babel: Add route metric smoothing

The Babel RTT extension employs metric smoothing to dampen route
oscillations in the face of varying RTT values between two peers[0].

This patch implements such dampening in Bird, roughly following the
implementation in babeld (i.e., using the same exponential function
definition). The main difference is that we calculate everything in the
native Bird microsecond time unit (and increase constants accordingly), and
that we split out the smoothed metric calculation in two function variants,
one that has no side effects and one that does.

  [0] https://arxiv.org/pdf/1403.3488.pdf

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

doc/bird.sgml

@@ -1865,6 +1865,7 @@ protocol babel [<name>] {
 	ipv4 { <channel config> };
 	ipv6 [sadr] { <channel config> };
         randomize router id <switch>;
+        metric decay <time>;
 	interface <interface pattern> {
 		type <wired|wireless|tunnel>;
 		rxcost <number>;
@@ -1914,6 +1915,17 @@ protocol babel [<name>] {
       router ID every time it starts up, which avoids this problem at the cost
       of not having stable router IDs in the network. Default: no.
 
+      <tag><label id="babel-metric-decay">metric decay <m/time/ s</tag>
+      The metric smoothing decay time. When route metrics vary (because of
+      varying quality of a wireless link, or varying RTT when timestamps are
+      enabled), Babel applies an exponential smoothing procedure to the metric
+      to dampen route oscillations. This parameter specifies the half-life of
+      the convergence of the smoothed metric to the actual metric of the route.
+      I.e., the distance between the smoothed and the actual metric will be
+      halfed for each time period specified here (until they converge). Set to 0
+      to disable metric smoothing; if set, the value must be in the interval
+      1-180 s. Default: 4 s
+
       <tag><label id="babel-type">type wired|wireless|tunnel </tag>
       This option specifies the interface type: Wired, wireless or tunnel. On
       wired interfaces a neighbor is considered unreachable after a small number

proto/babel/babel.c

@@ -134,6 +134,103 @@ babel_expire_sources(struct babel_proto *p, struct babel_entry *e)
   }
 }
 
+static u16
+babel_calc_smoothed_metric(struct babel_proto *p, struct babel_route *r, u8 update)
+{
+  struct babel_config *cf = (void *) p->p.cf;
+  uint metric = r->metric, smoothed_metric = r->smoothed_metric;
+  btime smoothed_time = r->smoothed_time, now = current_time();
+
+  if (!cf->metric_decay || metric == BABEL_INFINITY ||
+      metric == smoothed_metric || !smoothed_time)
+  {
+    smoothed_metric = metric;
+    smoothed_time = now;
+    goto out;
+  }
+
+  int diff = metric - smoothed_metric;
+
+  /*
+   * The decay defines the half-life of the metric convergence, so first iterate
+   * in halving steps
+   */
+  while (smoothed_time < now - cf->metric_decay && diff) {
+    smoothed_metric += diff/2;
+    smoothed_time += cf->metric_decay;
+    diff = metric - smoothed_metric;
+  }
+
+  /*
+   * Then, update remainder in BABEL_SMOOTHING_STEP intervals using the
+   * exponential function (approximated via the pre-computed reciprocal).
+   */
+  while (smoothed_time < now - BABEL_SMOOTHING_STEP && diff) {
+    smoothed_metric += (BABEL_SMOOTHING_STEP * diff *
+                        (cf->smooth_recp - BABEL_SMOOTHING_UNIT) / BABEL_SMOOTHING_UNIT);
+    smoothed_time += BABEL_SMOOTHING_STEP;
+    diff = metric - smoothed_metric;
+  }
+
+  /* Consider the metric converged once we're close enough */
+  if (ABS(diff) < BABEL_SMOOTHING_MIN_DIFF)
+    smoothed_metric = metric;
+
+out:
+  if (update) {
+    r->smoothed_metric = smoothed_metric;
+    r->smoothed_time = smoothed_time;
+  }
+
+  return smoothed_metric;
+}
+
+static u16
+babel_update_smoothed_metric(struct babel_proto *p, struct babel_route *r)
+{
+  if (!r->metric)
+    return 0;
+
+  if (!r->smoothed_metric) {
+    r->smoothed_metric = r->metric;
+    r->smoothed_time = current_time();
+    return r->smoothed_metric;
+  }
+
+  u16 smoothed = babel_calc_smoothed_metric(p, r, 1);
+  DBG("Updated smoothed metric for prefix %N: router-id %lR metric %d/%d\n",
+      r->e->n.addr, r->router_id, r->metric, smoothed);
+
+  return smoothed;
+}
+
+static u16
+babel_smoothed_metric(struct babel_proto *p, struct babel_route *r)
+{
+  return babel_calc_smoothed_metric(p, r, 0);
+}
+
+static void
+babel_update_metric(struct babel_proto *p, struct babel_route *r, u16 metric)
+{
+  babel_update_smoothed_metric(p, r);
+  r->metric = metric;
+}
+
+static inline u8
+babel_route_better(struct babel_proto *p, struct babel_route *mod,
+                   struct babel_route *best)
+{
+  if (!mod->feasible)
+    return 0;
+
+  if (!best)
+    return mod->metric < BABEL_INFINITY;
+
+  return (mod->metric < best->metric &&
+          babel_smoothed_metric(p, mod) < babel_smoothed_metric(p, best));
+}
+
 static struct babel_route *
 babel_find_route(struct babel_entry *e, struct babel_neighbor *n)
 {
@@ -151,8 +248,10 @@ babel_get_route(struct babel_proto *p, struct babel_entry *e, struct babel_neigh
 {
   struct babel_route *r = babel_find_route(e, nbr);
 
