Netgear FVS318v1 – Prosafe Wireless N VPN Firewall Reference Manual

Virtual Private Networking

D-5

SAs operate using modes. A mode is the method in which the IPSec protocol is applied to the 
packet. IPSec can be used in tunnel mode or transport mode. Typically, the tunnel mode is used for 
gateway-to-gateway IPSec tunnel protection, while transport mode is used for host-to-host IPSec 
tunnel protection. A gateway is a device that monitors and manages incoming and outgoing 
network traffic and routes the traffic accordingly. A host is a device that sends and receives 
network traffic.

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Transport Mode: The transport mode IPSec implementation encapsulates only the packet’s 
payload. The IP header is not changed. After the packet is processed with IPSec, the new IP 
packet contains the old IP header (with the source and destination IP addresses unchanged) 
and the processed packet payload. Transport mode does not shield the information in the IP 
header; therefore, an attacker can learn where the packet is coming from and where it is going 
to. The previous packet diagrams show a packet in transport mode.

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Tunnel Mode: The tunnel mode IPSec implementation encapsulates the entire IP packet. The 
entire packet becomes the payload of the packet that is processed with IPSec. A new IP header 
is created that contains the two IPSec gateway addresses. The gateways perform the 
encapsulation/decapsulation on behalf of the hosts. Tunnel mode ESP prevents an attacker 
from analyzing the data and deciphering it, as well as knowing who the packet is from and 
where it is going.

Note: AH and ESP can be used in both transport mode or tunnel mode.