IKE and IPsec packet processing

Internet Key Exchange (IKE) is the protocol used to set up SAs in IPsec negotiation. As described in Phase 1 parameters, you can optionally choose IKEv2 over IKEv1 if you configure a route-based IPsec VPN. IKEv2 simplifies the negotiation process, in that it provides no choice of Aggressive or Main mode in Phase 1. IKEv2 also uses less bandwidth.

The following sections identify how IKE versions 1 and 2 operate and differentiate.


Phase 1

A peer, identified in the IPsec policy configuration, begins the IKE negotiation process. This IKE Security Association (SA) agreement is known as Phase 1. The Phase 1 parameters identify the remote peer or clients and supports authentication through pre-shared key (PSK) or digital certificate. You can increase access security further using peer identifiers, certificate distinguished names, group names, or the FortiGate extended authentication (XAuth) option for authentication purposes. Basically, Phase 1 authenticates a remote peer and sets up a secure communication channel for establishing Phase 2, which negotiates the IPsec SA.

IKE Phase 1 can occur in either Main mode or Aggressive mode. For more information, see Phase 1 parameters.

IKE Phase 1 is successful only when the following are true:

  • Each peer negotiates a matching IKE SA policy.
  • Each peer is authenticated and their identities protected.
  • The Diffie-Hellman exchange is authenticated (the pre-shared secret keys match).

For more information on Phase 1, see Phase 1 parameters.

Phase 2

Phase 2 parameters define the algorithms that the FortiGate unit can use to encrypt and transfer data for the remainder of the session in an IPsec SA. The basic Phase 2 settings associate IPsec Phase 2 parameters with a Phase 1 configuration.

In Phase 2, the VPN peer or client and the FortiGate unit exchange keys again to establish a more secure communication channel. The Phase 2 Proposal parameters select the encryption and authentication algorithms needed to generate keys for protecting the implementation details of the SA. The keys are generated automatically using a Diffie-Hellman algorithm.

In Phase 2, Quick mode selectors determine which IP addresses can perform IKE negotiations to establish a tunnel. By only allowing authorized IP addresses access to the VPN tunnel, the network is more secure. For more information, see Phase 2 parameters.

IKE Phase 2 is successful only when the following are true:

  • The IPsec SA is established and protected by the IKE SA.
  • The IPsec SA is configured to renegotiate after set durations (see Phase 2 parameters and Phase 2 parameters).
  • Optional: Replay Detection is enabled. Replay attacks occur when an unauthorized party intercepts a series of IPsec packets and replays them back into the tunnel. See Phase 2 parameters.
  • Optional: Perfect Forward Secrecy (PFS) is enabled. PFS improves security by forcing a new Diffie-Hellman exchange whenever keylife expires. See Phase 2 parameters.

For more information on Phase 2, see Phase 2 parameters.

With Phase 2 established, the IPsec tunnel is fully negotiated and traffic between the peers is allowed until the SA terminates (for any number of reasons; time-out, interruption, disconnection, etc).

The entire IKEv1 process is demonstrated in the following diagram:


Phase 1

Unlike Phase 1 of IKEv1, IKEv2 does not provide options for Aggressive or Main mode. Furthermore, Phase 1 of IKEv2 begins immediately with an IKE SA initiation, consisting of only two packets (containing all the information typically contained in four packets for IKEv1), securing the channel such that all following transactions are encrypted (see Phase 1 parameters).

The encrypted transactions contain the IKE authentication, since remote peers have yet to be authenticated. This stage of IKE authentication in IKEv2 can loosely be called Phase 1.5.

Phase 1.5

As part of this phase, IKE authentication must occur. IKE authentication consists of the following:

  • The authentication payloads and Internet Security Association and Key Management Protocol (ISAKMP) identifier.
  • The authentication method (RSA, PSK, ECDSA, or EAP).
  • The IPsec SA parameters.

Due to the number of authentication methods potentially used, and SAs established, the overall IKEv2 negotiation can range from 4 packets (no EAP exchange at all) to many more.

At this point, both peers have a security association complete and ready to encrypt traffic.

Phase 2

In IKEv1, Phase 2 uses Quick mode to negotiate an IPsec SA between peers. In IKEv2, since the IPsec SA is already established, Phase 2 is essentially only used to negotiate “child” SAs, or to re-key an IPsec SA. That said, there are only two packets for each exchange of this type, similar to the exchange at the outset of Phase 1.5.

The entire IKEv2 process is demonstrated in the following diagram:

Support for IKEv2 session resumption

If a gateway loses connectivity to the network, clients can attempt to re-establish the lost session by presenting the ticket to the gateway (as described in RFC 5723). As a result, sessions can be resumed much faster, as DH exchange that is necessary to establish a brand new connection is skipped. This feature implements "ticket-by-value", whereby all information necessary to restore the state of a particular IKE SA is stored in the ticket and sent to the client.