[mle] derive address registration limit from runtime pool sizes

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
default of 4, discarding any addresses beyond that limit even when the
runtime pools are larger.

Author: suveshpratapa

src/core/config/ip6.h

@@ -58,7 +58,9 @@
  * 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- * `OPENTHREAD_CONFIG_IP6_MAX_EXT_MCAST_ADDRS` are no longer applicable or used.
+ * `OPENTHREAD_CONFIG_IP6_MAX_EXT_MCAST_ADDRS` are no longer applicable or used.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()`.
  */
 #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 +86,9 @@
  * @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()`.
  */
 #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 +3640,17 @@ 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 registration
+    // limit from the actual pool sizes rather than the compile-time config constant,
+    // since `OPENTHREAD_CONFIG_MLE_IP_ADDRS_TO_REGISTER` is not meaningful when pool
+    // sizes are not known at compile time.
+    const uint8_t maxAddrs = ClampToUint8(Get<ThreadNetif>().GetExtUnicastAddressPoolCapacity() +
+                                          Get<ThreadNetif>().GetExtMulticastAddressPoolCapacity());
+#else
+    const uint8_t maxAddrs = kMaxIpAddressesToRegister;
+#endif
+
     SuccessOrExit(error = Tlv::StartTlv(*this, Tlv::kAddressRegistration, tlvBookmark));
 
     // Prioritize ML-EID
@@ -3680,7 +3691,7 @@ 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 +3721,7 @@ 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);
         }
     }