[routing-manager] add minimal delay between RAs (#7473)

This commit adds minimal delay (3 seconds) between consecutive Router
Advertisements according to `6.2.6.  Processing Router Solicitations,
RFC4861`.

src/core/border_router/routing_manager.cpp

@@ -72,6 +72,7 @@ RoutingManager::RoutingManager(Instance &aInstance)
     , mDiscoveredPrefixInvalidTimer(aInstance, HandleDiscoveredPrefixInvalidTimer)
     , mDiscoveredPrefixStaleTimer(aInstance, HandleDiscoveredPrefixStaleTimer)
     , mRouterAdvertisementCount(0)
+    , mLastRouterAdvertisementSendTime(TimerMilli::GetNow())
 #if OPENTHREAD_CONFIG_BORDER_ROUTING_VICARIOUS_RS_ENABLE
     , mVicariousRouterSolicitTimer(aInstance, HandleVicariousRouterSolicitTimer)
 #endif
@@ -772,8 +773,15 @@ void RoutingManager::StartRoutingPolicyEvaluationJitter(uint32_t aJitterMilli)
 
 void RoutingManager::StartRoutingPolicyEvaluationDelay(uint32_t aDelayMilli)
 {
-    LogInfo("Start evaluating routing policy, scheduled in %u milliseconds", aDelayMilli);
-    mRoutingPolicyTimer.FireAtIfEarlier(TimerMilli::GetNow() + aDelayMilli);
+    TimeMilli now          = TimerMilli::GetNow();
+    TimeMilli evaluateTime = now + aDelayMilli;
+    TimeMilli earlestTime  = mLastRouterAdvertisementSendTime + kMinDelayBetweenRtrAdvs;
+
+    evaluateTime = OT_MAX(evaluateTime, earlestTime);
+
+    LogInfo("Start evaluating routing policy, scheduled in %u milliseconds", evaluateTime - now);
+
+    mRoutingPolicyTimer.FireAtIfEarlier(evaluateTime);
 }
 
 // starts sending Router Solicitations in random delay
@@ -928,6 +936,7 @@ void RoutingManager::SendRouterAdvertisement(const OmrPrefixArray &aNewOmrPrefix
 
         if (error == kErrorNone)
         {
+            mLastRouterAdvertisementSendTime = TimerMilli::GetNow();
             LogInfo("Sent Router Advertisement on interface %u", mInfraIfIndex);
             DumpDebg("[BR-CERT] direction=send | type=RA |", buffer, bufferLength);
         }

src/core/border_router/routing_manager.hpp

@@ -220,6 +220,7 @@ private:
     static constexpr uint32_t kMaxRtrSolicitationDelay     = 1;      // Max delay for initial solicitation. In sec.
     static constexpr uint32_t kRoutingPolicyEvaluationJitter = 1000; // Jitter for routing policy evaluation. In msec.
     static constexpr uint32_t kRtrSolicitationRetryDelay     = 60;   // The delay before retrying failed RS tx. In Sec.
+    static constexpr uint32_t kMinDelayBetweenRtrAdvs        = 3000; // Min delay (msec) between consecutive RAs.
 
     // The STALE_RA_TIME in seconds. The Routing Manager will consider the prefixes
     // and learned RA parameters STALE when they are not refreshed in STALE_RA_TIME
@@ -413,7 +414,8 @@ private:
     TimerMilli mDiscoveredPrefixInvalidTimer;
     TimerMilli mDiscoveredPrefixStaleTimer;
 
-    uint32_t mRouterAdvertisementCount;
+    uint32_t  mRouterAdvertisementCount;
+    TimeMilli mLastRouterAdvertisementSendTime;
 
 #if OPENTHREAD_CONFIG_BORDER_ROUTING_VICARIOUS_RS_ENABLE
     TimerMilli mVicariousRouterSolicitTimer;

tests/scripts/thread-cert/border_router/test_multi_thread_networks.py

@@ -108,6 +108,9 @@ class MultiThreadNetworks(thread_cert.TestCase):
         self.simulator.go(5)
         self.assertEqual('router', router2.get_state())
 
+        # Wait for network to stabilize
+        self.simulator.go(15)
+
         self.collect_ipaddrs()
 
         logging.info("BR1     addrs: %r", br1.get_addrs())