Performance

The first experiment of this section aims at learning a universal policy, that is, the “best” response for all possible instances of the network model using the topology consisting of 30 learning agents (5 teams). The system is trained for100000episodes; since this chapter investigates scalability and the number of agents will be increased, we leave the system to train for more episodes compared to the number of training episodes in the previous chapter. At the start of each episode a new network instance is generated i.e. we re-choose the legitimate users, attackers and their rates according to the model (described earlier in section 4.3). Attackers use the constant-rate attack pattern.

Our approach uses a linearly decreasing-greedy exploration strategy with an initial= 0.3, and the learning rate is set toα= 0.05; since the number of training episodes was increased to 100000, we use slightly lower values for bothandαthan those in the previous chapter. These parameters, despite not being optimised, are set in accordance to the reinforcement learning literature (Sutton & Barto 1998). As in the previous chapter, our goal is to investigate the new proposed CTL design and evaluate its performance and potential deployability. Parameter tuning is performed in Chapter 5 which deals with online learning.

Exploration is stopped after the80000th episode. Each episode runs for1000time steps and both legitimate and DDoS traffic starts at 0. The system training attempts to learn a universal policy for all network instances. MARL, Comm, ITL+Comm and CTL+Comm refer to the basic approach (described in section 3.2), hierarchical communication (described in section 4.1), independent and coordinated team learning (described in section 4.2) approaches respectively.

We plot the global reward at the last time step of each episode, averaged over ten repetitions (i.e. over ten universal policies). Training results for the four reinforcement learning-based ap- proaches are presented in Figures 4.3a - 4.3d. It is clear that the system learns and improves over time until it finally converges. Because of the probabilistic nature of the environment, different rewards will be yielded in each different episode and hence the shape of the graphs.

We evaluate our approach against the Baseline and the popular AIMD router throttling (Yau et al. 2005) approaches. Each reinforcement learning agent uses its policy learnt during the system training. For evaluation we randomly sample 100 episodes each of a duration of60time

Section 4.4 Offline Learning Experiments 89

(a) MARL (b) Comm

(c) ITL+Comm (d) CTL+Comm

Figure 4.3: Offline Learning for30RLs

steps. Legitimate traffic is started att= 0and stopped att= 60. Attack traffic lasts for50time steps; it is started att= 5and stopped att= 55. All approaches use an upper boundary for the victim ofUs= 62and the Baseline and AIMD also use a lower boundary ofLs= 56.

Evaluation is performed using the following five different patterns of attack dynamics of different sophistication levels. We emphasise that these patterns have not been previously seen by our system during the training period. The attack dynamics are described below (Mirkovic & Reiher 2004):

• Constant-rate attack: The maximum rate is achieved immediately when the attack is started.

• Increasing-rate attack:The maximum rate is achieved gradually over25time steps.

• Pulse attack:The attack rate oscillates between the maximum rate and zero. The duration of the active and inactive period is the same and represented byD. We create two different attacks namely the high and low pulse attacks which have a period ofD = 5andD = 2

Figure 4.4: Performance for Constant-rate Attack

• Group attack: Attackers are split into two groups and each group performs simultan- eously a different attack pattern. We choose the first group to perform a constant-rate and the second to perform a low pulse attack.

Figures 4.4 - 4.8 show the average performance over the 100 episodes for the 10 policies learnt during the system training, for the five types of attack rate dynamics respectively; error bars show the standard error around the mean. Performance is measured as the percentage of the legitimate traffic that reached the victim throughout an episode; the higher the value on the graph the better. For completeness, the figures also show the percentage of the DDoS traffic (note that the problem is caused by the attack volume and not the traffic content).

As expected, the AIMD approach outperforms the Baseline approach in all five scenarios. Furthermore, the basic MARL approach fails to perform well in this large-scale domain. The Comm approach offers great benefit to the system’s performance over the basic MARL approach. Recall that the goal of an agent in offline learning is to obtain a universal policy, that is, the “best” response for all possible situations the network might be found in. Hierarchical communication helps each agent to distinguish between the different situations and therefore to learn a better policy for similar ones.

ITL+Comm and CTL+Comm further improve the system performance as they use task de- composition and team rewards. As expected, CTL+Comm performs better since it allows a team

Section 4.4 Offline Learning Experiments 91

Figure 4.5: Performance for Increasing-rate Attack

Figure 4.7: Performance for Low Pulse attack

Section 4.4 Offline Learning Experiments 93

Figure 4.9: Scalability of MARL

leader’s load to exceed its hypothetical boundary as long as the victim router’s load is below its limit.

Most importantly, CTL+Comm outperforms the Baseline, AIMD and all other learning-based approaches in all scenarios. Specifically, it outperforms the Baseline and AIMD approaches about 24-33% and 5-17% respectively. The AIMD approach suffers the most in the highly dy- namic scenarios of high pulse, low pulse and group attacks.

CTL+Comm outperforms AIMD for the following two reasons. Firstly because CTL+Comm learns a better behaviour during the training phase, and secondly because our proposed approach is more adaptable and responsive to the attackers’ dynamics. These are examined in section 4.4.3.