feat: Disable PMTUD if using DA

Author: simon-lemay-unity

com.unity.netcode.gameobjects/Runtime/Transports/UTP/UnityTransport.cs

@@ -519,6 +519,14 @@ public NetworkSettings GetDefaultNetworkSettings()
                 sendQueueCapacity: m_MaxPacketQueueSize,
                 receiveQueueCapacity: m_MaxPacketQueueSize,
                 heartbeatTimeoutMS: m_HeartbeatTimeoutMS);
+            
+            // Disable PMTU discovery if using DA, leave it at its default otherwise.
+            if (OnCurrentTopology() == NetworkTopologyTypes.DistributedAuthority)
+            {
+                var config = settings.GetNetworkConfigParameters();
+                config.performPathMtuDiscovery = false;
+                settings.AddRawParameterStruct(ref config);
+            }
 
             // Configure Relay if in use.
             if (m_ProtocolType == ProtocolType.RelayUnityTransport)