[routing-manager] include Stub Router flag in emitted RAs by BR (#9486)

This commit updates `RoutingManager` to include the Flags Extension
Option with Stub Router flag in its emitted Router Advertisement
messages.

Config `STUB_ROUTER_FLAG_IN_EMITTED_RA_ENABLE` can be used to enable
or disable this behavior which is enabled by default.

5 files changed, 69 insertions, 2 deletions

diff --git a/src/core/border_router/routing_manager.cpp b/src/core/border_router/routing_manager.cpp
index 453f49549..97fbad55f 100644
--- a/src/core/border_router/routing_manager.cpp
+++ b/src/core/border_router/routing_manager.cpp
@@ -593,6 +593,7 @@ void RoutingManager::SendRouterAdvertisement(RouterAdvTxMode aRaTxMode)
     // RA message max length is derived to accommodate:
     //
     // - The RA header.
+    // - One RA Flags Extensions Option (with stub router flag).
     // - One PIO for current local on-link prefix.
     // - At most `kMaxOldPrefixes` for old deprecating on-link prefixes.
     // - At most twice `kMaxOnMeshPrefixes` RIO for on-mesh prefixes.
@@ -600,8 +601,8 @@ void RoutingManager::SendRouterAdvertisement(RouterAdvTxMode aRaTxMode)
     //   previous prefixes while adding new ones.
 
     static constexpr uint16_t kMaxRaLength =
-        sizeof(Ip6::Nd::RouterAdvertMessage::Header) + sizeof(Ip6::Nd::PrefixInfoOption) +
-        sizeof(Ip6::Nd::PrefixInfoOption) * OnLinkPrefixManager::kMaxOldPrefixes +
+        sizeof(Ip6::Nd::RouterAdvertMessage::Header) + sizeof(Ip6::Nd::RaFlagsExtOption) +
+        sizeof(Ip6::Nd::PrefixInfoOption) + sizeof(Ip6::Nd::PrefixInfoOption) * OnLinkPrefixManager::kMaxOldPrefixes +
         2 * kMaxOnMeshPrefixes * (sizeof(Ip6::Nd::RouteInfoOption) + sizeof(Ip6::Prefix));
 
     uint8_t                         buffer[kMaxRaLength];
@@ -611,6 +612,11 @@ void RoutingManager::SendRouterAdvertisement(RouterAdvTxMode aRaTxMode)
 
     LogInfo("Preparing RA");
 
+#if OPENTHREAD_CONFIG_BORDER_ROUTING_STUB_ROUTER_FLAG_IN_EMITTED_RA_ENABLE
+    SuccessOrAssert(raMsg.AppendFlagsExtensionOption(/* aStubRouterFlag */ true));
+    LogInfo("- FlagsExt - StubRouter:1");
+#endif
+
     // Append PIO for local on-link prefix if is either being
     // advertised or deprecated and for old prefix if is being
     // deprecated.
diff --git a/src/core/config/border_routing.h b/src/core/config/border_routing.h
index a7f57d1fe..4640a081a 100644
--- a/src/core/config/border_routing.h
+++ b/src/core/config/border_routing.h
@@ -127,6 +127,17 @@
 #endif
 
 /**
+ * @def OPENTHREAD_CONFIG_BORDER_ROUTING_STUB_ROUTER_FLAG_IN_EMITTED_RA_ENABLE
+ *
+ * Define to 1 so for the routing manager to include the Flags Extension Option with Stub Router flag in the emitted
+ * Router Advertisement messages from this Border Router.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_BORDER_ROUTING_STUB_ROUTER_FLAG_IN_EMITTED_RA_ENABLE
+#define OPENTHREAD_CONFIG_BORDER_ROUTING_STUB_ROUTER_FLAG_IN_EMITTED_RA_ENABLE 1
+#endif
+
+/**
  * @def OPENTHREAD_CONFIG_BORDER_ROUTING_DHCP6_PD_ENABLE
  *
  * Specifies whether to support handling platform generated ND messages.
diff --git a/src/core/net/nd6.cpp b/src/core/net/nd6.cpp
index 3d24b6ad7..e66fb043f 100644
--- a/src/core/net/nd6.cpp
+++ b/src/core/net/nd6.cpp
@@ -269,6 +269,25 @@ exit:
     return error;
 }
 
+Error RouterAdvertMessage::AppendFlagsExtensionOption(bool aStubRouterFlag)
+{
+    Error             error = kErrorNone;
+    RaFlagsExtOption *flagsOption;
+
+    flagsOption = static_cast<RaFlagsExtOption *>(AppendOption(sizeof(RaFlagsExtOption)));
+    VerifyOrExit(flagsOption != nullptr, error = kErrorNoBufs);
+
+    flagsOption->Init();
+
+    if (aStubRouterFlag)
+    {
+        flagsOption->SetStubRouterFlag();
+    }
+
+exit:
+    return error;
+}
+
 //----------------------------------------------------------------------------------------------------------------------
 // RouterSolicitMessage
 
diff --git a/src/core/net/nd6.hpp b/src/core/net/nd6.hpp
index bc116a450..bbf6a6ea7 100644
--- a/src/core/net/nd6.hpp
+++ b/src/core/net/nd6.hpp
@@ -762,6 +762,17 @@ public:
     Error AppendRouteInfoOption(const Prefix &aPrefix, uint32_t aRouteLifetime, RoutePreference aPreference);
 
     /**
+     * Appends a Flags Extension Option to the RA message.
+     *
+     * @param[in] aStubRouterFlag    The stub router flag.
+     *
+     * @retval kErrorNone    Option is appended successfully.
+     * @retval kErrorNoBufs  No more space in the buffer to append the option.
+     *
+     */
+    Error AppendFlagsExtensionOption(bool aStubRouterFlag);
+
+    /**
      * Indicates whether or not the RA message contains any options.
      *
      * @retval TRUE   If the RA message contains at least one option.
diff --git a/tests/unit/test_routing_manager.cpp b/tests/unit/test_routing_manager.cpp
index ae1d21e7b..c5cbb5cda 100644
--- a/tests/unit/test_routing_manager.cpp
+++ b/tests/unit/test_routing_manager.cpp
@@ -449,6 +449,17 @@ void ValidateRouterAdvert(const Icmp6Packet &aPacket)
             break;
         }
 
+#if OPENTHREAD_CONFIG_BORDER_ROUTING_STUB_ROUTER_FLAG_IN_EMITTED_RA_ENABLE
+        case Ip6::Nd::Option::kTypeRaFlagsExtension:
+        {
+            const Ip6::Nd::RaFlagsExtOption &flagsOption = static_cast<const Ip6::Nd::RaFlagsExtOption &>(option);
+
+            VerifyOrQuit(flagsOption.IsValid());
+            VerifyOrQuit(flagsOption.IsStubRouterFlagSet());
+            break;
+        }
+#endif
+
         default:
             VerifyOrQuit(false, "Unexpected option type in RA msg");
         }
@@ -545,6 +556,15 @@ void LogRouterAdvert(const Icmp6Packet &aPacket)
             break;
         }
 
+        case Ip6::Nd::Option::kTypeRaFlagsExtension:
+        {
+            const Ip6::Nd::RaFlagsExtOption &flagsOption = static_cast<const Ip6::Nd::RaFlagsExtOption &>(option);
+
+            VerifyOrQuit(flagsOption.IsValid());
+            Log("     FlagsExt - StubRouter:%u", flagsOption.IsStubRouterFlagSet());
+            break;
+        }
+
         default:
             VerifyOrQuit(false, "Bad option type in RA msg");
         }