Implement probe moderation and delaying algorithm.

This should meet the following requirements:
  - only one probe in flight,
  - not too often (500ms delay min),
  - increase SRTT on idle flows.

src/network/network.cc

@@ -137,6 +137,21 @@ Packet Connection::new_packet( Flow *flow, uint16_t flags, string &s_payload )
     flow->saved_timestamp_received_at = 0;
   }
 
+  if ( !server ) {
+    unsigned int delay = MAX( (int)flow->SRTT + 4 * flow->RTTVAR, MIN_PROBE_INTERVAL );
+    if ( flow->first_sent_message_since_reply <= flow->last_heard ) {
+      flow->first_sent_message_since_reply = now;
+    } else if ( delay < now - flow->first_sent_message_since_reply ) {
+      flow->SRTT = now - flow->first_sent_message_since_reply;
+      log_dbg( LOG_DEBUG_COMMON, "Flow %hu seems idle: SRTT = %dms.\n",
+               flow->flow_id, (int)flow->SRTT );
+    }
+
+    if ( flags & PROBE_FLAG ) {
+      flow->next_probe = now + delay;
+    }
+  }
+
   Packet p( flow->next_seq++, direction, timestamp16(), outgoing_timestamp_reply,
 	    flow->flow_id, flags, s_payload );
 
@@ -272,6 +287,9 @@ Connection::Flow::Flow( const Addr &s_src, const Addr &s_dst )
     expected_receiver_seq( defaults.expected_receiver_seq ),
     saved_timestamp( defaults.saved_timestamp ),
     saved_timestamp_received_at( defaults.saved_timestamp_received_at ),
+    first_sent_message_since_reply( defaults.first_sent_message_since_reply ),
+    last_heard( defaults.last_heard ),
+    next_probe( defaults.next_probe ),
     RTT_hit( defaults.RTT_hit ),
     SRTT( defaults.SRTT ),
     RTTVAR( defaults.RTTVAR ),
@@ -291,6 +309,9 @@ Connection::Flow::Flow( const Addr &s_src, const Addr &s_dst, uint16_t id )
     expected_receiver_seq( defaults.expected_receiver_seq ),
     saved_timestamp( defaults.saved_timestamp ),
     saved_timestamp_received_at( defaults.saved_timestamp_received_at ),
+    first_sent_message_since_reply( defaults.first_sent_message_since_reply ),
+    last_heard( defaults.last_heard ),
+    next_probe( defaults.next_probe ),
     RTT_hit( defaults.RTT_hit ),
     SRTT( defaults.SRTT ),
     RTTVAR( defaults.RTTVAR ),
@@ -582,8 +603,9 @@ void Connection::send_probes( void )
   for ( std::map< uint16_t, Flow* >::iterator it = flows.begin();
 	it != flows.end();
 	it++ ) {
-    if ( it->second != last_flow ) {
-      send_probe( it->second );
+    Flow *flow = it->second;
+    if ( flow != last_flow && flow->next_probe <= timestamp() ) {
+      send_probe( flow );
     }
   }
 }
@@ -1022,7 +1044,7 @@ string Connection::recv_one( int sock_to_recv )
     }
 
     /* auto-adjust to remote host */
-    last_heard = timestamp();
+    flow_info->last_heard = last_heard = timestamp();
 
     if ( server ) { /* only client can roam */
       bool has_roam = last_flow != flow_info;

src/network/network.h

@@ -104,6 +104,7 @@ namespace Network {
     static const unsigned int SERVER_ASSOCIATION_TIMEOUT = 40000;
     static const unsigned int PORT_HOP_INTERVAL          = 10000;
     static const unsigned int MAX_ADDR_REQUEST_INTERVAL  = 10000;
+    static const unsigned int MIN_PROBE_INTERVAL         = 500;
 
     static const unsigned int MAX_PORTS_OPEN             = 10;
     static const unsigned int MAX_OLD_SOCKET_AGE         = 60000;
@@ -116,6 +117,7 @@ namespace Network {
       Flow( void )
 	: src( Addr() ), dst( Addr() ), MTU( DEFAULT_SEND_MTU ), next_seq( 0 ),
 	expected_receiver_seq( 0 ), saved_timestamp( -1 ), saved_timestamp_received_at( 0 ),
+	first_sent_message_since_reply( 0 ), last_heard( 0 ), next_probe( 0 ),
 	RTT_hit( false ), SRTT( 1000 ), RTTVAR( 500 ), flow_id( 0 )
       {}
 
@@ -128,6 +130,9 @@ namespace Network {
       uint64_t expected_receiver_seq;
       uint16_t saved_timestamp;
       uint64_t saved_timestamp_received_at;
+      uint64_t first_sent_message_since_reply;
+      uint64_t last_heard;
+      uint64_t next_probe;
       bool RTT_hit;
       double SRTT;
       double RTTVAR;