[PATCH v3 4/4] tcp: Update data retransmission timeout

[Yumei Huang <yuhuang@redhat.com>]

According to RFC 2988 and RFC 6298, we should use an exponential
backoff timeout for data retransmission starting from one second
(see Appendix A in RFC 6298), and limit it to about 60 seconds
as allowed by the same RFC:

   (2.5) A maximum value MAY be placed on RTO provided it is at
         least 60 seconds.

Combine the macros defining the initial timeout for both SYN and ACK.
And add a macro ACK_RETRIES to limit the total timeout to about 60s.

Signed-off-by: Yumei Huang <yuhuang@redhat.com>
---
 tcp.c | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/tcp.c b/tcp.c
index 3ce3991..84da069 100644
--- a/tcp.c
+++ b/tcp.c
@@ -179,16 +179,12 @@
  *
  * Timeouts are implemented by means of timerfd timers, set based on flags:
  *
- * - 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
- *   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, eiher
+ *   during handshake(flag ACK_FROM_TAP_DUE without ESTABLISHED event) or 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 retry. If this persists for more than 
+ *   allowed 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
@@ -342,8 +338,7 @@ enum {
 #define WINDOW_DEFAULT			14600		/* RFC 6928 */
 
 #define ACK_INTERVAL			10		/* ms */
-#define SYN_TIMEOUT_INIT		1		/* s */
-#define ACK_TIMEOUT			2
+#define ACK_TIMEOUT_INIT		1		/* s, RFC 6298 */
 #define FIN_TIMEOUT			60
 #define ACT_TIMEOUT			7200
 
@@ -352,6 +347,11 @@ enum {
 
 #define ACK_IF_NEEDED	0		/* See tcp_send_flag() */
 
+/* Number of retries calculated from the exponential backoff formula, limited
+ * by a total timeout of about 60 seconds.
+ */
+#define ACK_RETRIES		5
+
 #define CONN_IS_CLOSING(conn)						\
 	(((conn)->events & ESTABLISHED) &&				\
 	 ((conn)->events & (SOCK_FIN_RCVD | TAP_FIN_RCVD)))
@@ -589,13 +589,13 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
 	} else if (conn->flags & ACK_FROM_TAP_DUE) {
 		if (!(conn->events & ESTABLISHED)) {
 			if (conn->retries < c->tcp.syn_linear_timeouts)
-				it.it_value.tv_sec = SYN_TIMEOUT_INIT;
+				it.it_value.tv_sec = ACK_TIMEOUT_INIT;
 			else
-				it.it_value.tv_sec = SYN_TIMEOUT_INIT <<
+				it.it_value.tv_sec = ACK_TIMEOUT_INIT <<
 					(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 {
@@ -2433,7 +2433,7 @@ 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->retries == TCP_MAX_RETRIES) {
+		} else if (conn->retries >= ACK_RETRIES) {
 			flow_dbg(conn, "retransmissions count exceeded");
 			tcp_rst(c, conn);
 		} else {
-- 
2.47.0