Is vanilla possible with this mod load, side tests.

fml/src/main/java/cpw/mods/fml/common/network/NetworkRegistry.java

@@ -285,8 +285,18 @@ public enum NetworkRegistry
     {
         NetworkModHolder networkModHolder = new NetworkModHolder(fmlModContainer, clazz, remoteVersionRange, asmHarvestedData);
         registry.put(fmlModContainer, networkModHolder);
+        networkModHolder.testVanillaAcceptance();
     }
 
+    public boolean isVanillaAccepted(Side from)
+    {
+        boolean result = true;
+        for (Entry<ModContainer, NetworkModHolder> e : registry.entrySet())
+        {
+            result &= e.getValue().acceptsVanilla(from);
+        }
+        return result;
+    }
     public Map<ModContainer,NetworkModHolder> registry()
     {
         return ImmutableMap.copyOf(registry);

fml/src/main/java/cpw/mods/fml/common/network/internal/NetworkModHolder.java

@@ -19,6 +19,7 @@ import org.apache.logging.log4j.Level;
 
 import com.google.common.base.Preconditions;
 import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.SetMultimap;
 
@@ -94,6 +95,9 @@ public class NetworkModHolder
 
     private NetworkChecker checker;
 
+    private boolean acceptsVanillaClient;
+    private boolean acceptsVanillaServer;
+
     public NetworkModHolder(ModContainer container)
     {
         this.container = container;
@@ -230,4 +234,12 @@ public class NetworkModHolder
         this.networkId = value;
     }
 
+    public void testVanillaAcceptance() {
+        acceptsVanillaClient = check(ImmutableMap.<String,String>of(), Side.CLIENT);
+        acceptsVanillaServer = check(ImmutableMap.<String,String>of(), Side.SERVER);
+    }
+    public boolean acceptsVanilla(Side from) {
+        return from == Side.CLIENT ? acceptsVanillaClient : acceptsVanillaServer;
+    }
+
 }