Tunneling
Protocols
Most VPNs use the concept of tunneling to create a private
network that extends across the Internet. Conceptually, it’s as if a secure
tunnel has been built between two end devices (routers, firewall, or VPN
device). Data can be directed into one end of the tunnel and it travels securely
to the other end. These end devices, or tunnel interfaces, are typically the
perimeter router firewalls for the LANs being connected.
Technically, no tunnel exists and the process doesn’t resemble a
tunnel, but the term “tunneling” somewhat describes the end result of traffic
being able to pass through a non-secure environment without concerns about
eavesdropping, data hijacking, or data manipulation. Tunneling
is a process of encapsulating an entire data packet as the payload within a
second packet, which is understood by the network and both end points. Depending
on the protocols used, the new payload—the original packet—can be encrypted. Figure 9-5
is a common graphical representation of Layer 3 tunneling technology.
The tunneling process requires three different protocols:
-
Carrier protocol The network protocol used
to transport the final encapsulation
-
Encapsulating protocol The protocol used
to provide the new packet around the original data packet. Examples: IPSec, GRE,
L2F, L2TP, PPTP
-
Passenger protocol The original data
packet that’s been encapsulated. Examples: IP, IPX, NetBEUI
Through tunneling techniques, you can pass non-IP packets or
private IP addressed packets through a public IP network. You can even route
NetBEUI—the famous non- routable protocol—once it’s been encapsulated for
tunneling through a VPN. What happens is the new data frame, or packet, is, in
fact, a legal packet with proper addressing to travel through the network.
Hidden safely within the payload portion of this new frame is the original
packet, which needs the assistance and/or protection.
L2F, L2TP, and PPTP are all three Layer 2 tunneling protocols that
support Access VPN solutions by tunneling PPP.
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