Network firewalls keep outside threats away from sensitive data available inside the network. Whenever different networks are joined together, there is always a threat that someone from outside of your network will break into your LAN. Such break-ins may result in private data being stolen and distributed, valuable data being altered or destroyed, or entire
You have upgraded to RouterOs 6.39.x and now your P2P mangle rules are no longer working. This issue is caused by MikroTik removing the P2P option/field from the mangle rules. There explanation is that this method of matching P2P traffic is obsolete. The image above shows what the error looks like. To fix this