TCP/IP Tutorial and Technical Overview - IBM Redbooks

Data
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Figure 22-55 L2TP packet flow through any IP cloud
22.14.3 L2TP security issues
IP UDP L2TP PPP Data
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IP Cloud
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Although L2TP provides cost-effective access, multiprotocol transport, and
remote LAN access, it does not provide cryptographically robust security
features. For example:
► Authentication is provided only for the identity of tunnel endpoints, but not for
each individual packet that flows inside the tunnel. This can expose the tunnel
to man-in-the-middle and spoofing attacks.
► Without per-packet integrity, it is possible to mount denial-of-service attacks
by generating bogus control messages that can terminate either the L2TP
tunnel or the underlying PPP connection.
► L2TP itself provides no facility to encrypt user data traffic. This can lead to
embarrassing exposures when data confidentiality is an issue.
► While the payload of the PPP packets can be encrypted, the PPP protocol
suite does not provide mechanisms for automatic key generation or for
automatic key refresh. This can lead to someone listening in on the wire to
finally break that key and gain access to the data being transmitted.
Realizing these shortcomings, the PPP Extensions Working Group of the IETF
considered how to remedy these shortfalls. Rather than duplicate work done
elsewhere, it was decided to recommend using IPSec within L2TP. This is
described in RFC 2888.
In summary, Layer 2 Tunnel Protocols are an excellent way of providing
cost-effective remote access. And when used in conjunction with IPSec, they are
an excellent technique for providing secure remote access. However, without
L2
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Chapter 22. TCP/IP security 879