Which IKE Version Should I Select When I Create a VPN Connection?

IKEv2 is recommended because IKEv1 is not secure. In addition, IKEv2 outperforms IKEv1 in connection negotiation and establishment, authentication methods, dead peer detection (DPD) timeout processing, and security association (SA) timeout processing.

IKEv2 will be widely used, and IKEv1 will gradually phase out.

Introduction to IKEv1 and IKEv2

As a hybrid protocol, IKEv1 brings some security and performance defects due to its complexity. As such, it has become a bottleneck in the IPsec system.
IKEv2 addresses the issues of IKEv1 while retaining basic functions of IKEv1. IKEv2 is more simplified, efficient, secure, and robust than IKEv1. Additionally, IKEv2 is defined by RFC 4306 in a single document, whereas IKEv1 are defined in multiple documents. By minimizing core functions and default password algorithms, IKEv2 greatly improves interoperability between different IPsec VPNs.

Security Risks of IKEv1

The cryptographic algorithms supported by IKEv1 have not been updated for more than 10 years. In addition, IKEv1 does not support strong cryptographic algorithms such as AES-GCM and ChaCha20-Poly1305. For IKEv1, the E (Encryption) bit in the ISALMP header specifies that the payloads following the ISALMP header are encrypted, but any data integrity verification of those payloads is handled by a separate hash payload. This separation of encryption from data integrity protection prevents the use of authenticated encryption (AES-GCM) with IKEv1.
IKEv1 is vulnerable to DoS amplification attacks and half-open connection attacks. After responding to spoofed packets, the responder maintains initiator-responder relationships, consuming a large number of system resources.
This defect is inherent to IKEv1 and is addressed in IKEv2.
The aggressive mode of IKEv1 is not secure. In this mode, information packets are not encrypted, posing risks of information leakage. There are also brute-force attacks targeting at the aggressive mode, such as man-in-the-middle attacks.

Differences Between IKEv1 and IKEv2

Negotiation process
- IKEv1 is complex and consumes a large amount of bandwidth. IKEv1 SA negotiation consists of two phases. In IKEv1 phase 1, an IKE SA is established in either main mode or aggressive mode. Main mode requires three exchanges between peers totaling six ISAKMP messages, whereas aggressive mode requires two exchanges totaling three ISAKMP messages. Aggressive mode is faster, but does not provide identity protection for peers as key exchange and identity authentication are performed simultaneously. In IKEv1 phase 2, IPsec SAs are established through three ISAKMP messages in quick mode.
- Compared with IKEv1, IKEv2 simplifies the SA negotiation process. IKEv2 requires only two exchanges, totaling four messages, to establish an IKE SA and a pair of IPsec SAs. To create multiple pairs of IPsec SAs, only one additional exchange is needed for each additional pair of SAs.
  
  For IKEv1 negotiation, its main mode involves nine (6+3) messages, and its aggressive mode involves six (3+3) messages. In contrast, IKEv2 negotiation requires only four (2+2) messages.
Authentication methods
- Only IKEv1 (requiring an encryption card) supports digital envelope authentication (HSS-DE).
- IKEv2 supports Extensible Authentication Protocol (EAP) authentication. IKEv2 can use an AAA server to remotely authenticate mobile and PC users and assign private IP addresses to these users. IKEv1 does not provide this function and must use L2TP to assign private IP addresses.
- Only IKEv2 supports IKE SA integrity algorithms.
DPD timeout processing
- Only IKEv1 supports the retry-interval parameter. If a device sends a DPD packet but receives no reply within the specified retry-interval, the device records a DPD failure event. When the number of DPD failure events reaches 5, both the IKE SA and IPsec SAs are deleted. IKE SA negotiation will start again only when there is traffic to be transmitted over the IPsec tunnel.
- In IKEv2, the retransmission interval increases from 1, 2, 4, 8, 16, 32 to 64, in seconds. If no reply is received within eight consecutive transmissions, the peer end is considered dead, and the IKE SA and IPsec SAs are deleted.
IKE SA timeout processing and IPsec SA timeout processing
In IKEv2, the IKE SA soft lifetime is 9/10 of the IKE SA hard lifetime plus or minus a random number. This reduces the likelihood that two ends initiate renegotiation simultaneously. Therefore, you do not manually set the soft lifetime in IKEv2.

Advantages of IKEv2 over IKEv1

Simplifies the SA negotiation process, improving efficiency.
Fixes many cryptographic security vulnerabilities, improving security.
Supports EAP authentication, improving authentication flexibility and scalability.
EAP is an authentication protocol that supports multiple authentication methods. The biggest advantage of EAP is its scalability. That is, new authentication methods can be added without changing the original authentication system. EAP authentication has been widely used in dial-up access networks.
Employs an Encrypted Payload on basis of ESP. This payload contains both an encryption algorithm and a data integrity algorithm. AES-GCM ensures confidentiality, integrity, and authentication, and works well with IKEv2.

Parent topic: Operations on the Console