The concept of "defense in depth" is over 1,000 years old (so we are sure that Al Gore did not invent it). I first came to fully appreciate the concept as a cadet at West Point. We studied the forts, cannon positions and forward lookouts that surround the main defenses at West Point. The structures combined with masterful use of the existing terrain created a tremendous obstacle to any Britsh ship attempting to head South from Canada into New York harbor. In doing this, George Washington's army was able to reduce the number of fronts it had to defend.
This basic and powerful concept can be applied to network security as long as the defenses are properly integrated into the "terrain" of the network. As new technologies appear in the network being secured, the defenses must be reexamined to ensure their effectiveness. As with the network of structures that comprise the defense system around West Point, each network security element is not in itself a total solution. Each element of a network security system must work together to cover the gaps and weaknesses of the other. The entire system then acts in concert to stop all manner of mulit-pronged, multi-tiered and multi-threat acts.
We must remember the Alamo. I live in Texas now and have visited and studied the defenses at the Alamo. The last stand at the Alamo was tragic, but it serves to illustrate this point: once the single wall of defenses is breached, the fortification will fall. This is analogous to the single-layer network security perimiter. The basic principle of that type of defense is that the single battlement will stop all threats. The major flaw with this concept is that if the threat can find one single weakness or hole, then there is no stopping a complete breach of the entire defense. Just like the German army breeched the Maginot Line in France during WWII, any defense the relies on a single layer and is inflexible to adapt to the attack will fail.
This historical perspective on combat techniques and the analogy between network secuirty and military defense serves as a lesson to those to would design a network security system. Here is a list of main lessons we must apply:
1. The system must have multiple layers.
2. Each layer must be mutually supportive.
3. The entire system must integrate into the technology "terrain".
4. The overall architecture must be flexible to adapt to changes.
