[mle-router] leader to handle "BR request" in an Address Solicit (#7578)

This commit adds support for new Status TLV values and updates
the processing of Address Solicit request messages on leader.

The new status `kBorderRouterRequest` can be used by a BR to request to
upgrade to router role. If there are fewer routers than "router
upgrade threshold" leader will accept the request. If the number of
routers is more than the threshold but less than max, the leader will
accept the BR request only if the number of current Border Routers
(as indicated by Network Data) that are acting as router is less than
two (thus allowing up to two BRs to use this status reason to become
router when already above the threshold).

The other change in this PR, is that when processing Address Solicit
request if the included status value is not known or invalid, a
response with `kUnrecognizedStatus` status is sent.

src/core/thread/mle_router.cpp

@@ -3803,11 +3803,12 @@ void MleRouter::HandleAddressSolicit(void *aContext, otMessage *aMessage, const
 
 void MleRouter::HandleAddressSolicit(Coap::Message &aMessage, const Ip6::MessageInfo &aMessageInfo)
 {
-    Error           error = kErrorNone;
-    Mac::ExtAddress extAddress;
-    uint16_t        rloc16;
-    uint8_t         status;
-    Router *        router = nullptr;
+    Error                   error          = kErrorNone;
+    ThreadStatusTlv::Status responseStatus = ThreadStatusTlv::kNoAddressAvailable;
+    Router *                router         = nullptr;
+    Mac::ExtAddress         extAddress;
+    uint16_t                rloc16;
+    uint8_t                 status;
 
     VerifyOrExit(aMessage.IsConfirmablePostRequest(), error = kErrorParse);
 
@@ -3816,6 +3817,17 @@ void MleRouter::HandleAddressSolicit(Coap::Message &aMessage, const Ip6::Message
     SuccessOrExit(error = Tlv::Find<ThreadExtMacAddressTlv>(aMessage, extAddress));
     SuccessOrExit(error = Tlv::Find<ThreadStatusTlv>(aMessage, status));
 
+    switch (Tlv::Find<ThreadRloc16Tlv>(aMessage, rloc16))
+    {
+    case kErrorNone:
+        break;
+    case kErrorNotFound:
+        rloc16 = Mac::kShortAddrInvalid;
+        break;
+    default:
+        ExitNow(error = kErrorParse);
+    }
+
 #if OPENTHREAD_CONFIG_TIME_SYNC_ENABLE
     {
         uint16_t xtalAccuracy;
@@ -3827,7 +3839,12 @@ void MleRouter::HandleAddressSolicit(Coap::Message &aMessage, const Ip6::Message
 
     // Check if allocation already exists
     router = mRouterTable.GetRouter(extAddress);
-    VerifyOrExit(router == nullptr);
+
+    if (router != nullptr)
+    {
+        responseStatus = ThreadStatusTlv::kSuccess;
+        ExitNow();
+    }
 
     switch (status)
     {
@@ -3839,61 +3856,61 @@ void MleRouter::HandleAddressSolicit(Coap::Message &aMessage, const Ip6::Message
     case ThreadStatusTlv::kParentPartitionChange:
         break;
 
+    case ThreadStatusTlv::kBorderRouterRequst:
+        if ((mRouterTable.GetActiveRouterCount() >= mRouterUpgradeThreshold) &&
+            (Get<NetworkData::Leader>().CountBorderRouters(NetworkData::kRouterRoleOnly) >=
+             kRouterUpgradeBorderRouterRequestThreshold))
+        {
+            LogInfo("Rejecting BR %s router role req - have %d BR routers", extAddress.ToString().AsCString(),
+                    kRouterUpgradeBorderRouterRequestThreshold);
+            ExitNow();
+        }
+        break;
+
     default:
-        ExitNow(error = kErrorParse);
+        responseStatus = ThreadStatusTlv::kUnrecognizedStatus;
+        ExitNow();
     }
 
