Compute the moving exponential average of the RTT

babeld.c

@@ -58,6 +58,10 @@ struct timeval now;
 unsigned char myid[8];
 int debug = 0;
 
+/* A higher value means we forget old RTT samples faster. Must be
+   between 1 and 256, inclusive. */
+unsigned int rtt_exponential_decay = 42;
+
 int link_detect = 0;
 int all_wireless = 0;
 int default_wireless_hello_interval = -1;

babeld.h

@@ -90,6 +90,7 @@ extern int do_daemonise;
 extern const char *logfile, *pidfile, *state_file;
 extern int link_detect;
 extern int all_wireless;
+extern unsigned int rtt_exponential_decay;
 
 extern unsigned char myid[8];
 

message.c

@@ -610,6 +610,16 @@ parse_packet(const unsigned char *from, struct interface *ifp,
         rtt = local_waiting_us - remote_waiting_us;
         debugf("RTT to %s on %s sample result: %d us.\n",
                format_address(from), ifp->name, rtt);
+
+        if (valid_rtt(neigh))
+            /* Running exponential average. */
+            neigh->rtt = (rtt_exponential_decay * MAX(rtt, 0)
+                          + (256 - rtt_exponential_decay) * neigh->rtt) / 256;
+        else
+            /* We prefer to be conservative with new neighbours
+               (higher RTT) */
+            neigh->rtt = MAX(2*rtt, 0);
+        neigh->rtt_time = now;
     }
     return;
 }

neighbour.c

@@ -99,6 +99,8 @@ find_neighbour(const unsigned char *address, struct interface *ifp)
     neigh->ihu_interval = 0;
     neigh->hello_send_us = 0;
     neigh->hello_rtt_receive_time = zero;
+    neigh->rtt = 0;
+    neigh->rtt_time = zero;
     neigh->ifp = ifp;
     neigh->next = neighs;
     neighs = neigh;
@@ -325,3 +327,9 @@ neighbour_cost(struct neighbour *neigh)
         return MIN((a * b + 128) >> 8, INFINITY);
     }
 }
+
+int
+valid_rtt(struct neighbour *neigh)
+{
+    return (timeval_minus_msec(&now, &neigh->rtt_time) < 180000) ? 1 : 0;
+}

neighbour.h

@@ -36,6 +36,8 @@ struct neighbour {
        according to remote clock. */
     unsigned int hello_send_us;
     struct timeval hello_rtt_receive_time;
+    unsigned int rtt;
+    struct timeval rtt_time;
     struct interface *ifp;
 };
 
@@ -53,3 +55,4 @@ unsigned check_neighbours(void);
 unsigned neighbour_txcost(struct neighbour *neigh);
 unsigned neighbour_rxcost(struct neighbour *neigh);
 unsigned neighbour_cost(struct neighbour *neigh);
+int valid_rtt(struct neighbour *neigh);