[mle] derive address registration limit for children from runtime pool

When `OPENTHREAD_CONFIG_IP6_INIT_EXT_ADDR_POOL_ENABLE` is set, the
external unicast and multicast address pool sizes are determined at
runtime by the caller of `otIp6Init()`, not at compile time. The
existing compile-time constant `kMaxIpAddressesToRegister`
(from `OPENTHREAD_CONFIG_MLE_IP_ADDRS_TO_REGISTER`) is therefore
not meaningful in this mode: a library compiled without knowledge of
the pool sizes will silently cap MLE Address Registration TLVs at the
compile-time limit, discarding any addresses beyond that limit even
when the runtime pools are larger.

Author: suveshpratapa

src/core/config/ip6.h

@@ -58,7 +58,10 @@
  * new OpenThread stack configuration to be built.
  *
  * When this feature is enabled, the configs `OPENTHREAD_CONFIG_IP6_MAX_EXT_UCAST_ADDRS` and
- * `OPENTHREAD_CONFIG_IP6_MAX_EXT_MCAST_ADDRS` are no longer applicable or used.
+ * `OPENTHREAD_CONFIG_IP6_MAX_EXT_MCAST_ADDRS` are no longer applicable or used. Similarly,
+ * `OPENTHREAD_CONFIG_MLE_IP_ADDRS_TO_REGISTER` is not used; the MLE address registration limit
++ * is instead derived at runtime from the pool sizes passed to `otIp6Init()` plus internal
++ * addresses (ML-EID, and optionally DUA and SLAAC).
  */
 #ifndef OPENTHREAD_CONFIG_IP6_INIT_EXT_ADDR_POOL_ENABLE
 #define OPENTHREAD_CONFIG_IP6_INIT_EXT_ADDR_POOL_ENABLE 0

src/core/config/mle.h

@@ -86,6 +86,10 @@
  * @def OPENTHREAD_CONFIG_MLE_IP_ADDRS_TO_REGISTER
  *
  * The maximum number of IPv6 address registrations for MTD.
+ *
+ * If `OPENTHREAD_CONFIG_IP6_INIT_EXT_ADDR_POOL_ENABLE` is enabled, this config is not used;
+ * the registration limit is derived at runtime from the pool sizes passed to `otIp6Init()`
+ * plus internal address slots (ML-EID, and optionally DUA and SLAAC).
  */
 #ifndef OPENTHREAD_CONFIG_MLE_IP_ADDRS_TO_REGISTER
 #define OPENTHREAD_CONFIG_MLE_IP_ADDRS_TO_REGISTER (OPENTHREAD_CONFIG_MLE_IP_ADDRS_PER_CHILD)

src/core/net/netif.hpp

@@ -355,6 +355,20 @@ class Netif : public InstanceLocator, private NonCopyable
      */
     bool IsInitialized(void) const { return mInitialized; }
 
+    /**
+     * Returns the capacity of the external unicast address pool.
+     *
+     * @returns The number of external unicast address entries in the pool passed to `Init()`.
+     */
+    uint16_t GetExtUnicastAddressPoolCapacity(void) const { return mExtUnicastAddressPool.GetSize(); }
+
+    /**
+     * Returns the capacity of the external multicast address pool.
+     *
+     * @returns The number of external multicast address entries in the pool passed to `Init()`.
+     */
+    uint16_t GetExtMulticastAddressPoolCapacity(void) const { return mExtMulticastAddressPool.GetSize(); }
+
 #endif // OPENTHREAD_CONFIG_IP6_INIT_EXT_ADDR_POOL_ENABLE
 
     /**

src/core/thread/mle.cpp

@@ -3640,6 +3640,23 @@ Error Mle::TxMessage::AppendAddressRegistrationTlv(AddressRegistrationMode aMode
     Tlv::Bookmark tlvBookmark;
     uint8_t       counter = 0;
 
+#if OPENTHREAD_CONFIG_IP6_INIT_EXT_ADDR_POOL_ENABLE
+    // When using runtime-configured external address pools, derive the total registration
+    // budget from the pool sizes plus compile-time internal address slots (ML-EID, DUA,
+    // SLAAC), since `OPENTHREAD_CONFIG_MLE_IP_ADDRS_TO_REGISTER` is not meaningful here.
+    const uint8_t maxAddrs = ClampToUint8(Get<ThreadNetif>().GetExtUnicastAddressPoolCapacity() +
+                                          Get<ThreadNetif>().GetExtMulticastAddressPoolCapacity() +
+#if OPENTHREAD_CONFIG_IP6_SLAAC_ENABLE
+                                          OPENTHREAD_CONFIG_IP6_SLAAC_NUM_ADDRESSES +
+#endif
+#if OPENTHREAD_CONFIG_DUA_ENABLE
+                                          1 + // Domain Unicast Address
+#endif
+                                          1); // ML-EID
+#else
+    const uint8_t maxAddrs = kMaxIpAddressesToRegister;
+#endif
+
     SuccessOrExit(error = Tlv::StartTlv(*this, Tlv::kAddressRegistration, tlvBookmark));
 
     // Prioritize ML-EID
@@ -3680,7 +3697,7 @@ Error Mle::TxMessage::AppendAddressRegistrationTlv(AddressRegistrationMode aMode
         SuccessOrExit(error = AppendAddressRegistrationEntry(addr.GetAddress()));
         counter++;
         // only continue to append if there is available entry.
-        VerifyOrExit(counter < kMaxIpAddressesToRegister);
+        VerifyOrExit(counter < maxAddrs);
     }
 
     // Append external multicast addresses.  For sleepy end device,
@@ -3710,7 +3727,7 @@ Error Mle::TxMessage::AppendAddressRegistrationTlv(AddressRegistrationMode aMode
             SuccessOrExit(error = AppendAddressRegistrationEntry(addr.GetAddress()));
             counter++;
             // only continue to append if there is available entry.
-            VerifyOrExit(counter < kMaxIpAddressesToRegister);
+            VerifyOrExit(counter < maxAddrs);
         }
     }