Tunnel Mode and Transport mode - IPSec through Firewall VPN Tutorial
Tunnel mode and Transport mode
When using ESP you can specify one of two modes, in which ESP operates in. Tunnel mode encrypts the whole packet. Tunnel mode is used for site to site VPN, when securing communication between security gateways, concentrators, firewalls, etc. Tunnel mode provides security for the entire original IP packet, that is the headers and the payload.
The other mode ESP can operate in is Transport mode, which is not as secure as it only encrypts the data portion and not the whole packet unlike tunel tunnel mode.
Transport mode encrypts the data portion of the packet. It works between two different workstations running some kind of VPN software. Transport mode protects payload of packet and the high layer protocols. Transport mode leaves the original IP addresses in open clear text. Using transport mode the final destination is not a gateway or router, generally the host itself. Transport mode provides security to the higher layer protocols only.
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