-  if (r)
+  if (r) {
+    babel_update_smoothed_metric(p, r);
     return r;
+  }
 
   r = sl_allocz(p->route_slab);
 
@@ -641,7 +740,7 @@ done:
     struct babel_route *r; node *n;
     WALK_LIST2(r, n, nbr->routes, neigh_route)
     {
-      r->metric = babel_compute_metric(nbr, r->advert_metric);
+      babel_update_metric(p, r, babel_compute_metric(nbr, r->advert_metric));
       babel_select_route(p, r->e, r);
     }
   }
@@ -765,8 +864,7 @@ babel_select_route(struct babel_proto *p, struct babel_entry *e, struct babel_ro
   /* Shortcut if only non-best was modified */
   if (mod && (mod != best))
   {
-    /* Either select modified route, or keep old best route */
-    if ((mod->metric < (best ? best->metric : BABEL_INFINITY)) && mod->feasible)
+    if (babel_route_better(p, mod, best))
       best = mod;
     else
       return;
@@ -777,17 +875,24 @@ babel_select_route(struct babel_proto *p, struct babel_entry *e, struct babel_ro
     if (!best || (best->metric == BABEL_INFINITY) || !best->feasible)
       best = NULL;
 
+    /* best will be compared to many routes below, make sure it's up-to-date */
+    if (best)
+      babel_update_smoothed_metric(p, best);
+
     /* Find the best feasible route from all routes */
     WALK_LIST(r, e->routes)
-      if ((r->metric < (best ? best->metric : BABEL_INFINITY)) && r->feasible)
+      if (babel_route_better(p, r, best))
 	best = r;
   }
 
   if (best)
   {
     if (best != e->selected)
-      TRACE(D_EVENTS, "Picked new route for prefix %N: router-id %lR metric %d",
-	    e->n.addr, best->router_id, best->metric);
+    {
+      u16 smoothed = babel_update_smoothed_metric(p, best);
+      TRACE(D_EVENTS, "Picked new route for prefix %N: router-id %lR metric %d/%d",
+	    e->n.addr, best->router_id, best->metric, smoothed);
+    }
   }
   else if (e->selected)
   {
@@ -1366,9 +1471,9 @@ babel_handle_update(union babel_msg *m, struct babel_iface *ifa)
     return;
 
