[PATCH v5 4/4] 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 | 27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

diff --git a/tcp.c b/tcp.c
index 71e2da0..31bba6f 100644
--- a/tcp.c
+++ b/tcp.c
@@ -179,16 +179,14 @@
  *
  * 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
+ * - 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. If this persists too many times
+ *   in a row, reset the connection: TCP_MAX_RETRIES for established
+ *   connections, or (tcp_syn_retries + tcp_syn_linear_timeouts) during the
+ *   handshake.
  *
  * - 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 +340,7 @@ enum {
 #define WINDOW_DEFAULT			14600		/* RFC 6928 */
 
 #define ACK_INTERVAL			10		/* ms */
-#define SYN_TIMEOUT_INIT		1		/* s */
-#define ACK_TIMEOUT			2
+#define RTO_INIT			1		/* s, RFC 6298 */
 #define FIN_TIMEOUT			60
 #define ACT_TIMEOUT			7200
 
@@ -588,13 +585,13 @@ 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 = RTO_INIT;
 			else
-				it.it_value.tv_sec = SYN_TIMEOUT_INIT <<
+				it.it_value.tv_sec = RTO_INIT <<
 					(conn->retries - c->tcp.syn_linear_timeouts);
 		}
 		else
-			it.it_value.tv_sec = ACK_TIMEOUT;
+			it.it_value.tv_sec = RTO_INIT << conn->retries;
 	} else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) {
 		it.it_value.tv_sec = FIN_TIMEOUT;
 	} else {
-- 
2.47.0