[routing-manager] learn and copy M & O flags from discovered rout…

…ers (openthread#9607)

This commit implements a mechanism in `RoutingManager` to learn the
Managed Address Config `M` and Other Config `O` flags in received RA
message from discovered routers on the infrastructure link and copy
the same flags in the emitted RA message from BR.

If any discovered router on the infrastructure that is not itself a
stub router (i.e., does not include the Stub Router flag) includes
the `M` or `O` flags, the same flag are included in the emitted RA
message. If a discovered router has failed to respond to the maximum
number of NS probe attempts, we consider it as offline and ignore its
flags.

This commit also adds a detailed test case in `test_routing_manager`
to validate the newly added mechanism.

src/core/border_router/routing_manager.cpp

@@ -620,6 +620,12 @@ void RoutingManager::SendRouterAdvertisement(RouterAdvTxMode aRaTxMode)
 
     LogInfo("Preparing RA");
 
+    mDiscoveredPrefixTable.DetermineAndSetFlags(raMsg);
+
+    LogInfo("- RA Header - flags - M:%u O:%u", raMsg.GetHeader().IsManagedAddressConfigFlagSet(),
+            raMsg.GetHeader().IsOtherConfigFlagSet());
+    LogInfo("- RA Header - default route - lifetime:%u", raMsg.GetHeader().GetRouterLifetime());
+
 #if OPENTHREAD_CONFIG_BORDER_ROUTING_STUB_ROUTER_FLAG_IN_EMITTED_RA_ENABLE
     SuccessOrAssert(raMsg.AppendFlagsExtensionOption(/* aStubRouterFlag */ true));
     LogInfo("- FlagsExt - StubRouter:1");
@@ -1242,7 +1248,7 @@ void RoutingManager::DiscoveredPrefixTable::ProcessRouterAdvertMessage(const Ip6
 
     UpdateRouterOnRx(*router);
 
-    RemoveRoutersWithNoEntries();
+    RemoveRoutersWithNoEntriesOrFlags();
 
 exit:
     return;
@@ -1472,7 +1478,7 @@ void RoutingManager::DiscoveredPrefixTable::RemovePrefix(const Entry::Matcher &a
     VerifyOrExit(!removedEntries.IsEmpty());
 
     FreeEntries(removedEntries);
-    RemoveRoutersWithNoEntries();
+    RemoveRoutersWithNoEntriesOrFlags();
 
     SignalTableChanged();
 
@@ -1482,21 +1488,17 @@ void RoutingManager::DiscoveredPrefixTable::RemovePrefix(const Entry::Matcher &a
 
 void RoutingManager::DiscoveredPrefixTable::RemoveAllEntries(void)
 {
-    // Remove all entries from the table and unpublish them
-    // from Network Data.
+    // Remove all entries from the table.
 
     for (Router &router : mRouters)
     {
-        if (!router.mEntries.IsEmpty())
-        {
-            SignalTableChanged();
-        }
-
         FreeEntries(router.mEntries);
     }
 
-    RemoveRoutersWithNoEntries();
+    FreeRouters(mRouters);
     mEntryTimer.Stop();
+
+    SignalTableChanged();
 }
 
 void RoutingManager::DiscoveredPrefixTable::RemoveOrDeprecateOldEntries(TimeMilli aTimeThreshold)
@@ -1592,11 +1594,11 @@ TimeMilli RoutingManager::DiscoveredPrefixTable::CalculateNextStaleTime(TimeMill
     return foundOnLink ? Min(onLinkStaleTime, routeStaleTime) : routeStaleTime;
 }
 
-void RoutingManager::DiscoveredPrefixTable::RemoveRoutersWithNoEntries(void)
+void RoutingManager::DiscoveredPrefixTable::RemoveRoutersWithNoEntriesOrFlags(void)
 {
     LinkedList<Router> routersToFree;
 
-    mRouters.RemoveAllMatching(Router::kContainsNoEntries, routersToFree);
+    mRouters.RemoveAllMatching(Router::kContainsNoEntriesOrFlags, routersToFree);
     FreeRouters(routersToFree);
 }
 
@@ -1668,7 +1670,7 @@ void RoutingManager::DiscoveredPrefixTable::RemoveExpiredEntries(void)
         router.mEntries.RemoveAllMatching(Entry::ExpirationChecker(now), expiredEntries);
     }
 
-    RemoveRoutersWithNoEntries();
+    RemoveRoutersWithNoEntriesOrFlags();
 
     if (!expiredEntries.IsEmpty())
     {
@@ -1792,6 +1794,42 @@ void RoutingManager::DiscoveredPrefixTable::SendNeighborSolicitToRouter(const Ro
     return;
 }
 
