Ipsec Protocols (Esp/Ah); The Ah Protocol - D-Link NetDefend DFL-210 User Manual

9.3.4. IPsec Protocols (ESP/AH)

One thing that has to be considered when using Pre-Shared Keys is key distribution. How are the
Pre-Shared Keys distributed to remote VPN clients and firewalls? This is a major issue, since the
security of a PSK system is based on the PSKs being secret. Should one PSK be compromised, the
configuration will need to be changed to use a new PSK.
Certificates
Each VPN firewall has its own certificate, and one or more trusted root certificates.
The authentication is based on several things:
• That each endpoint has the private key corresponding to the public key found in its certificate,
and that nobody else has access to the private key.
• That the certificate has been signed by someone that the remote endpoint trusts.
Advantages of Certificates
A principal advantage of certificates is added flexibility. Many VPN clients, for instance, can be
managed without having the same pre-shared key configured on all of them, which is often the case
when using pre-shared keys and roaming clients. Instead, should a client be compromised, the
client's certificate can simply be revoked. No need to reconfigure every client.
Disadvantages of Certificates
The principal disadvantage of certificates is the added complexity. Certificate-based authentication
may be used as part of a larger public key infrastructure, making all VPN clients and firewalls
dependent on third parties. In other words, there are more aspects that have to be configured, and
there is more that can go wrong.
9.3.4. IPsec Protocols (ESP/AH)
The IPsec protocols are the protocols used to protect the actual traffic being passed through the
VPN. The actual protocols used and the keys used with those protocols are negotiated by IKE.
There are two protocols associated with IPsec, AH and ESP. These are covered in the sections
below.
AH (Authentication Header)
AH is a protocol used for authenticating a data stream.
Figure 9.1. The AH protocol
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Chapter 9. VPN