Results of Our Virus Simulation
Figure 1 shows our simulated network with 10,000 vulnerable hosts. At initial time T1, Code Red is released and continues to infect vulnerable hosts until saturation. At some arbitrary point after the virus has reached steady state (with all 10,000 hosts infected), the vaccine is released at time T2. After release at time T2, the vaccine begins to compete successfully with Code Red for vulnerable hosts.
In the simulation depicted in Figure 1, the vaccine has not been attenuated. Both Code Red and the vaccine have been set to send two random probes per time step. Thus, both Code Red and the vaccine are at equal strength. However, because the vaccine patches systems that it infects, the vaccinated hosts are no longer vulnerable to Code Red. Thus, the number of Code Red infections eventually drops to zero, leaving only immunized (repaired) hosts that are no longer vulnerable to future infection.
Figure 2 shows an alternate method for vaccination. In this case, the vaccine is automatically released when a new strain of Code Red appears in the wild and begins spreading. In practice, this automatically "triggered" vaccine can be achieved through honeynets set to monitor the Internet for new global infections. A honeynet is a laboratory network of computers set up to attract and study Internet viruses and hackers in the wild.  For the simulation depicted in Figure 2, the vaccine is not attenuated; it's full strength and spreads as rapidly as Code Red itself. In fact, the vaccine is identical to Code Red in every respect, except that the vaccine patches vulnerable hosts that it infects, thus repairing the hosts and leaving them immune to future infection.
As shown in Figure 2, the vaccine begins to spread immediately when triggered by the appearance of a new Code Red infection at time T1. Because the vaccine is full strength and because it confers immunity, it rapidly outpaces Code Red. Thus, the epidemic is contained and is quickly eradicated. Once a steady state is reached, only protected, immunized hosts remain.
Now suppose we attenuate the vaccine. An attenuated vaccine should be less damaging to network resources. In other words, it should consume less overall bandwidth than a full-strength vaccine. For example, a vaccine that spreads more slowly than Code Red might use less total bandwidth, but it would still confer immunity. Figure 3 shows the results of a vaccine that has been attenuated by 25%. As in Figure 2, this vaccine is automatically triggered at initial time T1 when a new infection of Code Red is detected in the wild.
In the simulation depicted in Figure 3, the immune response is not as rapid because the replication speed of the vaccine is attenuated by 25% and thus spreads more slowly. However, the triggered, weakened vaccine still ameliorates the Code Red epidemic, albeit more slowly than the full-strength vaccine depicted in Figure 2. As seen in Figure 3, the Code Red outbreak is partially blunted by the vaccine. Meanwhile, the vaccine spreads more slowly, consuming less network resources yet still ultimately defeating and eradicating Code Red.
The simulation depicted in Figure 4 shows a triggered vaccine that has been further weakened. The replication speed of this vaccine has been attenuated by 50% relative to the speed of the Code Red virus. In this case, the greatly attenuated vaccine causes even less blunting of the Code Red outbreak.
The previous simulations demonstrated the effect of vaccines on ameliorating a Code Red epidemic. As shown in these simulations, attenuating a vaccine results in delayed efficacy against a Code Red outbreak. However, does attenuating a vaccine confer the advantage of reduced overall network bandwidth consumption? Figure 5 demonstrates the effect of attenuation on total bandwidth. As the vaccine becomes more attenuated, both the rate of new message generation and the magnitude of total messages per time step are reduced. For example, a vaccine with 25% attenuation generates fewer messages per time step than a vaccine with 0% attenuation. Similarly, a vaccine with 50% attenuation generates fewer messages per time step than a vaccine with 25% attenuation. Thus, greater vaccine attenuation leads to reduced total bandwidth consumption in the network.
A more controversial use of the vaccine is to release it in the interim after the vulnerability is found, but before the first virus appears in the wild. This strategy is more controversial because it involves releasing a self-replicating vaccine on the Internet (and thus damaging some small percentage of systems) before any immediate threat from a virus has appeared. Based on historical trends, we know that after a new vulnerability becomes known, it's only a matter of time before a virus writer creates and releases a virus to exploit that vulnerability.
In the case of Code Red, such a "prophylactic" vaccine could have been released in the three-week interim between the discovery of the vulnerability and the release of the first Code Red virus to exploit it. As in biology, the goal is to vaccinate as much of the population as possible before an outbreak occurs. In the first simulation (Figure 1), reversing the order to release the vaccine before the virus ever appears would make all hosts immune to the virus. Thus, the outbreak would never occur, which is the optimal goal.
However, what effect does releasing the vaccine before an outbreak have on total network bandwidth consumption? In the simulation depicted in Figure 6, the bandwidth used by the prophylactic vaccine is compared to previous simulations. In this example, the prophylactic vaccine is attenuated by 50%. The prophylactic bandwidth curve is seen in the lower right corner of the graph. In other words, the prophylactic vaccine has a more gentle impact on the network than any vaccine released after a virus outbreak.