Q: What TCP/IP Ports I need open in order to allow the L2TP packet going through?

A: Here are the ports involved in L2TP connection:

UDP 1701 - Used for L2TP protocol

UDP 500 & 4500   - Used for security association to negotiate the security method, mostly it is a password;

AH  - Authenticated Headers, Protocol ID 50 - it's a type of IP Packet, not a port;

ESP - Encapsulated Secure Payload, Protocol ID 51 - it's a type of IP packet, not a port;

Q: What is the encryption strength and authentication method used in Free L2TP VPN service?

For Encryption, it is: IPSEC AES 128
For Authentication, it is: MS CHAP V2