http://ssd.iykpqm7jiradoeezzkhj7c4b33g4hbgfwelht2evxxeicbpjy44c7ead.onion/zh-hans/module/understanding-and-circumventing-network-censorship
If the firewall can correctly recognize what kind of communication is happening or what technology is being used, it can be configured not to pass that communication along. For example, some networks might block the technologies used by certain VoIP (internet phone call), peer-to-peer file sharing software, or VPN applications. In this diagram, the router recognizes a computer attempting to connect to an HTTPS site, which uses Port 443.