Related to this, this an interesting paper that clarifies how the Great Firewall discovers Tor nodes by doing active probing: https://nymity.ch/active-probing/i

Related to this, this an interesting paper that clarifies how the Great Firewall discovers Tor nodes by doing active probing: https://nymity.ch/active-probing/imc2015.pdf https://nymity.ch/active-probing/imc2015.pdf. No deep packet inspection is necessary when you pretend to be a Tor user and can identify the typical response for a Tor node.