[rx-ra-tracker] add state to track initial router discovery process (#12080)

Adds a new state, `mInitialDiscoveryFinished`, to `RxRaTracker` to
track the completion of the initial router discovery (RS transmission)
process. A new method, `IsInitialRouterDiscoveryFinished()`, exposes
this state.

This new method replaces `IsRsTxInProgress()`, which previously
checked if any RS transmission was ongoing. The new model ensures
the initial discovery is tracked only once after `RxRaTracker`
starts, rather than every time RS messages are sent (e.g., due to
stale timer expiration).

Additionally, the `RoutingManager` now checks this state and ignores
incoming RS messages until the initial router discovery is complete.
This prevents the BR from replying to its own RS messages or sending
an RA prematurely with incomplete information before all routers
are discovered and decision factors are determined.

src/core/border_router/routing_manager.cpp

@@ -586,6 +586,13 @@ void RoutingManager::HandleRouterSolicit(const InfraIf::Icmp6Packet &aPacket, co
     Get<Ip6::Ip6>().GetBorderRoutingCounters().mRsRx++;
     LogInfo("Received RS from %s on %s", aSrcAddress.ToString().AsCString(), Get<InfraIf>().ToString().AsCString());
 
+    // We ignore a received Router Solicit (RS) message while
+    // `RxRaTracker` is performing its initial router discovery
+    // and sending RS. This ensures that we do not reply to our own
+    // RS and cause a premature routing policy evaluation.
+
+    VerifyOrExit(Get<RxRaTracker>().IsInitialRouterDiscoveryFinished());
+
     ScheduleRoutingPolicyEvaluation(kToReplyToRs);
 
 exit:
@@ -1288,7 +1295,7 @@ exit:
 
 void RoutingManager::OnLinkPrefixManager::Evaluate(void)
 {
-    VerifyOrExit(!Get<RxRaTracker>().IsRsTxInProgress());
+    VerifyOrExit(Get<RxRaTracker>().IsInitialRouterDiscoveryFinished());
 
     SetAilPrefix(Get<RxRaTracker>().GetFavoredOnLinkPrefix());
 

src/core/border_router/rx_ra_tracker.cpp

@@ -51,6 +51,7 @@ RxRaTracker::RxRaTracker(Instance &aInstance)
     , mRoutingManagerEnabled(false)
     , mMultiAilDetectorEnabled(false)
     , mIsRunning(false)
+    , mInitialDiscoveryFinished(false)
     , mRsSender(aInstance)
     , mExpirationTimer(aInstance)
     , mStaleTimer(aInstance)
@@ -92,7 +93,9 @@ void RxRaTracker::UpdateState(void)
 void RxRaTracker::Start(void)
 {
     VerifyOrExit(!mIsRunning);
-    mIsRunning = true;
+
+    mIsRunning                = true;
+    mInitialDiscoveryFinished = false;
 
     mRsSender.Start();
     HandleNetDataChange();
@@ -134,6 +137,8 @@ void RxRaTracker::HandleRsSenderFinished(TimeMilli aStartTime)
     // Solicitation.
 
     RemoveOrDeprecateOldEntries(aStartTime);
+
+    mInitialDiscoveryFinished = true;
     Get<RoutingManager>().ScheduleRoutingPolicyEvaluation(RoutingManager::kImmediately);
 }
 

src/core/border_router/rx_ra_tracker.hpp

@@ -105,18 +105,16 @@ public:
     void SetEnabled(bool aEnable, Requester aRequester);
 
     /**
-     * Indicates whether the Router Solicitation (RS) transmission process is in progress.
+     * Indicates whether the initial router discovery process (sending Router Solicitation (RS) message) is finished.
      *
-     * Upon `Start()`, the device performs the RS transmission process to discover routers on the infrastructure
-     * interface. The device sends three Router Solicitation (RS) messages every four seconds, starting with a random
-     * delay of up to one second for the first RS transmission. After sending the final RS message, the device waits
-     * one second before concluding the RS transmission process, at which point `IsRsTxInProgress()` returns `FALSE`.
-     * The RS transmission process is also performed if the stale timer for any discovered prefix expires.
+     * When `RxRaTracker` is started, it sends multiple RS messages to discover routers on the infrastructure interface.
+     * It sends three RS messages every four seconds, starting with a random delay of up to one second for the first
+     * RS transmission. After sending the final RS message, it waits one second before concluding discovery process.
      *
-     * @retval TRUE   If the Router Solicitation transmission process is in progress.
-     * @retval FALSE  If the Router Solicitation transmission process is not in progress.
+     * @retval TRUE   If the initial router discovery process is finished.
+     * @retval FALSE  If the initial router discovery process is not finished.
      */
-    bool IsRsTxInProgress(void) const { return mRsSender.IsInProgress(); }
+    bool IsInitialRouterDiscoveryFinished(void) const { return mInitialDiscoveryFinished; }
 
     /**
      * Initializes a `PrefixTableIterator`.
@@ -581,6 +579,7 @@ private:
     bool                 mRoutingManagerEnabled : 1;
     bool                 mMultiAilDetectorEnabled : 1;
     bool                 mIsRunning : 1;
+    bool                 mInitialDiscoveryFinished : 1;
     RsSender             mRsSender;
     DecisionFactors      mDecisionFactors;
     RouterList           mRouters;

tests/unit/test_routing_manager.cpp

@@ -318,6 +318,7 @@ otError otPlatInfraIfSendIcmp6Nd(uint32_t            aInfraIfIndex,
     case Ip6::Icmp::Header::kTypeRouterSolicit:
         Log("  Router Solicit message");
         sRsEmitted = true;
+        otPlatInfraIfRecvIcmp6Nd(sInstance, kInfraIfIndex, &sInfraIfAddress, aBuffer, aBufferLength);
         break;
 
     case Ip6::Icmp::Header::kTypeRouterAdvert: