Specifications

138 Chapter 5. Configuring Software Options
Additional IKE Settings
In addition to the peer identification and shared secret described earlier, IKE requires that the router be configured
with the following information:
¥ Session authentication
¥ Phase 1 IKE message authentication
¥ Phase 1 IKE message encryption
¥ One of the following for each IKE proposal:
ÑIPSec AH packet authentication
ÑIPSec ESP data authentication
ÑIPSec ESP data encryption
ÑIPSec ESP data authentication and data encryption
¥ Diffie-Hellman key generation group
¥ IPSec policy (filter) setup
¥ Policy and peer associations
¥ Policy and proposal associations
Security Associations (SAs)
A Security Association (SA) is an instance of security policy and keying material applied to a data flow. Both IKE
and IPSec use SAs. An IPSec SA is unidirectional, applying to only one direction of data flow. An IKE SA is bi-
directional, and thus, only one IKE SA is needed for a secure connection.
After an IKE SA is established, any number of IPSec SAs may be created. Although IPSec SAs can be configured
manually, most networks rely on IKE to set them up. IKE negotiates and establishes SAs on behalf of IPSec. SAs
are negotiated between the two endpoints of the tunnel and contain information on sequence numbering for anti-
replay.
IPSec SAs are unidirectional so a set of SAs is needed for a secure connection. For each security protocol used,
one SA is needed for each direction (inbound and outbound). Usually, only one protocol (ESP or AH) is used so
the connection would use two SAs (one inbound and one outbound). However, it is possible for a connection to
use four SAs if it uses two proposals and policies, one requiring the ESP protocol and the other requiring the AH
protocol.
IKE negotiates SAs in the following sequence:
Phase 1 IKE:
The session initiator creates a cookie and sends it to the responder, with a zero placeholder in the responder
cookie area. The responder then creates a cookie and fills in the zeros. All packets will contain these two
cookies until the Phase 1 SA expires. IKE Peer commands next establish the identity of local and remote
peers. Then IKE Proposal commands specify how packets will be encrypted and/or authenticated for the
initial exchange.
Phase 2 IKE
IKE IPSec Proposal commands specify how packets will be encrypted/authenticated for the final SA. Then
IKE IPSec Policy commands specify which packets will be encrypted/authenticated for the final SA.