[routing-manager] retry sending failed RS messages (#7038)

Current implement will start evaluating on-link prefix before
successfully sending kMaxRtrSolicitations=3 RS messages. This commit
fixes this issue by retrying sending failed RS messages in
kRtrSolicitationRetryFailDelay=60 seconds.

src/core/border_router/routing_manager.cpp

@@ -852,19 +852,25 @@ void RoutingManager::HandleRouterSolicitTimer(void)
         Error    error;
 
         error = SendRouterSolicitation();
-        ++mRouterSolicitCount;
 
         if (error == kErrorNone)
         {
             otLogDebgBr("Successfully sent %uth Router Solicitation", mRouterSolicitCount);
+            ++mRouterSolicitCount;
+            nextSolicitationDelay =
+                (mRouterSolicitCount == kMaxRtrSolicitations) ? kMaxRtrSolicitationDelay : kRtrSolicitationInterval;
         }
         else
         {
             otLogCritBr("Failed to send %uth Router Solicitation: %s", mRouterSolicitCount, ErrorToString(error));
-        }
 
-        nextSolicitationDelay =
-            (mRouterSolicitCount == kMaxRtrSolicitations) ? kMaxRtrSolicitationDelay : kRtrSolicitationInterval;
+            // It's unexpected that RS will fail and we will retry sending RS messages in 60 seconds.
+            // Notice that `mRouterSolicitCount` is not incremented for failed RS and thus we will
+            // not start configuring on-link prefixes before `kMaxRtrSolicitations` successful RS
+            // messages have been sent.
+            nextSolicitationDelay = kRtrSolicitationRetryDelay;
+            mRouterSolicitCount   = 0;
+        }
 
         otLogDebgBr("Router solicitation timer scheduled in %u seconds", nextSolicitationDelay);
         mRouterSolicitTimer.Start(Time::SecToMsec(nextSolicitationDelay));

src/core/border_router/routing_manager.hpp

@@ -198,6 +198,7 @@ private:
     static constexpr uint32_t kRtrSolicitationInterval     = 4;   // Interval between RSs. In sec.
     static constexpr uint32_t kMaxRtrSolicitationDelay     = 1;   // Max delay for initial solicitation. In sec.
     static constexpr uint32_t kMaxRoutingPolicyDelay       = 1;   // Max delay for routing policy evaluation. In sec.
+    static constexpr uint32_t kRtrSolicitationRetryDelay   = 60;  // The delay before retrying failed RS tx. In Sec.
 
     // 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

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

@@ -267,8 +267,9 @@ class SingleBorderRouter(thread_cert.TestCase):
 
         br.enable_ether()
 
-        # It takes around 10 seconds to start sending RA messages.
-        self.simulator.go(15)
+        # The routing manager may fail to send RS and will wait for 60 seconds
+        # before retrying.
+        self.simulator.go(80)
         self.collect_ipaddrs()
 
         logging.info("BR     addrs: %r", br.get_addrs())