-    switch (Tlv::Find<ThreadRloc16Tlv>(aMessage, rloc16))
+    if (rloc16 != Mac::kShortAddrInvalid)
     {
-    case kErrorNone:
         router = mRouterTable.Allocate(RouterIdFromRloc16(rloc16));
 
         if (router != nullptr)
         {
             LogInfo("Router id %d requested and provided!", RouterIdFromRloc16(rloc16));
-            break;
         }
+    }
 
-        OT_FALL_THROUGH;
-
-    case kErrorNotFound:
+    if (router == nullptr)
+    {
         router = mRouterTable.Allocate();
-        break;
-
-    default:
-        ExitNow(error = kErrorParse);
+        VerifyOrExit(router != nullptr);
     }
 
-    if (router != nullptr)
-    {
-        router->SetExtAddress(extAddress);
-    }
-    else
-    {
-        LogInfo("Router address unavailable!");
-    }
+    router->SetExtAddress(extAddress);
+    responseStatus = ThreadStatusTlv::kSuccess;
 
 exit:
     if (error == kErrorNone)
     {
-        SendAddressSolicitResponse(aMessage, router, aMessageInfo);
+        SendAddressSolicitResponse(aMessage, responseStatus, router, aMessageInfo);
     }
 }
 
 void MleRouter::SendAddressSolicitResponse(const Coap::Message &   aRequest,
+                                           ThreadStatusTlv::Status aResponseStatus,
                                            const Router *          aRouter,
                                            const Ip6::MessageInfo &aMessageInfo)
 {
-    Coap::Message *         message = Get<Tmf::Agent>().NewPriorityMessage();
-    ThreadStatusTlv::Status status;
+    Coap::Message *message = Get<Tmf::Agent>().NewPriorityMessage();
 
     VerifyOrExit(message != nullptr);
 
     SuccessOrExit(message->SetDefaultResponseHeader(aRequest));
     SuccessOrExit(message->SetPayloadMarker());
 
-    status = (aRouter == nullptr) ? ThreadStatusTlv::kNoAddressAvailable : ThreadStatusTlv::kSuccess;
-    SuccessOrExit(Tlv::Append<ThreadStatusTlv>(*message, status));
+    SuccessOrExit(Tlv::Append<ThreadStatusTlv>(*message, aResponseStatus));
 
     if (aRouter != nullptr)
     {

src/core/thread/mle_router.hpp

@@ -575,6 +575,11 @@ private:
     static constexpr uint32_t kStateUpdatePeriod             = 1000; // State update period (in msec).
     static constexpr uint16_t kUnsolicitedDataResponseJitter = 500;  // Max delay for unsol Data Response (in msec).
 
+    // Threshold to accept a router upgrade request with reason
+    // `kBorderRouterRequst` (number of BRs acting as router in
+    // Network Data).
+    static constexpr uint8_t kRouterUpgradeBorderRouterRequestThreshold = 2;
+
     Error AppendConnectivity(Message &aMessage);
     Error AppendChildAddresses(Message &aMessage, Child &aChild);
     Error AppendRoute(Message &aMessage, Neighbor *aNeighbor = nullptr);
@@ -616,6 +621,7 @@ private:
     Error SendAddressSolicit(ThreadStatusTlv::Status aStatus);
     void  SendAddressRelease(void);
     void  SendAddressSolicitResponse(const Coap::Message &   aRequest,
+                                     ThreadStatusTlv::Status aResponseStatus,
                                      const Router *          aRouter,
                                      const Ip6::MessageInfo &aMessageInfo);
     void  SendAdvertisement(void);

src/core/thread/thread_tlvs.hpp

@@ -166,6 +166,8 @@ public:
         kTooFewRouters         = 2, ///< Address Solicit due to too few routers.
         kHaveChildIdRequest    = 3, ///< Address Solicit due to child ID request.
         kParentPartitionChange = 4, ///< Address Solicit due to parent partition change
+        kBorderRouterRequst    = 5, ///< Address Solicit from Border Router request.
+        kUnrecognizedStatus    = 6, ///< The requested status is unrecognized or not meaningful in a request.
     };
 
     /**