sctp: Fix the RTO-doubling on idle-link heartbeats

SCTP incorrectly doubles rto ever time a Hearbeat chunk
is generated.   However RFC 4960 states:

   On an idle destination address that is allowed to heartbeat, it is
   recommended that a HEARTBEAT chunk is sent once per RTO of that
   destination address plus the protocol parameter 'HB.interval', with
   jittering of +/- 50% of the RTO value, and exponential backoff of the
   RTO if the previous HEARTBEAT is unanswered.

Essentially, of if the heartbean is unacknowledged, do we double the RTO.
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sctp/structs.h

@@ -910,8 +910,10 @@ struct sctp_transport {
 	 *		should be set. Every time the RTT
 	 *		calculation completes (i.e. the DATA chunk
 	 *		is SACK'd) clear this flag.
+	 * hb_sent : a flag that signals that we have a pending heartbeat.
 	 */
 	__u8 rto_pending;
+	__u8 hb_sent;
 
 	/* Flag to track the current fast recovery state */
 	__u8 fast_recovery;

net/sctp/sm_sideeffect.c

@@ -461,9 +461,15 @@ static void sctp_do_8_2_transport_strike(struct sctp_association *asoc,
 	 * expires, set RTO <- RTO * 2 ("back off the timer").  The
 	 * maximum value discussed in rule C7 above (RTO.max) may be
 	 * used to provide an upper bound to this doubling operation.
+	 *
+	 * Special Case:  the first HB doesn't trigger exponential backoff.
+	 * The first unacknowleged HB triggers it.  We do this with a flag
+	 * that indicates that we have an outstanding HB.
 	 */
-	transport->last_rto = transport->rto;
-	transport->rto = min((transport->rto * 2), transport->asoc->rto_max);
+	if (transport->hb_sent) {
+		transport->last_rto = transport->rto;
+		transport->rto = min((transport->rto * 2), transport->asoc->rto_max);
+	}
 }
 
 /* Worker routine to handle INIT command failure.  */
@@ -621,6 +627,11 @@ static void sctp_cmd_transport_on(sctp_cmd_seq_t *cmds,
 	t->error_count = 0;
 	t->asoc->overall_error_count = 0;
 
+	/* Clear the hb_sent flag to signal that we had a good
+	 * acknowledgement.
+	 */
+	t->hb_sent = 0;
+
 	/* Mark the destination transport address as active if it is not so
 	 * marked.
 	 */
@@ -657,6 +668,8 @@ static void sctp_cmd_transport_reset(sctp_cmd_seq_t *cmds,
 
 	/* Mark one strike against a transport.  */
 	sctp_do_8_2_transport_strike(asoc, t);
+
+	t->hb_sent = 1;
 }
 
 /* Helper function to process the process SACK command.  */

net/sctp/transport.c

@@ -79,6 +79,7 @@ static struct sctp_transport *sctp_transport_init(struct sctp_transport *peer,
 	peer->rttvar = 0;
 	peer->srtt = 0;
 	peer->rto_pending = 0;
+	peer->hb_sent = 0;
 	peer->fast_recovery = 0;
 
 	peer->last_time_heard = jiffies;
@@ -608,6 +609,7 @@ void sctp_transport_reset(struct sctp_transport *t)
 	t->flight_size = 0;
 	t->error_count = 0;
 	t->rto_pending = 0;
+	t->hb_sent = 0;
 	t->fast_recovery = 0;
 
 	/* Initialize the state information for SFR-CACC */