The Fives Steps of IPSec

IPSec, which is described in RFC 2401, is responsible for protecting your data with the necessary security protocols and algorithms. It is important to remember that IPSec is not a protocol; it is a framework of open standard protocol suites designed to give you CIA. If we look at the IPSec process, we can identify five high-level steps:

Step 1: Defining Interesting Traffic

Interesting traffic causes the IPSec process to start. Interesting traffic is usually defined by an access list called a crypto map. A permit means to encrypt, whereas a deny means to send it in cleartext. It does nothing to restrict the flow of traffic but only indicates what is encrypted or expected to be encrypted. Crypto maps are symmetrical, meaning that if you send the data encrypted, the other side needs to expect it to arrive that way, and vice versa.

Step 2: IKE Phase 1

IKE Phase 1 authenticates the IPSec peers, negotiates a matching IKE SA policy to protect the IKE exchange, performs an authenticated D-H exchange to produce matching shared secret keys, and then establishes a secure tunnel to negotiate Phase 2. In IKE Phase 1, IKE SAs are established. There are two modes in which Phase 1 is negotiated:

• Main mode The recommended mode for IKE. It is a touch slower than aggressive mode but more secure and reliable. It consists of six message exchanges, three in each direction.

• Aggressive mode Faster than main mode because it sends a total of three messages. The drawback is that information is exchanged before a secure channel is established.

IKE is used to dynamically build a secure channel for the IPSec parameters to be negotiated.

Step 3: IKE Phase 2

The purpose of IKE Phase 2 is to set up IPSec SAs. IKE negotiates IPSec SA parameters inside the secure channel built in Phase 1, establishes an SA through matching IPSec parameters, periodically renegotiates the SAs, and optionally performs additional D-H exchanges called Perfect Forward Secrecy (PFS) to refresh the keying material for greater security.

Step 4: IPSec Encrypted Tunnel

After Phase 2 occurs, you have a secure VPN tunnel set up to transmit your data. The VPN tunnel's security parameters were negotiated during IKE Phase 2.

Step 5: Tunnel Termination

After the interesting data is transmitted, the SAs end by being deleted or timing out. An SA has a lifetime measured either in seconds or in bytes: the amount of time an SA has been up or the total data that has been transferred. If you exceed your limits but you still need to transfer data, the keying material is refreshed dynamically and transparently, and the SA lifetimes start over.

The IKE tunnel protects the SA negotiation and the IPSec tunnel protects the data. Remember that each step is dependant upon the preceding step being completed and successful. This dependency actually makes it easier for troubleshooting because you can take a complicated process and break it down into smaller steps.

Cisco has been spearheading the IETF in a revision of IKE known as IKEv2. In fact, Cisco chairs the committee on IPSec and has been instrumental in producing a number of enhancements; one in particular is the ability to assign addresses remotely, which allows a gateway to download an IP address to the client as part of an IKE negotiation. IKEv2 was in final call (finished specification) in June 2003.