+void RoutingManager::DiscoveredPrefixTable::DetermineAndSetFlags(Ip6::Nd::RouterAdvertMessage &aRaMessage) const
+{
+    // Determine the `M` and `O` flags to include in the RA message
+    // header `aRaMessage` to be emitted.
+    //
+    // If any discovered router on infrastructure which is not itself a
+    // stub router (e.g., another Thread BR) includes the `M` or `O`
+    // flag, we also include the same flag.
+    //
+    // If a router has failed to respond to max number of NS probe
+    // attempts, we consider it as offline and ignore its flags.
+
+    for (const Router &router : mRouters)
+    {
+        if (router.mStubRouterFlag)
+        {
+            continue;
+        }
+
+        if (router.mNsProbeCount > Router::kMaxNsProbes)
+        {
+            continue;
+        }
+
+        if (router.mManagedAddressConfigFlag)
+        {
+            aRaMessage.GetHeader().SetManagedAddressConfigFlag();
+        }
+
+        if (router.mOtherConfigFlag)
+        {
+            aRaMessage.GetHeader().SetOtherConfigFlag();
+        }
+    }
+}
+
 void RoutingManager::DiscoveredPrefixTable::InitIterator(PrefixTableIterator &aIterator) const
 {
     static_cast<Iterator &>(aIterator).Init(mRouters);
@@ -1995,6 +2033,27 @@ uint32_t RoutingManager::DiscoveredPrefixTable::Entry::CalculateExpireDelay(uint
 //---------------------------------------------------------------------------------------------------------------------
 // DiscoveredPrefixTable::Router
 
+bool RoutingManager::DiscoveredPrefixTable::Router::Matches(EmptyChecker aChecker) const
+{
+    // Checks whether or not a `Router` instance has any useful info. An
+    // entry can be removed if it does not advertise M or O flags and
+    // also does not have any advertised prefix entries (RIO/PIO). If
+    // the router already failed to respond to max NS probe attempts,
+    // we consider it as offline and therefore do not consider its
+    // flags anymore.
+
+    OT_UNUSED_VARIABLE(aChecker);
+
+    bool hasFlags = false;
+
+    if (mNsProbeCount <= kMaxNsProbes)
+    {
+        hasFlags = (mManagedAddressConfigFlag || mOtherConfigFlag);
+    }
+
+    return !hasFlags && mEntries.IsEmpty();
+}
+
 void RoutingManager::DiscoveredPrefixTable::Router::CopyInfoTo(RouterEntry &aEntry) const
 {
     aEntry.mAddress                  = mAddress;

src/core/border_router/routing_manager.hpp

@@ -635,6 +635,8 @@ class RoutingManager : public InstanceLocator
 
         TimeMilli CalculateNextStaleTime(TimeMilli aNow) const;
 
+        void DetermineAndSetFlags(Ip6::Nd::RouterAdvertMessage &aRaMessage) const;
+
         void  InitIterator(PrefixTableIterator &aIterator) const;
         Error GetNextEntry(PrefixTableIterator &aIterator, PrefixTableEntry &aEntry) const;
         Error GetNextRouter(PrefixTableIterator &aIterator, RouterEntry &aEntry) const;
@@ -770,11 +772,11 @@ class RoutingManager : public InstanceLocator
 
             enum EmptyChecker : uint8_t
             {
-                kContainsNoEntries
+                kContainsNoEntriesOrFlags
             };
 
             bool Matches(const Ip6::Address &aAddress) const { return aAddress == mAddress; }
-            bool Matches(EmptyChecker) const { return mEntries.IsEmpty(); }
+            bool Matches(EmptyChecker aChecker) const;
             void CopyInfoTo(RouterEntry &aEntry) const;
 
             Router           *mNext;
@@ -816,7 +818,7 @@ class RoutingManager : public InstanceLocator
         bool         Contains(const Entry::Checker &aChecker) const;
         void         RemovePrefix(const Entry::Matcher &aMatcher);
         void         RemoveOrDeprecateEntriesFromInactiveRouters(void);
-        void         RemoveRoutersWithNoEntries(void);
+        void         RemoveRoutersWithNoEntriesOrFlags(void);
         void         FreeRouters(LinkedList<Router> &aRouters);
         void         FreeEntries(LinkedList<Entry> &aEntries);
         void         UpdateNetworkDataOnChangeTo(Entry &aEntry);

src/core/net/nd6.hpp

@@ -733,6 +733,14 @@ class RouterAdvertMessage
      */
     const Header &GetHeader(void) const { return *reinterpret_cast<const Header *>(mData.GetBytes()); }
 
+    /**
+     * Gets the RA message's header.
+     *
+     * @returns The RA message's header.
+     *
+     */
+    Header &GetHeader(void) { return *reinterpret_cast<Header *>(AsNonConst(mData.GetBytes())); }
+
     /**
      * Appends a Prefix Info Option to the RA message.
      *