by Michael H. Behringer, Cisco Systems
End-to-end security relies on protocols and mechanisms that are implemented exclusively on the endpoints of a connection. The most typical example is an HTTPS connection (based, for example, on Transport Layer Security (TLS) ) to a web server; IP Security (IPsec)  can also be used for end-to-end security, as was initially proposed as a default connection mechanism for IPv6.
There is a perception that end-to-end security is sufficient as a security solution, and that network-based security is obsolete in the presence of end-to-end security. This article outlines why in practice end-to-end security alone is not sufficient, and why network-based security is also required.
The traditional definition of an endpoint is a client or server. In this definition end-to-end security starts on the client and ends on the server. Given the multitude of applications running in parallel on an operating system, and given increasing virtualization, this definition is usually no longer precise enough. The operating system can establish a security association on either the session or application level. It can also be terminated on a front end, on behalf of numerous servers, as is the case in many TLS  deployments.
Because the main goal of this article is to understand why the network has a role to play in security, the precise definition of an endpoint is not relevant here. Abstractly seen, an endpoint is an entity that communicates over a network with another entity. This definition, albeit vague, is sufficient for the discussion at hand.M.
End-to-End Security Is Fundamental
Security on the endpoints (client-server, or client-client for peer-to-peer) is an absolute requirement for secure communications. Such a solution contains the following components:
For full end-to-end security, all of these components must be secure. In networks with end-to-end security, both ends can typically (depending on the protocols and algorithms used) rely on the fact that their communication is not visible to anyone else, and that no one else can modify the data in transit. End-to-end security is used successfully today, for example, in online banking applications. Correct and complete end-to-end security is required; without it, many applications such as online banking would not be possible.
However, a single security problem in any of the components can compromise the overall security for a connection. Today, most critical are implementation problems on endpoints, as well as human errors, specifically in handling exception cases.
Practical Shortcomings of End-to-End Security
Solutions that rely exclusively on end-to-end security have many potential problems, which fall into two broad categories: those that affect the end user and those that affect the network operator (the service provider, or the enterprise network operator, for example).
The End-User View
As reports on online crime and fraud demonstrate very clearly, even in the perceived presence of end-to-end security it is difficult to ensure that none of the components mentioned previously is "broken." Although protocols and algorithms in use tend to be secure and reliable, the main problems lie in the two main areas of endpoint security (secure implementation component) and lack of user education (secure operation component).
Endpoint security concerns include the presence of malware, as well as bugs in software. Even security professionals have difficulty determining whether a PC contains malware. Such malware can control the connection before it is secured, thereby achieving the ability to see the data, as well as potentially change it in real time. Although endpoint security software such as antivirus solutions as well as zero-day prevention solutions provides good security, they are not always installed, and antivirus software is often not up-to-date. Users also can temporarily disable the solutions. Therefore, the presence of malware remains a security concern. Bugs in software are also relevant, for example in the web browser or the operating system.
The lack of user education is the other important concern on the endpoint: Users must know how to identify a secured connection, for example by the little padlock in a web browser (although not even this security mechanism is completely secure). They must also know how to deal with exceptions such as expired or invalid certificates. Most average users do not entirely understand all these details, leading to breaches of security.
The Network Operator View
In the early days of IPv6 it was postulated that the protocol would come with IPsec end-to-end security built in and always "on," thereby eliminating all security problems. This assumption turned out to be wrong, because many problems remain on the network side—for example, general problems with end-to-end security—and they apply to all variants, such as IPsec, TLS, or Secure Sockets Layer (SSL).
Today, most enterprise network operators as well as service providers are skeptical about the ubiquitous use of end-to-end security solutions. The fundamental concern is that the endpoints generally cannot be trusted. The network operator, whether enterprise, university, or service provider, has an obligation to enforce certain policies on the endpoint, for example, to ensure that it does not spread worms, send spam mail, or attack servers. If, however, network operators cannot "see" the traffic of an endpoint because it is end-to-end secured, then they cannot comply with their obligations to control the endpoints. From a network operator's perspective it is therefore not generally desirable to use end-to-end security for all communications, but only for those that really need it.
Why Network-Based Security Is Essential
There are many examples where network-based security is essential, and where end-to-end security solutions not only do not help, but may actually present an additional problem. In all those cases it is essential to have strong network-based security solutions in place. Some examples explain this in more detail.
The Service Provider with DSL Customers
A service provider with DSL customers needs to control its users' traffic in various ways. However, the provider has no control over the endpoints, because those are the customers' property. Because they also cannot force their customers to use appropriate security software, there is always a certain percentage of infected PCs on any given service provider's network. Critical service provider concerns follow:
The Service Provider with Customers Under Attack
Service providers may also be required to help their customers when they are under attack. DoS attacks illustrate why endpoint security may not be sufficient, and network-based security is required. Under a DoS attack, a web server, for example, may receive more traffic than it can handle. Such attacks can also overload network resources, such as subscriber lines or routers; therefore, endpoint security is not able to solve such attacks. Massive overprovisioning would be the only way to handle DoS attacks, but this approach is commercially not generally feasible. Network-based solutions based on flow analysis and selective discard of flows are required to help in such situations.
The Enterprise Network
At first glance it seems that enterprises should have full control over the PCs in the enterprise. In such a case, it would be possible to rely completely on end-to-end security. However, this assumption is unrealistic. Numerous current shortcomings make this approach impractical today:
End-to-end security protocols and solutions are an essential cornerstone in network security. We cannot live without them. However, it is unrealistic in today's networks to assume that end-to-end security solutions alone will suffice. The fundamental underlying problem is that typically the network operator, where a PC is attached, has a need and often an obligation to monitor the behavior of the endpoint, and to control malicious activities emerging from that PC. All solutions to control endpoints, however, are by definition network-based. Therefore, network-based security mechanisms are also an essential component of overall network security: Overall security requires both endpoint security and network-based security.