Virtual Private Networks

Background
Virtual private networks (VPNs) are a fairly quixotic subject; there is no single defining product, nor
even much of a consensus among VPN vendors as to what comprises a VPN. Consequently, everyone
knows what a VPN is, but establishing a single definition can be remarkably difficult. Some definitions
are sufficiently broad as to enable one to claim that Frame Relay qualifies as a VPN when, in fact, it is
an overlay network. Although an overlay network secures transmissions through a public network, it
does so passively via logical separation of the data streams.
VPNs provide a more active form of security by either encrypting or encapsulating data for transmission
through an unsecured network. These two types of security—encryption and encapsulation—form the
foundation of virtual private networking. However, both encryption and encapsulation are generic terms
that describe a function that can be performed by a myriad of specific technologies. To add to the
confusion, these two sets of technologies can be combined in different implementation topologies. Thus,
VPNs can vary widely from vendor to vendor.
This chapter provides an overview of building VPNs using the Layer 2 Tunneling Protocol (L2TP), and
it explores the possible implementation topologies.

Layer 2 Tunneling Protocol
The Internet Engineering Task Force (IETF) was faced with competing proposals from Microsoft and
Cisco Systems for a protocol specification that would secure the transmission of IP datagrams through
uncontrolled and untrusted network domains. Microsoft’s proposal was an attempt to standardize the
Point-to-Point Tunneling Protocol (PPTP), which it had championed. Cisco, too, had a protocol designed
to perform a similar function. The IETF combined the best elements of each proposal and specified the
open standard L2TP.
The simplest description of L2TP’s functionality is that it carries the Point-to-Point Protocol (PPP)
through networks that aren’t point-to-point. PPP has become the most popular communications protocol
for remote access using circuit-switched transmission facilities such as POTS lines or ISDN to create a
temporary point-to-point connection between the calling device and its destination.
L2TP simulates a point-to-point connection by encapsulating PPP datagrams for transportation through
routed networks or internetworks. Upon arrival at their intended destination, the encapsulation is
removed, and the PPP datagrams are restored to their original format. Thus, a point-to-point
communications session can be supported through disparate networks. This technique is known as
tunneling.

Operational Mechanics
In a traditional remote access scenario, a remote user (or client) accesses a network by directly
connecting a network access server (NAS). Generally, the NAS provides several distinct functions: It
terminates the point-to-point communications session of the remote user, validates the identity of that
user, and then serves that user with access to the network. Although most remote access technologies
bundle these functions into a single device, L2TP separates them into two physically separate devices:
the L2TP Access Server (LAS) and the L2TP Network Server (LNS).
As its names imply, the L2TP Access Server supports authentication, and ingress. Upon successful
authentication, the remote user’s session is forwarded to the LNS, which lets that user into the network.
Their separation enables greater flexibility for implementation than other remote access technologies.

Implementation Topologies
L2TP can be implemented in two distinct topologies:
• Client-aware tunneling
• Client-transparent tunneling
The distinction between these two topologies is whether the client machine that is using L2TP to access
a remote network is aware that its connection is being tunneled.

Client-Aware Tunneling
The first implementation topology is known as client-aware tunneling. This name is derived from the
remote client initiating (hence, being “aware” of) the tunnel. In this scenario, the client establishes a
logical connection within a physical connection to the LAS. The client remains aware of the tunneled
connection all the way through to the LNS, and it can even determine which of its traffic goes through
the tunnel.

Client-Transparent Tunneling
Client-transparent tunneling features L2TP access concentrators (LACs) distributed geographically
close to the remote users. Such geographic dispersion is intended to reduce the long-distance telephone
charges that would otherwise be incurred by remote users dialing into a centrally located LAC.
TheremoteusersneednotsupportL2TPdirectly;theymerelyestablishapoint-to-pointcommunicatio
sessionwiththeLACusingPPP.Ostensibly,theuserwillbeencapsulatingIPdatagramsinPPPframe
The LAC exchanges PPP messages with the remote user and establishes an L2TP tunnel with the LN
through which the remote user’s PPP messages are passed.
The LNS is the remote user’s gateway to its home network. It is the terminus of the tunnel; it strips of
all L2TP encapsulation and serves up network access for the remote user.

Adding More Security
As useful as L2TP is, it is important to recognize that it is not a panacea. It enables flexibility in
delivering remote access, but it does not afford a high degree of security for data in transit. This is due
in large part to the relatively nonsecure nature of PPP. In fairness, PPP was designed explicitly for
point-to-point communications, so securing the connection should not have been a high priority.
An additional cause for concern stems from the fact that L2TP’s tunnels are not cryptographic. Their
datapayloadsaretransmittedintheclear,wrappedonlybyL2TPandPPPframing.However,additional
security may be afforded by implementing the IPSec protocols in conjunction with L2TP. The IPSec
protocols support strong authentication technologies as well as encryption.

Summary
VPNs offer a compelling vision of connectivity through foreign networks at greatly reduced operating
costs. However, the reduced costs are accompanied by increased risk. L2TP offers an open standard
approach for supporting a remote access VPN. When augmented by IPSec protocols, L2TP enables the
realization of the promise of a VPN: an open standard technology for securing remote access in a
virtually private network.