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Communication Security: Remote Access and Messaging • Chapter 3 131
sniff the network and gather information about end parties.Transport
mode is used in host-to-host VPNs.
■ Tunnel Mode Unlike transport mode where only the data is encrypted,
in tunnel mode (Figure 3.9) both the data and the IP headers are
encrypted.The advantage is that neither the payload nor any information
about end parties can be sniffed.The disadvantage is speed, since the size
of the encrypted packet increases.Tunnel mode is used in host-to-gateway
or gateway-to-gateway VPNs.
Figure 3.11 Using IPSec in Transport Mode Only Encrypts the Data Payload
IP Packet
IP Header Payload
IP Packet with IPSec
in transport mode
IP Header Payload
Figure 3.12 Using IPSec in Tunnel Mode Encrypts Both the Data and IP
Headers
IP Packet
IP Header Payload
IP Packet with IPSec
in tunnel mode
IP Header Payload
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