From 5896eefc2119b7e4c01855b82f5baa41f8d05071 Mon Sep 17 00:00:00 2001
From: Yi <xuyirio@users.noreply.github.com>
Date: Thu, 27 Jan 2022 13:27:08 +0800
Subject: [PATCH] [nat64] withdraw greater NAT64 prefix in network data (#7337)

This commit enables BR to withdraw its local NAT64 prefix if a smaller
one is found in network data.

It might happen when two BRs join the Thread network at merely the
same time and thus both advertise its local NAT64 prefix. The BR with
numerically greater prefix will withdraw its NAT64 prefix.
---
 src/core/border_router/routing_manager.cpp | 32 ++++++++++++++++++----
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/src/core/border_router/routing_manager.cpp b/src/core/border_router/routing_manager.cpp
index 0ea712cca..f861681e9 100644
--- a/src/core/border_router/routing_manager.cpp
+++ b/src/core/border_router/routing_manager.cpp
@@ -641,26 +641,46 @@ void RoutingManager::EvaluateNat64Prefix(void)
 
     NetworkData::Iterator            iterator = NetworkData::kIteratorInit;
     NetworkData::ExternalRouteConfig config;
-    bool                             hasAdvertisedNat64Prefix = false;
+    Ip6::Prefix                      smallestNat64Prefix;
 
     otLogInfoBr("Evaluating NAT64 prefix");
 
+    smallestNat64Prefix.Clear();
     while (Get<NetworkData::Leader>().GetNextExternalRoute(iterator, config) == kErrorNone)
     {
-        if (config.mNat64 && config.GetPrefix().IsValidNat64())
+        const Ip6::Prefix &prefix = config.GetPrefix();
+
+        if (config.mNat64 && prefix.IsValidNat64())
         {
-            hasAdvertisedNat64Prefix = true;
+            if (smallestNat64Prefix.GetLength() == 0 || prefix < smallestNat64Prefix)
+            {
+                smallestNat64Prefix = prefix;
+            }
         }
     }
 
-    if (!hasAdvertisedNat64Prefix && !mIsAdvertisingLocalNat64Prefix)
+    if (smallestNat64Prefix.GetLength() == 0 || smallestNat64Prefix == mLocalNat64Prefix)
     {
-        // TODO: when multiple prefixes exits, remove the ones with lower preference or numerically larger.
-        if (AddExternalRoute(mLocalNat64Prefix, NetworkData::kRoutePreferenceLow, /* aNat64= */ true) == kErrorNone)
+        otLogInfoBr("No NAT64 prefix in Network Data is smaller than the local NAT64 prefix %s",
+                    mLocalNat64Prefix.ToString().AsCString());
+
+        // Advertise local NAT64 prefix.
+        if (!mIsAdvertisingLocalNat64Prefix &&
+            AddExternalRoute(mLocalNat64Prefix, NetworkData::kRoutePreferenceLow, /* aNat64= */ true) == kErrorNone)
         {
             mIsAdvertisingLocalNat64Prefix = true;
         }
     }
+    else if (mIsAdvertisingLocalNat64Prefix && smallestNat64Prefix < mLocalNat64Prefix)
+    {
+        // Withdraw local NAT64 prefix if it's not the smallest one in Network Data.
+        // TODO: remove the prefix with lower preference after discovering upstream NAT64 prefix is supported
+        otLogNoteBr("Withdrawing local NAT64 prefix since a smaller one %s exists.",
+                    smallestNat64Prefix.ToString().AsCString());
+
+        RemoveExternalRoute(mLocalNat64Prefix);
+        mIsAdvertisingLocalNat64Prefix = false;
+    }
 }
 #endif