwd_out;
 	struct timespec timer_run;
 	size_t pipe_size;
+	uint8_t tcp_syn_retries;
+	uint8_t syn_linear_timeouts;
 };
 
 #endif /* TCP_H */
-- 
2.47.0

[PATCH v2 3/3] tcp: Update data retransmission timeout

[Yumei Huang]

Use an exponential backoff timeout with the initial timeout 1s
and total timeout 60s.

Signed-off-by: Yumei Huang <yuhuang@redhat.com>
---
 tcp.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/tcp.c b/tcp.c
index 85bbdac..0db7f30 100644
--- a/tcp.c
+++ b/tcp.c
@@ -185,10 +185,15 @@
  *   for more than TCP_MAX_RETRIES or (tcp_syn_retries +
  *   tcp_syn_linear_timeouts) times in a row, reset the connection
  *
- * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after sending
- *   data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data from the
- *   socket and reset sequence to what was acknowledged. If this persists for
- *   more than TCP_MAX_RETRIES times in a row, reset the connection
+ * - ACK_TIMEOUT_INIT: if no ACK segment was received from tap/guest, after
+ *   sending data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data
+ *   from the socket and reset sequence to what was acknowledged. It's the
+ *   starting timeout for the first retransmission. If this persists for more
+ *   than TCP_MAX_RETRIES times in a row, reset the connection
+ *
+ * - ACK_TIMEOUT_TOTAL: if no ACK segment was received from tap/guest after
+ *   retransmitting data repeatedly from the socket within this time, reset
+ *   the connection
  *
  * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (flag ACK_FROM_TAP_DUE
  *   with TAP_FIN_SENT event), and no ACK is received within this time, reset
@@ -344,7 +349,8 @@ enum {
 
 #define ACK_INTERVAL			10		/* ms */
 #define SYN_TIMEOUT_INIT		1		/* s */
-#define ACK_TIMEOUT			2
+#define ACK_TIMEOUT_INIT		1
+#define ACK_TIMEOUT_TOTAL		60
 #define FIN_TIMEOUT			60
 #define ACT_TIMEOUT			7200
 
@@ -358,6 +364,9 @@ enum {
 	 ((conn)->events & (SOCK_FIN_RCVD | TAP_FIN_RCVD)))
 #define CONN_HAS(conn, set)	(((conn)->events & (set)) == (set))
 
+#define RETRY_ELAPSED(timeout_init, retries)		\
+	((timeout_init) * ((1 << ((retries) + 1)) - 2))
+
 /* Buffers to migrate pending data from send and receive queues. No, they don't
  * use memory if we don't use them. And we're going away after this, so splurge.
  */
@@ -596,7 +605,7 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
 					(conn->retries - c->tcp.syn_linear_timeouts);
 		}
 		else
-			it.it_value.tv_sec = ACK_TIMEOUT;
+			it.it_value.tv_sec = ACK_TIMEOUT_INIT << conn->retries;
 	} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
 		it.it_value.tv_sec = FIN_TIMEOUT;
 	} else {
@@ -2438,6 +2447,10 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref)
 		} else if (conn->retries == TCP_MAX_RETRIES) {
 			flow_dbg(conn, "retransmissions count exceeded");
 			tcp_rst(c, conn);
+		} else if(RETRY_ELAPSED(ACK_TIMEOUT_INIT, conn->retries) >=
+			ACK_TIMEOUT_TOTAL) {
+			flow_dbg(conn, "retransmissions timeout exceeded");
+			tcp_rst(c, conn);
 		} else {
 			flow_dbg(conn, "ACK timeout, retry");
 
-- 
2.47.0