# An Analysis of MPTCP Congestion Control
## 7 MPTCP Congestion Control
### Experimental Validation of Simulation Results
Figure 19: LIA, OLIA and BALIA Throughput Share—Disjoint Paths with RTT1 = 50 ms and RTT2 = 150 ms.  
LIA performs reasonably well when the path has a low bandwidth delay product (both RTT and bandwidth is low). In this case MPTCP is almost able to fairly compete on the first path and is able to attain more than two thirds of the bandwidth required for fair sharing. BALIA is next best in attaining a reasonable proportion of the expected throughput. In these cases, CC performance is not heavily degraded as the second path RTT increases. This is primarily due to the queueing delay being larger than the path RTT, resulting in the actual difference in path characteristics being less pronounced than those configured. This is further evidenced with results for larger RTT (50 ms, 100 ms, 150 ms) and for larger bandwidth configurations, where the queueing delay has less of an impact on the path RTT. In all cases, the lack of aggression in growing _cavnd_ of the coupled CC algorithm results in the first subflow not being able to acquire its fair share of the available bandwidth. This is particularly pronounced for large BDP and/or large RTT differences. In these cases, the current coupled CC algorithms can result in the first subflow giving up almost all of its fair share of bandwidth on the shared path. Confirming the simulation results, the experiments show that MPTCP coupled CC is unable to attain its share of bandwidth in disjoint path scenarios, particularly when path characteristics are not similar. We next examine whether the problem of diverse path characteristics is also problematic in shared bottleneck scenarios, a situation that the coupled CC algorithms were particularly designed to address. The experiments are repeated using the topology in Figure 20. All settings are the same as for the previous set of experiments with regards to bandwidth and RTT configurations, the only difference is that the two MPTCP subflows are now sharing a bottleneck with the TCP flow. For this scenario, we would expect the TCP flow to attain half of the available bandwidth, and the MPTCP subflows to share the remaining half of the available bandwidth. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**Path Bandwidth—24 Mbps**} \\ \hline  & **RTT Path 1** & \multicolumn{3}{c}{**10 ms**} & \multicolumn{3}{c}{**50 ms**} \\ \cline{2-7}  & **RTT Path 2** & **10 ms** & **20 ms** & **30 ms** & **50 ms** & **100 ms** & **150 ms** \\ \hline \multirow{2}{*}{LIA} & TCP & 151.3\% & 186.7\% & 192.3\% & 192.8\% & 196.8\% & 196.7\% \\ \cline{2-7}  & MPTCP Subflow 1 & 46.8\% & 11.5\% & 5.8\% & 5.0\% & 1.4\% & 1.4\% \\ \hline \multirow{2}{*}{OLIA} & TCP & 192.6\% & 192.8\% & 192.9\% & 191.9\% & 196.4\% & 196.0\% \\ \cline{2-7}  & MPTCP Subflow 1 & 5.4\% & 5.4\% & 5.2\% & 6.0\% & 1.7\% & 2.1\% \\ \hline \multirow{2}{*}{BALIA} & TCP & 171.5\% & 182.9\% & 198.1\% & 189.8\% & 194.9\% & 194.5\% \\ \cline{2-7}  & MPTCP Subflow 1 & 26.4\% & 15.2\% & 12.2\% & 7.8\% & 3.2\% & 3.6\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Share of 24 Mbps bandwidth attained at Switch One-Disjoint Paths. Figure 20: Experimental Topology for MPTCP Shared Path. Figure 21 displays the box plot of throughput achieved for each of TCP, MPTCP subflow 1 and MPTCP subflow 2 for a bandwidth of 24 Mbps and RTT of (50 ms, 100 ms). In all cases we can see that the coupled CC algorithms preferred the path with lower RTT, with minimal throughput attained on the larger RTT path. Despite this, the combined throughput of both MPTCP subflows was still lower than that attained by TCP for all three algorithms. In this case, loss events on the path with higher RTT are impacting on the ability for _cvond_ growth on the path with lower RTT and therefore the opportunity for MPTCP to attain a fair bandwidth share. This plot is similar for all throughput plots where the RTT difference is high. Results for all experimental configurations are recorded in Tables 4 and 5.