[PATCH v7 4/5] tcp: Update data retransmission timeout

[Yumei Huang <yuhuang@redhat.com>]

Use an exponential backoff timeout for data retransmission according
to RFC 2988 and RFC 6298. Set the initial RTO to one second as discussed
in Appendix A of RFC 6298.

Also combine the macros defining the initial RTO for both SYN and ACK.

Signed-off-by: Yumei Huang <yuhuang@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c | 30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)

diff --git a/tcp.c b/tcp.c
index bada88a..96ee56a 100644
--- a/tcp.c
+++ b/tcp.c
@@ -179,16 +179,13 @@
  *
  * 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. Retry for
- *   TCP_MAX_RETRIES or (tcp_syn_retries + tcp_syn_linear_timeouts) times,
- *   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
+ * - RTO_INIT: if no ACK segment was received from tap/guest, either 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. This is the
+ *   timeout for the first retry, in seconds. Retry for TCP_MAX_RETRIES times
+ *   for established connections, or (tcp_syn_retries +
+ *   tcp_syn_linear_timeouts) times during the handshake, 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 +339,7 @@ enum {
 #define WINDOW_DEFAULT			14600		/* RFC 6928 */
 
 #define ACK_INTERVAL			10		/* ms */
-#define SYN_TIMEOUT_INIT		1		/* s, RFC 6928 */
-#define ACK_TIMEOUT			2
+#define RTO_INIT			1		/* s, RFC 6298 */
 #define FIN_TIMEOUT			60
 #define ACT_TIMEOUT			7200
 
@@ -589,12 +585,10 @@ 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)) {
-			int exp = conn->retries - c->tcp.syn_linear_timeouts;
-			it.it_value.tv_sec = SYN_TIMEOUT_INIT << MAX(exp, 0);
-		}
-		else
-			it.it_value.tv_sec = ACK_TIMEOUT;
+		int exp = conn->retries;
+		if (!(conn->events & ESTABLISHED))
+			exp -= c->tcp.syn_linear_timeouts;
+		it.it_value.tv_sec = RTO_INIT << MAX(exp, 0);
 	} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
 		it.it_value.tv_sec = FIN_TIMEOUT;
 	} else {
-- 
2.49.0