Home > Articles > Networking

Virtual Private Networks

In this chapter, we will agree on a definition of VPN and acknowledge that even with our definition, there can still be disagreement about whether a particular technology is a VPN or merely a secure application. We will see how this definition applies to PPTP and L2TP and examine the extent to which those protocols provide a robust VPN.
This chapter is from the book

5.1 Introduction

In Part 2, we discuss virtual private networks (VPNs). Chapter 4 had some examples of VPNs, but we were interested mainly in their tunneling aspects and didn't dwell on their security and authentication features.

In this chapter, we define VPN and briefly revisit the VPNs from Chapter 4. In the rest of Part 2, we study several types of VPNs, see how they are used, and take note of their strengths and weaknesses. As we shall see, these VPNs can operate at any layer in the TCP/IP stack. As usual, we will be less concerned with the administrative details of configuring the VPNs than with developing an appreciation for the protocols themselves and the manifestation of those protocols on the wire.

Before beginning our discussion of VPNs, we should agree on a definition for them. We already have an implicit definition from our study of MPLS VPNs in Chapter 4. We might say that according to that definition, a VPN is a method of using tunneling to build a private overlay network on top of a public network in such a way that the security of the private network is equivalent to that provided by leased lines. But this definition suffers from a lack of precision as to the meaning of "security equivalent to that provided by leased lines" and is a bit too general for our purposes.

Instead, let us say that a virtual private network is an overlay network built with tunnels in which the tunnel payloads are encrypted and authenticated. Given that we use robust encryption and authentication, such a VPN would certainly provide security as good as or better than that provided by leased lines, so this definition is consistent, if more restrictive, than that for MPLS VPNs. The underlying notion of both definitions is that we are trying to create the illusion of a private network while using a public network, such as the Internet.

It's worth dwelling, for a moment, on the differences between a "real" private network and a virtual private network. As a first approximation, we could say that real private networks provide security by physical separation of the underlying communication media. Separate leased lines are dedicated to the network, and these leased lines carry traffic only for that network. This means that short of a physical wiretap, an attacker does not have access to network traffic.

As we saw in our study of MPLS VPNs, the security of real private networks does not necessarily depend on the actual separation of physical media. We can also achieve segregation of network traffic through routing, or by multiplexing several data channels onto a single physical cable.

MPLS VPNs are an example of providing a private network by using routing to ensure that a private network's traffic is delivered only to intended recipients. Because the assignment of the MPLS label, and thus the route, takes place within the MPLS cloud, an attacker on the edge of the MPLS network has no way to capture traffic from another private network or to inject packets into it.

In a typical transcontinental leased-line deployment, the customer is provided with a partial or whole T1 line. Even if the entire T1 line is dedicated to a single private network, the traffic from the user's T1 line is multiplexed onto a higher-bandwidth backbone, such as a T3 or OC4 line, for transport across the continent to the other endpoint's T1 line. Thus, traffic from the private network is carried on the same physical media as traffic from other networks. Nevertheless, from the point of view of a user of the private network, this data is inaccessible and as a practical matter does not exist.

Despite the realities of the previous paragraphs, our normal conceptual model for a leased-line connection is a dedicated wire from one site to another. This model includes the notion of a physical connection, and when we're told that the network is down, we imagine that a physical event, perhaps involving a backhoe, has taken place. A virtual private network, on the other hand, is just that: virtual.

As with a TCP connection, a VPN's tunnel is a purely notional construct consisting of shared state at the tunnel endpoints. When told that the VPN is down, our first thought is not that a cable has been cut but that the shared state has become desynchronized. Once one of the VPN's packets enters the Internet, it is like any other IP datagram in the Internet. A malefactor can use a flooding attack to cause a router to drop it or can inject phony packets into the VPN by forging some of the packet's header fields. To protect itself from these and other attacks, a VPN relies on encryption and authentication to secure its data.

The advantages of a VPN over an actual private network should be clear. Instead of expensive leased lines or other infrastructure, we can make use of the relatively inexpensive, high-bandwidth Internet. More important in many instances is the ubiquity of the Internet. In most developed areas, access to the Internet is readily available without special provisioning or long waiting times. Given a VPN with robust cryptographic primitives and protocols, we could argue that a VPN is, in fact, more secure than a dedicated leased line, even if we accept our conceptual model of such a line as real.

In our definition of VPN, we said that the tunnel payload is protected by encryption and authentication. As we study the various types of VPNs, we will see that the meaning of payload depends on the class of VPN. In Chapter 6, for example, we study SSL tunnels, which operate primarily at the application layer. Thus, the payloads that they encrypt and authenticate are usually application data. At the other end of the spectrum, tunnel-mode ESP in IPsec (Chapter 12) operates at the network layer, so its payloads are entire IP datagrams.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020