# JSSTR: A Joint Server Selection and Traffic Routing Algorithm for the Software-Defined Data Center
## 6 Evaluation
### Evaluation Results and Analysis  
(1) Routing path hop degree: The routing path hop degrees of JSSTR, WLC-GA and Travs are calculated and shown in Figure 5. From that figure, it can be observed that JSSTR performs wonderfully when calculating the routing path. More precisely, for WLC-GA, the mean routing path hop degree was about 2.248, while for JSSTR, the mean routing path hop degree was about 1.850, which means that compared with the WLC-GA, JSSTR can decrease routing path hop degree by about 22 percent. For the Travs method, the routing path hop degree was 1.848, which is very close to that of the JSSTR  method. The above results were caused by the fact that the WLC-GA algorithm calculates the destination server without considering its impact on routing path calculation. More specifically, when using WLC-GA, the destination server is calculated by the WLC algorithm, and the weighted server status is the only factor considered in this process. After the destination server has been chosen, the GA algorithm can only calculate the routing path between that server and the user. This means that even it there is one more efficient routing path in the underlying network, if the server connected to that path is not chosen as the destination server, that path will not be taken as the final routing path. In contrast, when using the JSSTR algorithm, the destination server and routing path are calculated jointly. Thus, the destination server will be calculated based on the server status and network status. Therefore, the length of the routing path made by WLC-GA is bigger than that of JSSTR. Additionally, from that figure, it can be seen that the routing path hop degree of the JSTR algorithm is almost the same as the Travs method, which indicates that the JSSTR algorithm can almost get the optimal result as the traversal-based method. In addition, as the routing path degree made by JSSTR and Travs is less than WLC-GA, JSSTR and Travs can accept more users than WLC-GA. In the evaluation, when using JSSTR and Travs, the maximum acceptable number of users is 180. As mentioned above, all servers can serve at most 200 users in this evaluation; thus, it is the upper limit of users that the network can carry. Once the number of arrived users is larger than 180, even if the servers can serve more users, the subsequent arrived users will not be served as the network cannot transfer more traffic. For WLC-GA, the maximum acceptable number of users is 147, which decreases by about 22 percent compared to JSSTR and Travs. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Usage** & **Parameter** & **Value** \\ \hline \multirow{3}{*}{Evaluation network} & Number of users & 200 \\  & Number of switches & 22 \\  & Number of servers & 16 \\ \hline \multirow{3}{*}{\begin{tabular}{c} User request generation \\ \end{tabular} } & The arriving process of users & \multicolumn{2}{c}{Poisson distribution} \\  & Distribution of intervals of users’ arriving time & Exponential Distribution with mean set as 1.939 s \\ \multirow{3}{*}{\
\begin{tabular}{c} User request generation \\ \end{tabular} } & Distribution of each user sending http get requests & \multicolumn{2}{c}{t-distribution with mean, standard deviation} \\  & & and degree of freedom set as 1.938 s, 0.245 s and 2.086 \\ \hline \multirow{3}{*}{Server capacity} & High-end server & \multicolumn{2}{c}{1 server that can serve at most 30 users} \\  & Mid-end servers & \multicolumn{2}{c}{2 servers, each server can serve at most 20 users} \\  & Low-end servers & \multicolumn{2}{c}{13 servers, each server can serve at most 10 users} \\  & Total capacity of all servers & \multicolumn{2}{c}{200 users} \\ \hline \multirow{3}{*}{
\begin{tabular}{c} SFLA \\ \end{tabular} } & Number of server frogs & 20 \\  & Number of memeplexes of each group & 2 \\  & \(maximum\_time\_loop\_num\) & 5 \\  & \(fitness\_diff\_threshold\) & 0.001 \\  & \(maximum\_evolution\_num\) & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Important parameters.