   /* Last paragraph above - update the entry */
+  babel_update_metric(p, r, metric);
   r->feasible = feasible;
   r->seqno = msg->seqno;
-  r->metric = metric;
   r->advert_metric = msg->metric;
   r->router_id = msg->router_id;
   r->next_hop = msg->next_hop;

proto/babel/babel.h

@@ -60,6 +60,18 @@
 #define BABEL_RTT_MAX			(120 MS_)
 #define BABEL_RTT_DECAY			42
 
+/*
+ * Constants for calculating metric smoothing. Chosen so that:
+ * log(2) = BABEL_SMOOTHING_CONSTANT / BABEL_SMOOTHING_UNIT, which means that
+ * log(2)/x can be calculated as BABEL_SMOOTHING_UNIT + BABEL_SMOOTHING_CONSTANT / x
+ */
+#define BABEL_SMOOTHING_UNIT		0x10000000
+#define BABEL_SMOOTHING_CONSTANT	186065279
+#define BABEL_SMOOTHING_STEP		(1 S_)		/* smoothing calculated in this step size */
+#define BABEL_SMOOTHING_MIN_DIFF	4		/* metric diff beneath this is converged */
+#define BABEL_SMOOTHING_DECAY		(4 S_)
+#define BABEL_SMOOTHING_DECAY_MAX	(180 S_)
+
 /* Max interval that will not overflow when carried as 16-bit centiseconds */
 #define BABEL_TIME_UNITS		10000	/* On-wire times are counted in centiseconds */
 #define BABEL_MIN_INTERVAL		(0x0001 * BABEL_TIME_UNITS)
@@ -127,6 +139,8 @@ enum babel_ae_type {
 struct babel_config {
   struct proto_config c;
   list iface_list;			/* List of iface configs (struct babel_iface_config) */
+  btime metric_decay;
+  uint smooth_recp;			/* Reciprocal for exponential metric smoothing */
   uint hold_time;			/* Time to hold stale entries and unreachable routes */
   u8 randomize_router_id;
 
@@ -280,9 +294,11 @@ struct babel_route {
   u16 seqno;
   u16 metric;
   u16 advert_metric;
+  u16 smoothed_metric;
   u64 router_id;
   ip_addr next_hop;
   btime refresh_time;
+  btime smoothed_time;
   btime expires;
 };
 

proto/babel/config.Y

@@ -37,6 +37,13 @@ babel_proto_start: proto_start BABEL
   this_proto = proto_config_new(&proto_babel, $1);
   init_list(&BABEL_CFG->iface_list);
   BABEL_CFG->hold_time = 1 S_;
+  BABEL_CFG->metric_decay = BABEL_SMOOTHING_DECAY;
+};
+
+babel_proto_finish:
+{
+  if (BABEL_CFG->metric_decay)
+    BABEL_CFG->smooth_recp = BABEL_SMOOTHING_UNIT + BABEL_SMOOTHING_CONSTANT / BABEL_CFG->metric_decay;
 };
 
 babel_proto_item:
@@ -44,6 +51,7 @@ babel_proto_item:
  | proto_channel
  | INTERFACE babel_iface
  | RANDOMIZE ROUTER ID bool { BABEL_CFG->randomize_router_id = $4; }
+ | METRIC DECAY expr_us { BABEL_CFG->metric_decay = $3; if ($3 && (($3 < BABEL_SMOOTHING_STEP) || ($3 > BABEL_SMOOTHING_DECAY_MAX))) cf_error("Metric decay must be 0, or between 1-180s"); }
  ;
 
 babel_proto_opts:
@@ -52,7 +60,7 @@ babel_proto_opts:
  ;
 
 babel_proto:
-   babel_proto_start proto_name '{' babel_proto_opts '}';
+   babel_proto_start proto_name '{' babel_proto_opts '}' babel_proto_finish;
 
 
 babel_iface_start: