[PATCH v7 5/5] tcp: Clamp the retry timeout

[Yumei Huang <yuhuang@redhat.com>]

Clamp the TCP retry timeout as Linux kernel does. If RTO is less
than 3 seconds, re-initialize it to 3 seconds for data retransmissions
according to RFC 6298.

Suggested-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Yumei Huang <yuhuang@redhat.com>
---
 tcp.c | 25 ++++++++++++++++++++-----
 tcp.h |  2 ++
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/tcp.c b/tcp.c
index 96ee56a..84a6700 100644
--- a/tcp.c
+++ b/tcp.c
@@ -187,6 +187,9 @@
  *   for established connections, or (tcp_syn_retries +
  *   tcp_syn_linear_timeouts) times during the handshake, reset the connection
  *
+ * - RTO_INIT_ACK: if the RTO is less than this, re-initialize RTO to this for
+ *   data retransmissions.
+ *
  * - 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
  *   the connection
@@ -340,6 +343,7 @@ enum {
 
 #define ACK_INTERVAL			10		/* ms */
 #define RTO_INIT			1		/* s, RFC 6298 */
+#define RTO_INIT_ACK			3		/* s, RFC 6298 */
 #define FIN_TIMEOUT			60
 #define ACT_TIMEOUT			7200
 
@@ -365,9 +369,11 @@ uint8_t tcp_migrate_rcv_queue		[TCP_MIGRATE_RCV_QUEUE_MAX];
 
 #define TCP_SYN_RETRIES		"/proc/sys/net/ipv4/tcp_syn_retries"
 #define TCP_SYN_LINEAR_TIMEOUTS	"/proc/sys/net/ipv4/tcp_syn_linear_timeouts"
+#define TCP_RTO_MAX_MS			"/proc/sys/net/ipv4/tcp_rto_max_ms"
 
 #define TCP_SYN_RETRIES_DEFAULT		6
 #define TCP_SYN_LINEAR_TIMEOUTS_DEFAULT	4
+#define TCP_RTO_MAX_MS_DEFAULT			120000
 
 /* "Extended" data (not stored in the flow table) for TCP flow migration */
 static struct tcp_tap_transfer_ext migrate_ext[FLOW_MAX];
@@ -585,10 +591,13 @@ 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) {
-		int exp = conn->retries;
+		int exp = conn->retries, timeout = RTO_INIT;
 		if (!(conn->events & ESTABLISHED))
 			exp -= c->tcp.syn_linear_timeouts;
-		it.it_value.tv_sec = RTO_INIT << MAX(exp, 0);
+		else
+			timeout = MAX(timeout, RTO_INIT_ACK);
+		timeout <<= MAX(exp, 0);
+		it.it_value.tv_sec = MIN(timeout, c->tcp.tcp_rto_max);
 	} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
 		it.it_value.tv_sec = FIN_TIMEOUT;
 	} else {
@@ -2785,18 +2794,24 @@ static socklen_t tcp_probe_tcp_info(void)
 */
 void tcp_get_rto_params(struct ctx *c)
 {
-	intmax_t tcp_syn_retries, syn_linear_timeouts;
+	intmax_t tcp_syn_retries, syn_linear_timeouts, tcp_rto_max_ms;
 
 	tcp_syn_retries = read_file_integer(
 		TCP_SYN_RETRIES, TCP_SYN_RETRIES_DEFAULT);
 	syn_linear_timeouts = read_file_integer(
 		TCP_SYN_LINEAR_TIMEOUTS, TCP_SYN_LINEAR_TIMEOUTS_DEFAULT);
+	tcp_rto_max_ms = read_file_integer(
+		TCP_RTO_MAX_MS, TCP_RTO_MAX_MS_DEFAULT);
 
 	c->tcp.tcp_syn_retries = MIN(tcp_syn_retries, UINT8_MAX);
 	c->tcp.syn_linear_timeouts = MIN(syn_linear_timeouts, UINT8_MAX);
+	c->tcp.tcp_rto_max = MIN(
+		DIV_ROUND_CLOSEST(tcp_rto_max_ms, 1000), SIZE_MAX);
 
-	debug("Read sysctl values tcp_syn_retries: %"PRIu8", linear_timeouts: %"PRIu8,
-	      c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts);
+	debug("Read sysctl values tcp_syn_retries: %"PRIu8
+	      ", linear_timeouts: %"PRIu8", tcp_rto_max: %zu",
+	      c->tcp.tcp_syn_retries, c->tcp.syn_linear_timeouts,
+	      c->tcp.tcp_rto_max);
 }
 
 /**
diff --git a/tcp.h b/tcp.h
index befedde..a238bb7 100644
--- a/tcp.h
+++ b/tcp.h
@@ -59,6 +59,7 @@ union tcp_listen_epoll_ref {
  * @fwd_out:		Port forwarding configuration for outbound packets
  * @timer_run:		Timestamp of most recent timer run
  * @pipe_size:		Size of pipes for spliced connections
+ * @tcp_rto_max:	Maximal retry timeout (in s)
  * @tcp_syn_retries:	SYN retries using exponential backoff timeout
  * @syn_linear_timeouts: SYN retries before using exponential backoff timeout
  */
@@ -67,6 +68,7 @@ struct tcp_ctx {
 	struct fwd_ports fwd_out;
 	struct timespec timer_run;
 	size_t pipe_size;
+	size_t tcp_rto_max;
 	uint8_t tcp_syn_retries;
 	uint8_t syn_linear_timeouts;
 };
-- 
2.49.0