Page 891 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

protocol). The envelope is delivered through the postal service (the
untrusted intermediary network) to its intended recipient. You can use

tunneling in many situations, such as when you’re bypassing firewalls,
gateways, proxies, or other traffic control devices. The bypass is
achieved by encapsulating the restricted content inside packets that
are authorized for transmission. The tunneling process prevents the
traffic control devices from blocking or dropping the communication
because such devices don’t know what the packets actually contain.

Tunneling is often used to enable communications between otherwise
disconnected systems. If two systems are separated by a lack of
network connectivity, a communication link can be established by a
modem dial-up link or other remote access or wide area network
(WAN) networking service. The actual LAN traffic is encapsulated in
whatever communication protocol is used by the temporary
connection, such as Point-to-Point Protocol in the case of modem dial-
up. If two networks are connected by a network employing a different

protocol, the protocol of the separated networks can often be
encapsulated within the intermediary network’s protocol to provide a
communication pathway.

Regardless of the actual situation, tunneling protects the contents of
the inner protocol and traffic packets by encasing, or wrapping, it in an
authorized protocol used by the intermediary network or connection.

Tunneling can be used if the primary protocol is not routable and to
keep the total number of protocols supported on the network to a
minimum.

The Proliferation of Tunneling

Tunneling is such a common activity within communication
systems that many of us use tunneling on a regular basis without
even recognizing it. For example, every time you access a website
using a secured SSL or TLS connection, you are using tunneling.
Your plaintext web communications are being tunneled within an
SSL or TLS session. Also, if you use internet telephone or VoIP