The "C" bit was set to 1 if the Teredo client is located behind a cone NAT, 0 otherwise, but RFC 5991 changed it to always be 0 to avoid revealing this fact to strangers.
“C”位设为1，如果Teredo客户端位于一个锥形NAT后面，否则为0；但RFC 5991将它改为始终为0以避免向陌生人暴露此情况。