ZyXEL Communications Prestige 792H, User Manual

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Prestige 792H User’s Guide
13-6 Introduction to IPSec
computing its own hash value, and complain that the hash value appended to the received packet doesn't
match. The VPN device at the receiving end doesn't know about the NAT in the middle, so it assumes that
the data has been maliciously altered.
IPSec using
ESP in Tunnel mode encapsulates the entire original packet (including headers) in a new IP
packet. The new IP packet's source address is the outbound address of the sending VPN gateway, and its
destination address is the inbound address of the VPN device at the receiving end. When using
ESP protocol
with authentication, the packet contents (in this case, the entire original packet) are encrypted. The encrypted
contents, but not the new headers, are signed with a hash value appended to the packet.
Tunnel mode ESP with authentication is compatible with NAT because integrity checks are performed over
the combination of the "original header plus original payload," which is unchanged by a NAT device.
Transport mode ESP with authentication is not compatible with NAT.
Table 13-1 VPN and NAT
SECURITY PROTOCOL MODE NAT
AH Transport N
AH Tunnel N
ESP Transport N
ESP Tunnel Y