# On Cross-Layer Interactions for Congestion Control in the Internet
## 5 Performance Evaluation  
The purpose of the simulations was to verify how the most commonly used TCP congestion control algorithms interact with the state-of-the-art Active Queue Management approaches. Simulations were carried out using ns-3, release 3.30 with added TCP Cubic [32], and TCP Compound [33] implementations. The simulation network was configured based on the topology and scenarios presented in [17] and described in detail below. The standard dumbbell topology with two routers and six nodes (N1-N6) was used (see Figure 1). The simulations were carried out using two basic scenarios presented in [17], i.e., Data Center and Access Link. The Data Center scenario modeled a high-speed network with low delays. All links had a capacity of 1 Gbps. Links between nodes N1, N2, N4, and the router had 10 us delay, links between nodes N3, N5, N6, and the router had 100 delay; the central link introduced no delay (0 ms). The average RTT of all nine connections was around 200 us. The recommended buffer size should absorb 10 us of traffic which, assuming 1500 B sized packets, was 900 packets. In the Access Link scenario, every link rate was set to 100 Mbps, the propagation delay between routers A-B was set to 2 ms. The one-way propagation delays were distributed from 0 ms to 75 ms. The RTT values for all connections are gathered in Table 1. According to the work in [17], this is a representative distribution of RTTs on the central link. The buffer size was set according to the BDP rule (using the mean RTT), which was 900 packets. TCP sources were located in nodes N1-N3, nodes N4-N6 acted as data recipients. All TCP flows used 1500-byte packets. The total number of TCP flows was set according to the assumed congestion scenario. Three congestion scenarios were considered: the uncongested network (9 flows), mild congestion (90 flows), and heavy congestion (900 flows). Flows were started randomly during the first three seconds. Simulations were carried out for 100 s.  
For every scenario, the simulations were performed for a single TCP version (common TCP) or all TCP versions simultaneously (mixed TCPs). In the common TCP scenario sets, the same TCP congestion control mechanism for all nodes was used throughout the simulation. Simulations were carried out for each TCP algorithm in combination with every buffer management strategy and every congestion state. In the mixed TCPs scenario \begin{table}
\begin{tabular}{c c c c c c} \hline N1–N4 & 8 & N2–N4 & 32 & N3–N4 & 58 \\ \hline N1–N5 & 78 & N2–N5 & 102 & N3–N5 & 128 \\ \hline N1–N6 & 154 & N2–N6 & 178 & N3–N6 & 204 \\ \hline \end{tabular}
\end{table} 
Table 1: Values of RTT [ms] between all node pairs. Figure 1: The dumbbell topology. set, node N1 used TCP New Reno, node N2 used TCP Compound, and node N3 used TCP Cubic. Simulations were executed for each AQM and DT pair under varying congestion state. Although PIE and CoDel should not require any configuration, according to the work in [29], even in 100 Mbps, the default reference value of delay \(ref_{del}\) set to 20 ms results in PIE acting as a simple DT, which was verified through the simulation. Therefore, in 1 Gbps and 100 Mbps networks, the target delay variable of PIE was set to 2 ms and 5 ms, respectively. Moreover, to provide comparable results for CoDel in the Data Center scenario, the sojourn time was also set to 2 ms.  
To compare the obtained results, the overall throughput of the bottleneck link was monitored, as well as the mean queue length, the standard deviation of queue length, and the drop ratio at router A. The simulations lasted for 100 s, but the first 30 s were used for stabilization. Therefore, the queue measurements were taken from the 30 th second of the simulation. The mean throughput for each flow was also calculated. [PERSON]'s index was computed to evaluate the fairness of link sharing between competing flows. The Jain's index calculated for different flow combinations is interpreted as inter-, intra-, or RTT-fairness.