| 6.3 |  | IPSec |
| 6.3.6 |  | Five steps of IPSec |

| The goal of IPSec is to protect the desired data with the necessary security and algorithms . The operation of IPSec can be broken down into five primary steps:

1. Interesting traffic initiates the IPSec process. Traffic is deemed interesting when a packet triggers an access list that defines traffic to be protected.
2. During IKE Phase One, IKE authenticates IPSec peers and negotiates IKE SAs, setting up a secure communications channel for negotiating IPSec SAs in phase two.
3. During IKE Phase Two, IKE negotiates IPSec SA parameters and sets up matching IPSec SAs in the peers. These security parameters are used to protect data and messages exchanged between endpoints.
4. During the data transfer phase, data is transferred between IPSec peers based on the IPSec parameters and keys stored in the SA database.
5. During IPSec tunnel termination, IPSec SAs terminate through deletion or by timing out.

The events within an IKE session happen in following order:

In IKE Phase One (main or aggressive mode) the peers will:

- Negotiate an IKE protection suite
- Authenticate each other
- Exchange keying material to protect the IKE session
- Establish the IKE SA

Then in IKE Phase Two (quick mode) peers:

- Negotiate IPsec policies
- Exchange keying material of IPsec SAs
- Establish IPsec SAs

IKE Phase One runs in main or aggressive mode. The mode used is implementation and situation dependent. The purpose of IKE Phase One is the negotiation of an IKE protection suite, the authentication of peers, the exchange of keying material to protect the IKE session, and finally the establishment of an IKE SA, which defines the parameters of the secure IKE channel.

The IKE main mode is the first mode that negotiates protection suites between peers. ISAKMP uses six messages to establish the IKE SA. These messages include SA negotiation, a Diffie-Hellman key exchange, and the authentication of peers. IKE main mode hides the identity of IKE peers from eavesdroppers, and can use the protocol’s negotiation capabilities to the fullest.

Like the IKE main mode, the IKE aggressive mode negotiates protection suites between peers. The major difference between the main and the aggressive mode is that the aggressive mode takes half the number of messages as the main mode and consequently offers less negotiating flexibility for the IKE session protection. The initiating peer proposes a list of policies, and the responder accepts a policy or rejects the offers with no further negotiation of protection details. The aggressive mode does protection suite negotiation, authentication of peers, and generates keying material as the main mode does, but because of limited capabilities it does not provide peer identity protection (For example, an eavesdropper can determine the identity of negotiating peers).
Because only three messages are needed to establish IKE SA, an IKE aggressive mode exchange is also much faster than an IKE main mode exchange. It is used mainly when security policies are well known on both peers, and there is no need to use the full IKE negotiation capabilities to establish an IKE SA as quickly as possible.

IKE Phase Two is used to negotiate and establish SAs of other protocols (IPsec’s AH and ESP, IP PCP (payload compression protocol), etc). Phase Two needs an established IKE SA (produced in IKE Phase One to protect the IKE session) to operate, and only operates in one defined mode, the quick mode. The IKE initiator presents a list of (IPsec) policy proposals and the IKE responder chooses an acceptable proposal according to its locally defined policy. When the policy between peers is agreed upon, the keying material is agreed upon, and IPsec SAs are established.

IKE quick mode is quite fast, with almost no noticeable delay associated with it and if no Perfect Forward Secrecy (PFS)* functionality is used with IPsec. Once an IKE SA is in place (established via Main Mode or Aggressive Mode), only quick mode exchanges are used to negotiate additional IPsec SAs or to rekey established IPsec SAs, which are about to expire.

| Media Activity Demonstration Activity: Five Steps of IPSec In this activity, drag and drop the five steps of IPSec into the appropriate order.